300b lovers

If you do direct-coupling correctly, it can be very reliable. I don’t think it wise to direct couple throughout the circuit though!

The advantage of a direct-coupled cathode follower driver tube are several. First, there is more current available to provide linearity if grid current is produced by the output tube(s) (which allows for class A2 or A3 operation since both of those classes produce grid current), and no worries about Miller Effect, so very wide bandwidth (+30MHz!) is very easy (we bandwidth limit our OTL amps in the voltage amplifier section). The stability of the circuit is very high- in our OTLs (where one driver tube is controlling many triode power tube grids), bias adjustment only needs checking once every 6 months or so; often no adjustment needed. Distortion is kept in check in a way not possible if a cathode follower with coupling cap were used. We went to this circuit because its more reliable and prior to its introduction to the marketplace, OTLs generally had a reputation for being unreliable. This approach was key to making OTLs as reliable as any other tube amplifier; one of the reasons we are still in business after more than 45 years.

How this might work in an SET is a different story- yes, you need a B- supply to really pull it off, but if you want to do it with transformers you’ll have to pay that price too, since decent interstage transformers are not cheap- in fact, several times more expensive than a B- supply.

The other advantages of using a direct coupled cathode follower driver are that the driver prevents output tube conduction until the driver has warmed up and stabilized. This is likely not a concern when DHTs are used, but it is if indirectly heated, since cathode stripping can occur if the B+ is applied while the power tube is warming up.

Another advantage, probably the big one, is that small value coupling caps can be used between the voltage amplifier and the driver tube. Since coupling caps have inductance (since they are wound) no matter what materials are used they will always introduce some coloration on this account. By minimizing their value while still allowing for very wide bandwidth, the coloration they might cause is minimized and the inductive influence octaves above the audio band.

If course you could use an interstage transformer for this latter function as well; the advantage being that’s the least expensive place one could be used and most likely to win the most performance from the transformer.

At any rate, by using this approach you wind up with a very simple circuit that offers excellent linearity. One idea I’ve had for a while for an SET is taking advantage of the B- supply and building a differential amplifier for the input voltage amplifier. This would allow a proper balanced input as well as reducing distortion in that gain stage due to harmonic cancellation.

Of course if you already have a fully differential/balanced amplifier design from input to output, the advantages multiply.

We’ve been building a balanced preamp with a direct-coupled output (for which we have several patents) that uses servo control to prevent DC offsets at its output. The inputs of the preamp are direct coupled as well, but coupling caps are used between stages. The DC servo has proven to be one of the most reliable aspects of the circuit, which has been in production since 1989. During that time we simply have not seen any servo failures! The output of the preamp uses a Circlotron, like our OTLs, so there are no excessive voltages produced as the preamp warms up.

The obvious advantage is a lack of coloration at the output of the preamp (most tube preamps use a large coupling cap, from which, as pointed out earlier, colorations are unavoidable) and very wide bandwidth; to the latter point the direct coupling allows for reproduction of extremely low bass that most tube preamps won’t even acknowledge. Since the preamp uses no feedback, controlling phase shift can only be done by having bandwidth down to 1/10th the lowest frequency to be reproduced, so its good to 2Hz. If phase shift is present, the ear perceives it as a lack of impact, even though the preamp might otherwise be perfectly flat at 20Hz. By direct coupling that problem and the often tubby bass that can occur are both avoided.

So while direct coupling (with or without a servo) might seem tricky, it offers great advantages and can be extremely reliable if properly designed.

@stephenr First off, thanks for you polite post on this thread. I bet your 12B4 preamp sounds great. I have tried removing the Raven preamp and going direct to the amps from the Lampi Pacific DAC via xlr. It sounds very good. It doesn’t sound nearly as good as with the Raven active preamp in the chain though. Over the years I have experimented with various passive preamps built with top notch volume controls and none were as satisfying as a really high end active tube preamp in my system to my ear.

As for the direct coupling, no I didn’t experiment with it in this project. I have heard it in other amps. As Lynn has described above, there were reasons for avoiding it in this particular project. I am sure you can make it work well in your amp though and I suspect it sounds very good.

Don and I are moving towards production of the Blackbird 300B amplifier. We’re now using triode-connected KT88 drivers running at twice the current (56~64 mA per KT88) and one-third the plate impedance (Rp = 700 ohms) of the previous drivers. Not only that, owners will be free to use matched pairs of their favorite 6L6, KT66, KT77, 6550, or KT88’s, with no bias adjustments needed.

The tube lineup is: 6SN7 input, KT88 drivers, 300B outputs, with VR105 shunt regulators. As before, fully balanced Class A from input to output. Special-design Cinemag input and interstage transformers, with custom Monolith output and power transformers. 18"/457mm full-width monoblock chassis with two isolated audio-circuit and power-supply sections, a slow-start circuit, surge protection, and a 12V trigger option. (No AC power goes to the front panel, just DC trigger voltage for the slow-start circuit.)

Don and I anticipate Spatial Audio (of Salt Lake City) will start manufacturing in November 2023.

Amazing to watch the arc of the development and completion of these stellar monobock amps.  Just wonderful.  

Don and I were talking on the phone today, and I realized that, of the four to five key people involved, we might have two centuries of professional design experience in audio. I think Don and I might have a century; I filed my initial Document of Disclosure for Shadow Vector in 1973, and Don’s been doing vacuum tube amps since he was sixteen.

And the development arc has been interesting: the PP Karna 300B of 2003 has merged with Don’s PP KT88 Kootenai amplifier.

