Let’s talk a bit about distortion, particularly in a DHT driver. It defeats the purpose of using a very low distortion 45, 2A3, 300B, or 845 if the driver has more distortion than the DHT itself. You might as well use a much cheaper power pentode or beam tetrode and save yourself a lot of trouble. Not only that, but pentode and ultralinear-connected output sections requires feedback across the whole amplifier, which cleans up the mess from the input and driver sections. This is why the driver section is an afterthought in Golden Age PP pentode amplifiers. In the Dynaco circuit, there isn’t even a driver ... the power tubes are driven from the split-load inverter, or "concertina" stage.
But ... if the goal is the most linear possible amplifying stage, with zero feedback, a completely different approach is required. True, push-pull substantially reduces distortion, but in reality it only reduces even-order distortion ... 2nd harmonic, 4th harmonic, 6th harmonic. etc. It has no effect on odd-order distortion ... 3rd harmonic, 5th harmonic, 7th harmonic, etc. Most tubes have dominant 2nd harmonic distortion. But not all. The 6DJ8/6922, an RF tube, has dominant 3rd harmonic. Tubes designed for video amps can also have dominant 3rd harmonics, since distortion in a video tube has almost no effect on the picture (even 5% distortion would be barely visible).
Aside from the subjective sonics of dominant 3rd harmonic, this has consequences for a PP amplifier. The PP circuit will have no effect on 3rd harmonic; it will not reduce it even 1 dB. So tubes with dominant 3rd harmonic should be avoided.
What about more "normal" tubes with dominant 2nd harmonic? Well, one of the requirements for PP cancellation is the magnitudes and phase of the cancelled harmonics line up with each other. The magnitudes of the (even) harmonics from each tube should be within 1 dB of each other, and the phase of the (even) harmonics should be within 45 degrees of each other. Normally this would be of no concern with reasonably matched tube sections, but it does matter if the capacitive loads are not the same. The odd harmonics, of course, do not cancel.
The notion of harmonics having phase might seem a little odd, but keep in mind that square waves and triangle waves have identical magnitudes of harmonics; the only difference between the two signals is the phase of the harmonics. So the phase of the harmonics is not insignificant, and actually reflects a different transfer curve. In graphical terms, you want complementary transfer curves; if kinks appear in the curve of one tube but not the other, that kink will not cancel.
The full scope of the driver requirements for DHTs is probably now coming into view.
* Three times as much swing is required, compared to pentode or beam tetrode.
* Linearity should be better than the DHT itself.
* The load is mostly capacitive, consisting of the 60 to 80 pF Miller capacitance of the DHT grid. Reactive loads increase distortion at high frequencies, where it is most audible, and also reflects the power back to the plates of the driver tube.
* If the driver is PP, 2nd harmonic should be at least 10 to 20 dB greater than 3rd harmonic, and the two loads should be symmetric to keep the phase of the driver harmonics the same.
* There should be at least 3 dB of driver headroom so the amplifier doesn’t all clip at once. More headroom is desirable.
* Momentary sags, or program-correlated noise, in the power tube B+ supply should not interact with the driver tube B+ supply.
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The Statement ... or Blackbird ... or whatever it’s called, is pretty sensitive. Don told me 1.5 Vrms to full clipping, and my gain calculations indicated a bit more sensitive than that. Since there is no RC loading anywhere, and all the cathodes are bypassed, all tubes run at full gain.
For a 6SN7 that’s a mu=20, and for a triode-connected 6V6 that’s a mu=8. Since the 300B grids take 80 volts to reach zero bias, and can go positive to +20 above that, that’s 100 volts of swing for each side, about 3x what a pentode requires. Since this is a PP amplifier, full power requires +100 volts on one grid, while the other receives -100 volts. The input+driver provide 20 * 8 = 160 gain, excluding transformer losses, which are typically 5% or less.
Imagine those 200 volt peaks at 30 kHz, with no local or global feedback, at less than 1% distortion, with another 3 dB of headroom above that. This is why driver design for DHTs is difficult, and not in the Radiotron Designers Handbook.
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Great response, Ralph, answering questions I have long wondered about. My Sachs monos put out a fair amount of heat so I mounted a couple of noiseless computer fans at the back of my rack to dissipate the heat to a degree.
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1. Does it matter what volume a power tube is played at? Does that effect tube life?
@markusthenaimnut
If the tube is operating class A1, the power its making won't matter. If operating class A2 or A3 the higher power levels will probably affect tube life. If the tubes are running class AB then they will run cooler, which could translate to longer life, but higher power will shorten that. You didn't mention the load but that affects things too- its rare that the speaker actually loads the output transformer correctly for a given tap; transformers transform impedance in both directions so a load too low on the output transformer will be a load too low on the output tube(s) as well. That will reduce tube life as more of the power made by the output section will be dissipated in the tube(s) rather than the load!
2. Does it matter if a tube is cooled, say, by a small fan nearby?
It helps! During WW2, 6L6s were used due to shortages to get amazing power levels by being water cooled.
3. If a tube is powered up but not making music does that "cost" tube life just as if you were playing music through it?
Always- how much depends on the class of operation and other variables such as dust on the envelope and so on.
