The old "Black Beauty" coupling caps on the Dyna Stereo 70 get bad with age.
They become leaky, and overcome the adjustable fixed bias voltage to the EL34 grids.
Replace them.
Beauty is one thing. Leaky is another.
I had one channel with 100mA (1.56V across the 1.56 Ohm resistor).
But one EL34 was at 40mA, and the other EL34 was at 60mA (its grid bias was less negative than the bias setting).
And that caused saturation of the output transformer laminations. Bad!
And that brings up another problem. Back then, people used a 1.56V carbon cell battery.
They compared that reading, and set the meter to the same reading by adjusting the bias pot.
Today, we have inexpensive accurate DMMs.
Put a 10 Ohm resistor from one EL34 cathode to ground.
Put another 10 Ohm resistor from the other EL34 cathode to ground.
Adjust the bias pot for 50mV across each resistor.
Does not match?
Surprise!
You are not using Properly matched tubes at the operating voltages and currents of your amplifier.
Either use properly matched tubes;
Or add bias pots (4 total for the stereo amp);
Or modify the amp to use Individual Self Bias at each cathode (resistor and bypass cap; 4 sets per stereo amp).
Each of these 3 solutions has tradeoffs; but decide to make it right by using one of them.
They become leaky, and overcome the adjustable fixed bias voltage to the EL34 grids.
Replace them.
Beauty is one thing. Leaky is another.
I had one channel with 100mA (1.56V across the 1.56 Ohm resistor).
But one EL34 was at 40mA, and the other EL34 was at 60mA (its grid bias was less negative than the bias setting).
And that caused saturation of the output transformer laminations. Bad!
And that brings up another problem. Back then, people used a 1.56V carbon cell battery.
They compared that reading, and set the meter to the same reading by adjusting the bias pot.
Today, we have inexpensive accurate DMMs.
Put a 10 Ohm resistor from one EL34 cathode to ground.
Put another 10 Ohm resistor from the other EL34 cathode to ground.
Adjust the bias pot for 50mV across each resistor.
Does not match?
Surprise!
You are not using Properly matched tubes at the operating voltages and currents of your amplifier.
Either use properly matched tubes;
Or add bias pots (4 total for the stereo amp);
Or modify the amp to use Individual Self Bias at each cathode (resistor and bypass cap; 4 sets per stereo amp).
Each of these 3 solutions has tradeoffs; but decide to make it right by using one of them.
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Thank you for your attention! Well, these modifications looks like are well beyond my abilities, but worth a try.
I purchased a set of Ampex matched Electro-harmonix, and the bias been very steady since it set. The sound thats coming out of the amp now differs, but my ears and brain are getting used to it.
I just managed to buy a set of unobtanium NOS Valvo gear, and put it on for only a short test, but when I going to leave them in, I must replace those caps as well.
I purchased a set of Ampex matched Electro-harmonix, and the bias been very steady since it set. The sound thats coming out of the amp now differs, but my ears and brain are getting used to it.
I just managed to buy a set of unobtanium NOS Valvo gear, and put it on for only a short test, but when I going to leave them in, I must replace those caps as well.
By the way before I had a pair of EH EL34 like these and I could bias them for higher current, but they were slightly mismatched. 37ma on one and 33ma on another.
Your bias adjustment range is too small.
Surrounding the bias adjustment pot is 2 resistors 10k and 10k.
As you want less negative voltage i suggest you reduce the resistor
closest to ground to 5k.
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