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#1 2018-02-27 11:42:10

Tony
New member
Registered: 2018-02-27
Posts: 8

EL34's bias balanced except when in high power output

Hope someone can help me. I have converted the VT-22 V4 to EL34's on two circuits. I am using only two EL34's now (one on each transformer side. I have added the 1k ohm 10 watt grid screen resistors. (I will change them to 5 watt resistors when they arrive), set bias at 29ma each tube.

My question is... when I watch the bias current meters, (a meter monitoring plate current on each of the two tubes) both on idle at 29ma each (545vdc), when playing at low to medium volume, bias seems fine (current fluctuates properly). But when I play loud, (more volume) the bias currents become unbalanced.

Watching both meters, one will fluctuate between 29 to about 95ma while the other fluctuates from 29 past 200ma. I have swapped and changed tubes of different manufacturers and it is definitely the same side of the transformer and not the tubes themselves. I had verified the meters also. I have checked the component values and all are fine, within <5% values.

Then, when I convert back to the 7027A's, re-adjusting the bias (using the other un-modded two tube circuits)the grid currents on both tubes are equal, both reaching together from 29ma to 95ma.

It does not matter which "balanced" EL34's I use, the same side of the transformer current value always excede the other by at least double.

I am using the appropriate speaker output impedance. Two tubes, 4 ohm switch setting for 8 ohm speaker load.

Anyone know what is going on?

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