Dual Bias Supply (how?)

[Justin]

I was wondering if anyone might have some thoughts on designing a dual bias setup for the output tubes on the Express. As far as I know, I think the THD Flexi 50 is the only production amp that has this feature that enables the user to bias each power tube separately so that you don’t have to use matched output tubes. Are there any disadvantages to this style of bias circuit? What would be the best way to incorporate this into the express design? Would you have to have a custom power transformer designed with two bias taps or could it be done with a single bias tap? Any information on circuit design and/or a diagram would be greatly appreciated.

Justin

[loverocker]

Kevin O'Connor (www.londonpower.com) has already done the hard work for you in the TUT series of books. Not sure which one you want, but the relevant schematics are in TUT3 and TUT5, and it'd be a breeze to add it to the Express. Slight problem: Most people who have bought the books (which are not cheap but are great value) do not share the schematics.

> advantages?

How about running dissimilar output valves (one EL34 and one 6V6, for example)?

[seulb-rd]

if you are asking how set up separate bias adjustment for each power tube, a bias supply individually adjustable for each power tube is pretty much straight forward. e.g. on the A1 schemo just duplicate the 47k and 20k pot. there will be some interaction between the 2 adjustments, but still it will be easy to dial in both tubes.

hth


ciao...dr. blues