Hi,
I'm going to be starting up a build around an AC30 power section. However, I may want to incorporate a higher HV primary that is on the PT (600v) in addition to the standard (560v) tap.
Question: How much additional resistance would I need in series with the single/shared 10ohm cathode bias resistor to be able to get current down to 25-30 milliamps from (what I understand to be) the stock 47ma if I want to experiment? Would a 10-ohm rheostat in series with the stock resistor be enough to experiment and then replace with a fixed resistance, or would it need more than that to slow the flow?
Also, would that 600v tap, rebiased as necessary, necessitate lower than AC30 standard screen voltage?
Thanks!
Joe
EL84 Cathode Bias Adjustment Question
Moderators: pompeiisneaks, Colossal
Re: EL84 Cathode Bias Adjustment Question
What are you hoping to achieve with the higher HT / lower idle current?
The issue is that successful cathode bias p-p amps generally operate close to class A.
Too far from class A and there will be excessive bias shift with signal, resulting in yucky tone.
Assuming that you want more power output, I think it would be way better to switch to fixed bias. That would make another ~15V of HT available and avoid the above bias shift, without messing with the HT winding taps / rectification.
The issue is that successful cathode bias p-p amps generally operate close to class A.
Too far from class A and there will be excessive bias shift with signal, resulting in yucky tone.
Assuming that you want more power output, I think it would be way better to switch to fixed bias. That would make another ~15V of HT available and avoid the above bias shift, without messing with the HT winding taps / rectification.
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Re: EL84 Cathode Bias Adjustment Question
The amount of current at any one point in a series circuit is the total of all the current drawn through that circuit beyond that point. Resistance won't reduce current. However, it will affect the voltage dropped across the resistor.JoeTele wrote:Hi,
I'm going to be starting up a build around an AC30 power section. However, I may want to incorporate a higher HV primary that is on the PT (600v) in addition to the standard (560v) tap.
Question: How much additional resistance would I need in series with the single/shared 10ohm cathode bias resistor to be able to get current down to 25-30 milliamps from (what I understand to be) the stock 47ma if I want to experiment? Would a 10-ohm rheostat in series with the stock resistor be enough to experiment and then replace with a fixed resistance, or would it need more than that to slow the flow?
He who dies with the most tubes... wins
-
gingertube
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- Joined: Mon Nov 14, 2011 2:29 am
- Location: Adelaide, South Oz
Re: EL84 Cathode Bias Adjustment Question
Cathode bias resistors are typically 200 to 270 Ohms for each EL84 (6BQ5).
The AC 30 used a single cathode bias resistor of 50 Ohms for all 4 EL84s bypassed with 220uF. BUT they ran the tubes really hot. You should use a resistor for each pair of output tubes or better yet a resistor on each EL84 for best results.
Recently rebuilt a blown up AC30 Output Stage. EACH EL84 got a 235 Ohm bias resistor ( 2 x 470 Ohm 2 Watt resistors in parallel) EACH with its own bypass cap of 47uF/50V.
The owner says he can't tell any difference in sound from the original. It is now a lot more robust and much less prone to melting its output stage.
If running really high B+ then take each tubes cathode bias resistor up to a 270 Ohm 5 Watt and increase screen resistors to at least 2K2 2 watt.
Cheers,
Ian
The AC 30 used a single cathode bias resistor of 50 Ohms for all 4 EL84s bypassed with 220uF. BUT they ran the tubes really hot. You should use a resistor for each pair of output tubes or better yet a resistor on each EL84 for best results.
Recently rebuilt a blown up AC30 Output Stage. EACH EL84 got a 235 Ohm bias resistor ( 2 x 470 Ohm 2 Watt resistors in parallel) EACH with its own bypass cap of 47uF/50V.
The owner says he can't tell any difference in sound from the original. It is now a lot more robust and much less prone to melting its output stage.
If running really high B+ then take each tubes cathode bias resistor up to a 270 Ohm 5 Watt and increase screen resistors to at least 2K2 2 watt.
Cheers,
Ian