Standby switching in VVR'd or Power Scaled amps

[Gaz]

I was just reading Kevin O'Connor's article on standby switches http://www.londonpower.com/standby-switch, and noticed this short paragraph:

"In a Power Scaled amp, the standby should be rewired into the screen circuit after the Power Scale regulator. This is especially important in amps with a ground-lift standby in the centertap of the plate winding."

Any clue as to why this is a bad idea? I've had a standby arrangement like this in a VVR'd amp (not sure, of course, how much Power Scaling (TM) differs), and have had no problems, but don't want to put any unnecessary stress on anything because of it!

[John_P_WI]

Kevin is big on reducing the stress load to the switches. Of course, placing the switch after the power tube plates reduces the power tube caps loading and OT inductive load.

IF I were to guess why with the center tap standby switching method as questioned, it would have to do with the PT losing it's ground reference when the center tap is lifted possibly allowing a momentary voltage spike that exceeds the pass regulator ratings OR the possibility that the source pin of the pass regulator could have a higher voltage on it than the drain as the power scaling pot will be pulling the drain to ground.

Anyhow, standby is not needed with power scaling - just turn the "voltage down".

[R.G.]

Not sure why he's after that. But if it's a question of reducing switch stress, the switch stress can be reduced to nearly zero with this technique:
http://www.geofex.com/FX_images/Return% ... Fol....pdf

If it's surge stress on caps, the current clamp eliminates that, too.
http://www.geofex.com/Article_Folders/m ... 0Clamp.pdf

The current clamp and switching can both be done on the negative side, too.

[teemuk]

Interrupting the plate supply is not really that good idea in the first place. Yes, it prevents cathode stripping in transmitter tubes that work with plate voltages in excess of 1KV, but "guitar amp tubes" are not transmitter tubes and B+ voltages are never that high so cathode stripping is really no issue.

Cathode poisoning, on the other hand, is. And tubes become suspect to it when they are heated without plate voltage. Ouch. The usual B+ interruption standby not only fails to increase tube life, it actually decreases it.

If you put standby switch to screen supply the issue is overcome but you still have to struggle with switching-related surge currents of a high voltage supply.

If you put standby switch to power tube cathode circuit (e.g. in a fixed bias amp, toggling the standby "opens" the cathode circuit and disables the power tubes) you overcome both issues. In practice, do not "open" the cathodes, just switch from short circuit to, say, 100K resistance (this actually increases power tube lifetime while they are on standby). You can switch with MOSFETs or relays and put all kinds of neat stuff to their driver circuitry in addition to that standby a.ka. mute switch (e.g. delayed mute on startup, tube overcurrent protection, etc.)

[Gaz]

Interrupting the plate supply is not really that good idea in the first place. Yes, it prevents cathode stripping in transmitter tubes that work with plate voltages in excess of 1KV, but "guitar amp tubes" are not transmitter tubes and B+ voltages are never that high so cathode stripping is really no issue.

Cathode poisoning, on the other hand, is. And tubes become suspect to it when they are heated without plate voltage. Ouch. The usual B+ interruption standby not only fails to increase tube life, it actually decreases it.

If you put standby switch to screen supply the issue is overcome but you still have to struggle with switching-related surge currents of a high voltage supply.

If you put standby switch to power tube cathode circuit (e.g. in a fixed bias amp, toggling the standby "opens" the cathode circuit and disables the power tubes) you overcome both issues. In practice, do not "open" the cathodes, just switch from short circuit to, say, 100K resistance (this actually increases power tube lifetime while they are on standby). You can switch with MOSFETs or relays and put all kinds of neat stuff to their driver circuitry in addition to that standby a.ka. mute switch (e.g. delayed mute on startup, tube overcurrent protection, etc.)

Thanks for the reply, and I'm sure someone else will find this info useful, but it's really a dead horse around here, and many have paraphrased Merlin's book ad nausem on the subject. The amp already has a standby, and unlike the cathode or screen switching, it actually mutes the sounds completely.

Kevin is big on reducing the stress load to the switches. Of course, placing the switch after the power tube plates reduces the power tube caps loading and OT inductive load.

IF I were to guess why with the center tap standby switching method as questioned, it would have to do with the PT losing it's ground reference when the center tap is lifted possibly allowing a momentary voltage spike that exceeds the pass regulator ratings OR the possibility that the source pin of the pass regulator could have a higher voltage on it than the drain as the power scaling pot will be pulling the drain to ground.

Anyhow, standby is not needed with power scaling - just turn the "voltage down".

Thanks for the theory on that. It's definitely beyond my ken (I really only do VVR paint by numbers), but I was thinkng it had to be something specific to having the regulator there, not just a general statement about reducing stress on the switch. But after seeing some schematics for actual Power Scaling circuits, I think the only reason he says that is because the scaling circuit is actually in series with the PT's CT. I think he's merely saying 'you can't have it there because the scaling circuit is there now!'