OTL amps, my pick for the tube used.

The 6LF6 used in my Futterman H3aa's is capable of decent current delivery. This is a TV tube designed for use as a horizontal deflection amplifier in b&w TV's.

Those who are fans of the Futterman H3aa should keep an eye out for a new OTL designed and built by Roger Modjeski that uses the 26DQ5 which is also a sweep tube designed for very high peak currents. If I remember correctly Roger mentioned the 26DQ5 tube is capable of 1 amp of current and in this circuit, which he designed as a tribute to Futterman, uses 6 output tubes per monoblock, 3 to pull up and 3 to pull down, giving an output current of 3 amps into any load.

giving an output current of 3 amps into any load.

I know this is a good figure for an OTL, but I have to laugh when I look at the flagship models of my amps (100-amps continuous)

https://ibb.co/59M4ZZp

https://ibb.co/VHgjcXJ

We had a pair of these here https://img.ukaudiomart.com/uploads/large/1760474-jbl-array-1400-speakers.jpg driven by the big Fluterman OTL’s, and they were sucked dry on transients when we wanted some medium level out of them.

Cheers George

Obviously people named George need not apply. This will be an amp for more efficient loudspeakers and for people who have some kind of emotional attachment to Futterman. I trust Roger will be able to build one that won't blow up. 
George, you should build one with your 6C33C tube and see how it goes.
Atma-Sphere corners the Market in OTLs as far as this guy is concerned. I have no interest in any amp that can not do at least 200 watts.  
Mike

I’ve always been interested in these which use the 6C33C but at $22k/pair they are out of my ballpark:

http://www.atma-sphere.com/Products/#Novacronhttp://

Those 6C33s in the OP are too closely spaced for reliable operation. The 6C33 is a very hot running tube on account of its prodigious filament.

We re-introduced the Novacron by popular demand, but its a simple fact that other tubes are a lot easier to live with. 

I know this is a good figure for an OTL, but I have to laugh when I look at the flagship models of my amps (100-amps continuous)

Just to be clear, that's 100 **watts** not amps, per the manufacturer's website. With 150 watts into 4 ohms, that's about 6.2 amps total. Our MA-2 can do more than that.

I have yet to talk with the owner of a product that uses 6C33 tubes that hasnt had to live with much higher than average problems with these tubes during the first 50 or so hours of use. Perhaps this is no longer the case. 

Just to be clear, that’s 100 **watts** not amps, per the manufacturer’s website. With 150 watts into 4 ohms, that’s about 6.2 amps total. Our MA-2 can do more than that.

Hi Ralph,

Before George points it out, the datasheet shown at the second link in his last post does in fact say "output current: 100 amps (continuous)," for the ME 1400/ME 1500 amplifiers referred to in that post (as opposed to the OTL amp referred to in his OP).

However, as best as I can tell from some somewhat blurry rear panel photos I found on the net there is a 5 amp fuse located immediately adjacent to the ME amp’s pendant power cord. Assuming 240 VAC (as is used down under) that would correspond to a maximum AC input of significantly less than 240 x 5 = 1200 watts, assuming reasonable derating of the fuse. Which in addition to calling into question the specified "typical" output power capabilities (e.g., 800 watts per channel into 2 ohms and 1500 watts per channel into 1 ohm, these figures stated to be "continuous and with both channels driven"), would mean that 100 amps could be continuously supplied into a load impedance of no more than 1200/(100 x 100) = 0.12 ohms. And actually much less impedance than that, given reasonable derating of the fuse and reasonable efficiency assumptions, and if two channels are to be driven.

In other words, a usage scenario that would never exist in practice.

Best regards,

-- Al

Beautiful amps, I can't afford the tubes let alone the amps though.

I have no interest in any amp that can not do at least 200 watts.
Mike

And hey! I also have no interest in any amp that can’t drive speaker loads that can vary from 2-32 ohms without becoming a tone control.
I just started this post with those pics as they are more a work of art to me rather than listened to, been down that OTL path and with SE transmission tube as well, I will never go back there again.

Cheers George

Before George points it out, the datasheet shown at the second link in his last post does in fact say "output current: 100 amps (continuous)," for the ME 1400/ME 1500 amplifiers referred to in that post (as opposed to the OTL amp referred to in his OP).

