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Restoring an old Twin reverb

Discussion in 'Amp Tech Center' started by Blue Bill, Oct 25, 2020.

  1. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    I finally got motivated to take this on. I have so many questions. I am grateful for any suggestions, opinions, or comments. Back story: I traded a Datsun B210 for a Twin Reverb, maybe 35 years ago. I bought the car for $200 and ran it for a year, including a memorable trip to Chicago, where I got to buy John Lee Hooker a drink.

    The serial number, stamped on the chassis is A04069, which indicates 1965 ( http://www.ggjaguar.com/fendamp6.htm ) The Tube chart is stamped with date code PG, which indicates July 1966, so it's a '65 or '66.

    It's nearly factory stock, just a few replaced parts and new speakers. The old speakers were mis-matched, I can't remember what they were. My first question is: Is it better to just go ahead and replace all the caps, or test each one and only replace the ones that act funny? The amp has been mostly babied. It lived in a recording studio for 15 or so years. It's been on a shelf for the last 15 years, unused.

    I don't mind replacing the caps, but am wondering about two things: Will it affect the sound? The amp always sounded really great, clean, punchy, and exciting, really fun to play through. I don't see any caps that are leaky or burnt looking.

    Also, will it affect the value of the amp? Is it worth more with vintage components, than new, shiny ones? One reason I'm restoring it, is that I've been GAS-ing for a Boogie, an since I haven't used the Twin in ages, it's so loud and a DR is much easier to transport, I'm hoping to not damage it's value by fixing it up too much, so I can sell it or trade it.

    Some things I'm planning on:

    1. Replace the original 2-prong plug setup, including the death cap, with a standard 3-prong plug and safe wiring.

    2. Use the ground reverse switch hole to install a half-power switch, based on Rob Robinette's scheme.

    3. Replace the old 2-hole AC outlet with a grounded one.

    4. Add a bias trim pot, also based on Robrob's diagrams.

    5. Major cleaning.

    Some pictures:

    123_0450.JPG 123_0443.JPG 123_0445.JPG 123_0485.JPG
     
  2. hepular

    hepular Tele-Holic

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    it is NOT stock. 3 of those e-caps have been replaced, & those screen (?) resistors on the power tubes weren't there when it left the factory.

    Still, if it's mostly been sitting around for 15 years replace ALL the e-caps. coupling caps only if you feel like it or can test their esr rating effectively.
     
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  3. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    123_0490.JPG 123_0499.JPG 123_0501.JPG Screen Shot 2020-10-23 at 1.46.43 AM.png Next question: the power tube resistors, the 470-Ohm, 1W ones, appear to have been replaced with blue ones, which I think are 480-Ohm, 3Watt, 2% ones. I ordered some 470W, 1W, 1% resistors from Mouser; also some 1500-Ohm, 1/2-W, 1% ones.

    Is it better to use the 1-Watt 470-Ohm ones, or the 3-Watt 480-Ohm ones. I assume the lower power ones will act as a crude fuse if the current spikes. Also, I noticed on Rob Robinette's site ( https://robrobinette.com/How_The_AB763_Deluxe_Reverb_Works.htm ), he shows a 3-Watt, 470-Ohm resistor, although the Fender schematic indicates a 1-Watt resistor. Or, I could order some 470-Ohm, 3-W resistors. I must have switched to the blue ones at some point, but I can't remember when or why.

    More pictures:
     
  4. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Hi Hep. So, those 3 GE "ALUMALYTICS" are not original? I was typing when you posted; as I mentioned, I think I must have replaced those resistors at some point. I have not touched the dog house, I'm pretty sure.

    I just checked the Transtronics site. An ESR tester is $224, yikes. Is there any other way to test ESR?



    Thanks for checking in.
     
  5. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    What do you mean by bias trim pot ? That amp has a bias adjustment pot already are you wanting to add a bias balance pot? Just my opinion but I wouldn't trade the worst Twin I've ever heard for the best Boogie in the world.
     
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  6. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Next question: What should I replace the E-caps with? Again, I'm looking for resale value, so I'm thinking orange drops or other modern-looking ones may turn off some people. I've used the black F&Ts on other amps. Are the blue Sprague Atom's better? I don't mind spending some extra money to get the best performance and appearance.
     
  7. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    LOL, Viejo! There's a decent chance I will fall back in love with this amp after it's fixed up. You know how GAS works, it can be fickle. I've never owned a Mesa. I've tried out a couple, but not for a few years.

    I may be wrong, but I thought the existing bias pot was the balance pot. On the picture below, lifted from Rob Robinette's site, it shows the added trim pot, for adjusting the bias voltage, soldered to the back of the factory balance pot. I'll figure out the correct info before I pull the trigger. I like the idea of being able to easily adjust the voltage, so I can experiment with raising and lowering it a bit, to get the best sound.

