Max voltage ???

[jurgen]

Ok experts,

When a tube manufacturer specs a max plate voltage,
say 300V for a 12AX7, that means plate to cathode and
not necessarily plate to ground.

Do I have this right?

[Ears]

yes

Edit: answering doesn't imply I'm an expert though.

[tele_player]

Correct.

[jurgen]

Thanks Guys

Guess it should have been obvious but I'm all the way tired of discovering my bad assumptions the hard way

[David Root]

Same thing applies to resistors. A 400V rated resistor that has less than 400V between its ends is also OK, BUT you have to check the current thru it to see that you don't exceed the power rating of the resistor either.

For example a 400V rated 100K plate resistor that has B+ 350V and plate voltage of 150V is OK voltage wise, but if it is rated 1/2 watt and you have more than 2 mA running thru it you're up against the 1/2 watt rating and looking for a failure. (0.002*0.002*100,000=0.4 W). Especially if it's a carbon comp type.

Roughly speaking, I use a 50% rule. If the average power in service is more than 50% of the chosen or installed resistor's power rating I go up to the next higher power rating. Except for cathode bias resistors where I use minimum 4x the average power generated in the resistor, or a 25% rule.

[mhuss]

Remember, though, that audio tubes also have a max cathode-to-heater spec, something like 75v for a 12AX7 IIRC. This mainly comes into play when doing SRPP circuits and the like. In this case you may need to elevate the heater center tap (to say, 75v vs. the normal ground)

--mark