4. What is harder on a power tube? Turning it on and off, let's say twice during a day (two listening sessions totaling three hours) or letting it stay on, let's say for an eight hour period?
Tubes wear out no matter what you do. They also draw power... The longer the off time the easier it is to answer a question like this. If we're talking about an indirectly heated power tube, the turn on should include time where no B+ is applied until the power tube cathode is properly warmed up. We supply a Standby switch for this purpose on our amps. If the tube is directly heated it won't matter. So if it were me I'd shut the amp down when not in use, even if for only an hour.
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Here's a question for @atmasphere - Though I'd welcome the comments from anyone else on this thread - these are all related to the question of what causes a power tube to wear out -
1. Does it matter what volume a power tube is played at? Does that effect tube life?
2. Does it matter if a tube is cooled, say, by a small fan nearby?
3. If a tube is powered up but not making music does that "cost" tube life just as if you were playing music through it?
4. What is harder on a power tube? Turning it on and off, let's say twice during a day (two listening sessions totaling three hours) or letting it stay on, let's say for an eight hour period?
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For example, floating paraphase phase inverters instead of split-load inverters or Mullard long-tail pairs. The phase division isn’t as precise, which is why they dropped out of favor, but the drive capability is much stronger than the other two types.
@lynn_olson A simple way to improve this circuit is to use the bias supply for the KT88s as a B- voltage and then use a 2 stage constant current source to set the operating point of the differential driver. This improves the differential effect quite a lot and gets rid of the need for slightly different plate load resistors- they can be matched instead.
To this end we use bipolar supplies in our amps; B+ and B- have the same absolute value. No balancing is required in the differential amplifier and often the plate voltages are within 3-4 volts of each other. This improves the CMRR quite a lot which in turn reduces distortion (there is the additional benefit of more gain and wider bandwidth as well). If the input stage is built in a similar manner, even orders will cancel throughout the circuit, resulting in a dominant 3rd harmonic which can mask higher orders.
The harmonics will be found to fall off at a faster rate (than seen in SET circuits) as the order of the harmonic is increased; they will follow an exponential decay based on a cubic function. This works really well for the human ear (smoother sound and greater detail, both on account of reduced open loop distortion)!
To eliminate a frequency pole caused by a coupling capacitor, we’ve been using a differential cascode circuit as the sole source of gain in our OTLs. Because the gain is increased in that single stage of gain, so is the CMRR and overall differential effect. This allows one to use a cathode follower driver direct coupled to the output section. In our OTLs the power tubes are in turn direct coupled to the loudspeaker. If you were to use an output transformer, a pair of DHTs are easily driven- linearity is such that no feedback need be used. Bias is obtained from the driver circuit, so if bias controls are used, they are in the grid of the driver tube rather than the power tubes.
If feedback is desired, it can be wrapped around the circuit and applied in a manner identical to how its done in opamps- using resistor divider networks that mix the feedback with the incoming signal at the grids of the input stage. This technique vastly reduces distortion that the feedback signal would otherwise encounter, which in turn means the feedback is more effective at its job, generating less higher ordered harmonics (caused by non-linearities in the feedback nodes traditionally used in both tube and solid state amplifiers). By doing this a wider range of speakers can be used.
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I have a question to @lynn_olson
I have 300B SET 6sn7 input, 6f6g in triod driver with RC coupling. One friend of mine who reads DIY forums and tries all this thing by himself, recommended me to enlarge cathode capacitors up to 100,000uF in input and driver tubes.
So I did 10,000uf in input first and because I like the results I did 100,000uf on input and 50,000uf on driver. So I get even more improvement - deeper and more controlled bass, bigger soundstage, more low level details, transparency, bigger more dynamic sound.
Then I increased driver capacity to 87,000 uF and it gave a big improvement in dynamics and sound quality. Despite intuitively it shouldn't do too much!
In all cases I use a bunch of 10000uf-15000uf (CDE, Nichicon KA) medium quality capacitors bypassed by one 50uf AN Kaisei NP.
Can you explain why these huge value cathode capacitors work?
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As for the tech, none of this is difficult. They will know how to connect and bias the floating paraphase phase inverter ... plenty of old schematics out there ... and the rest is a matter of chassis space and heater current for the new drivers.
"Floating Paraphase" sounds more scary than it is. The secret of any phase inverter is finding what the lower grid is connected to.
In a Mullard circuit, the lower grid is grounded through a coupling cap to ground, but is connected to the other grid through a high-value resistor. This sets the bias point the same as the other side, but the grid does nothing and is simply a reference. The common cathode does the phase splitting. The long-tail is either a high-value resistor or a current source .., yes, I know, transistors. The main drawback of the Mullard circuit is restricted peak current delivery to the power tubes. (But the split-load inverter is much worse in that respect. Split-load inverters do not like to deliver current ... they go out of balance. Ideally, they should be buffered with cathode followers.)
The floating paraphase has the upper tube drive the upper power tube through a totally normal RC coupling. Nothing to see here, sir, move along. The lower tube is often drawn in an opaque way, but what’s going on is the lower grid is connected to a pair of resistors midway between the power tube grids, after the RC coupling, What this weird-looking connection does is local feedback that forces the lower, phase-inverting tube to act as a unity-gain inverter, or plate follower. Phase inversion isn’t quite as pretty as the other two methods, but ... more power is available to drive the power tubes, which is what really counts. And you can ditch the 12AU7 and use real power tubes, because, why not? Just a matter of another socket and heater power.