The Kootenai is no slouch: a classic Mullard circuit (as used by Marantz and others) with 6SN7 input and drivers, PP KT88’s biased into near Class A, advanced B+ regulators, all on a compact stereo chassis. A clear step up from restored Marantz, McIntosh, and Citation II amplifiers, and the best Golden Age style amp I've heard.

@whitestix 

Amazing to watch the arc of the development and completion of these stellar monobock amps.  Just wonderful

Agreed! 
I wish them the upmost success. It been marvelous following this exceptionally interesting and educational thread.

Charles

Hi @lynn_olson ​​​​@donsachs 

What is changed in sound with the driver replacement from 6v6 to kt88?

On the one hand kt88 is much more powerful and low output impedance tube. On the other hand 6v6 is more linear. Did you try kt66 that is more linear and musical vs kt88.

The self-bias cathode resistor lets you choose anything between 6L6, KT66, 6550, and KT88. So far, the KT88 is winning, but the KT66 is very good also. We're shipping them with a matched quad of KT88's, but the user can use any matched quad in that family ... including NOS unobtainium tubes if they choose, because the operating point allows a life in excess of 10,000 hours.

I was reflecting on this approach vs simply using them as output tubes. Thanks to the gain of the 300B's, they're only using 1/3 to 1/4 of their potential swing, and the 300B's completely isolate them from the speaker load. So the only "load" is the 60 pF Miller capacitance of the 300B grids, not a loudspeaker. This puts them in the most linear operating region since they are essentially unloaded triode-mode power tubes, very different than the heavily loaded Class AB ultralinear of classical Golden Age amplifiers.

Good to see someone designing this way, I see too much premature tube death from a lot of high end audio designs.

If we chose, we could have a switch that would alter the quiescent current for the driver section, so 6V6’s could be accommodated as well. All of these tubes have the same pinout, but the optimal current for the 6V6 is in the 25~30 mA range, while the 6L6 and KT88 are happiest in the 50 to 65 mA range. (By comparison, the optimal range for a 6SN7 is 8 to 10 mA per plate.)

If the switch is in the wrong position, the 6L6 and KT88 would be under-biased and have increased distortion, but no harm done. If it is the wrong position with 6V6’s installed, though, they would burn up in minutes, and if one shorted, would take out the interstage transformer as well. So a visit back to the factory, all from one user mistake. You don’t want to give the user the power to damage the amplifier, just from a single switch setting.

A limited range switch to trim between 6L6 and KT88 would not harm either (in the wrong position), so that’s an option. But optimizing current for the KT88 still gives plenty of current for the 6L6, and we’ve found these types aren’t all that sensitive to current settings, just so long as there is enough.

A general rule-of-thumb in vacuum tube design is setting the plate dissipation somewhere between 50% and 75% of max rating, with 65% to 70% usually considered a good balance between tube life and overall performance. Unfortunately, many KT88 amps take the tubes right through their ratings, so new types like KT120’s have been created for these amps.

Let’s do a little quick math to see how hard the driver is working, compared to an output tube. The gain of a 300B is 3.9, according to this Western Electric data sheet. The interstage transformer has a moderate gain of 1:1.2, so the net voltage gain of the output section is 4.68. In power terms, that’s a ratio of 21.9, or 13.4 dB.

The driver is working at 0.2137 times the voltage swing of the output stage, into an open load that only has 60 pF of capacitance, and only draws current when the 300B grids pass into the A2 region at 80 volts swing. The driver can push the 300B grids at least 20 volts positive, and surprisingly, the 300B remains linear in this region, with no visible transition when it goes from A1 (negative bias) to A2 (positive bias).

There are no charge storage effects, unlike conventional RC coupling, so the transition between A1 and A2 is seamless, and recovery is immediate on departure from the A2 region. In addition, if there are any nonlinear grid impedances from the 300B as the drive voltage goes up and down, it is swamped by the orders-of-magnitude lower plate impedance (700 ohms) of the driver section.

There are further subtle benefits of transformer coupling. The 300B grids do not go into the positive-grid region at the same time: they take turns. This means both driver tubes are available to drive whichever grid needs current, not just one. And the paired drivers aren’t just paralleled, but in a Class A balanced circuit, so most distortion is cancelled.

This is important with an output tube with distortion as low as the 300B; the driver should be as clean as possible, yet capable of peak-to-peak 200 volt swings. At 30 kHz (which is a 38 V/uSec slew rate).

Last but not least, although four driver tubes are required, they don’t have to be matched quads. Matched pairs are fine. It doesn’t matter if the drivers in one amp are at 50 mA each and the drivers in the other amp are at 55 mA each. The difference can’t be measured and won’t be heard.

So if you have a stash of priceless MO-Valve KT66’s, which were manufactured in matched pair sets for the amps of the day, go ahead and use them! They will be operated very conservatively (far more so than most power amps).

There are a pair of pin jacks on the top panel to check if plate voltages match. Measure across the pin jacks with a basic DVM, set the DVM to measure DC, and if you see 3 volts or less, you’re good to go.

I see Lynn has spilled the beans on the driver tube change.  I have listened to the 6L6 (Russian 6P3S-E), the KT66 (Shuguang black treasure KT66), and the KT88 (My stash of a quad of the now unobtanium Shuguang WEKT88).  Of course I have spent almost two years listening to the 6V6 as the driver before this.  In my system the KT88 wins hands down.  It has midrange and bass that hits you in the chest at moderate listening levels.  That is in a good way.  It is vibrant and rich and tonally correct.  It has all the highs of the other tubes, but sounds less thin.  If you put any of these tubes, including the original 6V6 in this amp and had never heard any other driver, you would still think it was the best or one of the best amps you had ever heard anywhere.  But the KT88 is just superb.  That doesn't mean that in someone else's system they wouldn't prefer a KT66.  I also have EL34 to try, but haven't bothered because I think it the worst of the octal output tubes of that ilk and never understood why anyone liked them.  Mine are Mullards too....    

