Fender Hot Rod Deville Amp

[Chuck H]

dual post-deleated

[Chuck H]

Did you check the two plate resistors on the phase inverter tube as G1 suggested? The 82K is a common failure point in these amps.

Is it the common Fender PCB models mistake where they used 1/4W resistors where they should have used 1/2W? This rings familiar.

That 82k resistor is R57 on the schematic. I'd change both PI plate resistors to 1/2W or even 1W then.

[TwinBeech]

Success!! I found the plate R for V3A was 137K instead of 82K. Changed that and all the noise stopped. Thanks to all who took the time to respond..

[Chuck H]

Success!! I found the plate R for V3A was 137K instead of 82K. Changed that and all the noise stopped. Thanks to all who took the time to respond..

Hmmm... The Deville is a later HR model and the schem specs 1/2W resistors. That SHOULD eliminate value drift due to abuse, as might be the case with a 1/4W resistor there. Can you tell us the code spec marked on the resistor body? Is it marked 137k-ish or 82k? Glad it's performing. Don't forget to check the bias since you changed the power tubes. And 'good call' g1

[g1]

the schem specs 1/2W resistors. That SHOULD eliminate value drift due to abuse, as might be the case with a 1/4W resistor there.

I seem to recall some speculation about it possibly being more of a voltage issue than a wattage issue, but I think upping the wattage of resistor used usually ends up giving a higher voltage rated part anyway.

[Tom Phillips]

I agree that PI plate resistor failure is fairly common in many modern commercial amps. In this case I think that the upward resistance drift is just another symptom of the part going bad. The higher resistance itself would not cause the crackling noises if the resistor was otherwise good. I.e. if a good 137k Ohm resistor was put in its place. Of course we wouldn't do that and the correct fix has been done.

Thanks TwinBeech for reporting the outcome.

[Leo_Gnardo]

When I see PI plate resistors gone bad, mostly in the Hot Rod / Blues series Fenders, I just swap in ol' fashoned half watt carbon comps. If they work for decades in the old Fenders, should work for decades in the new ones.

Screen grid resistor report: just had a look inside a recent RI Twin. It has tiny size "modern" 470 ohm SG resistors, each about the size of a mouse turd. I'm guessing they're rated for 1 or 2 watts. Let's see how long they last. So far so good.

[52 Bill]

Agree with the carbon comp approach, but we usually just swap in a 1 watt flame proof and that seems to fix them.

I know that we've discussed this before, but I can remember replacing mostly only the 82K resistors.

[octal]

I had the same issue with a Hot Rod Deluxe, and it was also the 82K which failed. Just adding a data point.

[Enzo]

It is the same circuit, so not surprising.