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Unfortunately, revising the driver away from the usual single 12AU7 means another hole in the chassis, and a pair of octal sockets at that. And split power supplies means another power transformer and rectifiers, although it relaxes the endless search for Holy Grail rectifiers. When an amp is that sensitive to rectifier choice, frankly, there is a design error lurking in there somewhere.
Somewhat counter-intuitively, splitting the power supplies front to back sounds better than isolated stereo power supplies, where the gain is fairly minor. The front to back isolation is not minor. Another counter-intuitive thing is PP amps benefit more from power supply isolation than SE amps.
As for inverters, the split-load inverter plus input tube uses two sections, while Mullard and the floating paraphase use three sections. And the last two sections can be octal medium-power tubes, not a single 12AU7.
This will change the forward gain of the amplifier, not by by much, but some. Probably a few dB less gain thanks to different and lower-mu drivers. Maybe a small trim in the feedback network, but less gain is easy to adjust for (unlike more gain).
In practical terms, a new chassis. Sorry. Those old amps were engineered down to the last inch, with no room to spare, and not one part wasted. Flip one upside-down and the parts in there are really, really tight. Too tight, and prone to overheating, which does no favors to the capacitors.
Most of the engineering effort in the Golden Age, even for Marantz and McIntosh, was simply watts-per-dollar, and keeping chassis size down as a secondary goal. In the late Sixties, the Crown DC300 and Phase Linear 700 blew the watts-per-dollar tube amps out of the water, and forced them into the audiophile market, where they remain today.
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So, Lynn, given your comments, would an experienced technician know where to start to make these kinds of modifications? Or are they too obscure for the average tech to understand and execute...
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The interesting thing is that "conventional" amps like PP KT88’s can be substantially improved as well. For example, floating paraphase phase inverters instead of split-load inverters or Mullard long-tail pairs. The phase division isn’t as precise, which is why they dropped out of favor, but the drive capability is much stronger than the other two types. And they really sound better. That’s one thing.
Another is dual B+ power supplies per channel, and is especially worthwhile in any PP amp, more so than SE. Isolating the noise and distortion of the final section from the driver + input is very worthwhile and lifts the amp into another class. This applies to solid-state as well. That’s another thing.
The third thing is scaling up driver current two or three times, and choosing the most linear driver tube that is available. Typically, this will be a medium-power tube, not a video amplifier out of a 1958 color TV. Video amps are powerful but linearity was not a major consideration in the tube design. You might get a linear one, but you might not. And you really want matched pairs for equal and symmetric distortion spectra.
Combine these three things, and the PP KT88 is now on a different level, but still has the same power as before. The slew rate goes up, and the noise and distortion go down.
Am I going to design this? Nope. But folks in the medium to high-power PP KT88 arena should check it out. You can’t keep making copies of Marantz amplifiers forever.
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This thread has been informative and inspiring and I’m looking forward to hearing these amps at the PAF next month.
This thread has also caused me to reflect on the question of "what should I do" or "what should we do?" given the fact that Don & Lynn’s new amps are not available to purchase at this time and that, when they are, they are likely to be out of my budget.
My guess is that many of us who have followed this thread with great interest already have equipment that reflects the performance goals, priorities and values of Don and Lynn. I'm grateful that there are so many options out there right now - we seem to be living in a new "golden age" of hi-fi and music. So I'm not actually discontented with my equipment right now. But I've learned a lot on this thread that has provoked some new ideas and questions and for that I'm grateful.
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This is getting dangerously close to an infommercial😉 Really, I have enjoyed the thread and never expected it to have this sort of life. I just received the final version of the preamp output transformers from Dave Geren at Cinemag yesterday and installed them. They are the next level up. I expected a subtle improvement, but I am quite shocked. The prototypes were the best I had heard, but the final version has improvements to the core material to give even more detail and even better bass response. You can hear it right away. These will be in Seattle and again, I hope to get everyone's opinion who can stop by and listen.
I will state again that there are many paths to audio nirvana and I would never claim that Lynn and I are on the only true path. For those who love SETs please come hear these amps and let us know what you think. You may be surprised. My own personal bias (pun intended) is towards an all tube system. I can hear any solid state device in the signal path and don't like it. Yes, my Lampizator Pacific DAC has a solid state chip set doing the DAC part of the job, but that feeds the output tube grids directly and from that DAC chip set to the speaker is all tubes. I would always choose a tube friendly speaker so that I can use a tube amp. I realize there are those folks who love speakers that are difficult loads and require big solid state amps to drive them. So be it. There is no right or wrong. There is what we like. If you can visit the Pacific Audio Fest in Seattle you will hear the sort of system I favor. All tubes, these amps and preamp, and some wonderful all passive open baffle speakers from Spatial Audio Labs. There will be no DSP or powered subs or anything like that in the signal path. The only solid state device in the chain will be the DAC portion of the Lampizator Pacific, then it is all tubes and passive speakers. We hope you like it!
