I am reviving an old WEM guitar amplifier which has push pull EL84s in what is rated (optimistically I thought) as an 18watt effort. It is a conventional setup where the EL84 anodes are directly connected to the OP transformer while the screen grids run from HT via a 1.5K resistor. The EL84 cathodes are commoned to ground via a single 100R bypassed by 50uF. To avoid having to use matched EL84s I would like to institute separate cathode biasing. My question is should I merely duplicate the existing for each valve or should resistor values be changed? I am afraid I do not really know how to calculate this and would be grateful if it could be explained. Thank you.
The bias voltage is to remain the same for both tubes as it is now. But with the existing resistor you have twice as much current with two bias currents. So the calculation uses Ohms Law. Ohm's Law can be calculated as Voltage /Current = Resistance. Or it can be arranged as Current x Resistance = Voltage. Since we want the same voltage and we are going to use half the current through the resistor, to have the same voltage the resistance has to double. You can get 220 ohm resistors for each cathode.
Can I vote to not bother? You don't NEED to use matched pairs, certainly the original amp maker never did. ANy small differences in tube current won't hurt the amp. Remember, it is just a guitar amp, not a hifi. So balancing the power tubes is more of a hifi thing, and unless everything else about the power amp stage is "balanced" doing separate tubes won't accomplish that.
Unless you plan to make those cathode resistors adjustable, making them separate won't balance the amp, you'd still "need" to use matched pairs of tubes.
Unless you plan to make those cathode resistors adjustable, making them separate won't balance the amp, you'd still "need" to use matched pairs of tubes.
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