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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 agree. Without a Variac and a few hours to watch things, I don't want to take the chance. I haven't heard back from fenderjunkie73 yet, re the cap kit.

    Thanks, KF. It's way more fun doing this with the company of other guitar and amp enthusiasts. Are you a fan of all the Kings, or one in particular?
     
  2. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Interesting comment, Powdog. I think the sell-it-as-is ship has sailed; I'm really psyched up to fix it up and use it as my rehearsal amp. I can just park it at our rehearsal space where we practice every week.

    Most opinions I've heard indicate that the F&Ts are superior to the Spraque Atoms; thanks for the heads up!
     
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  3. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Rather than the Variac, have you considered powering it up with a high-value series resistor to check current draw?

    From R.G. Keen:
    This could help tell you within a couple hours whether one of the caps wants to blow.
     
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  4. tubegeek

    tubegeek Friend of Leo's

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    Forgot about this trick - thanks for citing RG.

    BTW: I asked him once: "RG" doesn't stand for anything. Not sure how that works but there it is.
     
  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Interesting! He renders it as "R.G." on a lot of his site, so I figured they were initials for something. Like he was a Robert that didn't want to be "Bob'ed"... :D
     
  6. Wally

    Wally Telefied Ad Free Member

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    Diodes...IN4007s do the job, iirc. Bias cap...I use a 100mfd/100vdc electrolytic there. You do not want to go beyond 100mfds due to the time constant concerns. regarding changing the capacitance in the first stage of filtering, one has options that have sonic differences. The stock 2x70mfds in series up yields a 35mfd cap. 2x80mfds would yield a 40mfd cap....for all purposes very much the same. 2x100mfds yield a 50mfd cap. That is approaching a 50% increase in filtering and will yield a different sonic....tighter and more articulate especially in the low end. 2x220s would yield a 110mfd cap, and this stiffens things up considerably. One might consider the target sound aspect before making a big change.

    Power tubes...very personal thing, imho. IF one has the bucks and wants what I consider to be the best tube for a big TR sound, buy some NOS 7581s. I would not use these tubes as ‘daily drivers’....once you price them you will understand why. For a recording studio application, this would be THE power tube for a TR, imho/ime. I made the mistake of putting a price on my last BF TR...a 965 with EVs and NOS 7581 that had less than 20 hours. The fellow had already paid me for two very nice SFTRs and then asked about my personal TR. I said I had never put a price on it...this was whenBFTRs had taken the fall from the top of the hill consider value for a BF Fender. I priced it at. A point at which I figured he would NOT ant to pay for it. He had heard it in action, though. When I said “$2800”, he didn’t even blink...counted it out and I had to console my seller’s regret with Benjamins. It was a superlative amplifier.
    I like JJ 6L6s. Money on tubes would be well-spent for two or three NOS 12AX7s for V1,V2, and V4 in addition to the two 12AT7 drivers you are buying for V3 and V6. Any functioning 12AX7 works for the V5 position...even a microphonic tube since is not processing signal.
     
    Last edited: Oct 27, 2020
  7. tubegeek

    tubegeek Friend of Leo's

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    He might have been f'ing with me, who knows?
     
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  8. 24 track

    24 track Doctor of Teleocity Ad Free Member

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    • looks like a fun project, enjoy the rebuild , these are very loud amps ( I have a 65'BFRI)
     
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  9. Andy ZZ

    Andy ZZ TDPRI Member

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    "Just my opinion but I wouldn't trade the worst Twin I've ever heard for the best Boogie in the world."
    Man, that is true Gospel. That Twin is a sweet machine. If you plan to sell, everything you do will devalue it somewhat.
    Leave it, unless something is broke.
    2 cents only.
     
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  10. vintageampz

    vintageampz Tele-Meister

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    If you want to KILL its vintage value do all those stupid things, except for the 3-prong power cord conversion and leave the damn 2-prong on the chassis ALONE! ONLY replace the bad caps, resistor etc. If they ain't broken, DON'T break it.

