Fender Hot Rod Deluxe Blown rectifier diode and resistor on output tube

[jshag]

Fender Hot Rod Deluxe Blown rectifier diode(CR4 .25 amp/3000V) on the plate and 1 W 470R(R64 & R61 1/4 W) resistors on input plate for output tube were smoked! Even burnt pcb under the 1 watter.
Tubes check out good to excellent and amp still worked through gig although ugly distortion no clean sound.

Amp powers up . Any one seen this before?

[52 Bill]

I'd be suspicious of the power tube in that socket, even if it tests good.

Blown diode? Do you mean open, shorted, exploded?

Has R61 470R 1W gone open circuit?

What fuse is in the main fuse holder?

[jshag]

R61 is charred beyond recognition and burnt the pcb underneath. CR4 split in half and R64 also became open and severely charred.
82K resistor on PI measures within tolerance =81K

[jshag]

Fuse is 10A 250V

[g1]

Well the over-fusing would help explain some of the extensive damage. Hopefully no transformer issues.
How are R66 and CR3 ?

[jshag]

R66=1.3 ohms CR3=0.000V either direction

[52 Bill]

Ouch. Replace the fuse with the correct one. Power up the amp without the power tubes and see if the fuse holds. If it does, test the ac secondary voltages and then the dc power supply voltages.

Replace the charred and exploded parts. I'd probably at least test if not replace the other flyback diode CR5 and the other screen resistor R62. Clean up the pc board where it has burned. If the carbonizing is bad enough, you may have to get creative.

Check all of the voltages at the two power tube sockets before installing new power tubes. If you feel like testing fate reuse the old ones, but I'd suggest a lightbulb limiter if you do. Once they heat up, they may show signs of problems.

If you were hearing the odd distortion of only half of the power amp working the 82K resistor would be a good thing to test, but I think that your problem is that the screen grid resistor of V5 cooked and turned off the tube, giving you the same sound. Now only hope that the OT survived the meltdown.

[jshag]

Nice comment. I do use a lightbulb limiter. Curious when checking voltages originally bulb was near off but voltage readings were off B+ was down by more than 30 % Apply input AC power w/o limiter and got good readings all around.
Original problem was pcb mounted input jack #1 broke internally and loud hum(not just small 60 Hz sound) emanated from speakers. Simply replaced input jacks with good Switchcraft jacks. Used 1 Meg resistor to drain caps before going in. I like a good puzzle but this is clients amp had to use it at a Beatles Impersonation act. They needed clean. Good pros that they were they worked through it. 4 hour show with 10 amp fuse didn't help

[g1]

CR3 will read short in circuit as it is paralleled with R66. Lift one leg and check both again, it's probably ok but best to check it.

[jshag]

Will Do Thank you for your patience. Maybe spent more time as an AV design engineer than on the bench in recent years. Opened up a work shop. So far replaced a few Fenders back in service...still working.
Go slow and find the problem rather than replace burnt components otherwise I will be doing it again.

[Enzo]

Well, good luck to you then.

I offer the standard advice:

When you see hoofprints in the dirt, think horses, not zebras.
Never forget these are just guitar amplifiers, not pieces of NASA space probes. Redesigning is never the first choice.

ANd always...
Isolate the problem.

[g1]

You mentioned installing switchcraft jacks, did you isolate them from the chassis with insulating washers? The plastic stock jacks usually do not ground to the chassis so it's something to consider if you have any ground loop/hum issues.