Bias pot range

[Blindog]

Ok, I always get confused on bias circuits. I've rewired my ODS build to take El34's, and in the process I changed the bias circuit resistor value to get the EL34's where I wanted them. Right now I have a 2k7 feeding a 10k pot with a 27k to ground. I can bias just about any 6L6, El34, or 5881 to 70% dissipation just fine. I'm wanting to try a pair of KT88's, and with my plate voltage of 503v, I should be around 97mv for the pair, and I can only go as low as 109mv before I bottom out. Any suggestions to tweak , but still mantain the abilty to dial in the other tubes I like to use?

Thanks!

Mark

[tubeswell]

Any suggestions to tweak , but still mantain the abilty to dial in the other tubes I like to use?

Thanks!

Mark

Hi Mark

you could try decreasing the ground leg resistor to 10k-15k and increasing the range of the pot to 20k

Cheers

Pete

[Blindog]

Any suggestions to tweak , but still mantain the abilty to dial in the other tubes I like to use?

Thanks!

Mark

Hi Mark

you could try decreasing the ground leg resistor to 10k-15k and increasing the range of the pot to 20k

Cheers


Pete

Ok, that makes sense. Is there a way to do this without changing out the pot? Thanks for the response!

Mark

['67_Plexi]

Always best to limit the range of the bias for the tubes being used and use a switch and fixed resistors to alter the range for other tubes

The more range you have on the pot the more the bias tends to drift with temperature differences in the amp, even physical vibration can cause minor adjustments because the pot is so sensitive, plus it makes it a real pain in the ass to make a small increment adjustments.

Alan
Carol-Ann