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Restoring an old Twin reverb

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

  1. BobbyZ

    BobbyZ Doctor of Teleocity

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    I'm just skimming this thread, or should I say swimming in horse manure?
    Anyway, this tech wants to reform the original. electrolytic capacitors in a 1965 Fender Twin Reverb? ? ?
    Just replace the damned things, all of um. Trying to reform them is like trying to reform Charles Manson, it won't end well.
    I use F&T for the filter caps, a 100v-100uf F&T for the bias cap and Sprague 25v-25uf for the cathode bypass caps.
    Get a set of colored Sharpie permanent markers if you want different colors. Hell make them black F&Ts blue if you want! Be better than paying for the cheap little caps inside the expensive blue covers. ;)

    Trade a BFTR for a Mesa?
    Now that's just plain crazy talk.
     
  2. Jason Hutchinson

    Jason Hutchinson TDPRI Member

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    I'm 99.999% sure they can't be reformed. Those old caps are dried up and will no longer hold a charge. There is no reason to try to save them, especially the ones in the power supply. They are just power supply filter caps.

    If just one of the diodes fail the diode test, then they should all be replaced. You don't want mismatched rectifier diodes.

    The diodes and electrolytics are there to protect the circuit, and do not directly effect the tone. Carbon comp resistors in the power supply should also go. When they age, their resistance goes way up and could be putting a strain on the power transformer, which does show signs of overheating since some of the potting wax has melted out. A weak point in blackface amps are the transformers. One of the things that CBS ownership got right was to pot their transformers in epoxy rather than wax.
     
  3. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Wait a minute. Wait a minute. You posted a previous picture of a what you said in that post was a reverb transformer which had a 9th week of '66 stamp on it. That must have been a picture of the choke.
     
    Last edited: Oct 29, 2020
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  4. Wally

    Wally Telefied Ad Free Member

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    Blue Bill, I have heard too many before and after examples NOT to think that one should replace all electrolytics as a matter of proper care. I have had people comment that an amp sounded great with the original e-caps in it....until they heard the amp with fresh electrolytics. The loose low end with out of tune subharmonics was tightened up,solid, and accurate. The high end regained the sparkle and was smoother. And the amp’s soundstage regained that three-dimensional pattern with full, accurate harmonic content.
    These amps did not leave the factory with old e-caps. These amps did not sound the way they sound all,of these years later with old caps that cannot possibly meet the demands of the circuit the way fresh caps can. If one wants to hear these old amps sound as they should, fresh electrolytics are the first step, ime. Imho, half-step efforts yield an amp that does not sound like it should/could, and the door op is left open for repetitive trips to the repair bench. Go through the amp and do good service with all e-caps replaced. The amp will reward you with years of good, reliable service, ime.
     
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  5. Wally

    Wally Telefied Ad Free Member

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    in post #40, Blue Bill gave us this picture.....
    BFF174A3-2212-4568-B9BF-46F435F9EF97.jpeg

    This is a reverb driver transformer from the 43rd week of ?1973?
     
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  6. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    That was the post I was responding to.

    Edit: correction, go look at post #33
     
  7. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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  8. Wally

    Wally Telefied Ad Free Member

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    I agree that Blue Bill mistook that choke for the reverb tranny in that post #33.
     
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  9. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Anyway, just wanted to say something since I can't go back and edit my post where I said the reverb transformer is dated 9th week of 1966. Lol
     
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  10. tubegeek

    tubegeek Friend of Leo's

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    Manufacturer 606: Woodward-Schumacher according to a chart on Triode Electronics.

    http://www.triodeel.com/eiacode.htm

    Agreed, more likely 1973 than '63 or '83 or...

    Has anyone here done the experiment of listening to the reverb send by connecting a speaker directly? It should work just fine, but just be sure to put a dummy load on the actual speaker output if you do.
     
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  11. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I did it when I was troubleshooting the reverb on the '67 Super Reverb that I restored. I just hooked up the output to one of the loose CTS alnicos that was sitting near by.

    In that case, it turned out the reverb wasn't working because my test cables were bad!
     
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  12. tubegeek

    tubegeek Friend of Leo's

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    Did it sound like the main output or is it EQ'd differently enough that you can tell a difference?
     
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  13. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I was testing through just the driver. The amp and speaker were on the bench, and the speaker was connected to the reverb transformer secondary via alligator clips.

