voltage at power tube cathode?

[dmartn149]

I just finished building an amp based on a silvertone 1482 schematic, with some small mods to the mic input. All of the voltages are a little lower than the schem. except for the power tube cathodes. Voltage there is about 1.5v higher. I expected lower v throughout per the schem. for the radio that my PT came from. The amp sounds good, but I am wondering how I ended up with higher v there and should/can I do anything about it.
Thankssilvertone1482.pdf

[The Dude]

It's a cathode biased amp and likely just the power tubes you selected. A different set of tubes would likely yield a different voltage. It's nothing to worry about. If you want to bias hotter or colder, change the value of the cathode resistor.

[dmartn149]

Thanks Dude. I think it sounds pretty good as is. More clean headroom would be better, but I'm happy.

[pdf64]

Check that there’s no Vdc on the power tube control grids (terminal 5).
Grid stoppers there would be good practice. Probably omitted from the original design to reduce cost.

[Helmholtz]

...and lift one end of the 270R cathode resistor and measure resistance. Might have increased over time.

[dmartn149]

Check that there’s no Vdc on the power tube control grids (terminal 5).
Grid stoppers there would be good practice. Probably omitted from the original design to reduce cost.

I get 0v on p5 of the tube that's fed by p3 of the PI and 3v on the tube fed by pin 1.

[dmartn149]

...and lift one end of the 270R cathode resistor and measure resistance. Might have increased over time.

It's all new.

[dmartn149]

Those grid stoppers would be R23 & R25 in this schematic, right?
silvertone1472.pdf

[Helmholtz]

and 3v on the tube fed by pin 1.

That indicates a leaky 0.01µ coupling cap and would explain increased cathode current and voltage.

[dmartn149]

That indicates a leaky 0.01µ coupling cap and would explain increased cathode current and voltage.

yeah, I know the coupling caps are supposed to stop DC. I guess I got a defective part.

[Helmholtz]

yeah, I know the coupling caps are supposed to stop DC. I guess I got a defective part.

Just lift the coupling cap and measure grid voltage again.

[dmartn149]

Thanks, Helmholtz. I can't get to it now, but I'll do that asap

[pdf64]

Those grid stoppers would be R23 & R25 in this schematic, right?
[ATTACH=CONFIG]55525[/ATTACH]

Yes, but 330k is a far higher value than necessary.

[dmartn149]

Yes, but 330k is a far higher value than necessary.

maybe 1.5k like a 5e3 deluxe?

[dmartn149]

ok, I disconnected the coupling cap at pin 5 of the power tube. still getting an extra 1.5v at cathode AND still getting 3vdc at pin 5. So, is that a bad tube?