CBS Bandmaster with weird power tube extras.

[Randall]

I have a CBS Bandmaster in for a go thru. It's one of those odd circuits with both a bias adjustment and a 7w 150 ohm resistor on each of the 6L6 cathodes. I never understood why they did this, and this is only the second amp I have seen with it. It also has the 2000pF caps on the grids, which if I understand correctly was done to stop oscillation due to the poor CBS lead dress. But my real question is, there is a 25uF cap going between the cathodes. WHY? And should I remove it? It looks like in later silver face models they did remove it. Can't understand what this cap is for.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Jazz P Bass]

The 25uf capacitor is kind of like the bypass cap on the first 12AX7.
It helps to "pass" all frequencies.

[Jon Snell]

The 25u on the first stages stop signal leaking from the normal to the tremelo channels and vice versa.
25uF ... it is a 5uF.

The 5u 25v Non Polarized, is more interesting between the cathodes of the output valves. It helps to make the higher frequencies from the phase splitter more linear with respect to the misbalanced 12AT7 anode load resistors and helps with high frequency stability. Not very useful in more modern designs with better output transformers.

[Helmholtz]

The 25µ caps at the preamp cathodes are just the usual cathode bypass caps.
They roughly double gain and lower heater buzz.
The second triodes of normal and vibrato channels use a shared cathode resistor.
The bypass cap here also decouples the channels.

The 5µ cap between the 6L6 cathodes shorts differential cathode signals above maybe 200Hz.
When tubes are balanced, the effect is the same as with using 2 cathodes bypass caps of 10µ.
When tubes are unbalanced, the stronger tube helps the weaker one and thus balance is improved.
But I think this can only work at low output where both tubes conduct full time (class A region).

[Randall]

Guys, I am not asking about the cathode bypass caps in the preamp section. I am asking about the 25uF going between the cathodes of the output 6L6 tubes. Should it stay or should it go?

Glad to have the forum back.

[Helmholtz]

I am asking about the 25uF going between the cathodes of the output 6L6 tubes. Should it stay or should it go?

This is not a 25µ cap. You caused the confusion by calling it a 25µ cap.
Rather it's the 5µ/25V cap I was speaking of. At least that's what the schematic says.
It should stay. Why remove it?

[pdf64]

That arrangement gets even weirder with the parallel push pull models https://el34world.com/charts/Schemat...verb_ab568.pdf
ie 100uF between push pull cathodes, 5uF between the parallel cathodes.

[Randall]

Hemholtz, no sir it is indeed a 25uf/25v white Mallory electrolytic cap. I don't understand what it's purpose is other than maybe compensate for CBS inferior lead dress and thinner wiring..

[Helmholtz]

Hemholtz, no sir it is indeed a 25uf/25v white Mallory electrolytic cap. I don't understand what it's purpose is other than maybe compensate for CBS inferior lead dress and thinner wiring..

Well it looks like 5µ/25V from the schematic.
25µ/25V will work even better at lower frequency.
Just make sure it's non-polarized.
It's function I explained in post #4: It improves balance at low output and thus can lower ripple hum.

[Randall]

It is a polarized 25uf/25v electrolytic Mallory white cap. Believe it to be original.

[Helmholtz]

It is a polarized 25uf/25v electrolytic Mallory white cap. Believe it to be original.

This application requires a non-polar (NP) Ecap. Also see schematic.
A normal Ecap might not last long.
Funny thing is a bad cap might go unnoticed, because the amp will work with a shorted cap as well as with an open cap.

[Tom Phillips]

It is a polarized 25uf/25v electrolytic Mallory white cap. Believe it to be original.

This situation has a lot of characteristics pointing to a Fender assembly line mistake during the original build. It's not the first time that such things have been found.