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Old 30th Nov 2020, 9:15 pm   #1
chas
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Default Push pull EL84 amplifier cathode bias question

I am reviving an old 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.
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Old 30th Nov 2020, 9:23 pm   #2
Trevor
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Default Re: push pull EL84 amplifier cathode bias question

you will need to change the resistors to 220 ohm recommend 5 watt
fairly normal but expect it to sound different
Funny things guitar amps
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Old 30th Nov 2020, 9:26 pm   #3
Megatron
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Default Re: push pull EL84 amplifier cathode bias question

If you use individual cathode resistors you would have to double the value of each resistor 220 r being nearest preffered value. you need the cathode current (or voltage to be the same)
Also I think the bypass capacitor would need to be larger value bypassing each resistor, as there may be more signal across each resistor than if one were shared between two valves.
Ideally though the valves should still be matched.
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Old 30th Nov 2020, 9:29 pm   #4
G6Tanuki
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Default Re: push pull EL84 amplifier cathode bias question

The answer is "it depends".

A pair of EL84 in Class-AB1 can easily do 17 Watts. If you want to get more out of them you need to bias them back [a higher-value cathode resistor/resistors] and provide more drive to the grids.

The problem then comes that - whether you have a common cathode-resistor for the pair or separate cathode-resistors - once you move into driving the valves with anything that delivers aggregated variable cathode-currents, you get non-uniform cathode-voltages and unpredictability occurs.

There are 2 ways round this: either ground the cathodes and provide a 'stiff' bias-supply, or fit the cathodes [whether separately or commoned] with something like a Zener-diode which will keep the cathodes at a fixed voltage irrespective of increased-current at high volume.
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Old 30th Nov 2020, 10:15 pm   #5
chas
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Default Re: Push pull EL84 amplifier cathode bias question

Thanks for that. Was what I suspected.
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Old 1st Dec 2020, 12:07 am   #6
trobbins
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Default Re: Push pull EL84 amplifier cathode bias question

If each EL84 ends up having a significantly different cathode current, and hence dissipation level, then the individual cathode bias resistors can be tweaked in value to get a more balanced cathode idle current, and more equal dissipation. If you were keen, you could tweak to get a balanced anode current if your aim was extend bass response (depending on the output transformer primary inductance roll-off as a function of dc current imbalance).
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Old 1st Dec 2020, 6:48 am   #7
Diabolical Artificer
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Default Re: Push pull EL84 amplifier cathode bias question

Re the cathode bias caps,the formula is 1/(6.28 x R x C) R or C may be subbed by F which is frequency.
Example - 1/(6.28 x 220r x 15hz) = 0.000048 or 4.8u, nearest value is 4.7u but most folk would plonk a 10u in there,which would give a slightly lower bass rolloff. If you do the sum for 100r it's not that different.

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