Marshall Studio series. Increasing cathode resistor.

[Kuga]

My SC20H cathode biased amp looks like bias is very hot. Near Class A. Same ballpark like my Astoria Custom but It sounds better at high volume.

I think all Marshall Studio series have a 150R 5W cement resistor to bias It. Like my SC20H. Does anybody know If this it's true?
Does anybody moded his Studio series Marshall to get a cold bias?

 

[Jon Snell]

The cathode resistor should indeed be 150R.

 

[PelliX]

My SC20H cathode biased amp looks like bias is very hot. Near Class A. Same ballpark like my Astoria Custom but It sounds better at high volume.

Depends on the valves you have in there, too, of course. After a short period I moved to big glass in there, 6550's - and finally settled on KT88's. Of course it's not cheap to 'try', but I'm not alone in saying it's well worth it in that amp.

 

[Kuga]

The cathode resistor should indeed be 150R.

What values do you think I could Install? 170R? 190R?
Honesty I don't think I'll do It because I have more powerfull amps to do this job but it's good to know.

 

[Kuga]

Depends on the valves you have in there, too, of course. After a short period I moved to big glass in there, 6550's - and finally settled on KT88's. Of course it's not cheap to 'try', but I'm not alone in saying it's well worth it in that amp.

Good option but a resistor it's way cheaper than two valves.

 

[Pete Farrington]

My SC20H cathode biased amp looks like bias is very hot

Schematic?
What V DC do you measure at pins 1, 3, 4, what mV at pins5?
With respect to circuit common.
Also the V DC across each screen grid resistor, typically 1k 5W, and the actual screen grid resistor and cathode resistor resistances.

 

[Kuga]

Schematic?
What V DC do you measure at pins 1, 3, 4, what mV at pins5?
With respect to circuit common.
Also the V DC across each screen grid resistor, typically 1k 5W, and the actual screen grid resistor and cathode resistor resistances.

Power section it's like a SV20. Pin 1-3 266Vdc. Pin 1-4 258Vdc Pin 1-5 -17,4 Vdc. Screen grid resistors Pin4 have a 7,29 & 7,12 Vdc across each. Both are 1K 5W. Cathode resistor resistance it's 148,2 ohms.(150R 5W)
On High power mode. About 230Vac on the wall socket. Stock EL34 valves

 

[Helmholtz]

If I understand correctly you have:
Vak = 266V, Vk = 17.4V, Rk = 148R.
From this I get an average cathode current of 59mA per tube and a plate dissipation of 15.6W (or 62.5% for an EL34).

 

[Kuga]

If I understand correctly you have:
Vak = 266V, Vk = 17.4V, Rk = 148R.
From this I get an average cathode current of 59mA per tube and a plate dissipation of 15.6W (or 62.5% for an EL34).

Thank you. I can see what you mean. Is It a good plate dissipation for a cathode biased amp? Looks like it's ok If you compare It with a Classe A/B fixed bias amp.

 

[Helmholtz]

What made you think the bias was hot?

 

[Kuga]

What made you think the bias was hot?

Because how It sounds when I play It at high volume. I think It's not an amp to play at high volume with a lot of gain using a high output pickups.