Fender White Noise

[h4ck.b0x7]

I have DRRI that has white noise in both channels regardless of guitar plugged in or not. The noise is affected by the volume knob of both channels. It definitely reduces but does not go away 100% when the volume knobs are all the way down.

I have deduced there are no issues with the tubes as I have removed/swapped all sorts in and around with known good tubes and it doesn't make a difference.

Looking at the schematic the channels are separate and the volume knobs are after the tone stacks. To me this means that there has to be something in the circuit before the volume pots of each channel causing this. I'm guessing it would be related to the plate resistors. (I'm making the assumption that the Phase Inverter is not the cause because if so the volume knobs wouldn't affect it.)

From what I gather the applicable plate resistors are 100k ohm 1/4 watt located as follows:

V1a = R4
V1b = R11
V2a = R16
V2b = R23

What are the odds that these resistors are noisy junk from the factory??? The amp isn't old at all and both examples I've seen did this. I have done some mods to the amp but absolutely nothing in the way of changing much of the circuit. I upgraded caps with higher quality caps and removed the bright cap.

NOTE: IT DID THIS BEFORE I UPGRADED THE CAPS

[h4ck.b0x7]

Schematic:

[Leo_Gnardo]

What are the odds that these resistors are noisy junk from the factory??? The amp isn't old at all and both examples I've seen did this. I have done some mods to the amp but absolutely nothing in the way of changing much of the circuit. I upgraded caps with higher quality caps and removed the bright cap.

NOTE: IT DID THIS BEFORE I UPGRADED THE CAPS

Not too much trouble to swap in a couple of metal film plate resistors. And use 1W rated, although they don't generate much in the way of heat, the cooler they run, the less noise they'll generate.

ALSO consider the 68K resistors at the input jacks. They are noise generators too. Here's an area that can generate some controversy but can also help reduce your noise by a notable factor. It's good practice to use carbon comp as a "stopper" resistor, and place the resistor as close to the tube as possible, right on the tube socket. BUT Fender's standard input circuit has those two 68K's, and worse they're at the far end of the wiring between pre tube grid and input jack. IF you decide you're not going to need the #2 input, I'd suggest do away with the 68K's, wire the input end of that wire direct to the #1 jack, and insert a lower value "stopper" right on the tube socket, between the input wire and control grid lug. What value works best - I haven't a clear answer. Somewhere between 1K and 22K should work fine, and leave you with overall less noise. The "stopper" function is to "stop" radio noise pickup, and you can reduce it as far as you dare, but too low a value (or none at all) & you may start picking up a radio station. That's why there's no clear answer. You'll have to experiment.

[J M Fahey]

You can also ground the first triode plate using some cap you have available, .022 or .047 x 250/400/600V is fine, to check if it kills that noise.

If it does, then that plate resistor is the strongest suspect; you already eliminated the tube itself.