On the second DC3 I got, there was a bias adjust mod installed. It also has 1645 Hammond OT and a 159P Hammond choke installed, with a "Fender" mod on the clean channel that actually works! It does sound amazingly like a Fender clean. Not quite as creamy as the Rivera clean I have sitting next to it, but it would work very nicely for rhythm guitar in a funk band! The only drawback to this mod is it has to be cranked up to about 3 or higher to really sound right!
I don't have a schematic handy at the moment, but......
(sorry I don't have a photo handy yet either)
On the main board the two wires from the adjust pot disappear just under a pair of orange caps, close to the big blue power caps. Next to the orange caps, next to the edge of the board, there is a diode and a 33k R. The bias pot has replaced the 33k R. Only thing is, I'm not sure which 33k resistor that is on the schematic. And I can't see the traces well enough to tell you how the resistor and the diode are connected.
There are two test points, each one has a 3w 1ohm 1% resistor to ground. I'm told this effectively lets you measure mV and since it is 1 ohm, it will directly reflect the idle current for one pair of tubes. So if the meter reads 40mV, that means there is 20mA of idle current to each tube in that pair. If each test point measures the same then (according to the tech who installed the mod) you know your quad is matched.
I put in a quad of new supposedly tested and matched Mullards and let it warm up, then I set the first test point to 40mV. Surprised to see that the second test point then measured 20mV, which I thought might be a significant mismatch. The tube seller responds by saying anything within 10% is considered "matched" depending on the circuit and the tester, etc. So I swapped the two inside tubes, one from each pair. When I remeasured this simple change brought the first test point down to about 33mV and the second test point up to about 30mV. I figured that was as close as I could get it given these tubes. I wanted it to run a bit on the cool side, and 30mA for a pair would calculate out to about 6 watts per tube, assuming 400v plate......right? So maybe about 25 watts output?
(Interesting to note that the quad of JJ's that came with the amp match EXACTLY on both sides no matter how I pair them up. The mod installer sent the JJ's with the amp)
So this particular DC3 is running nicely now, and is much quieter with the new preamp tubes. I also hit all the pots and sockets with Deoxit while I was in there.
BUT...........it ain't even close to sounding half as good as the DC3 I had that was modded by Monsta-Tone!
Rivera R55 and K55, Frenzel 5E3-40, MIA Strat, Reverend 6 Gun, Weber, Reaper.