Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

crusty philtrum

crusty philtrum

Vintageologist
A while ago, forum member and fellow Aussie Gibson175 PM'd me about some issues with his Twin Reverb. Being 1,000 Kilometres away, there was not much I could do without seeing it.

So, after some discussion, i recieved this in the mail ....




A general inspection showed that the entire amp appeared mostly original with little sign of maintenance, poor-quality coupling caps and some unhealthy-looking solder joints. The first thing i did was install a new mains cable and tidied up some of the internal mains wiring ...(before and after pics) ...






While looking more closely at the condition of the internals, i saw a dodgey-looking solder joint in the bias supply ...



and sure enough, a gentle tug with a pair of pliers presented me with this ....



That was easily enough fixed with a soldering iron, but i'm sure it hadn't been helping with good performance in recent years.
 
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Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

This amp was wired all through with some kind of green wire (I suspect it was original Fender, although it was so bad i initially thought someone had replaced all the wire at some stage). I was sceptical about it's use for the tube heaters (filaments), particularly when they're wired in a long single string. I decided to rewire all the heaters, using several strings so that the current pulled through each string would be less. This involved creating a termination point, where the several strings of heater wiring would connect with the heater wires coming out of the power transformer. It's a long tedious job, but it pays off ....







 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

We'd discussed some changes Cam wanted to the circuit (shared reverb on both channels, and the two channels revoiced), so I put in an order for parts. While waiting for those parts, I continued to work on the wiring, and began to look at the component board. My thinking was to take the circuit from it's original CBS version back to the original s/f circuit without the CBS 'improvements', and then add the shared reverb/ tremolo and channel re-voicing.

The traditional Fender fibre board had distorted badly ... i discovered they can absorb moisture and 'grow' between the mounting screws, and also become slightly conductive ... not something one really wants in a high-voltage environment.









 
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Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

I found some people selling reproduction eyelet boards, but they use smaller eyelets, and they're in the USA, meaning freight costs would make getting a new board an expensive proposition.

I have a Japanese Guyatone s/f Fender clone, and that uses a very primitive copper-clad board with the copper on top, and it's a great alternative to the fibre board, much cleaner and easier to work with. I also have a very large sheet of copper-clad fibreglass board for making pcb's, and it's old, meaning the copper layer is very substantial. However, i never learnt about etching boards. I was beginning to think it was time to learn.

So I cut a piece of my fibreglass board to the right size and drilled it for the mounting screws.




I covered the copper side with masking tape, and began to duplicate the tracks from the board in my Guyatone onto the masking tape.







I sent Cam a pic of how i expected the new board to look once it was populated with components ...




Then i continued with copying the artwork until it was complete ...



 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

That's some piece of work you're taking on there Crusty, but I'd like to bet it'll be better than it came out of the factory once you're done. I'll look forward to seeing the end result on this.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

Next i had to drill all the little holes for the component leads (my drill bit was a little too big, meaning later i had to use a fair bit of solder in each joint)





Next task was to carefully cut away the masking tape where i wanted to keep the copper to form the tracks ...








Then I painted over the exposed copper with red nail polish, thusly ...

 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

and once that was dry, i removed the rest of the masking tape, leaving me with this ...






Next came some reading to learn what i'd need to do and buy to etch the board. That led to this ....




And after a couple of small test etches, it was time to go for it ...



After a few minutes, I ended up with this ...




Then a wipe with Acetone removed the red nail polish and left me with this ...





 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

Okay, now it was time to start populating the board (you'll notice that at some point i tinned all the bare copper tracks with solder) ...








You may notice some variations between the old and the new ... Cam wanted to have the 'Normal' channel voiced with the classic sparkle of a s/f, but the 'Tremolo' channel to be more round and warm sounding (I thought of it as his 'jazz' channel). Keep in mind that both channels were, hopefully, going to be able to access the reverb, and probably the tremolo.A few components were re-used from the old board, but most were new.

Here's a pic of the new board alongside the one that inspired it, inside my Guyatone s/f clone (The Guyatone has had the Normal channel components removed as the normal channel is being replaced by a tube effects loop). Then a pic of the board back in the Twin ...



 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

In the meantime, i'd also replaced the power valve sockets with new ceramic ones ...




The filter caps had already been replaced but i needed to change the value of the power supply resistors in there to get things back to pre-CBS specs ...

 
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Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

The new board needed to be wired back into the amp, so that process began (I had fitted a rubber insulation sheet between the underneath of the board and the metal chasis) ...












And then it was time to put the valves in, ready for the first testing ...



 
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Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

The initial tests showed that the Normal channel was behaving fine, as was the power stage, but the Tremolo channel was making a bunch of buzzing and humming sounds. Some detective work found the issues and got them fixed.

Overall, things were good ... both channels had access to reverb and tremolo, although the tremolo depth seemed a little compromised by the sharing. That may be able to be addressed, possibly by using a different value depth pot. Something i will need to look into.