    I have a '65 and have had to "re-cap" it, but I bought a big NOS supply of Caps and resistors (and tubes) decades ago for my shop. So my Twin Reverb (Super, Deluxe's, Princeton's, Bassman's, etc) are ALL original and sound that way, which is what I want for studeo gigs and when I have to sell them.

    Screw around with your Twin and you will be sorry. With it's value, you should ONLY be letting a qualified and VERY experienced Amp Tech make changes based on it's vintage value and sound.

    Fender Twin Reverb 1965  front.jpg
    1964 Fender Super Reverb front.jpg
     
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  11. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Leave the tubes if the amp sounds good, you have no noise, and the amp biases correctly. New manufacture tubes are hit or miss and in my experience and not very reliable.
     
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  12. Feramentum

    Feramentum TDPRI Member

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    Thoughts while you are at it. Replace the 100k plate load resistors. The old carbon ones has and with the reverb turned up they kind of attribute to the thunderstorm sounding amp "blow." (What you here when it's idling.) Replacing the power caps is a great idea and didn't affect the tone of my 65 super when I did all of them. Ran more reliably in my mind. You do this remember to rebias amp immediately afterwards. Caps that old may have dropped some peak voltage off the back side as it is developing that circuit voltage.
     
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  13. oldlefty

    oldlefty TDPRI Member

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    When building my current limiter I found- at Lowe's- a replacement power cord for appliances. It was in the extension cord section. Worked fine, beefy, BLACK, about $10.
     
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  14. Feramentum

    Feramentum TDPRI Member

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    The other deal save the parts. You can put them back in and let the bomb count down start again.
     
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  15. Bill Moore

    Bill Moore Tele-Afflicted

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  16. tubegeek

    tubegeek Friend of Leo's

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  17. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    LOL, yeah, ya wimps!

    I got enough blasts from these amps already to last a lifetime. The worst is on the lips, the first time you brush the mic, yeouch! For years, the first thing I did, after turning an amp on, is to touch the tuning keys to the mic and look for sparks. If it goes wap! wap! wap!, reverse the leads. I'm planning on removing the death cap and installing a 3 wire cord. I think I will leave the accessory outlet as it is, thanks for the scolding, vintageampz.
     
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  18. rschiller

    rschiller TDPRI Member

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    Do NOT replace any of the tone or coupling capacitors; they rarely go out. From what I can see, the cathode bypass caps are not leaking and are likely ok. Despite what others have indicated, the five filter caps are original to the era. Fender used whomever had caps at the time so the two different brands is no big deal and I suspect they are the same. The 3 20/500s look ok but should be replaced. The 2 70/350s appear to be starting to bulge and should be replaced; use 110/350 and also replace the 220k/1w in series resistors.

    Some work has been done on the amp; the 6L6 screen resistors are newer metal film resistors which were unavailable in 1965. I suggest checking value on all B+ drop resistors and cathodes resistors, likely they have gone out of value to the high side. If Cathodes are more than 5% out replace them. Even if B+ 100k resistors in both of the gain stages are near correct, replacing with metal-film will make the amp quiet.

    If the bias is near the factory -52v or slightly or slightly lower, a bias pot isn't necessary. Folks get overly nutty about setting bias exactly.
     
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  19. Mark1406

    Mark1406 TDPRI Member

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    A power transformer just doesn’t go pop, there is a fuse in place and the diodes would go before the power transformer, that’s just common sense. I can’t say I’ve seen old electros that were short circuit power caps.

    It’s not a common issue, if I draw on my own experience, I have bought cheap new electros for a pedal that were short circuit, and encountered cathode bypass electros that were short circuit in a Mesa Boogie SOB, and that is in twenty years of working on electronics professionally.

    My friend also fixes amps and we talk about repairs and he has never mentioned an amp with short circuit filter caps.

