Individual Bias POTS for an Express?
Moderators: pompeiisneaks, Colossal
Individual Bias POTS for an Express?
Has anyone done this, and can anyone provide a schematic ? I am using Spectrol trim pots. I have been refering to diagrams in the TUT books, but I can't seem to make it work. Thanks.
Re: Individual Bias POTS for an Express?
I don't own any of Kevin's books , but I can tell you that the express biasing is a little different than your standard Marshall setup. You aren't taking the bias voltage from the wiper of the pot. It is more like a Fender. The bias pot varies the resistance to ground and changes the voltage divider made up of the 15k, 47k and 25k pot. The bias is taken from the junction of the 15k and the 47k and the resistance to ground from that point is between 47k and 72k. To add a seperate pot for each tube it would be easier to change the circuit all together and then split the two 220k grid resistors apart and feed each one with it's own bias.
Re: Individual Bias POTS for an Express?
Perhaps you are thinking about what to do with unmatched tubes? Maybe you can work in a bias balance pot. That would probably be easier than building two bias supply circuits. Really, though, matched pairs are relatively easy to obtain, so this is extra stuff for not much reward...unless, of course you've got some old tubes made of unobtanium that you really want to use.
Re: Individual Bias POTS for an Express?
Individual bias supplies are technically "required" for fussy high-power tubes like 6550s and KT-88s. It it's only two ouput tubes, it's not that hard to implement. In the Trainwreck bias supply, remove the single 25K pot and replace the 47K resistor with two 25K cermet pots (the little blue thingies work fine). One end of each goes to the 10uF cap, the other ends go to a load resistor (47K is probably fine, but may need to be smaller to get -33VDC) and the wipers are your bias source for each tube. Split the common connection on the 220K grid return resistors and feed each one separately from the cermet pots' wipers. If you're paranoid, you can also connect another 220K resistor from the bias supply source end of the pots to the wipers -- if the pot fails, you won't lose negative bias voltage.
BTW, the grid return resistors need to be smaller for some tube types. 6550s in particular have a published maximum grid circuit reistance of only 50K (although you can get away with 150K - Marshall did).
Set up this way, you can get each tube to draw exactly the same current so you can use unmatched tubes more easily (unmatched for current -- transconductance is a whole 'nother story). The biggest advantage is that you control the DC balance of the OT so that each side has the same current flowing = near total 120Hz hum cancellation!
You can do this with four-tube amps too, but that requires 4 pots AND 4 coupling caps from the PI. You'll also have to recalculate the grid return resistor values and the cap values to maintain the same time constant.
BTW, the grid return resistors need to be smaller for some tube types. 6550s in particular have a published maximum grid circuit reistance of only 50K (although you can get away with 150K - Marshall did).
Set up this way, you can get each tube to draw exactly the same current so you can use unmatched tubes more easily (unmatched for current -- transconductance is a whole 'nother story). The biggest advantage is that you control the DC balance of the OT so that each side has the same current flowing = near total 120Hz hum cancellation!
You can do this with four-tube amps too, but that requires 4 pots AND 4 coupling caps from the PI. You'll also have to recalculate the grid return resistor values and the cap values to maintain the same time constant.