    The roll off is something like 300Hz going into the reverb driver also.
     
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  14. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    @tubegeek I guess I should add that the main speaker out was plugged into a dummy load.

    Anyway, you can imagine the sound of a 10" speaker with no baffle. It was just ... meep meep meep. :lol:
     
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  15. tubegeek

    tubegeek Friend of Leo's

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    Yeah, hard to assess the EQ on the low end with that setup no doubt!

    Sometimes it's even worse: with the driver sitting horizontally on the desk, listening to the back AND the front (edge-on) is a pretty terrible sound.
     
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  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    EDIT: I see you were typing your reasonable answer about e-caps below while I was just posting this short novel. You can ignore it, and go forward in peace. Since you ask about filter caps, I'll say almost no one now thinks blue Sprague Atoms are the 'superior' brand -- that ship sailed a while back. They're big and blue, but if you cut 'em open, they have a normal-sized, normal construction small e-cap inside. So if you want to impress buyers, or feel you chose the top (ordinary, non-tweaky) cap, most folks here like F&T for quality, not just size or price.... END EDIT

    @Blue Bill , your patience and tolerance for group input continue to impress me. :) Let me humbly suggest somewhere in the flood of advice above we may have latched onto an idea floating by that won't reliably keep us afloat.

    IIRC, early on someone smart said if you were hoping to demo the amp for a seller but NOT restore it, you might be able to try reforming the old e-caps on a variac. What you're now hearing from smart folks like @BobbyZ and @Jason Hutchinson and @Wally is that wouldn't be a good idea *if you intend to keep and play* the amp... or sell it as 'restored.' It may not work at all, and it certainly won't make the old caps reliable.

    There's a school (we've heard from a few members) that says leave old parts alone until they fail. But for e-caps, at least, the huge majority of pro restorers and techs I see here and on amp-specific websites says "just replace 'em." There is no logical, likely way their failings (or functionings, even) could actually improve the sound of the amp, much less its reliability.

    Again, I think we heard 'just leave them' or 'try reforming them' from several smart folks *when we thought you were all set to try to sell the amp* but hoped to make it slightly safer to turn on once or twice....
     
    Last edited: Oct 29, 2020
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  17. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    LOL. I am for sure replacing all the electrolyte caps. The only question is which caps to use. F&Ts seem to be the sensible choice. I'm still toying with the idea of hunting down a set of Sprague Atoms, not because of the color, it would be because they are better quality, i.e. last longer and add value. I haven't found a comparison between these two cap brands.

    More Boogie bashing, I see. No one likes Mesa amps?



    Thanks Jason, great info. What's the best way to test the diodes?



    Yep, I confused the RT for the choke. The RT was missing from the amp when I got it, the amp didn't work at all, IIRC, one or two of the screen grid resistors were blown. That's how I got it so cheap; I rolled my eyes a lot about what a piece of junk it was. I think it took about an hour to install the new RT and resistors, the amp worked like a charm for years after that.
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Great. I edited my long (and now needless) post above urging you to do just that. I just got confused when you said you were going to be able to borrow a variac to reform the caps.... Thanks for clarifying.
     
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  19. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Thanks Wally, I'm 100% convinced that the old e-caps are getting replaced and bagged up for posterity. I do appreciate your concern, amigo.



    I ordered the replacement RT from somewhere, it was before this internet thing was invented. I expect it was old inventory, hence the 1973 date stamp





    Meep meep, LOL. I haven't even started worrying about the reverb circuit and tank. I did order a new 12AT7. If it works when I get things back together, then I won't need to worry. If it doesn't work, I'll be bothering you guys all over again for more advice.
     
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  20. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Good morning, KF, I do understand and agree with the idea of leaving the old caps in place, if the plan is to sell it as a collector's piece. With this amp, my plan is to get it into gig-ready condition and play it. I'll probably use it as my rehearsal amp, that way I don't need to lift it very often and the loudness won't bother anyone, we practice in a warehouse.

    I just caught your edit... Thanks, I did not know that the SAs were "cosmetically enhanced"! Thanks for the heads up.

    My understanding of reforming is that even brand-new electrolyte caps should be brought up to working voltage gradually. That's why I was asking about a Variac.

    Have a great day gang, I gotta go to work so I can pay for all this stuff! :)
     
    Last edited: Oct 29, 2020
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