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Restoring an old Twin reverb

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

  1. tubegeek

    tubegeek Friend of Leo's

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    The consensus of every knowledgeable amp tech I know - including those who've checked in here - is that you replace electrolytics that are over 10 or 15 years old.

    You replace the less expensive short-lifetime parts to protect the expensive parts. Perhaps you haven't seen shorted e-caps take out a vintage transformer, but there are others who have and this is pretty common wisdom.

    There are exceptions to this rule - the typical failure mode of e-caps is worse when equipment has not been turned on for a long time - the reason e-caps are polarized has to do with the chemical reaction in the electrolyte which is maintained better with regular use, and when caps are said to be "dried out" it's because the electrolyte no longer is capable of depositing an insulation layer on the electrodes.

    So if an amp is in regular use, it is somewhat less likely to fail this way.

    Anyway I'm more of a theory guy than a working repairman, but the advice from working repairmen is pretty consistently to be safer than sorry on this point.

    The analogy I see a lot: "would you drive a '66 Mustang on its original tires?"

    This is where my comments are coming from.
     
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  2. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Thanks Wally, I expect I would have awful seller's regret if I sold this one. Some of the posts in this thread have made me lean toward minimal intervention. I'll replace the bias cap, but not the diodes (unless I detect a fault in the rectifier circuit). The existing bias cap is a 50mfd. I'm leaning toward replacing it with a similar value. I'll admit, I have no idea what my target sound aspect may be; as close to what I remember that is used to be would be the target, I guess.

    As for tubes, there is so many variables and opinions around, yikes. I figure a decent quad will cost $100 - $150, yet they may not be an improvement over the mismatch gang I have in there. It looks like NOS 7581s go for around $100 apiece. Tung-Sol has new 7581-As for $32 apiece, that sounds like a sensible compromise, if i go with the 7581 option. Decisions, decisions....

    123_0513.JPG




    Thanks 24, I appreciate you checking in. Loud is good.
     
    Last edited: Oct 28, 2020
  3. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    rschiller, hello. I agree, I'll leave everything but the electrolyte caps alone, unless I find something fishy with a multimeter. I'm pretty sure I installed those screen resistors myself, but it was 40 years ago and I can't remember why. I think I remember one or more of the being smoked. I'm trying to resist replacing components unless it is necessary; my new motto is: less intervention is better. This may change. I'm even wondering whether I should lift one leg of caps and resistors for testing, because the "original" solder joints have better resale value than re-flowed ones. At least that's what i hear from some smart-sound folks here.



    Thanks Mark. As I mentioned above, you guys are convincing me not to fix problems that may not exist. I believe replacing the electrolyte caps is a necessary "evil"; other than that, I'll do as little component replacement as I can get away with.
     
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  4. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    vintageampz, those look like perfect specimen, congrats! I have little to worry about regarding studio gigs, I'm a for-fun player. I think this one has gone past the collector's condition, and is a partscaster now. Although, I do respect your idea of not making unnecessary or gratuitous changes, thanks for the advice.



    JB, thanks for the thoughtful and generous post. I agree with all your suggestions, except, I'm planning to replace the ground switch with a 1/2 power switch, so I can turn it up without breaking windows. It's an easily reversable mod. I'm considering Sprague Atoms for the doghouse and bias cap. I'm vascillating on replacing the doghouse resistors. I'll test them and see if they are out of spec. Same with the diodes.
     
    Last edited: Oct 28, 2020
  5. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Bill I think it is a great idea to test the B+ resistors. Carbon comps are actually well suited for that duty in a tube amp because they take high voltage spikes well. On all my fenders those resistors have been out of spec though because of the heat. Also PM me if you want to change the courtesy outlet to the grounded 3 conductor style. I may be able to help you.
     
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  6. JIMMY JAZZMAN

    JIMMY JAZZMAN Tele-Holic

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    It appears that the amp is in very good shape; considering, but if you put all components back to original;
    it would never pass code. It's sure nice to see one of the best amps ever, once again. Those were the days.
     
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  7. Crunchmeister1435

    Crunchmeister1435 TDPRI Member

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    I'm an electrical engineer. I know they are electrolytic caps. Lol. Was just making sure you'd know what I was talking about.

