Some technical aspects of valve amps

[Noodnik]

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...

 

[Rheilly Phoull]

On 15/10/2010 6:38 PM, Noodnik wrote:

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...


Sell you are sort of re-inventing the wheel here, better to ask in

dedicated newsgroups ............. of which there are plenty.

Regards ....... Rheilly

 

[Phil Allison]

"Nood dick"

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?

** 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.

#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?

** 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

 

[Phil Allison]

"Rheilly Phoull"

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

** 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