@almarg If you look at their website, they claim 100 watts into 8 ohms and 150 into 4. So I am assuming that '100 amps' is a typo, not uncommon on the web... but just for fun, since Power = R x I squared, and giving the amp the benefit of the doubt by assuming a 1 ohm load, 100 amps would work out to an amplifier that makes 10,000 watts into 1 ohms, and 40,000 watts into 4... Obviously the amp can't do that, so I'm going with Occam's Razor on this one, IOW a typo is the simpler explanation :)

However, as best as I can tell from some somewhat blurry rear panel photos I found on the net there is a 5 amp fuse located immediately adjacent to the ME amp’s pendant power cord.

Here is the ME1500 spec sheet for you Al (bmp) a bit clearer maybe.
https://ibb.co/2Fq1fPZ
https://ibb.co/HCYVmfk
https://ibb.co/XpsGYZQ

Cheers George

^^ Clearly the 100 amp thing has nothing to do with its output power. That is likely the 10mS power supply short circuit current.

If you look at their website, they claim 100 watts into 8 ohms and 150 into 4. So I am assuming that ’100 amps’ is a typo, not uncommon on the web

Hi Ralph,

What amplifier are you referring to? What I was pointing out is that George’s statement about 100 amps referred to the solid state ME 1400 and ME 1500, which are not the amp referred to in the OP. And the ME 1400 and ME 1500 are specified as being far more powerful than 100 and 150 watts into 8 and 4 ohms, respectively. And what I was pointing out was that (a) the power ratings of the ME 1400 and ME 1500 into 2 and 1 ohm loads are not consistent with what appears to be the rating of their mains fuses; and (b) the 100 amp maximum continuous "output current" specification of the ME 1400 and ME 1500, even if accurate, would only apply in the case of absurdly low load impedances and is therefore not relevant to any real world usage of those amps. As I’m sure you agree.

Also, I couldn’t find any detailed specifications for output power or anything else for the OTL-150 at the Navison site (just some references to "coming soon"), although that has no relevance to my posts in this thread.

Best regards,
-- Al

Also, I couldn’t find any detailed specifications for output power or anything else for the OTL-150 at the Navison site (just some references to "coming soon"), although that has no relevance to my posts in this thread.

Same here, I have no idea what he's looking at, or the relevance to the ME1500 doing 100amps either????

Cheers George

I don't know about you folks out there but I'm getting dizzy. What does any of this have to do with how an amp sounds. It might have to do with how loud an amp will go but that is about it.

Looking at those spacings the word ‘Chernobyl’ keeps meandering into my subconscious.

Caveat emptor

I have a pair of Audio Mirror monoblocks that use the 6C33C tube and have more than 50 hours and no problems at all. I like the sound of these tubes - very clear, uncolored, balanced and almost no real sonic signature. Tubes with other amps I had always seemed to accentuate some part of the music - EL34s seemed slightly loose in the bass, but great for voices, KT120s were zippy on top, etc. etc. The 6C33Cs are auto-biased according to the Audio Mirror website. Maybe that is why I have had no tube issues yet. But I do have a backup quad! LOL.

From a glance at the manufacturer’s website your amp appears to be far more sensibly designed thermally.

I am lucky that my listening room has 2 vents in it for cooling and temperature is easily controlled by opening/closing the vents. Very cool in summer as I live in SC and it gets very hot here,  but heat from the amp is a non-issue as we have to run AC anyway. The amp gets hot but the heat sinks help plus I put Isoblocks under the footers to raise amps for circulation underneath. One vent across the room blows directly towards the amp. Just lucky, as my last amp was in same location. I really like this amp and think I am done with the search. 

What amplifier are you referring to? What I was pointing out is that George’s statement about 100 amps referred to the solid state ME 1400 and ME 1500

I just looked on the manufacturer's website- and didn't realize it shows a different model. Apparently the models you mention above are n.l.a.

moofoo
6C33C I like the sound of these tubes - very clear, uncolored, balanced and almost no real sonic signature.

That’s why I like them also when well implemented. Because it’s the closest to great Class-A solid state I have heard from a tube.
(better hide, sure I’ll get flamed for this little outburst).
https://dw8stlw9qt0iz.cloudfront.net/8helUHVPvP20LeKbM_SlMzQnfMw=/2000x2000/filters:format(jpeg):qua...