    Screen Shot 2020-10-25 at 9.41.56 PM.png
     
    Last edited: Oct 25, 2020
  8. 24 track

    24 track Doctor of Teleocity Silver Supporter

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    2 sugestions
    1) remove the death cap
    2) new complete cap job

     
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  9. avspecialist

    avspecialist Tele-Meister

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    Hi Blue Bill, I updated a few twin reverbs to date. I definitely would replace all the electrolytics, and save the old ones in a bag for a future buyer. I would leave the blue molded ajax caps in there is possible if they are not leaky. You could get a good esr meter on amazon from $65 to 120 dollars. I really like the C12N Jensen’s in there. The blue metal oxide resistors should be fine as well as the screen resistors if they actually measure 1500 ohms. I like a little less negative feedback and usually change the resister from 820 ohms to 4.7k, it makes the amp a little fuller. To me biasing the power tubes just right makes a world of difference. I’m fortunate to have a 100 meg scope. I bias the amp using 900Hz signal, bring the amp up to the start of clipping and the back off till I get a slight crossover notch. I double check my voltages to make sure I not really biasing too hot. I like the Weber bias calculator to check my numbers. I select preamp tubes to fill in the little holes in the sound I’m going for. You have a great amp, i hope you enjoy it
     
  10. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Hi 24. I've been reading old threads until my head is spinning. People say to carefully de-solder the death cap and put it up for sale, because they're highly sought-after, although, I can't remember what for. Any opinions on whether a Fender cord would be a better choice than a generic one, or an expensive one? I think I'll go for a 10-footer. Mercury Magnetics makes nice heavy-duty cords, but they are bright red. I wouldn't mind that if it's my amp, but I'm shooting for best resale value.

    So, you don't think a complete re-cap will hurt the "vintage mojo"? I'm probably way over-thinking this.
     
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  11. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    That is a later model twin your photo . Your Twin appears to be a AB763 Blackface Twin and already has a true bias adjustment pot ,it is possible to add a bias balance pot but with matched tubes being so prevalent that is probably not needed. Tune it up, clean it up( sockets and pots) take care of the grounding safety and fall back in love with one of the finest amps ever made.
     
  12. hepular

    hepular Tele-Holic

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    will the buyer lurv the smell of decaying electrolytics? will they love the gooey mess when one fails and explodes? The ship sailed on "mojo" with those metal screen resistors: get its circuit solid & the tone will sell it. I think Uncle Doug just uses any old power cord that can handle the specs.

    &, as for resistors, 2 or 3 watt is plenty--and probably 2 is better, maybe even 1 so that you get that fail mode protection.
     
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  13. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    AV, thanks for the info, very useful. I really like the idea of keeping the old caps for bona fides. I'll probably replace the 1500 Ohm screen resistors with the 1% ones, to balance the screen voltages; although, I'll admit, I don't know if this will improve anything or not. I got some cool-looking mil-spec ones from Mouser.

    I'll also read up on negative feedback, maybe I should build a substitution rigamajig and see what sounds better. I don't have a scope, nor a signal generator; If I get that far into it, I'll call around and find someone that has one.
     
  14. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Great tip, thanks!
     
  15. Engine Swap

    Engine Swap Tele-Holic

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    What makes you say the 3 electrolyics aren't stock? My 1966 Showman (essentially same circuit as Twin) has those exact caps.
     
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  16. hepular

    hepular Tele-Holic

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    yep, you could be right--which makes it even more imperative to replace them.
     
  17. Engine Swap

    Engine Swap Tele-Holic

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    You said they had been replaced - how can you tell?
     
    Last edited: Oct 25, 2020
  18. avspecialist

    avspecialist Tele-Meister

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    the 2 cardboard ones are original, the GE ones are probably from 1970/71. They are the same type that my 67 super reverb installed in that same date range when I had it repaired back then.
     
  19. Ten Over

    Ten Over Tele-Holic

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    If you are looking to sell this amplifier, then don't touch anything. ANYTHING. It is at its highest value right now. Anything you do will only devalue it.

    The electrolytics are all original including the doghouse. The new screen resistors are 470 ohms. There is a date code from 1966 on one of the capacitors and it really looks untouched.

    1. Only if you're going to keep it.

    2. Don't do that, even if you're going to keep it.

    3. Grounded convenience outlets are available, but outrageously overpriced. Don't do that, anyway.

    4. Doesn't need it.

    5. I wouldn't even do that.
     
  20. Wildeman

    Wildeman Tele-Meister

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    If it works now then I'd just sell it as is or trade it for what i want. Why throw time and money at it just to sell it? You won't get that money back.
     
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