Hot Rod Deluxe problem

[Speedsense]

I have a Hot Rod Deluxe (replaced the speaker with a Tone Tubby) that I used for jam sessions (mainly because it's so much lighter than my "66 Super Reverb Blackface).

So far the amp has been very reliable, but recently have incurred a problem, with the amp on and no guitar plugged in, volume off and on the clean channel the amp, pops loudly and makes sounds like frying bacon.
As a newbie to amps (though have ample experience with electronics) I looked in to the board and found one of the 470 ohm 5w resistors (R78) was found laying off the board and down below on the plate PC boards The leads were gone, completely not only from the resistor but even from the solder joints on the board and no where to be found inside the amp.
From the schematic, I find this is a channel switching circuit, which explains why I haven't had a drive channel for a while.
After replacing the resistor with a new one of the same value and visually looking at the other one (R79) observing no signs of heat, I put it back together and still have the same problem.
A good friend who is into fixing vintage amps and not a lot of experience with HR's or PC board amps, he thinks I have a cap problem and has suggested ditching the Illinios Caps in the HR and replacing them with Spragues. Though I can't find all the values in Srague's list and they are expensive in comparison. The odd ball ones are difficult to find and still stick with sprague through out.
Am I headed down the right path for fixing the amp and/or is it worth putting in better caps in this amp?
I've always like the tone I get, thanks to the tone tubby (even my Mexican Strat sounds great through this amp) Any thoughts or input would be greatly appreciated.

[Jazz P Bass]

First off.
Did replaceing the 5 watt resistor fix the channel switching?
As to the frying sound, that is usually indicative of a faulty plate resistor.
If you go down the line of tubes & compare actual plate voltages to the schematic, you very well may find the culprit.
As to wholesale replacement of the caps, I would recommend NOT.
It may well be a cap problem.
If it is it will show up as leaking dc volts to the next grid or ac volts ripple riding on a power supply voltage.
Please try to fix the problem at hand before you compound the issue with more unknowns.

[Speedsense]

First off.
Did replaceing the 5 watt resistor fix the channel switching?
As to the frying sound, that is usually indicative of a faulty plate resistor.
If you go down the line of tubes & compare actual plate voltages to the schematic, you very well may find the culprit.
As to wholesale replacement of the caps, I would recommend NOT.
It may well be a cap problem.
If it is it will show up as leaking dc volts to the next grid or ac volts ripple riding on a power supply voltage.
Please try to fix the problem at hand before you compound the issue with more unknowns.

Thanks for your response,
yes, the 5W resistor fixed the channel switching... plugging in a guitar into the amp, with the drive channel selected the problem isn't appearing in the same fashion, and is very unrepeatable and affected by volume. Switching back to the non drive channel and the popping and sizzling sounds and are unaffected by volume.
Also, one thing I forgot to mention, in the non drive channel, with no guitar plugged in, I pulled out the V1 tube, with no result, then pulled V2 and the sound stopped.
I then proceeded to use a chopstick to touch the plate resistors to try and get the sound to change. But nothing changed by touching the resistors, any of them....
So according to your comments the problem could be with V2, is that a safe assumption?
Again thanks for your reply

[Jazz P Bass]

Chopstickng will reveal a failing solder joint.
Pulling V2 pretty well killed the signal going through the amp, so that is not much help.
Pulling V1 did not affect the crackle. Now that helps.
I still would like to see someone test each tube stage.
Plate readings in dc volts & cathode readings in dc volts will tell if the stage is functioning.
Grid voltage readings will tell if a coupling cap is bad.
You could try to narrow down the issue.
How about plugging your guitar in the "power amp in jack".
If that does not crackle, the problem is for sure in the preamp.

[GregS]

Am I headed down the right path for fixing the amp and/or is it worth putting in better caps in this amp?
I've always like the tone I get, thanks to the tone tubby (even my Mexican Strat sounds great through this amp) Any thoughts or input would be greatly appreciated.

Problems like this are very typical in these Fender Blues and Hotrod Deluxes and Devilles. The channel switching problem (which you've already discovered) is due to the PS resistors associated with the channel switching circuit getting way too hot and the solder joints deteriorating over time. It is common to have burn marks right through to the other side of the board under these resistors. Most of the problems in these amps are due to bad electrical contacts. Many people simply go over all the solder joints and redo them rather than trying to track down a problem. It is not very likely to be the caps that are causing your problem.

[Alex R]

Noisy plate resistors are pretty common in these too, so it is worth trying to track down the problem to the stage that's causing it, I think. Tapping plate resistors won't necessarily cause changes in the noise they're causing, but freeze spray generally will.