I would love to see the bias mod for the DC-3 as well. Maybe monsta will stop by and enlighten us. I was going to just follow the directions he posted in his DC mods thread, but it looks like that is specific to the DC-5 and DC-10.
I'm going to do that mod on my DC 3.It should be prety much the same as any ajustable bias curcuit.I would think anyway.I'm gonna replace the 33k resistor with a 25 or 50k pot and go from there.Neg.12 volts is just way too hot of a bias for todays production tubes.I'm gonna shoot for around 14 to 16 neg.voltage.That should keep the tubes running hot enough for tonage and have them last a bit longer. I think modern tubes are gonna red plate at that bias more often then not.The PV c30 runs at -14 and you can throw anything in there and it works fine.
We do make an adjustable bias mod which consists of a small pcb board that houses a trim pot and a series resistor. It comes with complete instructions and should work fine on the DC3 and DC5. Some customers even mounted theirs external for biasing without pulling the chassis. They have been very popular with the Stileto and Lonestars so you can check those sections to read more from actual users. The only thing different with these amps is the phase inverter is also biased via a tap in the bias supply which is not the best design. You would basically remove the 33k resistor and install either our bias kit or for those who don't want to buy the kit you can install a trim pot in that spot. The advantage of the kit is cleaner installation with vitually no exposed leads to short out plus we wire the trim pot for safety and add a series resistance for safety. These can all be done without our kit but can get a bit sloppy without the pcb board. The most important thing when installing an adjustable bias on these amps is that the voltage on the phase inverter stays the same. In these amps the schematic calls for 2.3 volts on the cathode of the phase inverter tube. For those not familiar it is the last preamp tube closest to the power tubes and since these are dual triodes in one space the cathodes are pins 1 and 8. For anyone interested in the kit they can be purchased on our website at www.mansfieldguitars.com. Feel free to contact us with any questions.
Warning lethal voltages inside amp. Work on it at your own risk! If you dont know how to discharge and check the caps are drained dont work on it.
Check the PI bias voltage. Slap a 470K resistor atop the 120k. Check the voltage at pin 2 of the power tubes. Below -13? Good.
Recheck bias voltage at PI Only moved a bit? Done. Mine is at 2.6 vdc .3 above specs and sounds fine. Bob