Let's just say that with the KT88 you FEEL the music in a way you do not with any of the other driver tubes.  You sort of LIVE the performance.  It is quite striking, and the larger chassis of the final design, coupled with the very conservatively designed power supply, allows for the amps to easliy handle the roughly 14 watt dissipation of the drivers vs. the 7 watts of the original 6V6.  It still runs very cool for a class A 25 watt tube amp.

What I will say is the output section is basically a supercharged amplifier of the driver section sonics.  You clearly hear the nature of the driver tubes.  This is of course true in other amps, but I have never heard the effect like this.  Usually a driver tube change is audible, but not night and day.  Imagine all the things you love about your classic KT88 amp with far less distortion and that KT88 rich full sound on steroids.  You are inside the music in a way that none of the other drivers do in my system, in my living room.  You get that KT88 sound without all the nonsense of feedback, RC coupling, whatever.  Instead you get an effortless, breathless, KT88 rich sound with lightning fast transient response.  This is related to slew rate and I will let Lynn discuss that if he chooses.  What matters is how it sounds:)  So now you have an amp with all large plate tubes, no feedback, that is lightning fast and will produce 25 watts all day long with tons of current.  The previous 6V6 version has easily driven 85 dB speakers.  The KT88 version... :)

Just to be clear, this is my system so those of you wanting to get a feel for it can know what I am using:

Lampizator Pacific DAC with 46 DHT tubes run XLR.  Dac is modded to eliminate a cap between DAC board and output stage resulting in a very slight pop when it changes resolution at the beginning of a track.  I find this benign, and the clarity is increased.  Just saying so you know this is better than a stock Pacific.

This drives the Raven preamp and Blackbird amps, cabled with Paul's best Anticable XLR v. 5 something.....   Speakers are Spatial audio X5 with seriously updated crossovers.  I never heard the X5 wake up like this....

@Donsachs I'm interested in the specifics of the X5 crossover upgrades please.

I have a technical question for tube gurus @lynn_olson @donsachs @atmasphere

I put the intrastage transformer Hashimoto A-107 between the 6sn7 input and 6f6 driver tubes in my SET amplifier. The load resistor connected to the 6f6 grid was 520KOhm. I measured a square wave output and it had a big overshoot. I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and  47KHz.

Should I decrease the load resistor more to remove overshoot completely?

If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.

http://www.tube-amps.net/HP_A107.htm

I went through that with the Amity amplifier in the late Nineties. Nice square waves vs sonics. The amp sounded best with NO grid resistor, and the overshoot was a non-factor. Your amp may be different, of course. Keep in mind that music sources will never excite the overshoot, since it’s all in the far ultrasonic, and most studio mikes are all gone by 25 kHz.

The tube doesn't need that 510K resistor. The DC impedance of the IT is a few kohms at most, and that stabilizes the tube at low frequencies. If the driver is really unstable, maybe the 510K resistor helps, but that's usually the function of a grid stopper resistor, something quite different.

@tinear123 Hi.. write Cloud Sessions at Spatial audio lab and he will know what is in the crossover.  I am sure they can build something for you.

I just cannot resist.  This is a poor photo of the square wave on the Blackbird interstage transformer between the 6V6 and the 300b at 1 KHz and about 30% power (7 or 8 watts).  There are NO grid resistors or grid stoppers or networks of any kind.  The primary of the transformer is wired directly to the driver plates and the secondary is wired directed to the 300b grids.  The KT88 looks as good or better....  Lynn and I were quite impressed.  Dave Geren at Cinemag knows what he is doing.....   This is an all tube amp with no feedback anywhere.

Yeah, it sounds like it looks..... 

Now’s a good time to discus the difference between overshoot in feedback vs non-feedback amplifiers. Despite similar appearance on the scope, they are caused by completely different mechanisms.

* Overshoot in a feedback amplifier is quite malign, since it indicates the onset of oscillation, something that can destroy the amplifier and the speaker it is connected to. It is caused by the amplifier running out of phase margin, possibly the result of a reactive load, but also the result of design oversights in the feedback loop.

* Overshoot in a non-feedback amplifier is quite different. Now, it might be the result of high-transconductance tubes self-oscillating, but this can prevented by grid-stopper resistors and good layout practices. Normally, though, it is merely transformer overshoot, the result of phase shift at the edge of the passband, and has nothing to do with stability. That is what we are seeing here.

Something to be considered about ultrasonic behavior in the time domain: if there is no spectral content in the frequency range of the overshoot, it will never be stimulated in the first place. It never happens.

This is the difference between overshoot in a feedback amplifier and a non-feedback amplifier: in a feedback amplifier, it is a warning sign, like the LOW OIL light in a car. You ignore it at your peril. In a non-feedback amplifier, it has nothing to do with stability, since there is no feedback loop to induce oscillation. It is simply the behavior of passive parts, in this case, the input and interstage transformers.

As you can see, Cinemag has done a very nice job here. (Same photo as above, just tidied up a little.)

Hi @donsachs ,

Yes Dave Geren at Cinemag do great job!

I use Hashimoto A-305 IT between 6f6 (driver) and 300B. The square wave was almost perfect at 10KHz, 20KHz, 30KHz. I use an 80K Ohm grid resistor on 300B. But I’m not sure this resistor is needed! The -3dB frequency response is from 6Hz to 95KHz. This IT is extraordinary on measurement and sound.

