What tubes for a tube amp?

[leadbelly]

Originally Posted by Ty_Bower

Sorry, none intended. I see very little conversation about OTL. Always assumed there must be something inherently difficult/expensive/evil about it.

There a few OTL fans on here, but not a lot. There are some excellent OTL threads on here if you search. It's definately not for a newbie, unless a kit of course. Of course it's expensive, just think of the number of tubes required and iron needed to power them.

But to answer the original poster's question, IMHO an SE OTL is like hammering finish nails with a sledghammer. That many tubes, that much iron, that much power consumption, a big ugly nonlinear cap on the output, and a few watts of power. What is the point?

[markusA]

Actually "the Beast" looked like an interesting solution.
Except for the "beeing way to big" part.
But yeah, I kind of get the message. It's like crossing the river just to get some water...

[radiostar]

Try 6080 or 6336 ... Lovely stabilisator triodes for OTLs ...

[Bob_345]

you could use a 6as7. there are quite allot otl amps made with these tubes and they look nice

[atmasphere]

The 6AS7G is the most practical tube for making an OTL. 4 of them can do 20 watts into 8 ohms. 8 will do 60 watts if you run class A.

Another tube to use is the 6C33. it runs quite hot so they must be spaced carefully and the socket itself has to to be mounted properly for best air movement. 2 will do about 15 watts into 8 ohms, 4 will do about 60.

The 7241 is the OTL dream tube. 4 will do about 150 watts.

OTLs are in two basic camps. The totem-pole output (Futterman, Rosenblit and RCA tube manual circuits) and the Circlotron. I of course prefer the Circlotron.

You can also use the EL509. Considerations should be made for the plate cap. There is a variant that has the plate connection in the base, made by EI.

The class of operation, like any tube amp, is determined by plate voltage, bias voltage and the load. So it is possible to have a class A OTL, but OTLs are immune to crossover and notch distortions, so they are often biased class AB.

You have no reason to run a plate voltage over 200 volts! The more plate current you can run, the lower the output impedance will be within the limits of the tubes. The Circlotron is preferred as it has about 1/2 the output impedance of the totem-pole circuit. It also needs less feedback (I run often run zero feedback) due to the symmetrical nature of the circuit.

There is considerable advantage in getting rid of the output transformer. If properly built, and OTL can drive a wide range of speakers. A surprising amount of current can be supported by the power tubes- which is why I prefer the 6AS7. BTW, the Russians and the Chinese have both made variants called the 6H13C and the 6N13, respectively. Both last as long and perform about the same but IME the Russians are a bit more consistent. The American 6AS7A should be avoided unless you avoid class A operation. It has a smaller grid heatsink and will not support substantial grid current without overheating the grid.

[SY]

I've been fascinated with the idea of building a Circlotron- what sort of source impedances do you achieve?

[markusA]

It feels like I'm not contributing very much to my own thread but I do read and think about what is being said. It's just that I don't know enough yet to ask any smart questions.

One thing that popped into my head is the matter of feedback. Some use it and some don't, is it really that bad?

An other matter that keeps bothering me is output impedance. How low is usually accepted in designs like these? Sice my speakers are 5ohm I'm thinking it's kind of important to me. Preferably it should be less than 1ohm but I don't know what's feasible?

I guess I should revise my initial statement? Perhaps 40W with a 8ohm load would be a better goal? That should roughly give me my 20W with a 5ohm load?
My current SS monoblocks are rated at 200W@8ohm (1000W@1ohm or something like it). Tubes are a new and wonderful world to me.

[leadbelly]

Originally Posted by markusG

I guess I should revise my initial statement? Perhaps 40W with a 8ohm load would be a better goal? That should roughly give me my 20W with a 5ohm load?

Tubes are a new and wonderful world to me.

Then just forget it. If this is your 1st tube project then attempting 40W into 8 ohms OTL is outrageous and not recommended. Either buy Rosenblizt's kit or start with something with OPT's.

[atmasphere]

Originally Posted by leadbelly

Then just forget it. If this is your 1st tube project then attempting 40W into 8 ohms OTL is outrageous and not recommended. Either buy Rosenblizt's kit or start with something with OPT's.

I do not agree!! Its my guess that OTLs have some weird reputations for being tricky and stuff due to the Futterman amps.

A Circlotron OTL is an easy circuit to set up, but if you want power you will be wiring multiple tube sockets. That in itself is not all that hard; a lot depends on your chassis choice. A 40 watt OTL driving 8 ohms should be quite easy.

You will need 6 6AS7s to do the job for each channel. If you use EL509s you will need at least 4. If you use 6C33s, you will need 4 also, but you will get 60-70 watts based on your class of operation. I think the 6AS7 is the easiest, that's why we've been using them so long. They're cheap too.

BTW 6AS7Gs usually don't need a speaker protection fuse if you rate the B+ transformer fuse correctly, but 6C33s do- its usually a good idea to have *each one* with its own fuse.

Last month there was a great article on a DIY OTL in audioexpress. This particular design featured feedback-corrected DC Offset. I liked it for it succinct nature although I don't like feedback as it injects odd ordered harmonics into the output of any amp, guarantying a hifi sound rather than a real music sound.

So this will be a bigger project than an SET, but what you get out of it is a lot more capable amplifier if you set it up correctly.

I recommend a B+ transformer that has dual secondaries for the output tubes. You will find high current filament transformers easily enough, but the B+ might have to be made up special, although in my early prototypes I found some surplus transformers that did the job, but I had to run two of them per channel. OTOH the parts cost was under $350.00

[tympani1d]

Originally Posted by leadbelly

Then just forget it. If this is your 1st tube project then attempting 40W into 8 ohms OTL is outrageous and not recommended. Either buy Rosenblizt's kit or start with something with OPT's.

Now why do you say that??? Let us work up a nice 40-60w design. Why not use as Bruce Rozenblit does: 6c19p's? They are easily avail. and not killer prices. I think that a bunch of us here would build someting in the 40-80w mono bolck range if we had the designs to work from. Bruce has kits but how many can afford 3200.00 clams for a kit????
SY any chance of a work up here? Atmasphere? Others????
Thanks for the rant.
Back to lurker mode.