pentode/triode switch

[Randall]

Is there anything wrong with this method of pentode/triode switch? I have read where some say a resistor should be included for current limiting, is this true?

Also, would a 3A 125vac switch be suitable? I did this to my Bassman build, and so far so good, but I don't want to risk damage either.

[Chuck H]

That is how it's usually done. That is, keeping the existing screen grid resistors in the circuit.

[Randall]

Talk to me about switch rating.

[Leo_Gnardo]

Talk to me about switch rating.

After searching high & low for affordable toggle switches rated 1000V more or less, coming up with nothing, I just go with regular good quality Carling switches rated for 250VAC. Though they may not be rated for the DC voltage they're asked to handle in a triode/normal switch, I haven't had any go bad yet. But I have had to replace a couple of Mountain brand switches (Far East manufacture, natch) that failed. No more of them thanx.

Note the mighty Mesa uses Carling rocker switches, little rectangular ones also rated for 250VAC. Haven't seen any of them fail yet either. But I'm not gonna start cutting rectangle holes in chassis whether for mods or builds. Life is difficult enough.

[Chuck H]

I've used plain ol Alpha rotary switches and just doubled up the poles. By "electrical" standards it's not that demanding current wise. It's the voltage rating you need to pay attention to. I don't have a switch to recommend, but just look for a high voltage rating.

[Randall]

So, per my orignal post, the 3A 125vac switches I have on hand aren't going to be reliable, sounds like. I have one in my Bassman build, and I have not experienced any pops or sparks with it.

[tubeswell]

It’s the voltage ‘across the switch’ that is key. There isn’t much voltage difference between most screens and plates in most amps.

[Chuck H]

It’s the voltage ‘across the switch’ that is key. There isn’t much voltage difference between most screens and plates in most amps.

But because there's no shorting balance point there is NO voltage and then high voltage during the action of switching. Not to mention that the screen itself is disconnected from HV during the switching while it's likely conducting (so, rapidly draining current and voltage). I know that you're supposed to put the amp in standby before switching, but then there's the real world with real guitarists at the helm. I think a high voltage rated switch is a sage consideration in this circuit.

[tubeswell]

But because there's no shorting balance point there is NO voltage and then high voltage during the action of switching. Not to mention that the screen itself is disconnected from HV during the switching while it's likely conducting (so, rapidly draining current and voltage). I know that you're supposed to put the amp in standby before switching, but then there's the real world with real guitarists at the helm. I think a high voltage rated switch is a sage consideration in this circuit.

but if you keep a 1M permanently across the switch (as discussed in the previous posts), then the screen is not disconnected from HT at any time

[Randall]

"but if you keep a 1M permanently across the switch (as discussed in the previous posts), then the screen is not disconnected from HT at any time"

tubeswell can you elaborate on this? This has been the crux of my question.

[tubeswell]

here you go

[pdf64]

It’s the voltage ‘across the switch’ that is key. There isn’t much voltage difference between most screens and plates in most amps.

At idle maybe; it may cause problems if the switch was flipped whilst under load.

My concern is that at high signal levels there are kV between the power tube plates of inductively loaded amps; what's the breakdown voltage between the switch terminals and the chassis?
Plus the additional wiring etc kinda invites issues due to parasitic coupling from their radiated field.

Agree that regular quality brand switches seem to hold up in this application, but it's surely way beyond their spec / intended use.

But the biggest issue is that amps generally sound like ass when switched to triode mode, and may have a dollop of ring modulation added from HT ripple unless it's very well smoothed.

[HaroldBrooks]

At idle maybe; it may cause problems if the switch was flipped whilst under load.

My concern is that at high signal levels there are kV between the power tube plates of inductively loaded amps; what's the breakdown voltage between the switch terminals and the chassis?
Plus the additional wiring etc kinda invites issues due to parasitic coupling from their radiated field.

Agree that regular quality brand switches seem to hold up in this application, but it's surely way beyond their spec / intended use.

But the biggest issue is that amps generally sound like ass when switched to triode mode, and may have a dollop of ring modulation added from HT ripple unless it's very well smoothed.

I ran a test of switching from a pentode to a triode power tube mode on one of my single ended Valco amps, and reached a similar conclusion regarding the extra ripple and hum it created.

The guys on this board helped me to fix it, and I added more capacitance to the screen when it was attached to the plate. That quelled the extra hum a bit (but not completely)... but all in all the change in tone didn't impress me that much, and I finally abandoned the idea in favor or other mods. One thing I really can't stand are loud ghost notes created by poor ripple filtering.

I'm sure a triode switch works quite well in some instances and on some amps and it does change the response and tone noticeably, but as pdf64 indicated, you might bring along some other bad sonic or leakage effects for the ride.

[tubeswell]

At idle maybe; it may cause problems if the switch was flipped whilst under load.

My concern is that at high signal levels there are kV between the power tube plates of inductively loaded amps...

In theory there might be for an amp with plates sitting at 500V, but in practice amps aren’t that efficient. You can easily write off 10-20% of the hypothetical peak-to-peak swing once you factor in where p-pmax is on the load line. Also, the screen to plate voltage is in the middle of that plate peak-to-peak voltage, so you halve it again. Having a 1M resistor across the switch alleviates spikes. I agree that most mains power rated switches will cope.

...
Plus the additional wiring etc kinda invites issues due to parasitic coupling from their radiated field.

no argument from me on this. Although we’re talking guitar amps, so...

[The Dude]

I won't argue about specs, but I'll add this: Anyone ever look at the voltage rating on a typical Fender standby switch?

Just sayin', if it works there, it should be fine for a pentode/triode switch.