Cheers George

Clio99
Any additional info on Rogers new amp?.  Nothing on website

The 6C33 is a nice sounding tube. The real issue with it is 2-fold:1) its been out of production for about 20 years. The tooling was destroyed with the plant that made it so there won't be any reissues.
2) the tube was designed to use the socket for the Sylvania 3C33, which is a dual power triode with less than half the filament current. As a result, the 6C33 overloads the socket connections which can fail within the lifetime of the tube itself. The sockets that hold up the best are NOS; the Chinese socket which is the most common is not very reliable. As a result special precautions have to be taken to extend the socket life but sooner or later when the socket fails it simply has to be replaced. This may not be a trivial matter depending on how the amplifier is built. Because of the heat involved circuit boards should be avoided in the output section. 

A further issue that is a common problem with OTLs is how the output tubes are biased. Because with any OTL multiple output tubes are used, the grid capacitance tends to be quite high in the output section. This in turn leads to a fair amount of grid current compared to higher voltage power tubes. The result of all this is that controlling the output tubes properly has been a traditional problem in OTL design which has resulted in the reputation that OTLs are unreliable (in a good number of cases this is well justified). This is caused by the fact that to get bass out of the amp, larger coupling capacitors have to be employed in the output section, but the bigger you make them the more coloration they have. This is countered by the fact that the bias network has to employ fairly low values in order to actually control the power tubes (re. the grid current I previously mentioned); because these two factors run counter to each other, designers often fudge the numbers a bit- and often run into issues like blocking distortion (as seen in the Joule Electra amps, which will literally shut down for a few seconds if overloaded) or bias drift, such as seen in the Counterpoint SA-4. When the amp is properly designed, you should be able to easily overdrive it all day long without the amp oscillating (regardless of the speaker load), bias going off or anything like that- and it should recover instantly from overload.


When these factors are right, the amp is simultaneously more transparent and will have greater authority.

its been out of production for about 20 years. The tooling was destroyed with the plant
6C33 overloads the socket connections which can fail within the lifetime of the tube itself.

I’ve asked Mike Matthews for his views on this, from what I understood his factory in Russia has all the dies and tooling, and sell them on New Sensor and the sockets at reasonable prices much cheaper if you join and login.

https://shop.ehx.com/item/6c33/tubes-vacuum-sovtek/

https://shop.ehx.com/item/7big/

I prefer my OTLs of the non-cyclotron, low waste heat, non-autoformer variety.

@bobheinatz, here is what I can say at the moment. The design is past the prototype stage and a few pilot amps will be built for listening tests.These will be monoblocks with moderate power and so far have been tested on a pair of Quad 63 with great results. I am going to have the prototypes in my system shortly on my Quad 57s. Ideally these will work quite well with 16 and 8 ohm box speakers and can include an external autoformer if you have speakers with lower or more difficult loads.

I can say while Roger studied the H3aa circuit as part of his research, this OTL will have more in common with the earlier Futterman designs which were simpler and in the process Roger was able to further reduce the parts count in half. A Pentode/Triode switch will also be available.

IIRC Roger mentioned the Futterman amplifiers have 7 adjustments with two pages of instructions to set up the amp. Roger simplified this by installing a meter with a 6 position switch so you can view the current in each tube and easily see if one is becoming weak or running too hot. There is also just one bias adjustment as the tubes are supplied computer matched. The only other adjustment is the bias for the driver stage and that is done on the same meter.

The 633C initially became popular for its unusual shape and very low cost. Even in small quantities they could be had for $13 including socket. Very unusual to sell a tube with a socket. They actually recommenced replacing the socket with each tube renewal!

Now I see M Mathews is offering them for $142 so this is either new production or NOS. I was told that there were warehouses everywhere with this tube stacked to the ceiling. Don't worry, never will run out, I was told.

This tube is very close to our 6336 made only by Tungsol and equals 3 x 6AS7 in parallel. It has other features and I recommend take the time to read the 3 paragraph description. The number is strangely similar.

https://frank.pocnet.net/sheets/127/6/6336A.pdf

I recently worked on a VK-75 amp which uses 4 of the 6C33. When turning the amp over to work in it the tubes keept falling out of their sockets. Replacing the sockets in most 6C33 amps I have seen is not always easy... but it should be. 