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Don and I reflected on the optimum interface between the Raven preamp and Statement power amps. We obviously wanted the highest-quality interface directly between the two, while retaining compatibility with the rest of the audio world.
There’s an input switch on the Statement 300B that has two positions: XLR DIRECT and RCA XFMR. They do what they say: the direct switch goes straight to the 6SN7 grids through twisted-pair wiring, with the input transformer secondary completely disconnected from the circuit. The RCA position goes through the studio-grade transformer, which does the phase splitting and isolates the RCA grounds, preventing ground loops.
This provides optimal quality with the Raven preamp, which has transformer-coupled balanced outputs, and also offers compatibility with existing vacuum tube and solid-state preamps with balanced XLR outputs. For preamps with unbalanced RCA outputs, the input transformer provides accurate, low-distortion phase splitting as well as ground decoupling.
Don and I spent a surprising amount of time coming to this input topology. It has the cleanest interface to the Raven, while offering flexible connections to other components.
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Amazing answers - I just hope I can afford these!
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I think it’s very cool that there is a type of renaissance with DHT amps. I wish Don the best in this new venture
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It is evident to me that Don's new preamp is designed to mate in such a fashion with the mono blocks, connected with balanced cabling.
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The preamp, since it has balanced transformer-coupled inputs and outputs, breaks the ground connection between components, reducing hum, as well as RFI breakthrough and buzz. That’s why transformers are often used in noisy professional applications, and are used here.
This comes in handy for digital sources, since all ultrasonic noise (outside the 50 kHz bandwidth) is scraped off, as well as complete ground isolation. Rowland has been doing this for decades, and they're on the right track.
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To Lynn's comments about the need for big power for your speakers, that Class D amps get the job done, I would humbly add that an excellent tube preamp such as Don makes in the path adds a measure of bloom and euphonic sound that so many of us desire. I prefer to own speakers that mate well with tube amplification, but that is just me.
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It’s been fun writing about the zigs and zags that took Don, myself, and the team at Spatial to where we are now. More to come? Who knows? The preamp (with remote control thanks to Khozmo) and power amps are off to a good start.
Phono preamp? Ooof. Now that would be a challenge, not one I’d like to take. DACs? Definitely not. I leave those things to the pros. Speakers? Spatial already has a crackerjack in-house team, plus I have no clue how these dipole things work. Maybe they could teach me.
People ask me about 100-watt or even 200-watt tube amps. Designing around banks of KT90’s in massive parallelism, mostly likely in Class AB, is pretty much the polar opposite of the current project. Seriously, if you need Big Watts, just go Class D (with Bruno Putzey tech) or buy the Manley professional products. Both approaches are reliable and get the job done.
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I have never had a wooly or bloated 300b amplifier owned about 30 of the things not one had any bloat. All had a very wide frequency range when powering proper loudspeakers. Now if you don't use loudspeakers that are correct for SET or 300b amps they may have issues think that's where most of the bloated 300b comments come from. Since I see many SET or PP 300b etc users running speakers that are not best used with it.
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The evolution from the original Karna amplifier, built by Gary Pimm in 2002 and designed by yours truly, to where it is now, is a desire for rationality combined with reliable performance. The original was a proof-of-concept, the kind of thing I’ve been doing since Shadow Vector in 1975. I’m not so good at manufacturing engineering, which is where my collaborators come in ... Gary Pimm, Thom Mackris, and now, Don Sachs. These guys keep me on the straight and narrow and argue me away from my wilder flights of fancy.
The original Karna was based on a desire to get rid of all capacitors in the signal path, since I truly despise these things. But realistically, transformers are mostly problem solvers, so it’s better design practice to give them a problem to solve.
1) The input transformer accurately phase splits and offers ground isolation between preamp and power amp, if desired.
2) The interstage offers Class A1/A2 operation for the output section, a favorable load to the driver (compared to RC coupling), and balanced Class A drive for lowest distortion into a reactive load (the Miller capacitance of the 300B grids).
3) The output transformer multiplies current by about 28 times, so the peak currents the 300B offers (in the 200 to 300 mA range) now become many amps at the 4 and 8 ohm taps.
All three solve problems. The first interstage in the original Karna was mostly there for spite, to get rid of coupling caps once and for all, just to prove a point. But that required a low impedance tube, and I didn’t want to use a 6DJ8, which is woefully unsuited to power amplifier use. That left a family of industrial tubes that are reasonably linear (much more linear than a 6DJ8), the 5687/7044/7119 family. They worked reasonably well but I wasn’t entirely happy with the tone quality, but I was stuck, since the high plate impedance of the 6SN7 made it a no-go.
The founder of Tribute Transformers saw this as a challenge ... 15 K plate-to-plate ... and built four interstages with an 80 kHz bandwidth and perfect square waves. They’re in my Karna amps and Gary Dahl’s Amity amps now, and no, they weren’t free, both of us paid in full for them. So yes, it is possible to build an amp with two interstages and no coupling caps at all. But ... using a truly exotic interstage, a genuinely one-of-a-kind design from Tribute.
I wasn’t going to lay that burden on Don. We started with the greatly simplified Symmetric Reichert and improved it step-by-step, edging back to the original Karna, but certainly not using a four-box design with a quite exotic grounding system, aviation-grade Amphenol connectors, and four massive power supplies. You really have to draw the line somewhere.
