EL95 Parallel Push Pull design
 

I designed an Amp using four pairs of push pull EL95 tubes, four in Parallel on each side of the push pull configuration.

Raa for a Pair of EL95 tubes is approx 10K so I opted for an Raa transformer 2.5K since I would have four pairs in parallel (with 43% UL taps) - ok so far.

B+ was set to exactly 300volts using a 6AS7G tube regulator circuit.

The amplifier works perfectly, good frequency response flat about 20KHz and below 15Hz. The problem is the power output. What ever I do, I cannot get more than 7W rms out of it before simple symmetrical clipping (20V pk - pk across my 8ohm test load). This is one quarter of what I expected.

Sowter transformers made the output transformers for me, they are 40W at 18Hz capable, so a lot of iron here. Brian Sowter and I discussed the Raa requirements in detail prior to the order.

It can't be a coincidence the power is 1/4 of what I expect with four pairs, either my design assumption is wrong about Raa or the transformer is wrong. All tubes are working, since unplugging pairs, drops the power evenly. I am using simple cathode bias, approx 330R AC bypassed resistor on each tube. I have swapped the Cathodyne phase inverter with a long tailed pair, zero and approx 10db of feedback, all the same result 8W or 20v pk-pk.


Here is an EL95 datasheet, at 300v B+ and a 330R resistor I get around 11volts on the cathode, (I know this is slightly hot, but I have tried a 22mA current source, 470R resister, everything here, and it still clips at 20v pk-pk).

I am out of ideas... any advice would be appreciated...

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20 Vrms across 8 ohms is 50 watts. That is about what I got with 8 6AQ5's.

20volts peak-peak = approx 8volts RMS = 8 watts

just corrected the typo in my original post!

can you send me a schematic of your 6AQ5 design, what Raa was your output transformer....
6AQ5 looks like a nice tube, maybe I should have used those ...

You could measure AC from anode to anode with two 1:100 probes to check if the OT has the correct winding ratio.

I have some 1:10 probes and my scope is good for 400volts pk... or i could put my signal generator on the spare transformer i have and measure it that way I guess.... but i think I am missing something here...

The fact that it clipped at the same volyage with a different bias point suggests to me that the problem is in the phase splitter. Assuming a 300V rail then (at idle) I would expect to see about 80V on the cathode and 220 on the anode of the concertina triode, check that first.

Cheers,
Ian

I replaced the phase splitter with a long tailed pair from a donor leak Stereo twenty hot wired in. The phase splitter is having a hard time driving 4 pairs of tubes, and maybe I should up the 1K grid stoppers to something a lot more sensible. The concertina splitter is running off a seperate 250v supply... I'll reassemble the concertina splitter and put it back in circuit, but having got the same problem with a completely different known good splitter I am not convinced. As the input is turned up with the output clipping, the phase splitter drive carries on increasing, right up to the point where you even get crossover distortion appearing on the output despite the fact it is clipped. It seems like the OPT is incapable of giving more swing ...

In the OP post you were not sure if the OT was right. If its winding ratio is too large you would see too little voltage on the output.
Another question: does the signal on the grids of the EL95s clip the way the output does? Easier to see if you open the fb loop.

Ok, question answered.