Pro Reverb from Hell

Discussion in 'Amp Tech Center' started by Ruairi, Oct 21, 2021.

  1. Ruairi

    Ruairi NEW MEMBER!

    Oct 21, 2021
    Hi to all here. I am new to TDPRI but have lurked for years. Finally joined :)

    I'll start with a maddening post.. please excuse the length, and thank you all. Nice to be here

    I have recently rebuilt a vintage Fender Pro Reverb amplifier for a customer.
    This amp is either a 1965 or 1966 model blackface. From the stampings, I'd say 1965 but it had a stock bias capacitor of 50u/50v not a 25u/50v as shown on the AA165 schematic. Minor detail but that cap was changed to a 50/50 for the next
    circuit so in true Fender fashion, makes me think might be 1966 but every other component was stock except for the tremolo electrolytics which had been replaced at some point.

    This came to me in awful shape, not basketcase but almost.
    The indicator lamp and jewel was missing. Tolex was torn and the faceplate bent out of shape by someone's attempt to repair from outside the cabinet. Tremolo speed pot smashed in, no handle and no upper backplate on the cabinet.
    I replaced everything broken and missing to start and wired a 3 prong code primary cord and removed Death Cap and ground switch wiring to fire it up.
    When I did (after warm up) the rectifier was shorted. It was not stock but replaced years ago.
    One tall and one short RCA 6L6GC for power. The short one had no filament. None visible and never heated up although good 6vac at 2 and 7.
    Saved the three of them for customer. Replaced with JJ GZ34 and matched pair of Soviet (63PS?) tubes that I recommended. The thinking was first, save some money on what would be (and was indeed) a very costly repair. secondly, I wanted to put NOS tubes in initially (without breaking the bank) and was looking for something that would not melt while I did the rest of the amp. The tubes could be upgraded later if all else was good.
    V1-V3 were old stock RCAs. All three were horribly microphonic. Saved them, new ones went in (Tung-Sol CP)
    V4-V6 tested OK.

    I recommended a power supply cap job and customer thought about it, I think he wanted to get it up before sinking the money in a cap job.
    Fair enough. Now the real fun began. Biased up the power tubes and started tone and play testing.
    B+ about 460v. Biased tubes for 17.5w ea for 35w. Also installed 1 ohms on cathodes to measure bias (always account for screen I, yes)
    This should be high 6x% dissapation for those tubes.
    Proceeded to find numerous way out of tolerance resistors and several cracked solder joints. You folks know how time consuming that can be.
    Ended up replacing very noisy 0.1 caps with 715s on the PI output. 220Ks looked and tested fine. All power and signal tracing good at this point and noise was reduced to minimum noise (maybe -60-70dB) a little annoying hum but could be balanced out with each channel's volume.
    Old stock Utah speakers were shot, cone cry, noise the works. Replaced with Jensen C12Ns.

    I play tested for about an hour in the cradle, back in the cabinet bolted up with stock reverb pan and cables and new Jensens.
    I kept hearing an intermittent noise and when I went fret by fret found the noise only on E (any octave) and only with the reverb on.
    I figured a bad pan but while checking around I got the noise when moving the cables. Replaced cables and noise disappeared on E and everywhere
    else on the board. Still like to figure that out.

    All looked good. Brought it up to the studio. Practiced with it for two nights, good.
    Called the customer, explained the hefty bill line by line and brought it back to shop. I like to have the shop play test it to verify all in order when I drop them off. Counter man a guitar teacher and loves his rock. After jamming loudly for 20 minutes started to sound funky (not in a good way). I got down to check it out and see one power tube very red plated. Damn.
    Killed the HV, turned it off. Back to the shop with it.
    Usually when a tube red plates that hard, it lost bias -v. Tested all that again. Talked the customer into doing the cap job anyway to be sure all that was covered (I know, but it turned out one cap was leaking at least so good advice, nonetheless)
    I could not replicate the problem. All components and voltages checked out.

    Measured output transformer. Primary was about 155-160 ohms but slightly unbalanced about 78 vs. 86 ohms. hmmm.

    New production 125A6As spec about 185 ohms so hmmm. Figured maybe a partial short in the OT just breaking over? Ordered a Hammond 1750J just in case.
    I have used only Classic Tone transformers in the past but they (sniff) are COVID casualties so went Hammond for now.
    I hesitate to replace the OT because this is a vintage amp and there are no more stock 125A6As. So I went through everything 5 times. Components values. solder joints and continuity for perhaps everything from the PI input on. Everything checked out.
    While testing it I noticed very slight red plating (about 2mm spot on both tubes) that sounded very much OK to me while playing. I'm a big BF fan and if I could, this was exactly the amp I want. Better condition though.

    Those tubes are 26w max plate dissapation so at 460v maybe not up to it? Changed tubes to a brand new pair of Tung-Sol 6L6GC-STR current production, a tube I have built with and has never had any problems. Biased those to 18-20w each which is a about 65% for the Tung-Sols
    Played again for and hour or so in the studio, sounds great. At after about an hour hear funkness. Check back, red plated tube.