Don had a lot more experience on the practical side, as well as real-world production experience with top-quality, and most of all, reliable power supplies in his well-regarded KT88 amp. So off we went.
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I’m not surprised. The great enemy of transformers are high impedance, which decreases bandwidth on both ends of the spectrum, and increased insulation resistance, which takes up room on the winding stack. Both act to increase the size of the core, the winding stack, and the entire transformer.
When transformers get bigger, inter-winding capacitance goes up, and HF bandwidth decreases. Yes, little transformers are better, at least at high frequencies. To get the lows, though, you need more inductance, and that makes it bigger and heavier.
The technique to increase HF bandwidth are more complex interleaving schemes between primary and secondary, but this can cause ultrasonic resonances and poor square wave performance. The art of interleaving has fortunately been somewhat simplified by computer modeling, but there’s still plenty of art involved. You want a skilled builder with plenty of experience with audio transformers.
There ain’t no free lunch with transformers. They solve problems, but you really have to keep impedances low, and work closely with the transformer designer. They’ll tell you what they want, and you tell them what you want, and you work together to meet your goals. In my case, I wanted matched capacitances on the primaries, good phase match at 20 kHz, and the ability to tolerate a certain amount of imbalance current. With SE, of course, it’s all imbalance current, so a (very) large gapped core is required.
A problem with using old transformers is corona discharge where the enamel on the wire has thinned or cracked. Once a point of breakdown occurs, those windings are shorted to each other, and the problem cannot be solved unless the transformer is disassembled and completely rewound using the same interleaving scheme. New transformers are commonly tested with HIPOT, a test fixture that generates 5 kV between the windings and the core.
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My PP 845 Mono's are produced with a optimised performance for the 845, they are Low Wattage approx' 25 Watts when the VU Metre at Biasing is set to 12.5, I have been informed I can push the Watt Output Up if I increase the Bias, but the Trade Off is a reduction in the Head Room, in the past I played with Bias, but the Headroom when perceived is quite addictive as a trait of the Amp.
These Amps have Hand Wound Output and Power Tranx's, the designer/builder has informed me recently, that even though there are increased options for Tranx's since this Amp' was designed, there is not an off the shelf winding that they are aware of, that is readily available to suit the Amp's design.
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@lynn_olson
The original Karna had twointerstages, which was frankly over-the-top. Requirements for the first one bordered on impossible, since impedance from the first tube was much higher than the driver. When I switched over to the 6SN7, I decided enough was enough, and went with simpler inductor loading instead.
I love the candor.
Charles
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Since the 300B fits into the KT88 ecosystem, the same choices for power and output transformer apply, except you’re not messing with ultralinear connections, and some thought (well, a lot of thought) needs to be applied to the filament circuit, which is a very critical node sonically.
I agree about the output transformer. That’s the make-or-break part. Fortunately, we have many good PP output transformers, going back to the Partridge in 1948. Lots of good ones today, too, as long as they are PP and you specify the allowable offset current. The SE world has not quite as many choices, but there are still lots of vendors making good parts these days. Many more than the Nineties.
The interstage is tougher. There are fewer choices, and it is a more difficult design assignment, since impedances are higher than the output transformer. Don’s industry connections came in very handy here, so we had a custom unit designed for us, with outstanding performance.
The original Karna had two interstages, which was frankly over-the-top. Requirements for the first one bordered on impossible, since impedance from the first tube was much higher than the driver. When I switched over to the 6SN7, I decided enough was enough, and went with simpler inductor loading instead.
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A thread with this caliber of participants stimulates further inquiries from posters. Informed commentary is valued. 😀
@charles1dad Such is the nature of the internet I suppose, where fact and opinion freely co-mingle. The physical nature of SET transformers are governed by physical law FWIW and that law isn't interested or caring about opinion. It simply is.
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@donsachs
So many paths to audio nirvana. So many tubes, so little time:)
+1
Charles
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@atmasphere
I’ve said it many times in the past. The greatest limitation SETs have is getting bandwidth as the design is scaled for more power. The OPT is the defining issue.
Yes you have and with clear explanation. I was specifically asking @alexberger because he was quite emphatic. A thread with this caliber of participants stimulates further inquiries from posters. Informed commentary is valued. 😀
Charles
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@atmasphere So many paths to audio nirvana. So many tubes, so little time:)
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The 300B ... all of them ... quite happily accept at least 20 volts of positive grid drive. This is not secondhand info gleaned off the Internet, I’ve seen it for myself on a Tektronix scope screen back in the Nineties. I was frankly surprised, because there wasn’t even a trace of a kink or a glitch as it went from negative to positive grid drive. I was expected more drama from the Big Bad Positive Grid Drive, but nothing, no drama, and no signs of grid or plate overheating, either.
@lynn_olson Back in the 1990s we built an experimental OTL that used four 6300bs (a graphite plate variant of the 300b) per channel. The plate voltage was only 150V since higher than that is impractical in an OTL. To get the tubes to conduct properly we biased the tubes at +15V as their operating point. We played that amp at CES that year. The only reason we didn't produce that amp was it was impractical- that's a lot of money to spend on power tubes for a 15 Watt amplifier! We could get slightly less than double the power using four 6AS7Gs which could be had for less than the cost of one of those 6300bs.