On to the tube I use, the 26DQ5 has all the attributes one can desire in a OTL output tube. Those are

• High Current per tube
• High overload current. RCA advertised 400% for a few seconds.
• Low screen voltage, ability to run in Triode
• Octal socket holds the tube firmly. Much better than wirepins.

The thing that has always concerned me is the cathode flaking I see with Pass Tubes.That tells me they are not up for high peak currents like Sweep tubes are. I have to be very careful while curve tracing these Pass Tubes so as not to flake the cathode. It lowers the peak current and can cause quite a bias shift.

When you get right down to it Sweep Tubes are the only choice for making OTL amplifiers. 

They actually recommenced replacing the socket with each tube renewal!

The original Russian spec sheet called for 750 hours! In practice I've seen the tube outlast the socket.

Now I see M Mathews is offering them for $142 so this is either new production or NOS. I was told that there were warehouses everywhere with this tube stacked to the ceiling. Don't worry, never will run out, I was told.

I got this same story- but was also told by Mr. Mathews that the only supply there would ever be was NOS. So I'm not buying the story that New Sensor controls the tooling, but if that actually turns out to be the case (we've not seen any 6C33s made past the 1990s) it would be a simple matter to set the production up to make the tube for the socket used by the 7241- then the sockets would last decades instead of 100s of hours.

The thing that has always concerned me is the cathode flaking I see with Pass Tubes.

The 6C33 is a pass regulator tube but does not seem to have this problem. IME, the key to making **any** power tube last is 'pre-conditioning'. This is the practice of lighting up the filament circuit for an extended period (often several days) prior to applying B+. Each power tube type has a certain minimum amount of pre-conditioning time that is optimal for the type. Once properly pre-conditioned, the life of the tube can be doubled, and premature failure due to arcing (caused by cathode coating failure) is dramatically reduced.
In the construction of an OTL, there are several factors that should be considered in the choice of power tube, and they are fairly practical. The cost of the tube vs power output is one factor- closely related is availability (which should be considered in any design FWIW). Reliability is important as well as things like plate resistance (which will relate to how many tubes are needed to do the job) as well as the filament current the final tube bank is going to need to make it run.

Availability is a major concern because power tubes fail. While we can get 10,000 hour service lives out of our tubes, knowing that the tube will still be there down the road has been important, since our amps are pretty reliable. So since the inception of Atma-Sphere we've had an internal design rule called the 20-Year Rule, which simply states that since our gear is built to run about 20 years without service (other than tubes)- after which time the filter capacitors may need replacement, its a Good Idea to design the amp for tubes that are available and in production. Since we've now been in business 43 years, this rule has really worked well for us- we've seen competition that didn't take this idea into account and consequently had to close down. In the twilight of the tube era (which ironically has lasted longer than its heyday) it reasonable to expect that the diversity of tubes will dwindle so we've always been conservative about tube types. Its helpful when you have more than one source for tubes!

The ultimate OTL tube in many regards is the 7241 but it's expensive and rare- not a good choice. The 6C33 is a lot cheaper and with slightly over 1/2 the power output capacity of the 7241, still a good choice, but its prodigious filament requirements (and resulting heat production) causing it to eat sockets makes it less reliable.

Sweep tubes could be an excellent choice, hampered only by availability as they are decades long out of production- except for the PL509 and its variants. They have good filament requirements though and are otherwise pretty reliable, although linearity isn't great- but that can be improved by wiring them in triode. 

The 6AS7G is the tube we chose as most on this thread know. But rather than use the American types which indeed proved to have fragile cathode coatings, we used the Russian variant, the 6N13C (marked '6H13C' or sometimes '6AS7G'). This tube has reasonable filament requirements, is easy on the sockets, is inexpensive and over the 43 years we've been in business, proven pretty reliable- such that we have always had a 1-year warranty on the power tubes. Linearity is also good- it is a triode after all- and it has a fairly low plate resistance. Since we were uninterested in building an amplifier with a lot of feedback, linearity in the output section was/is pretty important to our design, as keeping distortion low without feedback is a bit of a trick.

I prefer my OTLs of the non-cyclotron, low waste heat, non-autoformer variety.

So do I. A cyclotron OTL would be a thing to behold though if it didn't make you blind :)