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OTL Help

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Hello guys,

I´m not sure which tube to use, have 6C33, 6336,
6AS7, 6C19. If another is better I will use this.

I know the Rozenblit Book, think I have it, but I
am still looking for a reliable, full symmetrical design, circlotron? Don´t like the asymmetrical futterman designs, because they are only working with a lot of feedback.

Thanks for your answers.

Ah, I don´t need much power, 10 Watt with some slam in the bass region is ok.

diyAudio Senior Member
Joined 2002

Hello Reinhard,

With OTL designs you inevitably have to use a certain number of output tubes to get low output impedance after applying global NFB.
So you inevitably end up in the 20 to 40 Watt range if you want to get good bass response into an 8 Ohm speaker.
As in Germany a lot of the speakers have a 4 Ohm impedance you would then need at least twice as much power.

If you need to stick to the Circlotron types of OTL then the only ones currently made (to my knowledge) are Atmasphere.
They have a good support group at this site:


Other possibilties exist: Bruce Rozenblitts' design appeared in Glass Audio and I've built a few based around that with twice the power.
It sounds very good and is pretty straightforward to build.
Maybe he published the circuit in his book as well.

As far as Croft goes you will not find any circuit diagrams drawn by himself but I can tell you that the principles are the same as used in the Rosenblitt design.
His latest designs even add HT regulation (with tubes) for the output stages.
I think Klimo used sweep tubes for the output.The Diefenbacher OTL I'm not familiar with.


Bruce Rosenblit DOES publish the complete schematic in his book Audio Reality. I own the Transcendent Stereo T8 OTL and it is the finest amp I've heard. The only drawback is that I'd like more power for one of my several pairs of speakers. Still, at 25 watts it acquits itself quite well at moderate volumes. Highly recommended.

diyAudio Senior Member
Joined 2002



Best thing for you seems to get another stereo amp and have both bridged for twice the power?
Does the book describe the circuit with the EL509/6KG6 or the one that was published in GA with the 6AS7s'?


For a fully symmetrical approach only the Atmasphere do that according to the Wiggins Circlotron principles.

Another candidate for OTL use is the 6528 if you can find it at reasonable price and quantities.

my otl!

Hello friends,
have build my own in the meantime, looks like the atmasphere, 2*6N1 as diff.cascode amplifier, cap coupled to a 6H30 Cathode followers, direct coupled to 12*6C19 triodes. Some 60 Watt into 8 Ohms.

Use a servo to correct the output DC, an idea of tubecad´s John Broskie, thanks for it.

Have a plus minus of 310 Volts for the driver and
two 180 Volt for the output stage.

At the time I have made first lissening sessions with great pleasure. I am more impressed about the bass than any other thing.

diyAudio Senior Member
Joined 2002



Have a plus minus of 310 Volts for the driver and two 180 Volt for the output stage

Is your amp PP or SE?

Re:the output stage: you didn't use a B+ and B- on anode and cathode on either side of the output tubes.

Use a servo to correct the output DC, an idea of tubecad´s John Broskie, thanks for it.

If you use the B+ and - Ht rails as described any DC is automatically nulled at the output if you take it between the tubes.
It makes me wonder if this wouldn't degrade the performace of the amp.

In the 16 years I use my OTL amps I never had any DC on the output even when pulling tubes out live.

The above is assuming your design is PP.

Can you tell us some more,how does it sound?

As far as I know full schematics for Atma-Spheres OTL amps are not online anywhere. I am currently building an sorta MA-1 clone but I am using 12SX7GT's for input tubes, 6BX7's for driver and 6336B's for outputs. I bought the manual directly from Atma-Sphere and that had a partial schematic and I was able to learn enough about the rest of the circuit by hanging around the ASOG forum at:

I will be using (14) 6336b's per channel.

diyAudio Senior Member
Joined 2002

Hi Dave,

I will be using (14) 6336b's per channel.

You can already get 140 W out of four of these into16 Ohm.

Your amp is going to be a mega-monster.

If you don't mind:

6336 : don't buy any of the Chinese NOS ones,they stink.

Select whatever you have carefully and if possible burn them in for a couple of days.
Not many tubetesters can handle this tube so better built something for this one.
Also keep in mind that this tube will need about 20 minutes of warm up before it sounds ok.

OTHO,you can turn the heating of.