Do you believe that the output transformers in these amplifiers is the overwhelming factor that informs your opinion?
@charles1dad I've said it many times in the past. The greatest limitation SETs have is getting bandwidth as the design is scaled for more power. The OPT is the defining issue.
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Hi @charles1dad ,
I don't have practical experience. But theoretically 300B is winn-winn vs 845.
1. Output transformer has wider bandwidth.
2. Easy to drive. Driver tube and driver transformer.
3. Easy to build a high quality PS for 400v vs 1000v.
The only one but significant advantage of 845 is more power. But it looks like a parallel 300B SET or 300B Push-pull can be a better solution if you need more power.
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To distill much of the above. What struck me (well, Clayton and folks at Spatial Audio as well) with the first build of the circuit in a stereo box with a CCS on the plate of each driver tube was the purity of the sound. It didn't have a sound per se. The amp just got out of the way. The first prototype of the Spatial X4 fully passive speaker with the new crossover was there. It was about 87 dB and 4 ohms. The amp drove it with ease. One of their techs, who is a lot younger than I, liked it LOUD. I went out to grab lunch and when I came back, Ryan had it cranked so loud I could not be in the room. No strain, no clipping, no nothing. It just got louder. He said it played as loud as his Schiit mono block SS amps, and sounded FAR better. It indeed sounds like a 60-100 watt tube amp for drive, but it sounds far better than any other amp I have heard. Of course we have spent a year improving that prototype considerably. There are a number of reasons and they are all covered above. Circuit, power supply topology, coupling methodology, inductance of one form or another between every tube plate and their power supply. Real world modern production tubes, with many NOS versions available. It all sums up. What you hear is what you don't hear... various sorts of subtle distortion from other topologies that give amps a certain grayness. Trust me, I have heard a LOT of other amps. You don't even realize that it is there until it is gone. That is what these amps sound like. Or don't sound like.... Been a fun journey with Lynn, working out real world builds of the amp and preamp circuits. If you can come to Seattle, please hear them and give your opinion!
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@alexberger
A well designed 845 SET will always be inferior to a well designed 300B SET in sound quality
I am a thoroughly happy long term 300b SET owner. However, I do know that there are excellent sounding high voltage transmission tube SET amplifiers. Do you believe that the output transformers in these amplifiers is the overwhelming factor that informs your opinion?
Charles
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The output transformer for 845 SET should be at least 10K Ohm. And it can't be as wide a bandwidth as a 3K output transformer for 300B. Another issue is that driving 845 tubes is much more difficult.
A well designed 845 SET will always be inferior to a well designed 300B SET in sound quality.
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@lynn_olson
The usage of 845,211,805,GM 70 and other high voltage transmission tubes have been manufactured and sold by very reputable and established brands for quite awhile now. They’ve been enjoyed by owners for years without reliability or hazard issues. So it appears they’ve addressed the real world problems you logically cautioned about.
I will say however that this new approach/design of utilizing 300b tubes in push-pull is highly interesting and frankly exciting. Certainly in terms of acquiring higher tube power (Thus expanding speaker choice flexibility). I’m really looking forward to reading listener feedback from those attending the Pacific Audio show.
Charles
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Best thread on Audiogon in a long, long, long time. Great information, and has me wishing I could attend the Seattle show.
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The 845 is not happy at 500 volts. It is a (low-power) transmitter tube, and is designed to work from 800 volts (minimum) to well over 1000 volts.
Once you go over 500 volts, construction, and the parts required, are a whole different world. It requires ham-radio transmitter technique. Parts are air-spaced, wires DO NOT lay on each other, circuit boards are out of the question, and special-order high-voltage power and output transformers are required. Electrolytic caps have a hard upper limit of 550 volts, and 1 kV film caps are industrial parts, not audiophile specials. In short everything is different. Consult a 1950’s American Radio Relay League (ARRL) handbook to see what safe construction technique looks like. It is nothing like audiophile practice.
Sure, the builder can ignore safe construction technique and build it the regular way, but that’s a very serious safety and fire hazard. You do NOT want an amplifier exploding and then catching on fire. Transmitter technique takes us out of consumer electronics and into the realm of professional high-voltage equipment ... interlocked chassis doors, special start-up techniques, status lights, etc.
Yes, I see audio equipment at shows with hard-core Eimac transmitter tubes that light up the room. I would never allow anything like that in my house, unless it was in an outbuilding. The companies that build these high-voltage amplifiers have no track record of building ham or pro radio gear ... they’re just winging it, despite the curved glasswork and the pretty CNC chassis.
By contrast, the 300B lives in KT88 territory, with similar voltages and operating currents. Standard hifi building technique, but still not a plaything. The voltages in the B+ caps are quite lethal, so no poking fingers where they don’t belong.
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I’m sure this will sound wonderful - but I can’t stop and think about why not simply run 845 SE, what are the sonic factors driving the adoption and development of 300b push-pull?
I do understand the desire to avoid working with high voltages however.