For 6sn7 I use Hashimoto A-107 that is not as perfect but has more primary inductance that is essential for higher input impedance 6sn7.

Hashimoto A-305 is designed specially for SET, but A-107 is universal IT that can be phase splitter or SET 1:1 or SET 1:2.

I just installed A-107 and it has zero break-in time. So I can’t comment how does it sound compared RC (V-Cap CuTF, AN Silver Tantalum) that were before. It need more break-in time.

@alexberger Please report back on the sonic diff between the A-107 and RC coupling.  Completely different amps, but I much preferred all IT coupling.  Curious to hear your impression

I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and 47KHz.

Should I decrease the load resistor more to remove overshoot completely?

If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.

@alexberger As you have noticed, if you are using a coupling (interstage) transformer, it will be needing proper loading to prevent ringing (distorting). You are nearly there with your technique so far; put a potentiometer across the output of the transformer, run a square wave through the active circuit prior (6SN7) and adjust the pot for minimum ringing (critical damping). IME its probably best if you leave a very slight amount of overshoot as opposed to rounding the square wave.

Its important that the driver to the transformer be active, since transformers transform impedance: Whatever impedance on the primary side, if it varies, will affect the critical damping value on the output side. Conversely, whatever is loading the output side will also load the input thru the ratio of the transformer. So you want to feed the squarewave to the active 6SN7 circuit so your loading value will be correct. Best to have the 6F6 running also for this very same reason, although the loading resistor will likely dominate that side of the transformer equation.

Once that is sorted out, you might find it interesting to measure the impedance at 1KHz on the primary side (you won’t need the 6SN7 in the circuit for that, but it would be a good idea to have the 6F6 in place and active) just to see what the load on the 6SN7 actually is. You may find that you have to adjust the operating point of the 6SN7 to obtain greater linearity (by adjusting the cathode resistor value if you have one); if you do that then you may have to readjust the loading value of the transformer since the source impedance of the 6SN7 varies a little with the operating point; in this way zeroing in on the optimal values.

Obviously you can stop at any point (call it ’good enough’), but in a zero feedback design I’ve found that the more you pay attention to refining things like this, the more it pays off in the end!

@lynn_olson I agree overshoot in a circuit using feedback is bad!

But if the feedback is applied properly you’ll get no overshoot at all. Our OTLs don’t exhibit any squarewave overshoot, being zero feedback and free of inductors in the signal path. The 10KHz waveforms are pretty good since they have wide bandwidth; our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback; the difference being the residual sine waveform imposed by the switching frequency.

@atmasphere 

our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback; 

This GaN  class D balanced amplifier with copious utilization of NFB  is literally at the opposite end of the audio design spectrum from the DHT (300b) balanced class A  zero NFB Blackbird under discussion on this thread. Talk about traveling different roads toward the destination of Rome.

The in depth information presented here concerning the Black bird amplifier has me exceedingly curious to hopefully hear it one day. Its development is a fascinating story.

Charles

Hi @atmasphere ,

How does excessive transformer ringing can influence on sound?

Does it make it too sharp or bright?

@charles1dad My point was addressing a comment made earlier by Lynn about overshoot in amps employing feedback; simply that if you do it right its not a problem. The OP mentioned using Spatial Audio Triode Masters who were a dealer of ours and have used our OTLs and class D on their speakers. It didn’t seem that off topic, especially if we discuss the issues of signal coupling, operating points and the use or lack of use of feedback.

As I understand it, you are particularly enamored of SETs; perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing.

How does excessive transformer ringing can influence on sound?

Does it make it too sharp or bright?

@alexberger Ringing contains higher ordered harmonics which can be heard as brightness and harshness. You also get lower orders which contribute to richness. Both are colorations and will obscure low level detail.

@atmasphere 

perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing

Different pathways to audio nirvana is something I’ve acknowledged long ago. It’s an undeniable individual journey with numerous successful outcomes. What I have found to be most pleasing and satisfying for me certainly may not be the choice for another.

I don’t believe my comments above contradict this perspective. I was merely comparing two earnest efforts to build amplifiers that are vastly different in concept, design and implementation.

Charles

As a minor diversion, I should describe the "Golden Age" amplifiers I keep referring to. This aren’t just the amplifiers made in the Fifties and Sixties; it describes the majority of PP tube amps made since then, including today.

There were only a few basic Golden Age circuits, or topologies, as we like to call them. (Topologies omit circuit values, but are easily worked out once you know the tubes.) The first was the Williamson of 1948, but it had the drawback of marginal stability. Still, it dominated the US market until 1955 or so, when the much simpler Dynaco variant came in. (The Dynaco topology simply omits the driver stage of the Williamson and uses the phase splitter to drive the output tubes. More distortion but more stable.)

The Mullard became the prototype of many tube amps as the better-performing alternative to the Dynaco circuit, and is still widely used today. Let’s walk through it.

There’s a high-gain input tube, typically either a 12AX7 or a pentode like an EF86. This is direct-coupled to one half of a differential stage, with the other grid AC-coupled through a cap to ground. Because the grid of the diff stage is at 150 volts or so, the cathode is a little bit higher, maybe 155 volts. This requires a large value resistor that goes all the way to ground, so the diff stage is frequently called a "long-tailed pair". A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

The diff pair are a pretty good phase splitter, and unlike the split-load inverter of the Dynaco circuit, audio-frequency balance is not too sensitive to load. It also has more drive capability than the split-load inverter, and unlike the split-load inverter, it has some gain, too. So a win all around.

And we’re not talking about a lot of parts here: 3 triode sections, and the output pair. A Dynaco is even simpler, with 2 triode sections, and the output pair. The only coupling caps with either circuit are between the grids of the output pair and the preceding circuit, so not really complex, and simple enough that a stereo chassis, running off a single B+ supply, is quite practical.

