What's the design purpose of hybrid fixed/cathode bias?

[Rhodesplyr]

I've been slowly working on a bit of a basket-case Magnatone M15A, so I've become very familiar with the circuit.

It uses a 2 x 7189A push-pull output stage with a -19V fixed bias supply, but instead of having their cathodes connected to ground, the pair of 7189As shares a single 100Ω unbypassed cathode resistor.



I read another discussion on this site where it was pointed out that this bias arrangement would introduce some compression. I can see that as the output stage gets pushed beyond Class A, the unbypassed 100Ω resistor becomes a negative feedback source, limiting output by allowing the cathode voltage to rise.

Is hybrid bias used in guitar amps like the Magnatone primarily as a dynamic voicing trick, or are there other reasons for doing it? It's not done very often. (And I'm aware of Fender's brief flirtation with it.)

Perhaps to ask the question another way, why not just have a cathode-biased output stage with an unbypassed cathode resistor? I have seen that in some old Premier amps.

[Chuck H]

Every designer has their own ideal. As does every player. The mix of bias methods does create a hybrid envelope. But probably more likely is that it offers a cushion for marginally matched or spec'd power tubes. The small addition of self bias was probably enough (back in the day) to allow a company to build boards and plug in whatever came in the box of tubes!?! It should be noted that the DC bias across the cathode resistor isn't dependent on the bypass cap OR not having one. So certainly one could have been used. But those things are EXPENSIVE! Or at least they were, relatively. So I think the idea was to maximize performance with the fixed bias aspect, but offer some leeway in assembly with the cathode bias. It's possible tone and response was a consideration, and that was likely ear tested before approval (?) but I don't think the tonal affect of the hybrid design was the primary purpose.

[Justin Thomas]

It's not the first time I've heard the explanation Chuck gave, to help with bad tube matching. I can't speak for your amp, but in 1965 Fender started with the Bias Balance instead of adjust on the AB165 Bassman. That's right, for 3 years a Blackface amp came with that "horrid" Bance...

In 68 they went to a "mixed-bias" (as I call it) for all the amps except the Champ. Supposedly these were those SF amps that sounded so bad they were all returned in droves and Fender went back to a pure Fixed Bias system but kept the Bias Balance on everything, in early 1969.

I got a January 69 Dual Showman Reverb that was all original excet for the power cord & 1 cap. I changed the electrolytics & fixed the reverb tank, and DAMN, that wasone of the BEST sounding amps I've ever owned. Maybe it was that "mixed-bias," maybe something else, but I've never heard another Fender like it. It was still "clean," but had a certain "slur" on the leading edge of notes that was just... Drool. I am occasionally tempted to mod my Bassman 100 to use that system, just to try to recapture it. I'm actually kinda sorry I sold it, but it was to a very good friend & it made her Jazz Bass sound just... Drool...

Anyway, I no longer ask "why." I ask "why not?" I loved it. I haven't had another 68 Fender to compare it to so I can't be sure that was it...

Justin

[Helmholtz]

I think it's essentially a hifi design optimized for lowest distortion, probably copied from some tube application handbook (also note UL operation).

As can be seen from the uneven spacings of the output characteristics, pentodes expand the signal with higher plate currents. An (unbypassed) cathode resistor can compensate for that effect.

Having both, fixed and cathode bias gives freedom to choose the cathode resistor value for best compensation, i.e. lowest distortion.

[Chuck H]

I think it's essentially a hifi design optimized for lowest distortion, probably copied from some tube application handbook (also note UL operation).

As can be seen from the uneven spacings of the output characteristics, pentodes expand the signal with higher plate currents. An (unbypassed) cathode resistor can compensate for that effect.

Having both, fixed and cathode bias gives freedom to choose the cathode resistor value for best compensation, i.e. lowest distortion.

Oh we do like having you around sir. Here's another

[Rhodesplyr]

I think it's essentially a hifi design optimized for lowest distortion, probably copied from some tube application handbook (also note UL operation).

I like this explanation, including taking UL operation into consideration. A huge proportion of the circuitry of this amp goes into Magnatone's complex stereo vibrato circuit, so it would make sense that they want you to hear that effect cleanly, without a lot of output stage distortion. Also, in this amp, the reverb tank is driven by one output stage