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Which I guess leaves the question why does the input stage have the topology it does. The quick-and-easy approach would be a Mullard-style long-tailed pair or differential stage, or maybe borrow from Williamson or Dynaco and direct couple a half-6SN7 input tube to a "concertina" or split-load inverter. It would certainly be cheaper, and is the approach of just about every Golden Age amplifier.
This is the intuition part. There is something wrong with the sound of Golden Age amplifiers ... hard to describe, and it’s not there in SE amps. Something to do with diminished low-level detail, subtly flattened tonality, and a lack of air and "shimmer". The folks at Sound Practices were confident that this "PP" coloration was inherent in push-pull itself, and that’s where I parted company with the common wisdom.
I became convinced the problem was the phase-splitter tube. For one thing, the three approaches to vacuum tube phase splitting (split-load inverter, long-tail pair, floating paraphase) sound quite different, and they all have varying levels of that "push-pull sound". So why not take a passive approach? Studio transformers have been around a long time, and if they are good enough, retain phase integrity through 20 kHz. Then the rest of the amplifier can simply be fully balanced, with none of the circuitry devoted to phase splitting, just amplification. Do one thing, and do it well.
Sure enough, even in the first version of the Amity in 1996, the coloration was gone. It didn’t sound push-pull, and it didn’t sound SE, either. It sounded like itself, and not like anything else. The rest of triode community went their own way, off in SE-land, and I did a lot of historical research for Glass Audio and Vacuum Tube Valley, while thinking of the next steps beyond the 2-stage Amity amplifier.
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The 300B ... all of them ... quite happily accept at least 20 volts of positive grid drive. This is not secondhand info gleaned off the Internet, I’ve seen it for myself on a Tektronix scope screen back in the Nineties. I was frankly surprised, because there wasn’t even a trace of a kink or a glitch as it went from negative to positive grid drive. I was expected more drama from the Big Bad Positive Grid Drive, but nothing, no drama, and no signs of grid or plate overheating, either. The 300B is a pretty tough tube, even the bargain-basement Chinese tubes of the day, back in the Nineties. The only reason I stopped at 20 volts is I lost my nerve at that point, and rolled back the gain. I actually have no idea how much power can be pushed into the 300B grid.
That’s when I realized why this amp, or rather the Amity precursor, sounded like a 60-watt tube amp, or a 150-watt transistor amp (I compared it to a Crown Macro Reference and it played just as loud). It just acts like a compressor when things get hot and heavy, and the separate B+ supply for the driver sails right through the heaviest output stage overload. Best of all, the interstage transformer recovers instantly from overload, nothing like RC coupling which requires the cap to re-charge once grid current flows.
An interstage transformer is certainly a benefit for a single-ended amp, thanks to the efficient power transfer and smooth entry into the A2 region, but the benefit is much greater for Class A push-pull. You see, in "normal" or Golden Age amplifiers, only one driver plate is available when the power tube grid goes into A2 and current starts to flow, or if heavy current is needed to overcome Miller capacitance.
The power tube grids never demand A2 at the same time; they take turns. This is important when a push-pull driver, coupled to a balanced interstage, needs to deliver A2 current into a 300B grid. Thanks to summing in the interstage, both sides of the driver circuit are available to push current at every moment, and not only that, because it is a symmetric circuit that always remains in Class A, it is far more linear than a single-ended driver. It really is a small, very linear power amplifier in its own right, so it shrugs off the demands of the DHT grid.
The key principle in a non-feedback amplifier is the lowest possible inherent distortion, coupled with immediate recovery from overload. A balanced, low-distortion driver that uses a well-balanced interstage transformer offers the greatest voltage swing, greatest linearity, and greatest immunity to a reactive load. And the requirements aren’t trivial: one grid needs to swing up 100 volts while the other grid swings down 100 volts, and this needs to happen at 30 kHz with no slewing, transformer saturation, or power supply sag.
This is far beyond the drive requirements of any pentode (35 volts typical), and double anything seen in a SE amp, unless it uses an 845 transmitter tube. (If you’re wondering why so many 300B amps sound mushy and dull, there’s your answer. It isn’t the 300B. It’s the driver falling short.)
That’s also why I discarded the SE driver -> PP output topology. I built an amplifier that used this approach, but the demands on the interstage transformer were too severe, and it never had the clarity or the sense of unlimited headroom of the balanced driver approach. That’s also when I switched from 5687/7044/7119 driver tubes to push-pull 45’s ... actual power tubes with 25~32 mA of current going through each one. Since the 6V6 was specifically designed to replace the 45 (they bias up pretty much the same) in an era when feedback was not universal, the performance of the 6V6 had to be good enough to replace the 45 in hundreds of thousands of radios in the mid-Thirties. The guitarists adopted it and the rest is history.
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This place needs more info like this, I almost resigned sludging through some pretty pathetic posts that all went south and should have been erased.
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Don,
Oh my, I was not aware that I had the good fortune to be the first in line for the mono's, but I do recall that I was a very early owner of your first preamp as well your DS2 preamp, both remarkable preamps. I am gratified to see revealed on this thread the thinking that went into the creation of these stellar monos and the collaboration that brought it about. The participation of Ralph and other knowledgeable audiophiles, along with Lynn for sure, has given us all an appreciation for the bounds of your joint inquisitiveness that has wrought such a stellar component. Obviously, it like nothing I have ever heard before, just as you said it would. The Spatial Triode Masters (which you encouraged me to buy nearly 6 years ago before you yourself had your own pair which was a stellar recommendation as all owners of them know) sound amazing with your amp and my soon-to-be-delivered Cube Audio Jazzon speakers clearly will love to be driven by your monos so expect a report back on that soon.