The two channels seemed to be in the ballpark for the desired outcome, but of course only Cam himself would really be able to say. So, finally convinced that i'd done as much as i could and that the amp was working solidly and reliably, I packed it up and mailed it back to him. He recieved it on Friday but had been busy, although tonight he has told me he has been able to spend some time with it.

At this stage I'll leave it to him to comment more. It was a big job, but you don't have to read here much to know that he loves his Twin. It would have helped if we lived closer together, but we don't, so I did what i could to get as close as possible to what he had in mind. Hopefully he'll keep this amp for a very long time and get many many hours of great tones from it.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

Nice! This is really inspiring for my amp aspirations. I've got a couple builds I need to work on. I found a great source for metal film 1% 1W resistors, but now I need to find a good source for power supply electrolytics. That, and a good organizer fill of Sozo or similar signal caps. Any advice there?

I am thinking of trying to make my own eyelet boards, using some plastics or formica for the board, and nickel plated brass eyelets.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

It's 1.30 a.m. here, I guess Cam has gone to bed and may not be able to contribute his thoughts here for a few hours. Hopefully he might even be able to post a clip or two later ... I couldn't do that here because lurking on my bench right behind his amp in most of the pics is the power supply for my tube mic preamps ... the phantom power supply needs attention, so i can't record until i get that sorted out.

But early comments from him earlier this evening are very favourable. Judging from what originally arrived here, he won't be hearing his old amp, it will be like having a completely new amp. He'll have to 're-learn' it. Hopefully soon he'll be able to tell you all about it.

Before I etched the board, i had the artwork scanned multiple times, so i can reproduce these boards quite easily. They are standard two-channel Fender layouts incorporating tremolo and reverb and line up perfectly with the various wires that attach along the edges of the board and use the same mounting holes as the original fibre boards. At least that way the tedious process of copying the layout from my Guyatone board was not simply for a one-off, it can be used again to make more boards a lot more quickly.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

trevorus said:
Nice! This is really inspiring for my amp aspirations. I've got a couple builds I need to work on. I found a great source for metal film 1% 1W resistors, but now I need to find a good source for power supply electrolytics. That, and a good organizer fill of Sozo or similar signal caps. Any advice there?

I am thinking of trying to make my own eyelet boards, using some plastics or formica for the board, and nickel plated brass eyelets.
Click to expand...

Metal film ? Any particular reason for using those ? I find them to be somewhat sterile-sounding. I mostly use carbon film as a compromise between metal film and carbon composite. A friend of mine bought a homebrew Deluxe clone that had been wired with all metal film, and it sounded too clinical, so i replaced them all with carbon film and better caps ... the amp sounded a lot more organic.

When i got to reading about the repro eyelet boards, the biggest issue seems to be for people to obtain the right size eyelets ... apparently most were too small, and as you may know, sometimes on Fender amps even their eyelets seem too small sometimes when you have to try to get the leads of 4 or even 5 components in there. That, and cost, led me to pursue the method i've shown here.

Btw, you can buy the blank fibre board ... just remember to get two pieces, so one can form the insulation layer underneath the one with eyelets installed. But from what I've learned, the fibre board may be best avoided for long-term durability and reliability.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

crusty philtrum said:
Metal film ? Any particular reason for using those ? I find them to be somewhat sterile-sounding. I mostly use carbon film as a compromise between metal film and carbon composite. A friend of mine bought a homebrew Deluxe clone that had been wired with all metal film, and it sounded too clinical, so i replaced them all with carbon film and better caps ... the amp sounded a lot more organic.

When i got to reading about the repro eyelet boards, the biggest issue seems to be for people to obtain the right size eyelets ... apparently most were too small, and as you may know, sometimes on Fender amps even their eyelets seem too small sometimes when you have to try to get the leads of 4 or even 5 components in there. That, and cost, led me to pursue the method i've shown here.

Btw, you can buy the blank fibre board ... just remember to get two pieces, so one can form the insulation layer underneath the one with eyelets installed. But from what I've learned, the fibre board may be best avoided for long-term durability and reliability.
Click to expand...

I like metal film for consistency. I've used both carbon and metal, but I found a good source for the metal film, so why not? If I can find the same sort of packs of carbon films or even carbon comps, I'll definitely get some.

I definitely will avoid fiber. I did plan on doing some hardwood for some pedal builds, but wood and the heat in amps don't mix that well. I haven't yet sourced eyelets, so we'll see what sizes I find.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

The fiber board issue is what surprised me the most. Although it makes sense, that happening just had not occurred to me.
 
Re: Restoring/Modifying Gibson175's Beloved Twin Reverb (lotsa pics)

Laughing Kookaburra said:
The fiber board issue is what surprised me the most. Although it makes sense, that happening just had not occurred to me.
Click to expand...

IIRC when I got my Bassman, this is what had happened to the original board. If zzmoore happens to pop in this thread, he could confirm. I remember that he told me at the time that it was fairly common for fenders of this vintage. He had to build me a new one too.
 
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