Happy building and you know where to find us,;)
I bought a case of NOS JAN Cetron 6336b's (the last case Fair Radio had in stock) and I will break them in carefully so hopefully I will not have a lot of infant mortality tubes. I have enough spares to last for years (I hope). I have each plate on each tube fused for protection as they do not have the internal fusing that the 6AS7G's have. I have the filaments for each PP pair in series so I can pull tubes out in pairs if I need less power. My Hickok can't test them so I will have to test them all in circuit (I will just keep lots of extra fuses on hand :) :)
Yes I am sure it will heat the 20x22' room up very nicely!

diyAudio Senior Member
Joined 2002

Hi Dave.

NOS JAN Cetron 6336b's

If the Cetrons have acid type etching on the bulb you should be O.K.
If not they're Chinese.

have each plate on each tube fused for protection as they do not have the internal fusing that the 6AS7G's have.

With the 6080/6AS7G I use just one fuse (2A FB) per HT rail,so 1 in B+ one in B-.
If these start blowing for no apparent reason there definitely will be a problem with a tube somewhere.
Fuses degrade the sound quality so I avoid them where feasable.
(non-linear device)

Also the quality of the fuses is very important,I already had mysterious fuses that popped for no apparent reason.
Since then I use the more expensive silver ones and have had no problems.
Another thing to keep in mind is that they can sometimes blow so violently that all that will be left in the fuseholder is shattered glass and that makes it a royal pain the get the endcaps out.
And if that happens to often the fuseholder will start to carbonize as well.
Quality is the keyword here.

By now I've probably scare all takers away but it is really not as bad as it sounds.

My experience with the 6336 is that you better match the triode halves or sooner or later you'll loose half the tube anyway.
Then you wonder who stole the bass.

One good way to make most OTL amps better is to use a seriously overrated HT rail CT Xformer for each side and overrate the rectifier bridges as well.
The more you apply the brute force type of PSU here using good quality caps the better the bass will get.
Do not use caps in series since even if you divide voltages with resistors thet will NEVER balance perfectly.
With currents involved here you don't want them to blow the security valves went one of them starts cooking.
That's a major mess.

Last note: if you're up to it regulate the B+ of the input tubes with a good shunt reg.(tubes of course!)

Caveat emptor.;)
Yes these are USA made 6336b's...I know about the Chinese tubes. I am also using high quality fuses and holders (Littlefuse mil spec) I also have a pin socket test point for each plate so I can measure the current across each 1 ohm plate resistor and that will aid in matching the tubes in circuit. My HT transformers are WAY over rated as are the bridges and caps (no caps in series either) I have beefed things up quite a bit over the Atma-Sphere design because of the 6336's.

diyAudio Senior Member
Joined 2002


Naturally I don't know what you got in your library but if there's one book no tube loving DIY should be without it is
the "Radiotron Designers Handbook".

It is even available on CD if you want to save the rainforrest and it is actually well worth the asking price.


Later this weekend I'll search my CD collection for info on a simple
OTL design but for the moment let's just keep the following in mind:

OTLs with a low tube count in the output stages will invariably have to rely on a fair bit of NFB to get Zout low enough to drive real world speakers.
Most fifties designs were optimized for a 16 Ohm load and often had a zobel network across the output to linearize the output impedance of the then popular FR speakers.

In the most common OTL topology as proposed by the RCA engineers you will find the typical arrangement of the output stage as one still often encounters with a lot of MOSFET amps.
However let's not forget tubes have no complementary counterparts.(NPN-PNP)

This inevitably puts the task down to the phase splitter which has to counteract this by providing dissimilar dephasing.

Just to give you a taster,:D
Answer to Frank

my amp is fully balanced (Atmasphere style).
The plus minus 310 Volts are for the input stage and driver. The two 180 Volt suplies are for the circlotron style output stage. There are also two switchable feedback passes to lower the output impedance.
The sound: you can exactly hear that the limiting output trans is not there. In the highs I can hear things never heard before, but they are on the software (CD and LP). The biggest wonder is that this little triodes are pulling out a great bass, never heard with tubes.

Let me say that the power supply is real diy and oversized. there are 6 pcs. 1000uf/200 Volt bypassesd with Epcos MKP´s for the output stage.

Never heard music like this.

diyAudio Senior Member
Joined 2002

Hi Bas,

Here's an interesting link about OTL topologies:


Are there some good (and simple) OTL designs out there...not necessarily from the well known manufacturers

Sure,but what is the wattage you're looking at?
In other words what is the effieciency and impedance you want to use it with?

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