The point of the high gain (in the input section) is to give feedback something to work with. Feedback requires "excess gain" to work its magic; you need 20 dB of excess gain to get 20 dB of feedback, which will reduce overall distortion tenfold. In a pentode or ultralinear connected amplifier, the output impedance is way too high to use with most speakers. The feedback really comes in handy here: 20 dB of feedback reduces output impedance tenfold.

What limits applicability of feedback is loss of stability if too much is used (I’m not going to get into Nyquist Stability Criteria here, nor phase margin, settling time, etc.) In other words, if we slap in another gain stage and try for 40 dB of feedback, it will just oscillate. At full power. And take out a tweeter before damaging itself and letting the smoke out.

A more clever approach is wrapping local feedback around the most distorted stages, like the output section, and then add overall global feedback on top of that. This was done in the McIntosh, Citation II, and a few other amplifiers. This really gets the distortion numbers down, but clipping can get ugly, and settling time from transients can be an issue. Multiple feedback amplifiers can be quite sensitive to operating conditions. It’s more often seen in modern transistor amps as "two-pole compensation", and is not trivial to design.

Note: To puzzle out a schematic, by convention, signal flow is left to right, just like you’re reading this. To see what a tube is doing, look what the grid (the dotted line) is connected to. Often, there will be a coupling cap, typically 0.1uF. If it is much smaller than that, like 30 mmF or 30 pF, it is bypassing RF or has something to do with stability. Larger caps are cathode bypasses or power supply. The plates (the flat-topped dingus) is the output of the tube and typically heads to the right side of the schematic.

You usually have to stare at a phase splitter quite a while before the function becomes obvious. One side is quite simple, coming directly from the input tube, but the other side can be pretty weird. A diff stage can be puzzling, because the DC connection is a high-value resistor going to the other grid, and the AC connection just goes to ground through a 0.1uF cap. The "other half" is actually driven from its cathode, not the grid.

What gives away a split-load inverter, or "concertina" stage, are the equal cathode and plate resistors. This is a dead giveaway you are looking at an inverter, since no other tube stage uses equal resistors ... for one thing, it’s kind of useless for anything else, since gain is a bit less than unity.

I leave the "floating paraphase" as an exercise for the reader. I kind of like them, actually, because current drive for the power tubes is pretty good, although balance is only so-so.

A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

Usually in a differential amplifier, the plate resistors are matched if both halves are driven. In the case circuit of the above description, only one side is driven. So if a CCS is not used, the plate resistors can't be matched; one side must have a slightly higher plate resistance to compensate for the mu (gain) of the tube of the un-driven side, so as to get equal outputs from each half.

A good CCS eliminates this problem (which is nice since in the real world you can't count on the mu of each section to be equal or matching that of the tube specs on paper). A good CCS is both inexpensive and reliable, allowing the tube to be removed from the circuit while active (hot plugged) without damage.

Further benefit can be had from placing a CCS in the output tube cathode circuit, if you control the output tube(s) bias using fixed grid bias. The cathodes are tied together and the CCS feeds them; thus improving the differential effect of the output section, which reduces distortion and makes it slightly easier to drive due to increased gain.

Of course, if you have the input tube be a differential amplifier too, it can accept a balanced or single-ended input and can have excellent performance if a CCS is used for this stage as well. But its not a good idea to direct couple both plates to the succeeding driver stage; its OK to do one but the other should be capacitively coupled so as to prevent DC offsets of the first stage of gain from causing distortion in the driver.

The original BAT VK-60 of the 1990s used a differential input direct coupled to a differential driver; to deal with the DC offsets a potentiometer in the cathode circuit of the input tube allowed the plate voltages to be equalized. I found this approach to be problematic (we had tried that back in the early 1980s; one obvious problem is that it requires the user to make this fairly critical adjustment...) and often causes more problems than it solves.

So in the quest to keep the number of coupling capacitors down but retain easy operation, we started using a differential cascode voltage amplifier. The advantage of this was that all the gain of the amplifier was in a single gain stage, consisting of three dual-section triode tubes, one for the input differential amplifier, one for the top of the cascode, being plate loads for the bottom tube sections, and finally a 2-stage Constant Current Source for the circuit, tied to a B- supply of equal potential to the B+ supply. The CCS prevented changes in the AC line voltage from affecting performance of the voltage amplifier from 107VAC to 126VAC the difference was only 17 parts per million. So you couldn't see any performance change on an oscilloscope over that range!

So that allowed for enough gain, low distortion (once the correct operating point was set up), and only one pair of matched coupling caps (of a small value, in our case only 0.1uf, further minimizing the sonic impact of the coupling caps). They drive a pair of cathode followers which are direct coupled to the output tubes. So the power tubes obtain their bias voltage from the driver; therefore the bias and DC Offset controls are in the grid circuit of the driver tube. This allows for instantaneous overload recovery and rock solid bias control of multiple high-capacitance triode grids, with low frequency response to 1 or 2Hz no problem at all.

If you use a coupling cap in the critical area of the grids of the output tubes, it must be large so as to get good bass response since the grid bias network must be of relatively low impedance to properly control the power tubes. This means that the driver tube has a difficult load to drive and the large coupling cap can cause blocking distortion and slow the overload recovery. While this really isn't much of a problem driving pentodes, using this topology to drive triodes is a bad idea IMO/IME.

