EL34 Push Pull in Class A

[richwalters]

My amp uses 2 parallel pairs per channel KT88's at 265V B+ running at 90ma per tube in UL fixed bias which gives roughly 30W+ o/p with it < 0.1% thd. The full load current only pushes the quies current by a mere 5mA per tube........so we can pretty well call it class A operation. I can recommend this lovely low operating voltage and the very smooth harmonics it creates. The 2K-A-A load could be reduced to 1.5K to get 35W.
Other tubes can be substituted with sim results and will have a long life.

As with other comments, I use 16dB global nfb and the low end response sounds "full".

The only disadvantage is the power supply which has to be substantially beefed up to take stereo operation.

richj

 

[SY]

come to think of it, i can not recall any class A push-pull design that ever came close to the popularity that the Dynaco ST70, a class AB1 design achieved.

i could be wrong though...

Tony, much of the popularity of the Dyna resulted from the low price. The most desired amplifier in those days, hands down, was the Marantz 9. And, guess what? Class A, EL34.

 

[Johan Potgieter]

Much of this goes to show that class was not the outstanding quality determining feature of amplifiers. Many designs were based on faith in what already existed (plus sometimes a modest helping of lore), rather than having sat down and determined by measurements which one of many options was actually superior.

I would still venture to say that the audible quality of any properly designed amplifier does not really depend on its class of operation.

 

[SY]

Johan, I would (once again) venture to agree with you.

 

[Johan Potgieter]

Oooooh, the risks you take, SY....

 

[arnoldc]

Presuming you mean tie the two cathodes together and then down to earth via a single unbypassed CCS.

Hi circlotron, or two balanced CCS - one per tube

My amp uses 2 parallel pairs per channel KT88's at 265V B+ running at 90ma per tube in UL fixed bias which gives roughly 30W+ o/p with it < 0.1% thd. The full load current only pushes the quies current by a mere 5mA per tube

Hi richj, well thank you for the op-points for the KT88. Should come in handy if I don't like the EL34

I would still venture to say that the audible quality of any properly designed amplifier does not really depend on its class of operation.

Hi johan, definitely not. I'm not going after Class A because of the "marketing bull" I've read about it, but for that single goal of using CCS on the cathode of the power tubes. If it sounds good, then I'd be happy. If not, then at least I did something I haven't done before.

SY, One (pair) of them diyAudio CCS is being planned to be in that position. Any thoughts?

 

[SY]

Yes, don't be shy about heatsinking.

 

[arnoldc]

He he he. Those efficient heatsinks costs much more than the boards + parts here. But they look good

Thanks SY!

 

[TonyTecson]

arnoldc,

st petersburg is about 9 hours by plane from where i'm at, an i haven't heard about that news. anyway tubes sounds foreign to local people around here, i have not met anyone who knows about tubes, all i get is a blank stare whenever i ask.

SY,
thanks for the heads up...

 

[Circlotron]

Hi circlotron, or two balanced CCS - one per tube

I have a patent on that one.
It works well

Your output stage is a long-tailed pair really. I wonder if the differential error voltage available at the cathodes is of any use for some kind of feedback? You couldn't just sent it back into any old kind of previous stage though.