Mesa Dual Rectifier Solo (3 channel) biasing question

[01redcobra]

Got in a Mesa Dual rectifier 3 channel that had a shorted 6l6gc and a blown screen resistor. Replaced the resistor and the customer requested that I install an adjustable bias so that he could run tubes other than the Mesa tubes. I installed a cermet pot and resistor and proceeded to set the bias voltage for -51 as on the Mesa schematic.
Checking the ma readings on the new tubes, I found that when the amp was in the SS rectifier mode and bold setting (459vdc plate), I was seeing 31ma. Moving it to the spongy setting (414vdc plate), only 26ma. With it in the tube rectifier mode, bold (433vdc plate) gave me 21ma and spongy (390vdc plate) only 18ma.
Adjusting the bias voltage down to -48, the readings were as follows:
(Plate voltages roughly the same in all settings, about 10vdc lower) SS Bold-40ma, SS spongy-35ma, Tube bold-26ma, and tube spongy-21ma.
I'm thinking I should be leaving that bias voltage closer to the -48. The bias readings with it at -51 seem awfully cold to me. Anybody have any thoughts on this?

[Jazz P Bass]

I would think that 'cold' is good in a high gain amp such as this.

And watch those EH5U4 rectifiers.

I have seen a few that shorted.

[01redcobra]

Thanks Jazz, I'll check the rectifiers on my tester, just to be sure. So you think it's ok to lean toward the -51 bias voltage? Even at the -48, the tubes are still running less than 60% (18w). With the -51, in some of the settings, they are probably running at 30% or less

[g1]

Did you check what kind of % bias the Boogie tubes were at with the factory bias?
Many amps are often biased much colder than you would expect, with no negligible effects, the upside being longer tube life.

[01redcobra]

G-one, I did not. I imagine I could still check at least two with 2 new tubes on the other side and see how much difference there is. Of course those Boogie tubes are pretty old.

[Pedro Vecino]

You can not set these four variables without automate settings. Most important in this model is (for me) the SS / tube rectifier bias relationship in bold mode. Once you have verified that the tubes have good symmetry you can adjust the bias in SS rectifier mode (460 V - 35mA per tube, no more) and replace the SS/Tube rectifier selector with a DPDT switch. One circuit for SS / tube selection and another (well synchronized, of course) to incorporate an extra resistor paralleled with bias circuit to compensate the adjustment with the tube rectifiers activated to obtain 38mA.
That is the most effective solution.

[01redcobra]

Thanks for your input, Pedro. The tubes are well matched (thank you Mojo!). I just want to get the bias set so that it doesn't cause any problems further down the road. If it were mine, I would consider re-engineering the bias switching to better compensate for the change from SS to tube rectification. But it is not and I just want to make (and keep) the customer happy and steer clear of "ever since" situations and warrantee issues later on.