Red Knob Twin. External fuse blowing. Burnt resistors/diode on filament supply. 6L6 socket burns. Bad 6L6?

[Whatizitman]

Ok, so I couldn’t leave well enough alone. I have it working with full wall power, two output tubes, replaced CT resistors, with a proper 4A mains fuse that has yet to blow. Instead of channel switching, I tried going through the parallel input, and can use both channels fine. Fuses blew when I used the switching input, and used the pedal to switch to the dirty channel. I would put on standby before switching. Taking off standby it would die. It has not done that if I use the parallel input, however.

Some googling found that it’s common in these amps to have issues with the switching ICs. Could that cause arcing on the tube or socket? Or perhaps just another compounding issue?

BTW, omg it sounds amazing with the new strat.

 

[Whatizitman]

Update. Replaced the burned socket. I let my buddy replace all the screen resistors, as they had all drifted up. He should have been a surgeon. My hands were just too tired and shaky by then. Screen stoppers were fine. Even the burned one was in spec, but I replaced that one. I had to drill out the chassis cutout a bit to fit. But it's better than having a socket that's too small, I guess.

Running now with all four (I believe) original 6L6s. I changed out V1 with a newer 12ax7, which definitely helped the sustain. Sounds fantastic with the mixed channels. Great SRV-ish tone on my new strat. There is some hum. Totally quiet on standby, though. I can live with that until I need to do any more surgery on it. The old 6L6s really should be replaced sooner than later. The caps seem newer to me - look like F&Ts. I have no idea if those were stock in the 80s Fenders. Anyone know?

It's definitely a keeper, considering what I paid to get it and repair (very, very little). I have no need for such a big loud amp, as I'm mainly a home noodler. But I sure as hell prefer to play on a big amp at home than on small amps. These still don't sell for a whole lot, anyway. So in the mannish cave it stays.

 

[corliss1]

Fender never uses F&Ts, so those have been replaced.

 

[Whatizitman]

Fender never uses F&Ts, so those have been replaced.

That's what I figured. I thought my '81 Pro Reverb had those old brownish paper mallory filter caps original when I had cap job in the early 90s. Did these late 80s, early 90s Twins have them, too? All the RK chassis pics I can find online are newer looking caps - mostly generic looking with the big pos/neg stripe. I assume they were all recapped, too.

EDIT: I take that back. This is more likely what my pro reverb caps looked like - silver aluminum. How long did Fender and/or Mallory caps look like this?


https://commons.wikimedia.org/wiki/...itors_(2013-08-02_13.25.51_by_Ian_Abbott).jpg

 

[corliss1]

Nope - the brown ones would have been phased out by the late 80s/early 90s. Not sure what the stock ones would have been in that era - probably the generic ones you're seeing.

 

[Lowerleftcoast]

The hum.
Several of the larger Fender amps came with a *hum dinger*. The two resistors may not find the equidistant *sweet spot* of a CT. You could replace the two CT resistors with a 500 Ohm pot to achieve this. Then dial out the hum. (If that is where the hum is coming from.)
(I noticed on the pic you supplied, the PCB looks messy around the filament wire connections on the board. If you haven't already, clear the board of the debris around those connections.)

The old tubes may also introduce hum.

 

[Whatizitman]

The hum.
Several of the larger Fender amps came with a *hum dinger*. The two resistors may not find the equidistant *sweet spot* of a CT. You could replace the two CT resistors with a 500 Ohm pot to achieve this. Then dial out the hum. (If that is where the hum is coming from.)
(I noticed on the pic you supplied, the PCB looks messy around the filament wire connections on the board. If you haven't already, clear the board of the debris around those connections.)

The old tubes may also introduce hum.

Could be the tubes. I swapped some newer tubes from my bassman 70 (two only), and the amp was a little quieter, IIRC.

As it is, it's not a huge issue. But good idea on the humdinger. The CT resistors I replaced may be underrated. The schem calls for 1/2watt, but all I had was 1/4watt. My last order I forgot to get 1/2watt replacements, so the 1/4 watt resistors stay. I cleaned it, but there is still some carbon around that area on the PCB that I couldn't seem to get. I haven't taken the board out and observed underneath, either. If and when I do caps and/or new tubes, I'll scope it out more thoroughly. It's working great now, and I'm thoroughly enjoying it.

 

[Lowerleftcoast]

The schem calls for 1/2watt, but all I had was 1/4watt.

The 1/4W will probably be OK. The resistors are just for a ground reference. For the most part very little current goes through them. The good thing is they act like a fuse, saving the PT, when they encounter this type of catastrophe. They are best placed with some distance from the board so when they burn the board is not damaged.