I have a problem which I can not figure out. I have biased the voltage on the speaker terminal on zero.

Now when I play music there is a negative DC component on the speaker terminals. Measurable with scope, and you can see the speaker is pulled backwards a little. A soon as the music stops the DC component is gone too.

I have changed swapped the el 509 around. Didi 8 tubes di 4 tubes DC component when music playing is still there.

I did now some voltage measurements:

I put a 50Hz signal of 745 mV on the input.

The values below are all measured in DC in relation to the grounbus.
All power voltages are perfect 170V and 160V plus and minus less then 1 volt difference.
Adjustable Bias is -41V
The -215V bias is -201V
Voltage on the V1 anodes both 81.4 V
Voltage cathode on V1 is 0.382V
Voltage on the V 2 anodes both 2.6 V
Voltage cathode on V2 is - 157V
The speaker output is -0.3V

The values below are all measured in AC in relation to the grounbus.
Input 745 mV 50 Hz signal sinus
The speaker output is 4.3 volt 50 Hz sinus.
grid V2 pin 2   700 mV 50 Hz sinus
grid V2 pin 7   280 mV 50 Hz sinus
I measure 9.16 V 50 Hz sinus on the output cap of V2 from pin 6
I measure 3 V 50 Hz sinus on the output cap of V2 from pin 1

I have difficulty understanding why the driver voltages to the EL banks are different? Are they not supposed to be the same voltage?

Any tips or ideas are appreciated.
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Bruce Rozenblit
You are not supposed to measure the DC offset when music is playing.  There is no problem here.
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Bruce Rozenblit
I'll give you a little more information.  What you are witnessing is an artifact of OTL operation.  The earlier models were much, much worse.  I've been able to almost design it out completely.  It doesn't affect the sound at all.  You just have to live with it and it doesn't harm the speakers.
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Robbert
Thanks Bruce, first time I am doing Push Pull, have always been single ended with transformer or without transformer. I reread your patent and the chapters in your first book and could not find any reference about the DC offset during an input signal..

As I stated earlier I learned so much about tubes from your designs, kits and books. Thanks again for making this audio hobby so engaging.
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