A client just brought one of these to me recently. Before it got to me it had already burned up one OT and ate three brand new 6L6GCs. Another tech had replaced it and the next time he used it the cathode resistor exploded. That's when it came to me.
This looks to be partly a problem of design choice for the power transformer and partly a problem of the consistency of new production power tubes.
I'm posting details here for reference and discussion.
5W cathode resistor was in crumbs and the heat apparently blew up the bypass cap which was mounted very close in // on the pc.
Existing tube was pulling something crazy around 157ma. Other two arced like mad. All new production.
I replaced cathode resistor and bypass cap and put in a used vintage 6L6 I had in the drawer.
It idled at 69ma in less than 5 min. Stock figures say 413 plate at 30.91 bias = almost 66ma = 2W = ok for 5W bias resistor.
69ma will eventually rise to 70-72ma = 3W = real hot and probably too hot for longevity. But, that's about the best figure I got.
Client went out and bought another brand new 6L6GC 30W 500V speced. It hummed right away and was drawing 80ma in less than 5 min. about 3.6W = cooking hot.
I managed to gather up 3 more assorted new production 6L6GCs, lightly used and all drew over 70ma.
And it's no wonder. PT is 800V CT unloaded. I can't see why they would even choose that high of voltage for this amp.
HV DCR is 563 ohms which charts to something like 55ma rating. I haven't done a hand test on it yet. You'd expect a little higher than a Champ PT so around 100ma.
Size comparison to Weber's Champ OT is interesting. Same dimensions excepting the bell width - Weber bell is 0.25" wider. Epiphone PT has only single filament presumably good for 1.2A. Weber has 3A rectifier and 4A of 6.3V plus 80ma HV. You would think the Epiphone would be closer to 100ma HV. It may be but that high of DCR on the sec at that voltage is usually in the 50-80ma range on most of the PTs I've measured.
Solution(s). It didn't make sense to have the client keep buying NON-refundable new prod tubes at 17-20 a pop in hopes of finding one that biased up to Epiphone's stated values . . . . and stayed there without tearing up. Vintage tubes might have did a little better but there were still other concerns.
A 10W cathode resistor would survive. The original OT apparently won't. You may need a bigger one depending on how much current the tube ends up pulling. And, the PT might be running too hot or way too hot.
There is room to install dropping resistors between the stand by switch and spliced into the pc board lead. Could drop about 40-50V with 750 ohms total.
You could install choke input to drop about 50V. Might be an interesting new sound.
I decided to just buy a Champ OT from Weber for 33 bucks. Has two HV taps at 600 and 660 plus several pri taps at 100, 120, and 125V to get something acceptable.
Now what do you do with an 800V PT with possibly only about 60ma of HV?
The other issue is that it has high tendency to oscillate when preamp vol is wide open - fine below 8 or 9 then it squeals past that. Seems to already be a number of complaints out there on that problem.
I had to ask why you would even want two volume controls on this amp. The lead dress is kind of long and now you have another grid wire sitting on the bullseye.
I think you can replace the 2nd pot with fixed resistor and a shielded lead run if needed. If 1M is still too much use 680K.
The amp is a good bargain and you could easily gut it and install tag board and start from scratch. The cab is a keeper.
This looks to be partly a problem of design choice for the power transformer and partly a problem of the consistency of new production power tubes.
I'm posting details here for reference and discussion.
5W cathode resistor was in crumbs and the heat apparently blew up the bypass cap which was mounted very close in // on the pc.
Existing tube was pulling something crazy around 157ma. Other two arced like mad. All new production.
I replaced cathode resistor and bypass cap and put in a used vintage 6L6 I had in the drawer.
It idled at 69ma in less than 5 min. Stock figures say 413 plate at 30.91 bias = almost 66ma = 2W = ok for 5W bias resistor.
69ma will eventually rise to 70-72ma = 3W = real hot and probably too hot for longevity. But, that's about the best figure I got.
Client went out and bought another brand new 6L6GC 30W 500V speced. It hummed right away and was drawing 80ma in less than 5 min. about 3.6W = cooking hot.
I managed to gather up 3 more assorted new production 6L6GCs, lightly used and all drew over 70ma.
And it's no wonder. PT is 800V CT unloaded. I can't see why they would even choose that high of voltage for this amp.
HV DCR is 563 ohms which charts to something like 55ma rating. I haven't done a hand test on it yet. You'd expect a little higher than a Champ PT so around 100ma.
Size comparison to Weber's Champ OT is interesting. Same dimensions excepting the bell width - Weber bell is 0.25" wider. Epiphone PT has only single filament presumably good for 1.2A. Weber has 3A rectifier and 4A of 6.3V plus 80ma HV. You would think the Epiphone would be closer to 100ma HV. It may be but that high of DCR on the sec at that voltage is usually in the 50-80ma range on most of the PTs I've measured.
Solution(s). It didn't make sense to have the client keep buying NON-refundable new prod tubes at 17-20 a pop in hopes of finding one that biased up to Epiphone's stated values . . . . and stayed there without tearing up. Vintage tubes might have did a little better but there were still other concerns.
A 10W cathode resistor would survive. The original OT apparently won't. You may need a bigger one depending on how much current the tube ends up pulling. And, the PT might be running too hot or way too hot.
There is room to install dropping resistors between the stand by switch and spliced into the pc board lead. Could drop about 40-50V with 750 ohms total.
You could install choke input to drop about 50V. Might be an interesting new sound.
I decided to just buy a Champ OT from Weber for 33 bucks. Has two HV taps at 600 and 660 plus several pri taps at 100, 120, and 125V to get something acceptable.
Now what do you do with an 800V PT with possibly only about 60ma of HV?
The other issue is that it has high tendency to oscillate when preamp vol is wide open - fine below 8 or 9 then it squeals past that. Seems to already be a number of complaints out there on that problem.
I had to ask why you would even want two volume controls on this amp. The lead dress is kind of long and now you have another grid wire sitting on the bullseye.
I think you can replace the 2nd pot with fixed resistor and a shielded lead run if needed. If 1M is still too much use 680K.
The amp is a good bargain and you could easily gut it and install tag board and start from scratch. The cab is a keeper.