I've been describing how our OTLs work but obviously this would work well with a 300b too. We've managed to get our OTLs to 0.5% THD which is pretty low distortion for a zero feedback circuit! SETs by contrast tend to be about 10% THD at clipping which might be only 7 Watts. Since the OTLs tend to be much higher power capacity, the tendency is, for any power level the SET might have, the OTL has distortion that might be 2 orders of magnitude lower or more at the same power level. This is why they tend to be so much more transparent than SETs.  I've no reason to think this cannot be applied to a 300b circuit with similar results; SETs have the distortion they do out of the topology rather than the power tube that is used. So a pair of 300bs could be used to much greater advantage!

For those that might want to see more about how our OTLs work (and how this might be a topology for a 300b amplifier), there is a DIYaudio.com thread from several years ago that has a schematic and discussion. A lot of this would work very nicely with a 300b; for example the Circlotron output can be transformer coupled of course and have all the advantages (such as zero DC saturation of the output transformer) it offers.

Hi Lynn & Don... Will you be at Capital Audio Fest with the Blackbirds?

@tinear123  No.  Production starts in late November when I go to Salt Lake and teach the guys at Spatial the builds.  I would expect a review by late spring and perhaps an audio show in the west somewhere next summer.  That is kind of going to be the schedule I think.   There will be a review pair that could potentially end up at a show in the east next year, but that would be Spatial Audio Lab's call.  I really wish that folks could hear what I am listening to in my living room.  It would be fun to have any of you over.  There will be a complete setup in Salt Lake City area by January and I am sure audition arrangements could be made for anyone who wants to hear it along with a top end Spatial Audio Lab speaker.  Of course this doesn't help any of you out East....     

It is definitely the plan to have a review setup of both preamp and amps and have it reviewed by legitimate reviewers next year as early as possible.  Perhaps sometime next year we can have them appear at a show in the east somewhere.

And I am very much looking forward to fellow enthusiasts hearing the Raven preamp and Blackbird power amp. I’ve been a voice in the wilderness for about twenty-five years ... neither a member of the SET fraternity (well, maybe on the edge of it) nor mainstream Audio Research/Jadis/Conrad-Johnson push-pull pentode big-watt amplifiers dominating the hifi shows. A handful of people built the Karna amps, but many abandoned the difficult project halfway through.

Don was one of the very few who persevered through two years of building prototypes that were far off the beaten path of mainstream tube gear. He’s had plenty of hands-on experience with the fiendishly difficult Citation II, the most complex amplifier of the Golden Age, and his own designs, the Valhalla (6L6) and Kootenai (KT88).

Of all the people I know in the industry, Don is the most qualified to honestly tell me what is unrealistic and pie-in-the-sky, and what is practical and a good solution. He’s been there and done that. Oh, and he has good taste, too, which isn’t that common in the industry.

You might think I’m being snarky about the "good taste" but I am perfectly serious. The industry has plenty of competent engineers, and whole hifi shows filled with high-powered marketers, but good taste? It’s not all that common, and I’ve been in the industry since 1973.

Well... Lynn has lots of wild ideas, but the thing is that they are very well thought out, and they are based on years of technical experience with Tektronix.  I learned many years ago in academia, and as part of my main career in forest ecology research, to listen to really smart people with wild ideas.  Many of them were simply ahead of the mainstream thinking.  The mainstream often ends up there.....eventually.  

Lynn suggests improvements and where practical, I try them.  We finally ended up with a larger chassis that could accommodate all the things we wished to try.  They are still under 19 inches so they fit any rack, since 19 inch is the traditional rack width.  To my ear, it worked beautifully....  The final touch was the addition of old school gas VR tubes to further isolate the input tube supply from the drivers, and of course the move to KT66/6L6 or KT88 drivers.  I think we are done finally.

I agree. This is a stable topology, taking full advantage of specialty transformers designed by two of the world’s top designers, and using vacuum tubes that are in current production as well as ample NOS stocks.

As mentioned earlier, it’s a very simple signal path, with only transformers and vacuum tubes, and fully balanced from input to output. Zero feedback, with the audio signal only propagating in the forward direction.

Another walk down Memory Lane. This time, we’ll go into the late Forties, when the Williamson burst on the scene. This English design wiped out all other designs in the USA until about 1955 or so, with the exception of the McIntosh and a few others.

How does it work? There’s an input tube, typically a triode like the 6SN7, direct-coupled to a split-load inverter, also called a "concertina" stage. This always has identical plate and cathode resistors, and gain a bit lower than unity. The plate output drives the upper half of the push-pull amplifier, while the cathode drives the lower half. Despite appearances, the voltages on top and bottom are equal and opposite ... provided the total loads match, as well.

The inverter is then cap-coupled to a separate push-pull driver stage, which is sometimes also set up as a differential stage, depending on the resistance presented to the common cathodes. High impedances move it towards a differential stage, with the limit being modern constant-current sources. 6SN7’s were typically used here, with later designs replacing them with 12AU7’s (which typically have more distortion).

The drivers are then RC cap-coupled to the output tubes in the usual way. The drawback of a classical Williamson are the two stages of cap coupling, which can introduce low-frequency instability unless the output transformer has extremely wide bandwidth. The Partridge transformer specified for the original design had one of the widest bandwidths of any output transformer ever made ... but lesser transformers introduced stability problems, sometimes "motorboating" at low frequencies, but more commonly long recovery times from overload.

The Dynaco, introduced in the mid-Fifties, took the drastic step of deleting the driver stage and its associated RC coupling, and driving the output tubes from the RC-coupled phase inverter. Although the open-loop performance was quite poor, rolling off around 100 Hz and 7 kHz, the 20 dB of feedback nicely corrected it, since the input section used a high-gain pentode and there was plenty of "excess gain" to drive the feedback network.

