Okay, going to try to explain how you go about center-tapping and elevating the heaters of the stock DarkVoice.
Please read in full before making any changes, be sure to understand what you are doing before you do it. If you are unsure if what you have done is correct, take a photo and PM me or post in the thread and tag me. Carry out these modifications at your own risk, there is ~200VDC inside the amplifier, working with high voltages carries a shock and safety hazard.
Heater Center Tap
What you will need:
- 2x 100ohm 1/2 resistors (type does not matter, metal film would be fine).
- 1x 2K 1/4W resistor
- 1x length of 20AWG wire (solid or stranded does not matter)
1) Find the two points on the main PCB where the heater wiring is soldered, here.
2) Solder one end of a 100ohm 1/2W resistor to each heater winding pad. Twist together the opposite ends and solder them together, like the illustration below. This is the center tap point!
3) Break the ground connection between the 6SN7 heater and the ground bus below. This wire can simply be cut or desoldered and discarded.
If you are elevating the heaters, skip to step 6. If you are not, go to step 4.
4) Now connect a piece of wire from the new center tap point to the ground bus. This is the long, thin length of bare wire that runs throughout the chassis.
5) Because the power LED is run from the heater supply and we have just halved the voltage it sees, we must adjust the series resistance before the power LED. On the power LED PCB, there are two 2K resistors, one on top and one on the bottom. Add the additional 2K resistor in parallel with the top 2K resistor. This means the leads of the new resistor will be soldered to the same pads as the existing 2K resistor.
If you are not elevating the heaters, you are done, the hum should be improved but might not be completely eradicated.
Elevated Heater Supply
You should have just finished step 3 and created the virtual center tap and broken the heater ground connection at the 6SN7 socket. To elevate the heater supply, we must make a voltage divider from the B+ supply to reduce its voltage to our desired elevation voltage. Anything from 30V-60V will work fine. This elevated voltage will be applied to the center tap point.
Additional items you will need:
6) Find the below power supply capacitor, the second to last in the supply (note this can be done with either capacitor in this position, they are all in parallel). This is the point at which you will connect the voltage divider. The B+ at this point should be
roughly 150V, although I would not be surprised if it varied between amplifiers given the quality of the transformer, the end result will be the same.
7) To the newly created center tap point (the junction of the two 100ohm resistors), solder one end of the 220K resistor, one end of the 100K resistor, the
positive end of the 10uF electrolytic capacitor, and one end of a 20AWG length of wire.
8) Solder the other end of the 100K resistor and the
negative end of the electrolytic capacitor to the ground bus, shown in step 4.
9) The other end of the 220K resistor must be connected to the
positive end of the power supply capacitor shown in step 6. If the leads are long enough to reach from the center tap point to the positive terminal, it can be done this way. Otherwise, connect the other end using an additional length of 20 AWG wire.
10) Disconnect the power LED PCB from the ground bus, shown below. The wire can simply be removed from the PCB and the excess clipped.
11) The power LED PCB ground must now be brought to the center tap point so the power LED does not see the elevated supply DC offset. Connect the length of wire connected to the heater center tap to the power LED ground PCB pad.
The wiring should look like below, another crude illustration.
12) Lastly, as in step 5, the series resistance before the power LED must be reduced so the LED will light up. On the power LED PCB, there are two 2K resistors, one on top and one on the bottom. Add the additional 2K resistor in parallel with the top 2K resistor. This means the leads of the new resistor will be soldered to the same pads as the existing 2K resistor.
That's it, you should now have ~45VDC heater elevation, the hum present on any noisy tubes should now be gone.
I'm sure it's this tube, the bottom has the burned glass at the bottom. I almost threw it out last year. If it goes out again after the Holidays...I will totally freak.
