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?
5e5-a low plate voltage
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Re: 5e5-a low plate voltage
Well, without any discrete specifics there is not enough info provided to draw any conclusion one way or the other.sixstringer wrote: I'm adjusting the bias circuit values, what do you make of the plate voltage?
TM
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Re: 5e5-a low plate voltage
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.
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Re: 5e5-a low plate voltage
Maybe I should re-phrase..... What would cause low plate voltage?
Re: 5e5-a low plate voltage
The power string design? The transformer? With all tubes removed, what is your B+ voltage? What's your wall voltage?
I build and repair tube amps. http://amps.monkeymatic.com
Re: 5e5-a low plate voltage
sixstringer wrote: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
Re: 5e5-a low plate voltage
sixstringer wrote: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
Re: 5e5-a low plate voltage
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.
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.
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Re: 5e5-a low plate voltage
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.
I'm sure they will come up a little as they warm up for a few minutes.
Thanks for your input and help.