Tech question: Help me understand how this works...

[tubby]

Hi all,

so I had this Fender CBS Super Reverb Master Vol on the bench. Problem was that vibrato and reverb were also on the normal channel when switched on in the same way as they are on the vibrato channel.

In the end I found that the previous tech, who had replaced all the electrolytics, had put the new 25uF cathode cap - which is supposed to go across the 820R of V1B/V2B - across the 470R of V3 (see attached schematic).
Aforementioned resistors are next to each other on the board, that's probably why they got switched up.

When I had put the cap in its proper place, vibrato and reverb were gone from the normal channel. If someone could pls explain how that misplaced cap can be responsible for reverb and vibrato appearing on the normal channel, because I'm not sure I understand. Thank you!

cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

[Jon Snell]

All that will do in mix the valves V1b and V2b making either valve interfere with the other and mixing the sound sources.
Voltage changes on V1b will also occur on V2b as the cathodes modulate each other.

[tubby]

OK, so the signal on v1b will modulate whatever is on the grid of v2b because of the common unbypassed cathode. I get it that far.
But when testing I only used the normal channel. The vibrato channel is thus grounded and there is no signal on the grid of v2b which could be modulated to begin with. I still don't see how that would cause the vibrato to appear on the normal channel.

[g1]

You can modulate (or apply signal) through the cathode or the grid.

[Dave H]

when testing I only used the normal channel. The vibrato channel is thus grounded and there is no signal on the grid of v2b which could be modulated to begin with. I still don't see how that would cause the vibrato to appear on the normal channel.

There's no signal on the grid but there is signal on the cathode which effectively changes (modulates) the grid to cathode voltage and therefore plate current.

[Helmholtz]

A cathode that is not (AC) grounded can act as a non-inverting input or as an non-inverting output. The cathode cap provides an AC ground.

[Leo_Gnardo]

A cathode that is not (AC) grounded can act as a non-inverting input or as an non-inverting output. The cathode cap provides an AC ground.

Indeed - check the inverter stage design for an example. In this circuit, the signal is deliberately transferred from the first triode's cathode to the second triode's cathode simply by connecting them together, while holding the second triode's grid at AC ground with a 0.1 uF cap.

[Justin Thomas]

I <FINALLY> GET HOW THAT STUPID THING WORKS NOW! THANKS, LEO!

No I'm not being sarcastic.

Justin & Jusrin

[Leo_Gnardo]

I <FINALLY> GET HOW THAT STUPID THING WORKS NOW! THANKS, LEO!

No I'm not being sarcastic.

No worries Justin, it took me a long time to "get it" too. Another one of the amazing things you can do with vacuum tubes.

[drewl]

Nice!

You see some weird things like that on old amps or cheap ones trying to get as much as they can with the least amount of parts.

[tubby]

Thanks everybody for explaining! Makes sense now...