Gibson GA-30RV Off/Standby/On switch

[Randall]

Customer asks if I can install a Gibson style on/standby switch on his 5E3 Deluxe. I understand how the primary side of the of the DP3T switch works, but I'm a little unsure how the upper standby side of the switch is working. Is it connecting the two grids of the power tubes, cancelling them out? Is this the basic theory behind the post PI volume control?

And does anyone see any reason this could not be done to a 5E3?

http://www.ampwares.com/schematics/gibson/ga30rv.pdf

[The Dude]

Yes. That's how it works- phase cancellation. I see no reason you couldn't do the same with a 5E3. If your 5E3 has a ground polarity switch, you could remove the death cap and use the existing switch for a standby switch and save some trouble and parts cost.

[Helmholtz]

The term "phase cancellation" makes no sense to me.
The standby switch shorts 2 opposite polarity signal voltages. So we have differential signal cancellation.

[g1]

The term "phase cancellation" makes no sense to me.
The standby switch shorts 2 opposite polarity signal voltages. So we have differential signal cancellation.

The term is used a little loosely. Opposite polarity signals being 180 degrees 'out of phase'.

[The Dude]

Frankly, I don't see what's so ambiguous about it. IMO, it seems to fit the situation pretty well.

The definition of phase cancellation:

Phase cancellation occurs when two signals of the same frequency are out of phase with each other resulting in a net reduction in the overall level of the combined signal. If two identical signals are 100% or 180 degrees out of phase they will completely cancel one another if combined.

[Helmholtz]

The term is used a little loosely. Opposite polarity signals being 180 degrees 'out of phase'.

Sure, but connecting 2 "out of phase" signals means a short. And a short cancels voltages, not phase. (Zero phase wouldn't mean zero voltage.)
More exactly a short cancels any differential voltage.

BTW, a short always causes increased currents, without changing their phase.

I think the misleading term "phase cancellation" is used to describe signal cancellation/attenuation caused by 180° phase difference of the signals when adding/mixing.

But shorting two (out of phase) signal voltages is not a constructive way of mixing signals.

[The Dude]

We're into a semantic argument here and I'm sure everyone understands how the circuit works, so it's a bit of a silly detour, but I'll say this. As you say, the short cancels voltages, but only because the two signals are out of phase. If they were in phase they would add together when shorted, so the more important point for the given objective is the phase relationship- not the short, IMO. And, again, by definition, this is phase cancellation. I'll move on now.

[loudthud]

In a 5E3, with it's Concertina type phase inverter, the two phases won't cancel completely, but because of the short, the same signal is applied to both power tubes and that will look like a common mode signal. So the result is a pretty quiet output depending on the match of the tubes and the output transformer.

[Helmholtz]

the short cancels voltages, but only because the two signals are out of phase. If they were in phase they would add together when shorted, so the more important point for the given objective is the phase relationship- not the short, IMO.

Sorry, no.

A short cancels any (differential) voltage, regardless of phase relationship of components.


The difference when shorting between in phase and out of phase components would only show in the common mode signal.
While shorting 2 balanced out of phase signals will result in zero common mode signal, shorting 2 in phase (100% common mode) signals would not change the common mode signal level.
(Paralleling 2 equal voltages does not add them.)

In other words, shorting the signals kills any differential voltages but doesn't change common mode voltages.

[SoulFetish]

Sorry, no.

A short cancels any (differential) voltage, regardless of phase relationship of components.


The difference when shorting between in phase and out of phase components would only show in the common mode signal.
While shorting 2 balanced out of phase signals will result in zero common mode signal, shorting 2 in phase (100% common mode) signals would not change the common mode signal level.
(Paralleling 2 equal voltages does not add them.)

In other words, shorting the signals kills any differential voltages but doesn't change common mode voltages.

I think when the dude says they "add" together, he is not using the term in a strict mathematical sense. I think in the context he is simply saying that the 2 AC voltages will "combined".
But, to your point, is it not the superposition of the two AC voltages which causes the nulling effect??... actually, I see it now. Yeah, I think you might be correct in this example.

[Helmholtz]

I think when the dude says they "add" together, he is not using the term in a strict mathematical sense. I think in the context he is simply saying that the 2 AC voltages will "combined".
But, to your point, is it not the superposition of the two AC voltages which causes the nulling effect??... actually, I see it now. Yeah, I think you might be correct in this example.

Not sure if there is a question involved.
Combining two voltages is not a precise technical term. Results depend on how the sources are wired and where the reference point is.

Maybe these explanations help:

Differential voltage is the voltage difference between the PI output/ power tube grid signals. If the PI is perfectly balanced, the differential voltage is twice the signal output (wrt ground) of each PI plate as the 2 opposite polarity voltages appear wired in series.
With a perfectly balanced power stage (tubes, OT), only the differential voltage can produce speaker output.

Connecting the power tube grids shorts/eliminates the differential voltage ( there can't be any voltage between directly connected nodes) but maybe leaves a small common mode signal at both grids.
Common mode signal is caused by imperfections of the PI and is measured between both (connected) power tube grids and ground.

[jack briggs]

Customer asks if I can install a Gibson style on/standby switch on his 5E3 Deluxe. I understand how the primary side of the of the DP3T switch works, but I'm a little unsure how the upper standby side of the switch is working. Is it connecting the two grids of the power tubes, cancelling them out? Is this the basic theory behind the post PI volume control?

And does anyone see any reason this could not be done to a 5E3?

http://www.ampwares.com/schematics/gibson/ga30rv.pdf

did it to mine