    What I suggest is, if the amp had been working, then it is probable that it is still working. There isn’t any sign of a fault, which would be heat and burning. Perhaps the owner can use his multimeter to make a few checks for short circuits around the power supply and then diagnose the amp, rather than fixing non existent faults people assume to be present.

    I hope that clarifies things.
     
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  20. Southboundsuarez

    Southboundsuarez TDPRI Member

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    I know that this is true, about orig . speakers.... but to me it just seems so crazy that a set of non orig. speakers can devalue an otherwise all orig. amp sometimes by as much as 50% !
    If you intend to bring this amp up to turnkey ready play vintage desirable goodness, for top saleability and all around attractiveness....
    1. take it apart only as far down as needed to give her good douching and good scrubbing off of all the grime. But be gentle.... dont polish away the patina. Only the dirt and grime.
    2. remove the "death cap" place it in a bag with other replaced components.
    Replace the 2conductor mains power cord with a good quality 3 conductor 9 foot 18 guage power cable with appropriate grounded 3 prong molded plug. Vintage Bonus Points for cable that has paper Underwriters Laboratory label/tag.
    Do not remove or replace the outlet or polarity switch.
    Use the tabs of polarity switch as a tag board to wire the mains with both poles of the switch wired the same so the switch remains on and in the same polarity in either direction and no ground lift. Make sure that you have the correct polarity on the accessory convience outlet. ( I have seen these with both with the wide and narrow keyed slots and also with the older nonkeyed same sized slots. Either way, make sure that the nuetral is wired correct.
    3.replace filter caps in the doghouse. Many folks appreciate seeing big old Spraque brand caps here. I use whatever high quality 105°c low esr 500v or better cap that fits but people like to see Spraque. *(bonus trick- it is 2020 and we are well into the new millienium futurewise as far as technology..... There are many offshore brands of high quality caps that are produced that greatly exceed the value of the old vintage and at a fraction of the physical size. With some effort and ingenuity, a person can easily retrofit the new caps inside of the body of the old cap and maintain the vintage correct look.) I believe those old GE's to be stock correct and it looks ad though the other two caps were also just old factory inventory from pre-cbs.
    If not worried about vintage "look" I would replace the resistors next to those caps with "flameproof" metal oxide style resistors. Otherwiise test them and if over 10% tolerance replace with Allen Bradley carbon comps.
    3 It wouldnt be a bad idea to replace the rectification diodes. You can wire most any quality modern diode here. All modern diodes exceed the equivalency of what is installed and you can actually use fewer diodes.. (IIRC there is seven stock including bias and you can reduce the b+ bridged diodes from six to four or just replace like you see it.) Those balance resistors on the rectifier board can optionally be replaced with metal oxide flame proof) The electrolytic bias cap should also be replaced . Use a Spraque for vintage correct capacitance value or a modern cap with a slightly higher value.
    4. as for the bias circuit I would leave it as it is.... While your amp is post cbs,,, technically it still subscribes to blackface circuit topology and indeed has the preferred global bias adjustment. Dont add any bias balance circuitry modification if you wish to naintain vintage Blackface value/esthetics and overall collectable appeal. (less mods or deviations the better)
    5. I have no problem with metal oxide screen resistors and actually prefer to see them here. Replacement plate resistors is a pro / con thing with Carbon Comp vs Metal Oxide .... for purely under the hood looks Carbon Comps for tone there is possible edge to the Carbon Comps, for performance, reliability and low thermal noise Metal Oxide.

    Hmm .... seems I had a couple of other suggestions but it also appears that you ard getting plenty of other advice in the time it has taken me to write with akk my distractions......
    But I think the main point are less is more....
    typically with vintage amps it is expected that the polarized aluminum electrolyrics will be changed out. grid stopper resistors often changed along with plate resistors and it is normal to no longer see the orig mains power cord.
    But no outright modifications , no no no for resale value.
     
    Last edited: Oct 27, 2020
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