Here's a photo of the cathode bias conversion I performed on my Deluxe Reverb... Look for the additional terminal strip soldered to the chassis with the 560 ohm/5 watt resistor paralleled with a 25uf/25volt electrolytic capacitor...Next in series before the ground connection is a 1 ohm/2 watt resistor for measuring the bias current. The red wire from the terminal strip connects to the cathodes of both power tubes (Pin 8.)
A complete detailed rundown of the conversion and disconnect of the feedback loop follows the photo.
<img src="http://img26.photobucket.com/albums/v79/Electricpie/Amp%20stuff/DlxRvbCathodeBias.jpg" alt=" - " />
CATHODE BIAS MOD:
1. Find the wire that feeds from the bias pot to the two 220K resistors in your bias network on the circuit board. Disconnect the wire from the bias pot and connect it to ground.
NOTE: This is assuming that your Deluxe (or other Fender) has an adjustable bias circuit and not a 70's-style hum-balance circuit.
2. Unsolder the ground from Pin 8 (the cathode) on each power tube.
3. Solder a jumper wire from Pin 8 of one power tube to Pin 8 of the other power tube.
4. For easy access, I soldered a short terminal strip to the chassis for mounting the cathode resistor and bypass cap as you'll see in the photo. The terminal strip originally had two screw down ends (ground), but one broke, so it is holding fine with one end soldered to the chassis.
5. Solder another wire from the right power tube Pin 8 to a terminal (NON-GROUNDED) on the strip.
6. Solder in place a 25mfd/25v electrolytic cap - negative end towards ground and the positive end connected to the wire from the power tubes' Pin 8 cathodes.
7. Solder a bias resistor (value to be determined) in parallel with the capacitor to ground on one end and the cathodes on the other.
8. ADDITIONAL RESISTOR PICTURED- This is a millivolt bias measure resistor, a 1 ohm/2-watt value. It is connected between the cap/bias resistor combo and ground. This will allow you to get a current draw reading of both tubes. (You can leave this out and connect the bias resistor/cap to ground)
That's it!! It's a bit of work initially, but if you want to reconnect the "fixed" bias to compare, just reconnect the wire from the 2-220K resistors back to the center post on the bias pot and then connect your new cathode wire back to ground...It's back to original.
Now...to determine the proper value bias resistor - The most accurate way to get your bias measurements is by the shunt method. If you're good at it, great, otherwise, let someone else do it, as I'm sure you know the scoop that it's very dangerous for you and the amp (stuff can blow up and people can get hurt.)
You should be able to bias it by ear... The resistor value should fall somewhere between 400 - 600 ohms...A safe level that shouldn't draw too much current. Your plate voltage (on a BF DeluxeRvb at least) should be around 410, maybe higher. Your plate current (per tube) should be safe around 30ma plus or minus, I'd imagine. That would give a reading of around 60 + across the 1ohm bias resistor.
I chose a 560 ohm resistor and here are the readings. My Deluxe Rvb possibly has a hotter plate voltage than most...It was 450v on fixed bias.
Bias Resistor value: 560 ohms/5 watt
(shunt method reading): 29ma per side (approx)
Plate voltage: 420v (approx)
Cathode 1 ohm resistor reading: 63.8 (average at idle)
TO DISCONNECT THE NEGATIVE FEEDBACK CIRCUIT:
Find the wire that goes from the external speaker jack to the circuit board. Disconnect the wire from the speaker jack and cap it or tape it up. This should be the only wire running from the speaker jacks to the circuit board. The other wires are coming from the output transformer.
There you have it! BaBoom - Big sound.
You can also refer to the AB763 Circuit layout..Here's the link (courtesy of Ampwares.com)
Good luck!! Anyone else who does it, let us know what you think!