silverface Fender Twin Reverb rebuild?

[kristianf]

Hi all!

I recently acquired an early-mid 70s pull master- silverface Fender Twin Reverb. I bought the amp in non-functioning condition, in part of a backline clearance.
The reason i bought the amp was because of the JBL D120F speakers, to pair with my silverface Bassman for pedalsteel use.
I got a pretty good deal on this Twin, and I already knew that it hasn't functioned properly for 15 years or so.
Due to that, I decided to pull the chassis and take a proper visual inspection before firing the amp up.

I have to admit that i was pretty shocked by what I saw. The amp had tells of previous modifications, as it has four different fuse holders on the back panel. The stock turret board and it's components looks mostly original, as far as I can tell, but the work done to it later is hazardous and pretty much the worst I've ever seen. Absolutely mind boggling that what I assume is a professional technician can do such terrible work.
The power tube sockets and components looks pretty damn shitty, and the more I poke around inside of this amp, the less faith I have in it's current condition. There are a lot of damaged insulation and wiring.
There's a lot of dodgy solderjoints all over the place. The unused taps of the export power transformer has been disconnected from the original rotary switch and was just bundled in one piece of electrical tape, no separate heat shrinking or anything. These are leads are cut very short, but I hope that I can manage to blind the unused taps in a safe fashion. Would these short tap leads pose any noise related/proximity issues in this way?

I don't have a lot of experience working on amps outside of bias adjustments and normal routine stuff, but I am an maritime electrician by trade so I have an understanding of electronics, schematics and luckily already have pretty good soldering skills, so I have considered getting into the DIY amp building for a long time.
I have been studying amps for the last while and think that I'm ready to begin DIY'ing the coming months.
After looking inside of this amp, I think the best solution would be to gut it, re-use the transformers and build something like a Hoffman single channel AB763, probably without reverb and tremolo.
The filter capacitors are the originals from the 70s, so it would have to be recapped anyway.
What do you guys think? Is it even worth trying to safe an amp in this condition or should I just gut the thing asap and start a DIY project?
I haven't had the time to measure the transformers to confirm that they are OK, but I will get to it shortly.
To be honest, I have a feeling that this amp will be a potential gold mine of trouble.

[vampwizzard]

First of all, welcome! This is a great post.. pictures, good description, not trying to bite off too much for the first go round, etc.

Youll be fine with the soldering, schematic reading etc.. i wouldnt be concerned with this as a first project, especially if you do go with a one channel AB763. There may be other folks with quad 6l6 schematics, but heres a lot of info on removing effects from that particular circuit https://robrobinette.com/Build_an_AB763_Amp.htm

[vampwizzard]

More inspiration: Tim posted gut shots of his own pedal steel amp. https://bb.steelguitarforum.com/viewtopic.php?t=242134&sid=76b84d4352fb5a2fddbf32f80754ba42

[Latole]

vampwizzard had a good advice.

Or you rebuilt as original; Is it a 100 W or 135 W ??

http://ampwares.com/schematics/100W_Mast_Vol_SF_Twin_Rev.pdf


http://ampwares.com/amplifiers/fender-silverface-twin-reverb/

Before power ON best is to build and use a bulb light limiter ; this will protect the amp and save fuse in case of malfunction:

It is a must when working on a unknow amp.



Mere about :

[PRR]

> JBL D120F speakers, ....the work done to it later is hazardous and pretty much the worst I've ever seen. ... what I assume is a professional technician .....a lot of damaged insulation and wiring.
There's a lot of dodgy solderjoints all over the place. The unused taps .........
> .... I am an maritime electrician by trade ...

If the 120s are healthy, this deal owes you nothing.

Bar-players can tolerate electrics which would be real darn stupid 60 miles out to sea.

OK, extra fuses.... what do they do?

[vampwizzard]

OK, extra fuses.... what do they do?

From the looks of it, melt wires! 

[DummyLoad]

looks like the 4 fuses are: AC line, B+, Inv OT 1/2 primary, Non-Inv OT 1/2 primary. possibly AR overkill?

replace the white electrolytics, esp. in the bias supply. replace all the electrolytic caps under the topside filter cover.



--pete

[kristianf]

Thank you very much all, great to be a part of this forum. I've been lurking and reading for quite a while, a lot of great information and knowledge over here, much appreciated.

I'll definitely make a bulb load as a part of my work station. I'll make a proper workbench in the garage before really getting into repairs, and that current limiter is on the short list.
I'm also trying to see if I can get a hold of the repair equipment for the now defunct electrical repair shop from a couple of years ago. I know that they had oscilloscopes and some other good tools, so if they're out of use, maybe I'll be able to buy them for a reasonable price.

The extra fuses are for the AC line, B+ and both OT blue/brown, correct.
As of now I'm thinking that the easiest thing is rewire it back to original spec, recap it and take it gradually from there.
It will probably take a while to get the right parts over here in Norway, due to pandemic related delays.

The extra fuses and the cabling is damaged and will have to be replaced anyway, do you guys think that I should just remove the extra OT fuses all together and just rewire it per to Fenders original spec?
The brown lead from the OT has damaged insulation, and as the fuses are already mounted, it might be an alternative to use the fuseholder to junction the damaged OT leads, as they won't reach tube sockets V8 and V9 directly as they are.
The heaters for the power tubes will all have to be replaced also, due to burnt isolation and bad solder joints.

[vampwizzard]

Oh lord. Good luck on the shipping! Im sure you wont have too many problems.

As far as extending wires, Id use crimped connectors over extra random connection points. If you need to make extra connections use a dedicated terminal strip. The goal should be to make life simpler for the next person who opens this thing up to work on it... save them from your troubles.

[Latole]

As of now I'm thinking that the easiest thing is rewire it back to original spec, recap it and take it gradually from there.
 

I agree.