How does this Marshall Hi/Lo power switch work?

[Randall]

Marshall JCM 900 SLX 100W 2100.

http://www.thetubestore.com/lib/thet...-Schematic.pdf

How does this Ho/Lo switch work? I see in one position the screens see plate voltage through 1K 3W resistors, but I'm not getting what is going on the the other position. Customer believes one setting is 100W and the other is 50W, I don't think is the case, and I need a solid stance to present to him.

[The Dude]

It's a triode/pentode switch. In one position, the 1K screen resistors are fed from the B+ (plate node) of the PS. In the other position, the resistors are fed from another PS node.

[Enzo]

In one position, the screens are connected to the screen supply in the B+ string. In the other position, the screens are connected to the tube plates, making the tube a triode instead of a pentode. That substantially reduces output, but I leave it to wiser people to determine the amount of reduction.

[The Dude]

I don't think it would be quite half power. Even if so, it wouldn't equate to half volume. 2 different things.

[Jazz P Bass]

Measure the output to see exactly what the difference in output power is.

[alexradium]

I retubed a 4100 recently, same deal, Power was half in triode mode, biggest change was the sound not volume.
IMHO that mode eats some type of tubes faster.

[Gregg]

...biggest change was the sound not volume.

+1
It's not so much a "power switch" as a "tone switch".
In triode mode the sound becomes "rounder" but to me it sounds dull. I prefer the pentode tone.

[Enzo]

So many people equate power with loudness. If we cut the power output in half it only reduces the loudness by 3 decibels, not much. SO yes the result is more a matter of tone and feel.

[diydidi]

There is only a 3db reduction in volume. Not really effective. The output transformer then isn't driven enough, which equates to tonal differences.
First prize would be to get a smaller amp. Second prize, install a ppimv.

[pdf64]

I suspect that the most significant contributor to the tonal difference between triode and pentode modes is due to the lower gain and output impedance of triode mode.

[Gingertube]

In the half power position then output tubes are in triode mode as guys have said above.
With the tubes in triode mode you get a VERY large increase in Miller Capacitance at the output tube grids (coz the screen grid is no longer effective in shielding the anode from the grid) and lose top end (the phase splitter can't drive that capacitance as well).
It is similar in effect as if you had wired about 470pF from the output tube grids to 0V.
Often described as a "darker" tone - in fact that is the usual criticism of triode mode in Git Amps.
Cheers,
Ian

[Chuck H]

According to my J.L.Hood book the ideal OT primary impedance is considerably lower when pentodes are wired for triode operation. Assuming this is true (as I must because it's J.L.Hood) some of the power reduction (and tonal change/detriment?) may be due to mismatch from an ideal load on the power tubes. And I have to wonder what difference idealizing the load might make both for power and tone. Especially considering the popularity of triode operated pentodes in hi fi amps.?. Just an observation that I can't reconcile with my tech limitations.

[Leo_Gnardo]

In the half power position then output tubes are in triode mode as guys have said above.
With the tubes in triode mode you get a VERY large increase in Miller Capacitance at the output tube grids (coz the screen grid is no longer effective in shielding the anode from the grid) and lose top end (the phase splitter can't drive that capacitance as well).
It is similar in effect as if you had wired about 470pF from the output tube grids to 0V.
Often described as a "darker" tone - in fact that is the usual criticism of triode mode in Git Amps.
Cheers,
Ian

Thanks for the explanation Ian. That triode "darker tone" - some of my customers love it. Some, not so much.

I'm sure the hi fi amps that use triode have OT's made to match impedance. Whether there's some treble pre-emphasis (or feedback compensation) to make up for the Miller effect treble loss, that's optional and I'm sure what makes the difference one brand of amp to another. Here's another place for the smart custom amp builder to use his/her brain & ears.

[darkfenriz]

Miller effect or lack of treble boost over voice coil inductance (present in pentode current drive, less present in 'mixed' triode drive)?

[Mick Bailey]

Some of my Hifi builds use switchable pentode/triode, though the OTs are for pentode use. I use a local feedback loop to help linearize operation. There isn't that much tonal difference overall - quite subtle, though there's a slight increase in volume in pentode mode. I think the main difference is that hifi amps don't intentionally distort so a guitar amp emphasizes the different characteristics a lot more.