5e5-a low plate voltage

[sixstringer]

I'm finishing a friend's 5e5-a (tweed pro) build and find the plate voltage is 297v. I was expecting more. He added an adjustable bias but it won't get anywhere near 70mA, which is what I calculated the proper bias should be.
I'm adjusting the bias circuit values, what do you make of the plate voltage?

[ToneMerc]

I'm adjusting the bias circuit values, what do you make of the plate voltage?

Well, without any discrete specifics there is not enough info provided to draw any conclusion one way or the other.

TM

[sixstringer]

What else would u need to know? Except for the adjustable bias circuit, it appears to be stock 5e5-a circuit using the fender spec-ed transformers and choke.

[sixstringer]

Maybe I should re-phrase..... What would cause low plate voltage?

[xtian]

The power string design? The transformer? With all tubes removed, what is your B+ voltage? What's your wall voltage?

[NickC]

What else would u need to know? Except for the adjustable bias circuit, it appears to be stock 5e5-a circuit using the fender spec-ed transformers and choke.

This might be of interest. Some controversy regarding PT:

http://www.fenderforum.com/forum.html?d ... ber=666303

[ToneMerc]

What else would u need to know? Except for the adjustable bias circuit, it appears to be stock 5e5-a circuit using the fender spec-ed transformers and choke.

So what are the specs for this particular "fender spec-ed" power transformer?

What are the voltages with no tubes installed?

TM

[Phil_S]

Plate voltage will vary with current draw. As you draw more current, voltage will decrease as vice-versa. Cold bias = higher plate voltage.

That having been said, as others indicated, we don't have much to go on. I suggest a set of voltage readings that include the PI, power tubes, and B+ supply ladder. Include VAC at the rectifier input.

The schematic for the 5E5-A says 385V at the B+ node that feeds both the OT and the screens (weird). I'd expect 380-ish at the plates. Other things being equal and no mods, 297 is low.

What is your bias voltage? Schematic says -32V.

[sixstringer]

It was a bad rectifier tube, replaced with a known good tube, the plate voltages measure 375, which is what I was expecting from the beginning.
I'm sure they will come up a little as they warm up for a few minutes.
Thanks for your input and help.