The Dynaco had the advantage of being the cheapest of all to build; a combined pentode/triode, the 7199, took care of the entire front end, and all that was left were a pair of EL34 output tubes and an output transformer. In addition to Dynaco, many receivers used this approach as well. It was simple, saved money, and saved space, which was at a real premium in a low-profile AM/FM stereo receiver.

Receivers in the early Sixties (Fisher, Scott, Sherwood, Harman-Kardon, etc.) all had Bass and Treble tone controls, an AM and FM tuner with two different IF strips, an FM multiplex stereo decoder, a stereo power amp with at least 20 to 35 watts/channel, and last but not least, a stereo phono preamp. All with vacuum tubes, in a very crowded chassis, with marginal ventilation and caps of much lower quality than we have today.

We don’t see many Williamson amplifiers today. The dominant PP-pentode designs are Mullards and Dynacos, depending how price-sensitive the amplifier is. The monster tube amps with 4, 6, or 8 output tubes per channel typically throw in a dedicated cathode-follower section to drive all those grids ... sometimes one cathode follower to drive them all at once, or preferably, each output tube gets its own cathode follower. The RC coupling is then moved to the input side of the cathode follower, and the CF directly drives the grids of the output tube(s). This easily provides independent biasing of each of the output tubes, which is important when that many tubes are used.

The Dynaco, introduced in the mid-Fifties, took the drastic step of deleting the driver stage and its associated RC coupling, and driving the output tubes from the RC-coupled phase inverter. Although the open-loop performance was quite poor, rolling off around 100 Hz and 7 kHz

Actually the Dynaco has ~ 6Hz LF cutoff (-3dB) running open loop. Its distortion rivaled that of the Marantz 8B which was and is well respected, for a lot less money. You can reduce the distortion easily by obtaining a socket adapter off of eBay, which allows you to replace the 7199 driver tube (which is rare) with the much more common (and cheaper) 6GH8A. No other changes are required.

Actually I have rebuilt 3 or 4 of the ST70 amps.   The best version is to skip the 7199 or 6GH8 types and to one install of the octal driver replacement boards.  I really liked the 3 6sn7 tube version that tubes4hifi used to sell and perhaps still does.   It makes the amp considerably better.  Really in every way.  That said, they still run out of steam under load because the power supply is only adequate and there is no room in that tiny chassis to install a better supply.  They are great amps for the money.  But they are still not particularly great.  However, there is little else you will find at that price point to touch it.

All classic vintage PP amps from 50-60 have very weak PS in terms of chokes, power transformers and especially capacitors values. If you use the same power supply for SET with no feedback with a such PS will be a disaster. But many of these classic PP still sound fine! Why? Because feedback or because PP has PS noise cancelation?  

@alexberger Fine is a relative term.  If you improve the power supply in a citation II the amp is greatly improved.  That is why the KT88 amps I used to build have FAR better power supplies than vintage amps.  If you build a vintage amp with the sort of power supply you are using in your SET that vintage amp would sound much better.

@donsachs 

If you build a vintage amp with the sort of power supply you are using in your SET that vintage amp would sound much better.

+1 

Charles

The three-tube 6SN7 circuit board for the Dyna ST70 converts it to a Mullard circuit, with lower distortion and stronger drivers. Since nearly all the ST70 circuitry is on the single circuit board (for both channels), swapping that board basically gives you a new amplifier ... while retaining the power supply, chassis, and transformers. Lots of ST70 variants, since so many were made and are still kicking around. And the output transformers are pretty good.

Of course, if you are replacing the power transformer and upgrading the power supply, you might as well build on a new chassis, and have an all-new amplifier. Nothing wrong with a 6SN7 Mullard circuit and modern power supplies ... that will take you into the $3000 to $10,000 quality bracket right there.

Actually, the Kootenay, and the the Valhalla amps use a similar circuit for input and drivers (with judicious use of a CCS), and the power supplies are the basis for what we used in the Blackbird 300b amps.  We improved the power supply design for the Blackbird, but the same basic ideas are in the Kootenay KT88 power amp and Valhalla 6L6 integrated amps.   Long tailed pair with CCS and really good power supplies and iron.  As Lynn said, it makes for a really good tube amp.  Not the level of the 300b project, but very nice indeed.

But many of these classic PP still sound fine! Why? Because feedback or because PP has PS noise cancelation?  

Feedback allows the amplifier rejection of that which is not the signal, so generally speaking, yes.

I really liked the 3 6sn7 tube version that tubes4hifi used to sell and perhaps still does.   It makes the amp considerably better. 

The problem with any mod that adds tubes to the circuit is the extra load on a power transformer that might already be 65 years old. Dynaco strikes me as being pretty precious about their transformer ratings- I don't like to take chances with them, especially in light of their age.

This is just me of course but if I'm going to modify a vintage piece I follow two simple rules. The first is don't add any extra load to the power transformer. The second is don't do anything that does not fit very easily into the existing chassis. Violate these and you're likely better off doing the whole thing from scratch.

The really glaring weakness in the ST70 is it should have been designed with dual rectifiers; as a result the 5AR4 is the most likely tube to fail in the amp. Triode Electronics of Chicago has a beefed up power transformer that is a drop in replacement that allows you to add a second 5AR4, thereby keeping the correct B+ operating point and so not stressing the output transformers as well. But you have to find room beneath the chassis for some 500V filter caps. It starts to get a bit ridiculous- at that point why not just do your own chassis so you can lay out things properly?

