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Some technical aspects of valve amps

Reacquainting myself with valve technology, a mate has asked for some advice on building a valve guitar amp. Couple of things I've picked up on:

#1: Many of the classic circuits have the standby switch directly on the output of the HT power supply, which can be around 400 volts unloaded for some of the more powerful circuits. I can see why it's been done this way, alternatives could be messy, but unless the switch is specifically rated for the voltage, that could be a bit on the marginal side. Does anyone know whether specifically rated switches are used here, or is it a case of she'll be right?

#2: It seems that power supplies transitioned to solid state rectifiers pretty quickly as soon as silicon diodes became available. That leads to the situation where the HT appears immediately on switchon, but the cathodes of the valves are still cold at that stage, so there's no current through the valves, and the entire upper end of the circuit will go up to the full, no load HT voltage. Presumably the situation is handled by ensuring that ratings such as caps are adequate, however I see that the max voltage rating for a 12AX7 is 330 volts, lower than the peak HT voltage on say a 100W amp might be. Once again, is something being done to ensure this situation is handled correctly, or is it she'll be right again?

Pesonally, I'd buy a MOSFET kit, but a lot of musos still swear by the valve sound...

Reply to
Noodnik
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Sell you are sort of re-inventing the wheel here, better to ask in dedicated newsgroups ............. of which there are plenty.

Regards ....... Rheilly

Reply to
Rheilly Phoull

"Nood dick"

** Any switch that is rated for 240 VAC mains power operation normally works fine.

However, the miniature kind should be avoided as they lack sufficient clearance between open contacts to break arcing under some circumstances.

** The maximum sustainable plate voltage of a 12AX7 is way higher than 330 volts - plus there is nearly always a plate resistor of 100 kohms or more limiting any current flow.

Valves do not have max ( never to be exceeded) voltage ratings in the same way that semiconductors do so the figures in data books have to be interpreted quite differently.

Often, the max plate voltage figure quoted is actually the recommended DC * SUPPLY * voltage and has nothing to do with what may safely appear on the plate under low or no current conditions.

FYI - the insides of a valve is at a high vacuum and the only high voltage break down mechanism is insulation failure.

..... Phil

Reply to
Phil Allison

"Rheilly Phoull"

** Valves are electronic devices and perfectly on-topic here.

I for one happen to be very familiar with them, unlike all the compewter wankers and trolls.

..... Phil

Reply to
Phil Allison

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