Redplating Fender Super Twin Reverb.

[Mljbone]

Hi,

New here, but have had success in the past with other forums!! I have a '78 Fender Super Twin Reverb (blackface) that has a power tube redplating. I know it's a beast, and some say to put it out on the curb. But it's loud if not heavy. The first tube from the power side is redplating. I pulled the chassis, and the 2 resistors on # 3 are burned to a crips. Everything I've read pertain to 4 power tube amps, this has 6. So I want to replace the resistors and see what happens, maybe still redplating. I quess my question is where can I find replacement resistors?. RadioShack? Thanks, Mark.

[Randall]

Antique Electronics Supply for all your amp needs https://www.tubesandmore.com/

Or Mouser when you need to go a little deeper https://www.mouser.com/

Consider replacing that tube.

[Mljbone]

Thanks Randall, is it weird that the tube with the bad resistors isn't redplating? I've 2 new tubes!

[Justin Thomas]

Just an FYI, don't let the cosmetics fool you: this thing is about as far from a BlackFace amp as you can get. It's not a Super "Twin Reverb." It's a "Super Twin" WITH Reverb. The first gain stage is different, the Tone Stacks are different, the output is different, etc. About the only thing it has in common with a Twin Reverb is the 2x12" speakers.

This thing is more akin to the 400PS, 300PS, & PA160.

Justin

[Jazz P Bass]

Here are a few sites using Google Shopping:
https://www.google.com/search?q=1500...w=1360&bih=689

https://www.google.com/search?q=470+...&bih=689&dpr=1

And the schematic: Fender-Super-Twin-Reverb-180W-Schematic.pdf

[Jazz P Bass]

Thanks Randall, is it weird that the tube with the bad resistors isn't redplating? I've 2 new tubes!

If the 470 ohm screen resistor is open, then the tube will not conduct.
Hence, no red plating.

[bob p]

I never thought of the first gain stage cap making all that much difference; whenever i work on any of those amps I end up diode biasing that stage and making them all sound the same.

I can't hang my hat on that treble cap in the tonestack making all that much difference either. Yeah, it's different, but not like it's night and day.

I always thought that the additional gain stage is what made the preamp different, in a way that isn't particularly that appealing. What else is different? Perhaps most significant to me is that the STR is a single channel amp that lacks tremolo, while the TR is a conventional Fender dual channel tremolo amp. That tremolo difference is HUGE!

The STR is an amp that appeals to an entirely different kind of player than a BFTR, IMO.

[Mljbone]

Thanks for the input! I picked up 470 ohm and 1500 resistors to replace on that power tube socket. But they are 2 watt , I'm not sure if that makes a difference I think they need to be 1 Watt. Also is there a way to tighten up the power tube connections inside the socket, some of them look like they're open. I was told loose ones could cause my red plating problem. THANKS!

[nevetslab]

When you look down the tube socket insets, are they wrap-around or fork-style terminals? The wrap-around type are easy to re-tension, using a very tiny bladed screwdriver to somewhat close down the opening without preventing pin insertion in the process. The forked terminals require a much smaller but stiff tool that can enter in the narrow slot that keeps them captive and aligned for mating with the tube pin. They're a bit tricky to bend in a bit in the attempt to make a tighter fit again.

With regards to the wattage on the screen resistors, I normally use Metal Oxide power resistors. There are two sizes available for each wattage up to 5W. MO2 is the larger 2W body size, and MO2S is a smaller 2W body. A MO2S looks like a MO1 in size. The real difference is surface temperature under load. A MO2S will run hotter than a MO2

[Mljbone]

Thanks nevetslab, they're wrap around.

[Randall]

With regards to your resistor wattage question, you can always go up in wattage with resistors. A 1 watt is built to withstand 1 watt of power, where a 2 watt will withstand 2 watts of power, so in your case, not a bad thing to go to 2 watts.

[Mljbone]

Thanks Randall!

[Mljbone]

So I replaced the resistors on the 3rd power tube socket. Plugged the head in to check output on pin number 5 on all sockets. Started with the first 3 with -53.6v the others at -49.1v. Waited a minute or two and went back to check output. They climbed up to -59.1v and -53.4. Then the resistors on first socket zorched and the fuse burned out. This socket is the one with red plating issues. Any ideas?

[Enzo]

You have an obvious arc, if there were no tubes. The resistor has no circuit, it goes to a socket pin and stops. Even with a tube in the socket, it goes to a grid, which is a wire hanging in space. To get that resistor to burn up, it has to have current from somewher, and most likely is pin 4 next to is, several hundred volts different. I'd replace that socket.

[Mljbone]

Thanks for the information Enzo!