Finally, as others have said, this has been among the most informative and civil threads I have ever participated in on the forum and I am among many who are excited to hear your room in Seattle next month.
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Many roads lead to Rome...
As to DHT shootout, using the same driver may suit certain output DHT tubes better than the other ones. Different DHTs also need different output transformers, different B+ voltages and filament supplies, etc. So AB test is not that easy as more than one elements have to be changed for each DHT output tube to have a chance to perform at its best potential. That said, there is no bad DHT in my personal view - assuming adequate power output. So, many good choices are around!
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FYI, the guy who published DH tube comparison more than 20 years ago uses the same driver and same speakers as well the same signal source during his blind listening tests.
I can confirm his report that PX25's bottom is much solid and deeper than 300B. 300Bs could not even match 7A1's deepness. The common sense is 300B could not be driven into A2 region w/o distortion, so I consider 300B is a good candidate for pre-amp/headphone amp which always worked at A1 region. Also, the measurements of output impedance from 300Bs is way higher than PX25s. FYI, my 6336 SE amp beats my 300BXL (beefed up 300B) amp hands down at bottom spectrum.
You are right about driving stage quality is the bottleneck of MOST commercial SE DH amps. but the reason is not what you stated. The real secret of making best SE amplifier is to SHUNT REGULATE POWER SUPPLY for the driving stage. Push Pull amps are less critical of regulated power supply for its driving stage.
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You want a technical presentation, here’s my talk at the 2004 European Triode Festival:
2004 ETF Presentation
As for the lengthy previous post, I am not sure about the impact of dynamic variations in output impedance. One way to measure the impact would be to measure distortion harmonics at a constant voltage output, compare 4 and 16 ohm loads, then repeat again into a reactive loads.
The key point stands: Push-pull Class A triode has the most linear interface to loudspeakers. Nearly all loudspeakers generate significant back-EMFs, which are resonant in character, that reflect back into the output devices. Other topologies have nonlinearities or discontinuities that interact with these resonances, which exaggerates speaker coloration.
There are planar-type speakers that present essentially resistive loads, but the reason for this flat impedance curve is very low magnetic coupling between the diaphragm and the magnetic system. As the magnetic coupling (BL product) becomes more intense and efficiency increases, resonances appear in the impedance curve, as well as smaller narrowband divots that reflect stored energy. Regrettably, speakers are inherently resonant, particularly as efficiency goes up. It has to be kept in mind that most speakers are stupendously inefficient, ranging between 0.3% and 1.0%.
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Fascinating read. I'm definitely stopping by the Spatial room at PAF
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I just scanned through this thread and while some is over my head, it is extremely informative. I love reading the journey behind the development of a new product, especially when the main driver is to create the best sound instead of the most profit. I am only sad because I was planning on going to PAF this year since I missed the first one, but I now have a work event that will not permit me to attend. If I were to attend I think I may be signing up to order these amps/preamp. Hoping and expecting those who attend will post their reactions here, thanks.
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@donsachs @lynn_olson @thom_at_galibier_design
I confirmed a theory I had about OPTs over the weekend. You might give this one a try; its inexpensive. Replace the bolts that hold the OPT together. Typically these are made of steel and are insulated from the transformer by fiber or plastic shoulder washers.
You can get non-magnetic stainless bolts to replace the steel bolts. Its not a big change, but in the case of SETs or lower powered tube amps in general, every drop counts. I measured about a 7% increase in amplifier power. I suspect this will vary depending on the transformer design as well as the specific alloy of stainless bolt used.
The shoulder washers are supposed to take care of the problem of a magnetic short of course and for the most part they do. But they don't do the job perfectly and I suspect that since this technique is 70-80 years old, tradition has set in and caused no-one to look into it further.
I found out decades ago that the mounting bolt in a toroid transformer, commonly made of common steel, would heat up more than the transformer because the actual toroidal mag field was sloppier than theory. So the bolt was a short to the field. By replacing it with non-magnetic stainless the transformer ran at a lower temperature.
I've been working on a low power PP tube amp recently so decided to give this a try.
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Conventional wisdom is that caps etc take a while to run in so do you have to run in the whole design for a a while to get the true sense of it and then tinker with the component parts thereafter.... and then wait for them to run in again before making an evaluation?
@whitestix Usually a thing like a coupling cap will reveal its character fairly quickly. We've been doing this since the 1970s and in that time have yet to see a coupling cap change so dramatically during break-in that it exceeds the character of another, better sounding cap with the same time on it. So you can audition them easily right out of the box. So far the Teflon dielectrics have proven themselves over and over again. A regard paper and oil as very nice sounding parts as well, but they can develop a voltage drop across them which can throw off operating points in the design; IMO not worth the return shipping and frustration!
@lynn_olson I've been harping about the audibility of the higher orders for a very long time. Nice to see some agreement in this regard.
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