Question on Biasing my Cameron Modded Marshall

[Drew]

Based on recommendations from a thread a few weeks ago, I purchased some Winged C's EL34's for my Cameron modded Marshall (100 watt 1977 JMP). I had my tech bias it and he said the plate voltage was 500 and he set the bias to 32. He said before he adjusted it, it was around 39

The amp still doesn't sound quite right and another friend suggested that 32 was too cold and believed 34 to 35 was more in line with where it should be.

Psychodave or anybody else, what's your experience on where this should be?

 

[Death by Uberschall]

EL34 with 500v at 39ma is 78% is dissipation and really hot.
EL34 with 500v at 35ma is 70% dissipation and hot and the industry accepted max.
EL34 with 500v at 32ma is 64% dissipation.
EL34 with 500v at 30ma is 60% dissipation and warm.
EL34 with 500v at 25ma is 50% dissipation and cold.

So he's got you set in between warm and hot which is not bad at all. But it needs to be set where it sounds the best. Just keep in mind, as you go up, tube life goes down.

 

[Drew]

EL34 with 500v at 39ma is 78% is dissipation and really hot.
EL34 with 500v at 35ma is 70% dissipation and hot and the industry accepted max.
EL34 with 500v at 32ma is 64% dissipation.
EL34 with 500v at 30ma is 60% dissipation and warm.
EL34 with 500v at 25ma is 50% dissipation and cold.

So he's got you set in between warm and hot which is not bad at all. But it needs to be set where it sounds the best. Just keep in mind, as you go up, tube life goes down.

Thanks. Good info. So he set it right.

 

[Death by Uberschall]

Couple of things I forgot to mention and ask about;

was the 39ma with the old tubes or the new ones?
was the 500v measured at 39ma or 32ma?
after setting the bias, did he recheck the voltage?

The thing with the bias adjustment and voltage is that as you increase the bias and run the tubes hotter, the voltage comes down. So after every adjustment you have to run the formula for ma vs. voltage to get the dissipation percentage.

 

[Drew]

Couple of things I forgot to mention and ask about;

was the 39ma with the old tubes or the new ones?
was the 500v measured at 39ma or 32ma?
after setting the bias, did he recheck the voltage?

The thing with the bias adjustment and voltage is that as you increase the bias and run the tubes hotter, the voltage comes down. So after every adjustment you have to run the formula for ma vs. voltage to get the dissipation percentage.

Good questions. I'll have to ask him.

 

[jcj]

Based on recommendations from a thread a few weeks ago, I purchased some Winged C's EL34's for my Cameron modded Marshall (100 watt 1977 JMP). I had my tech bias it and he said the plate voltage was 500 and he set the bias to 32. He said before he adjusted it, it was around 39

The amp still doesn't sound quite right and another friend suggested that 32 was too cold and believed 34 to 35 was more in line with where it should be.

Psychodave or anybody else, what's your experience on where this should be?

To be totally honest, they don't all sound great.

 

[dawnofdreamx97]

id go to 40 mv personally

Tubes Wont last as long but it be viscious and much faster. But i think you should set it by ear to some degree. I know Mark does this sometimes.