     
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  8. Wally

    Wally Telefied Ad Free Member

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    okay, now I know that you are far more qualified in the electronic tech world than am I. This leads me to observe that in order to decrease the possibility for confusion among those of us without your formal training that instead of saying ‘the Orange caps’—-which in the informal language of these tube amps these caps are NOT, imho an EE would have used a more definitive term for the reference to those caps...maybe ‘those electrolytic bypass caps’?? Also, imho, IF one understood that those were electrolytic bypass caps and understood the circuit, it would be a foregone assumption that those leads were ground leads and in this ?Fender amp go to ground on the brass ground plate that sits at the front of the chassis.
    The reason I responded to your post was to make sure that OTHERS understood what was meant in your post. I am not an EE, but I understand that the first question above is unnecessary IF one has the training or experience to understand what those ‘Orange caps’ are and how they are placed in the circuit. I certainly understood what those bypass caps were even though I am a self-taught tech with only 25 years experience, but there are those here who do not understand what they are or what their leads from the negative end do. Hence my first query about your post. I wanted to dispel confusion.

    Best to you.
     
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  9. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    ^ I think there have been some crossed signals in this thread since a few people have referred to the filter caps by their color, including the OP.

    Blue or black are perfectly acceptable colors to see in replacement filter caps, IMO. :D
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Bill, your good thread has somehow called in just about every possible opinion on restoring old amps. As I said, I admire your positive attitude and unfailing courtesy. Please update us on what you end up doing, and as I also said, have fun along the way. :)
     
    Last edited: Oct 28, 2020
  11. Wally

    Wally Telefied Ad Free Member

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    and hence my first query.... I like the black caps for Electrolytics. Tone and coupling??? Orange, yellow...anything but red.
    It just occurred to me....what about how color-blind brothers and sisters??? How to describe things to them??? Oh...do we REALLY have to revert to accurate and actual defining terms????
    Good Moooooorning, wonderWorld!!!,
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Heh, Wally, accurate terms??? that would totally violate the Prime Directive of internet forums: Never use exact nomenclature or cite a reputable source... :D
     
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  13. OldManTele

    OldManTele NEW MEMBER!

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    In my experience, value is determined by what somebody wants it for. People who demand 100%-original equipment will wind up making fixes anyway if they want to play or enjoy it, so I think that "maintenance" fixes that don't change the sound or look will not have a big effect on value to somebody who wants that amp. I have an ancient Bassmaster and if I wanted to get it a Twin Reverb buddy I would gladly pay full value if it had been maintained but still had the original sound and type of component. Replaced burnt-out tubes and caps would not bother me. Keith Jagger once said it was all about the beat. But in this case, it's all about the sound. It ain't furniture, it's a piece of electronic sound equipment.
     
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  14. Engine Swap

    Engine Swap Tele-Afflicted

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    You should consider replacing the diodes. I believe the originals are rated for 600v which doesn't give much of a cushion.

    I had one fail in my Showman. Replaced all with 1000v jobbies.
     
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  15. dickey

    dickey Friend of Leo's

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    I agree with Viejo...You couldn't give me a Boogie, and to swap a BFTR for one is about par with the time I swapped an original Broadcaster straight for an SG Custom.
    Well, maybe not THAT bad.
     
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  16. Ten Over

    Ten Over Tele-Holic

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    600V apiece? I've always wondered about that. If they are, three in series would be 1800V. Whatever they are, they have worked fine in countless amps for decades.

    Anything can fail at any time including new 1N4007's. This one falls into the "if it ain't broke, don't fix it" column.
     
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  17. Ten Over

    Ten Over Tele-Holic

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    Well then you should have called them big brown k'dealy bobbers.

    Did you know that you recommended doing something that would have disabled the amplifier?
     
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  18. tubegeek

    tubegeek Friend of Leo's

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    That's a sure-fire way to guarantee consistent performance!
     
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  19. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    LOL! That’s true.
     
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  20. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Jeepers, I leave you kids alone for a few hours, and all heck breaks loose! :lol:

    You both make valid points. I started out thinking I would update the rectifier circuit, now I think I'll poke everything with a multi-meter, and leave anything that looks OK.

    Not much love for Mesa products around here, is there?

    I ordered a bunch of stuff from amprepairparts.com. I'll replace the speaker plug with an original style with the plastic F on it. The cap supplier, that 24Track recommended, contacted me, he runs the vintagefenderamprepair.com website. I look forward to hearing his recommendations.

    I spoke with the guitar tech at my local music store, he offered to let me use his Variac to re-form the caps, great news.

    I got some more cleaning in today. Honestly I'm spending more time reading and typing, than restoring. o_O I'm enjoying very much the conversations.
     
    Last edited: Oct 28, 2020
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