Herbert: BIAS out of range when mixing 4x EL34 and 2x 6550

[chris_1984]

as the title says, i tried putting in a pair of Sovtek 6550WEs in my Herbert yesterday.

The amp was running on 6x TAD EL34BHTs. after adjusting th 6550s bias to 80-100mA (had to be turned almost completely off counter-clockwise), the bias current on the remaining four EL34s dropped drastically down to 10-20mA.

With the 6550 pair (tubes 3&4) set to 100mA, i can only adjust the other two EL34 pairs to about 50-55mA by maxing (!!!) the bias pot.

I've tried putting one pair of JJ KT88s from my VH4 before, and hadn't had any issues with the bias going out of range like here.

Anyway, the amp runs normal, no tube failure whatsoever. The sound also changed quite drastically. The amp feels a lot drier and like it has less gain now. Haven't tried it at full band volume yet, but i kind of like it though my isocab. It makes the Herbert feel stiffer and tighter in general. More like a VH4, if that makes any sense to you...

 

[racerevlon]

Try moving the 6550s to a different socket pair (1,6?) and see if you get the same result?

 

[chris_1984]

Try moving the 6550s to a different socket pair (1,6?) and see if you get the same result?

yep, it's the same. as expected

 

[chris_1984]

i just tried another quad of EL34s (also TAD B-STR). with these the bias range is fine can i adjusted them to 60mA per pair