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05-28-2023, 12:34 PM
Hi Guys
Champ81: Does your amp have a feedback loop around the power amp? Likely not if it is the typical 18W or 20W with EL-84s.
In any case, we are looking at the cathode-biased output stage in isolation. The note regarding how an MV might be positioned here is for completeness, as many of these amps do have an MV right at the input of the output tubes.
With the stock ground-referenced MV the only caps between the splitter and the output tube grids are the plate coupling caps. Were we to want to retain the ground-referenced MV while adding the bias control system, we have to add a second set of caps to isolate the MV from the bias voltage on the grids which is no longer simply zero volts.
Note that the bias pots are 16mm panel pots BUT they are mounted on the tube plane of the chassis. They are then as accessible and as protected as the tubes. Panel pots are far more reliable than trimpots, the latter which should never be used for bias controls in tube amps.
Yes, safety resistors would be good to add. In the case of the bias pots themselves, the resistor is tied from the pot wiper to the raw bias voltage, which in this case is ground. In a fixed-bias amp, the tie is from wiper to maximum raw bias voltage (which might be pot-0). AND when adding the MV as suggested, a safety resistor would tie to the MV pot between wiper and pot-0.
Safety resistors across bias/MV pots are 10x the pot value.
Increasing Rk will not change the stability of the circuit, it simply reduces idle current. Therefore, there is no limit to how high the Rk value is except for practical considerations.
Suppose the stock Rk=250R. Measure the voltage across it to calculate the idle current. Note this down as the maximum value NOT to exceed.
We might go to 330R to add the bias adjust circuit. You could make this change first simply to read Vk and calculate the current before adding the rest of the circuitry. Say you decide to go higher, to 470R. Again, there will be decreased current. The higher Rk drops more voltage per mA, so Vk may not change a lot, but now there is leeway to adjust the individual and net tube currents back towards stock while hum-balancing the output stage.
As Ik moves towards the original value, the voltage across the new Rk will head higher than the voltage drop across the stock Rk, so you may want to calculate that maximum voltage based on the original calculation of stock idle current. Then make sure the new Rk can dissipate the related power.
Make sure Ck has a voltage rating to accommodate whatever the maximum Rk voltage may be. For example, going from 250R to 500R generates twice the Vk value. Will the stock Ck handle this? If not, change it for a higher-voltage rated cap.
Champ81: Does your amp have a feedback loop around the power amp? Likely not if it is the typical 18W or 20W with EL-84s.
In any case, we are looking at the cathode-biased output stage in isolation. The note regarding how an MV might be positioned here is for completeness, as many of these amps do have an MV right at the input of the output tubes.
With the stock ground-referenced MV the only caps between the splitter and the output tube grids are the plate coupling caps. Were we to want to retain the ground-referenced MV while adding the bias control system, we have to add a second set of caps to isolate the MV from the bias voltage on the grids which is no longer simply zero volts.
Note that the bias pots are 16mm panel pots BUT they are mounted on the tube plane of the chassis. They are then as accessible and as protected as the tubes. Panel pots are far more reliable than trimpots, the latter which should never be used for bias controls in tube amps.
Yes, safety resistors would be good to add. In the case of the bias pots themselves, the resistor is tied from the pot wiper to the raw bias voltage, which in this case is ground. In a fixed-bias amp, the tie is from wiper to maximum raw bias voltage (which might be pot-0). AND when adding the MV as suggested, a safety resistor would tie to the MV pot between wiper and pot-0.
Safety resistors across bias/MV pots are 10x the pot value.
Increasing Rk will not change the stability of the circuit, it simply reduces idle current. Therefore, there is no limit to how high the Rk value is except for practical considerations.
Suppose the stock Rk=250R. Measure the voltage across it to calculate the idle current. Note this down as the maximum value NOT to exceed.
We might go to 330R to add the bias adjust circuit. You could make this change first simply to read Vk and calculate the current before adding the rest of the circuitry. Say you decide to go higher, to 470R. Again, there will be decreased current. The higher Rk drops more voltage per mA, so Vk may not change a lot, but now there is leeway to adjust the individual and net tube currents back towards stock while hum-balancing the output stage.
As Ik moves towards the original value, the voltage across the new Rk will head higher than the voltage drop across the stock Rk, so you may want to calculate that maximum voltage based on the original calculation of stock idle current. Then make sure the new Rk can dissipate the related power.
Make sure Ck has a voltage rating to accommodate whatever the maximum Rk voltage may be. For example, going from 250R to 500R generates twice the Vk value. Will the stock Ck handle this? If not, change it for a higher-voltage rated cap.