If an ST70 is properly refurbished but pretty well the stock circuit, it can be surprisingly good against a lot of modern PP and SET amps. Since it really does not have enough feedback, you have to help it along with good quality coupling caps and resistors in the voltage amplifier and driver circuit. The second thing to understand about this amp is because of its power supply weakness, you really should not push it hard (which is better for sound but also keeping that 5AR4 alive). CE Distribution in Arizona makes a drop in replacement filter can that features an 80uf section, which should be deployed after the choke, for the plates of the power tubes. That's about as much extra capacity as you can safely add to this amp without stressing the 5AR4.

The octal boards for ST70 are direct fit and do not stress the power supply.  You can use the diode replacement for rectifiers, but it sounds different.  Basically, the amp is not bad, and a great deal for the money when restored and updated.  A decent preamp and some reasonably efficient speakers and a person can have a very pleasing stereo.  That said, an ST70 is nothing special because there really isn't room to make it special.  I think we agree on that.  You might as well start over and do it right without the compromises.

Back in 1993 when I was trying many different amplifiers on the newly completed Ariel speakers, I came to the conclusion that a stock ST70 was the minimum acceptable standard for hifi. Most transistor amps fell below this mark, and most tube amps exceeded it. Restored Mullard designs did very well, as anyone might expect.

What I did not expect was the performance of the Ongaku and the Herb Reichert Silver 300B. I expected all SETs to be terrible, but those two were the best sound ever on the Ariels (which are 92 dB/meter efficient). After hearing several other SETs, I was struck how variable they were. A few were superlative, almost otherworldly, but many others were pretty bad. One was so terrible that Karna and I just burst out laughing ... it sounded like a 1960 transistor radio left out in the sun too long. That’s how variable SETs are ... all over the place.

By contrast, a competently engineered Mullard with a decent power supply is almost guaranteed to sound pretty good. And the best ones are superb.

And of course a part-Mullard circuit is perfectly acceptable for a PP 300B amplifier. Unlike a PP pentode amplifier, though, you need about two to three times as much voltage swing in the driver, so a Dynaco circuit is definitely not the right choice.

A Mullard PP 300B works as follows: input tube direct-coupled to a long-tail pair (or CCS) of triode-connected 6V6 drivers. These in turn are connected to a PP interstage transformer with a modest step-up ratio, between 1:1.4 and 1:2. The interstage then drives the PP 300B grids. This would be a non-feedback amplifier, so good power supplies are required. I would imagine a number of the PP 300B amplifiers already on the market use this topology.

@lynn_olson

I expected all SETs to be terrible, but those two were the best sound ever on the Ariels (which are 92 dB/meter efficient). After hearing several other SETs, I was struck how variable they were. A few were superlative,

Lynn,

What do you attribute to this wide spectrum of SET performance from superlative to terrible amongst them? Given their relatively simple circuit, is it primarily part quality (Output transformer?) or power supply quality and design?

Charles 

There's no feedback, and there's gobs of 2nd-harmonic distortion. Parts coloration works for or against that 2nd-harmonic distortion. Also, because power-supply rejection is zero, power supply coloration is right in your face.

Driving DHTs takes two to three times the swing of pentodes or beam tetrodes. This exposes driver nonlinearity as well. Even worse, in some designs, driver and output distortion partially cancels ... but only at certain power levels. So the distortion signature is strongly level-dependent, which is very undesirable.

A Mullard PP 300B works as follows: input tube direct-coupled to a long-tail pair (or CCS) of triode-connected 6V6 drivers. These in turn are connected to a PP interstage transformer with a modest step-up ratio, between 1:1.4 and 1:2. The interstage then drives the PP 300B grids. This would be a non-feedback amplifier, so good power supplies are required. I would imagine a number of the PP 300B amplifiers already on the market use this topology.

I suggested this earlier in this thread; and added to it that with the input circuit also being differential there is additional benefit. A fully differential circuit has harmonic cancellation at every stage of gain; not just at the output. This results in the 3rd harmonic being dominant; it is treated much like the 2nd by the human ear in that its innocuous. But compared to an amp that does not use this topology, the 3rd is at a lower amplitude, and succeeding harmonics fall off at a faster rate (than seen in an SET) on an exponential curve. So the 3rd is thus more effective at masking higher orders and since distortion is lower, the circuit can be smoother and more transparent.

The problem with two successive stages that are balanced and DC-coupled to each other is that DC drift is a big deal. A 1 volt shift on a 150 volt plate is normally inconsequential, but becomes a serious concern when the grids of the following stage have a 1 volt offset between them ... which is what DC coupling does.

A Mullard sidesteps this by direct-connecting the plate of the SE input stage to ONE driver grid. The other grid (of the driver) is AC-connected to ground through a 0.1uF cap and DC-connected to the other grid via a 100K ~ 220K resistor. As a result, the two driver grids always DC-track each other.

By contrast, if the Mullard input section is replaced with a DC-connected balanced or diff stage, then DC balance and drifting of the first stage becomes critical, requiring a servo circuit to always keep the plates of the input tube exactly matched. No thanks.

The Blackbird is fully balanced, input, driver, and output, with DC balance issues resolved by using transformer coupling. Transformers are incapable of passing DC from primary to secondary, since the coupling is magnetic. Charge/discharge issues associated with capacitors, as well as potential coloration, are also avoided since cap coupling is not used anywhere in the forward path.

The hard part is getting transformers of high enough quality ... this is where working directly with the transformer designer, making them a part of the design team, is essential. These are not off-the-shelf parts.

A minor side benefit is avoiding turn-on pops and clicks, since the circuit remains balanced in all modes of operation, without relying on servos to maintain balance.

As mentioned above, a part-Mullard is great way to build a PP DHT amplifier. Not too complex, a well-known circuit that behaves predictably, and capable of scaling up the driver so it has enough power to motivate DHT grids.