Express 6V6 bias

[ampmodster]

I just finished an Express clone with Ceriatone chassis/board and Moose transformers. Using EL34s I can bias the amp fine. I use 70% of max power dissipation method for the tube by measuring current at the cathode.

But, using a 6V6 I can only set the minimum power dissipation to 12W, which I think is too high for a 6V6.

An EL34 can dissipate more power than a 6V6, is everyone using the same bias for an EL34 as a 6V6?

Thanks for any help.

[paulruby]

Turn 'em down to 23mA or so.

[ampmodster]

Hey Paul,

I agree, I want to lower the current to the 21-23mA range, but I have no more bias pot adjustment. I can change a resistor (lower the 220K) in the bias circuit to give me lower current.

I guess my question is, is no one else having this problem or does everyone just bias the 6V6 extremely hot.

[hairyandy]

I don't have that problem, mine will bias below 21-23 if I want. Maybe you should change that resistor to give you more range on the pot. I'm taking a stab here but did you factor the value of the pot into the total value of the bias resistor?

Andy

[ampmodster]

Andy, thanks for the reply. I will change the 220K resistor to give me lower current (a greater negative bias voltage).

When biasing the EL34s the bias pot is in the middle of the range so I believe everthing is working right. It looks like the bias circuit was designed to bias EL34s.

Kent

[paulruby]

Yeah... Reduce the resistor... I use JTM45-style bias control with a 50K bias pot and I tweak the bias resistor until my max negative voltage is about -45V. Then the 50k pot gives a large bias range for pretty much any tube.

FYI, for a JTM45 with higher B+ (up around 480 to 500V), I do the same but shoot for -55V max negative voltage. That's what a KT66 will need to keep it biased cool. -45V is enough for the lower B+ in a wreck.

[ampmodster]

Sounds like a good plan. I can only get -38V with my circuit.

The reason I brought this up is because I am using the same circuit that is posted on the Express schematics. So I assume others would run into this problem also.

Kent

[mdroberts1243]

I just finished an Express clone with Ceriatone chassis/board and Moose transformers. Using EL34s I can bias the amp fine. I use 70% of max power dissipation method for the tube by measuring current at the cathode.

But, using a 6V6 I can only set the minimum power dissipation to 12W, which I think is too high for a 6V6.

An EL34 can dissipate more power than a 6V6, is everyone using the same bias for an EL34 as a 6V6?

Thanks for any help.

I had the same problem... but I figured on 14Watts for a pair of 6V6GT in Class AB and was able to bias for that fine.

[Rick]

I had the same problem, I lowered the 220K resistor by paralleling a 470K or something. Oddly enough, after I got all my 115AC primary PT problems straightened out, the bias range was workable without the lowering of the 220K.

Rick