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Re: Cathode Bias Idling Current

1/2/1998 1:52 PM
stephen	Re: Cathode Bias Idling Current Doc, You could fit turret pins, but the edge of the phenolic tube base is brittle. I just drill a small holes and glue in little loops of stiff wire. Alternatively, Mouser sells a small test-point loop with a glass bead collar for about 40c. Your reference to balancing cathode-biassed o/p stages by including small adj resistor networks is a good one. Rather than go into details here, I suggest anyone interested could look through some of the "dusty files" at triodeel.com. There are examples of several variants for dc and/or ac balancing. BTW, if I may compress and oversimplify here...I would say that DC balance is usually a good thing, but if this can be achieved together with a little AC UN-balance, the result can sound nice for guitar. One way of getting this (doesn't always work) is to fit a non-matched output pair and then bias them separately to balance their idling currents. There's a good chance the rest of the curves _won't_ match exactly...so you get balanced DC in the OT (and low hum where you need it most for low playing levels) but an unknown quantity of unmatching at higher levels....unsymmetrical + and - halves of the signal...lots of 2nd harmonic if you are lucky..and if you like that kind of thing. Of course, you could also just fit unequal R's in the PI ... but I mention the idea to illustrate that moderately unmatched o/p pairs of rare and desirable tubes don't have to sit on the shelf waiting for a third which matches on of the other two If you can approx balance the idle currents and get them both into about the right area, then give it a try - you may like it. And ...yes I often fine-tune the separate bias points of o/p pairs in small guitar amps by going for minimum hum from the speaker. As long as the idle currents then don't measure much different from each other, and the waveform looks ok on a scope and sounds ok, I'll go for least hum rather than exactly equal cathode currents. They don't always coincide exactly...maybe different curves, or slightly different _screen_ currents, but I've never looked into it. Separate bias resistors do work...may sound different when the amp gets close to clipping, and the rules for bypass caps (or not) are different. Usually you need bypass caps on separate Rk's. Separate Rk's _does_ have the big advantage that one tube turning off from old age won't fry the other one at double its intended current, and of course that you can set the idle currents separately (without additional balance networks) so moderately unmatched tubes may be fine. As always, please throw any contrary opinions back this way......discussion welcome Cheers...Stephen

1/2/1998 1:52 PM

stephen

Re: Cathode Bias Idling Current

Doc,
You could fit turret pins, but the edge of the phenolic tube base is brittle. I just drill a small holes and glue in little loops of stiff wire. Alternatively, Mouser sells a small test-point loop with a glass bead collar for about 40c.

Your reference to balancing cathode-biassed o/p stages by including small adj resistor networks is a good one. Rather than go into details here, I suggest anyone interested could look through some of the "dusty files" at triodeel.com. There are examples of several variants for dc and/or ac balancing.
BTW, if I may compress and oversimplify here...I would say that DC balance is usually a good thing, but if this can be achieved together with a little AC UN-balance, the result can sound nice for guitar. One way of getting this (doesn't always work) is to fit a non-matched output pair and then bias them separately to balance their idling currents. There's a good chance the rest of the curves _won't_ match exactly...so you get balanced DC in the OT (and low hum where you need it most for low playing levels) but an unknown quantity of unmatching at higher levels....unsymmetrical + and - halves of the signal...lots of 2nd harmonic if you are lucky..and if you like that kind of thing.
Of course, you could also just fit unequal R's in the PI :-)

... but I mention the idea to illustrate that moderately unmatched o/p pairs of rare and desirable tubes don't have to sit on the shelf waiting for a third which matches on of the other two :-)

If you can approx balance the idle currents and get them both into about the right area, then give it a try - you may like it.

And ...yes I often fine-tune the separate bias points of o/p pairs in small guitar amps by going for minimum hum from the speaker. As long as the idle currents then don't measure much different from each other, and the waveform looks ok on a scope and sounds ok, I'll go for least hum rather than exactly equal cathode currents. They don't always coincide exactly...maybe different curves, or slightly different _screen_ currents, but I've never looked into it.

Separate bias resistors do work...may sound different when the amp gets close to clipping, and the rules for bypass caps (or not) are different. Usually you need bypass caps on separate Rk's.

Separate Rk's _does_ have the big advantage that one tube turning off from old age won't fry the other one at double its intended current, and of course that you can set the idle currents separately (without additional balance networks) so moderately unmatched tubes may be fine.

As always, please throw any contrary opinions back this way......discussion welcome :-)

Cheers...Stephen