Epiphone EA-12RVT Bias Question

[eman-blues]

I'm working on a vintage EA-12RVT Tube Amp. It is a fixed bias design without a potentiometer to adjust the bias. What resistors do I need to change/tweak to adjust the bias current down?
Here's the schematic
Thanks for the help!

[loudthud]

Decrease the 390K to 330K 2W , then fine tune with the 7.5K and the 51K.

[Helmholtz]

The bias voltage is essentially proportional to the value of the 51k resistor, so adding 5k would give an increase of roughly 10%.
Or use a 20k trimpot in series.

[eman-blues]

Thanks for the suggestions. I changed the 51k to 33K in series with a 50K trim pot so I can tweak the bias. But, the most important thing, the schematic for the amp is incorrect. The polarity of both 25uf caps was backwards in the schematic. Turning those around brought the Plate current down from 115ma to 30ma.

So if you ever see one of these amps, don't follow the schematic for the polarity of these two caps!

BTW, this is a 1962(?) Gibson GA-30RVT (the older version that only has tone and not TMB). The EA-12RVT schematic matches the circuit of this amp. Only difference is the GA-30RVT has 2 speakers and the EA-12RVT has one.

Thanks again for your help!

[g1]

Were the original caps backwards, or were they replaced according to the schematic?
Usually they blow up or are permanently damaged if backwards, so I doubt the originals would have lasted. The ones you had in backwards should be replaced with new ones after being run with reverse voltage, as they are no longer reliable.

[eman-blues]

The originals were bad, but installed correctly (and not according to the schematic). They measured about 28 pf each vs. 25 uf. When I initially replaced them, I followed the schematic and put in new ones backwards, and the tubes were super hot like they were with the original caps. When I discovered the schematic was wrong, I replaced the caps with new caps and installed the correctly (opposite of the schematic). I wanted to document the saga here in case anyone else is looking at that schematic.

The initial symptoms, tubes and power transformer were super hot and the tremolo didn't work. Now it is correctly biased, and the tremolo sounds great.

I'm pretty new at tube amp repair; so I really appreciate all the help from experts here and on the Telecaster forum! Thanks!!

[DrGonz78]

Well you learned a very well known lesson on old Gibson schematics. They are full of errors and revisions. I mean these guys designing these things were really knowledgeable but they even left mistakes behind. As long as the amp was tested as working production was onto something else.