    Back to the bench, now ready to replace the OT but want to see the problem in the shop first. Wanted to make sure I wasn't running them too hot so biased to about 17.5w each (62-63%?) and jammed away Loud for quite awhile. Nothing.
    Tired of playing, plugged in some loud music from the hard drive and ran for a few hours while keeping an eye on the tubes. Nothing.
    With both channels from 6-8 and loud enough to begin to bother the ears (even with the cabinet facing away) still nothing and still sounds good.
    If nothing else burning his new speakers in.

    So that's where I'm at now. I have a few ideas to bounce off the learned folk here:

    1. I have a hunch that I have a flaky tube socket. One odd and unexplained part of this odyssey (not mentioned) was, when I tested that above mentioned short bottle RCA on the tube tester it was fine. When I put the pair back in, that tube still had no filaments. All continuity checks still good.
    No problems with the filament supply anywhere. ?
    In addition, almost all maddening intermittents are caused by heat. The transformers are warm, not hot. The tube sockets (especially on a Fender) take a beating. I hesitate to replace the tube socket without further evidence because of all the wiring. I recently replaced a tube socket on an old vintage Gibson that had cracked in two pieces. That was a breeze compared to this Fender. Apparently Leo paid $10 a foot for wire. Everything is stretched to the max, if I have to cut anything at all, it'll have to be rewired completely. At what point is this no longer a vintage Fender? I could have built my customer a new one (almost) by now.
    Still, a flaky Pin 5 that heats up would lose the -V bias.

    2. I still suspect the OT. However, as above, this is a 55 yo amp. Everything is this short of crapping out. I'm trying to not move too much around or stress anything not currently malfunctioning. The vintage OT is a big part of the mojo here. Hate to lose it.
    Current status: Amp is back in the cabinet all wired up. I lowered the bias a tad on the Tung-Sols, now just a crack over 60%. Back in the cabinet with hours of LOUD music through the amp, no problem. The only thing I have not done is replace the upper back panel; heat retention?

    Just looking to see what the builders here think. If you got through this whole post then lol thank you.
    If you've worked on something for this long, it's time to consider alternate perspectives.

    Thanks to everyone in advance.

  2. W.L.Weller

    W.L.Weller Tele-Afflicted Silver Supporter

    May 20, 2014
    Couldn't you try re-tensioning the tube sockets? It's a bit of a pain finding something thin and strong enough to push the individual contacts back together. I have a tiny flat-blade screwdriver I've managed to get into the sockets to gently pry the halves of the contacts towards the center.

    Also, welcome aboard! If you've been reading for a while, you already know this is a pretty decent place to hang out.
    tubedude likes this.
  3. Mexitele Blues

    Mexitele Blues Tele-Afflicted

    Mar 7, 2018
    Westminster, CO
    Pro Reverbs are fantastic amps, and much of the magic is indeed in the OT. The OT in mine is also a few ohms off side to side. I don't believe it is anything to worry about.

    I agree to retension the sockets. I use one of those glasses repair screwdrivers from the checkout line.
    W.L.Weller and Timbresmith1 like this.
  4. tubedude

    tubedude Tele-Holic Silver Supporter

    Jul 26, 2013
    east georgia
    A long thin sewing needle will slide in there easily to retension the sockets also.
    dougstrum and W.L.Weller like this.
  5. bebopbrain

    bebopbrain Tele-Meister

    Feb 5, 2021
    New York City
    > Measured output transformer. Primary was ... slightly unbalanced about 78 vs. 86 ohms.

    Probably sides of the primary have the same number of turns. But each turn is a different length, which accounts for the difference in ohms.

    > The vintage OT is a big part of the mojo here.

    Agree completely.
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Dec 30, 2019
    Welcome to TDPRI.


    A couple years ago, I had a similar experience with a blackface Bassman. It came to me with a blown fuse and bad power tube. It needed a cap job... the usual stuff, but it had the same kind of intermittent trouble. I carefully examined and then retentioned the sockets. I suspected the amp may have been oscillating which caused the amp to run away, but I could not catch it oscillating. It worked fine for a week so, I told the owner what I experienced with the amp and to watch for redplating. I told him to bring it back if it had any trouble. I have heard nothing of any trouble from the owner. Later, I did find I had a problem with my multimeter so maybe I was fooled by the readings I took. I have never felt comfortable with that repair.
  7. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    Another vote for tube socket here. Amp runs, amp gets hot, contact in the socket gets weird. If you can let it run and make it happen outside the amp, you'd likely even be able to catch it.

    While it's redplating, grab something insulated (rubber, shirt, whatever) and rock the tube around. Does that fix the redplating? If so, definitely socket.
  8. Ruairi

    Ruairi NEW MEMBER!

    Oct 21, 2021
    Thanks to everyone here
    I agree about the socket. Usually I would replace both power tube sockets but vintage etc.
    My inability to catch it failing is what's driving me nuts here
    Lowerleftcoast's feeling of discomfort is the worm in my brain

    thanks all
  9. zook

    zook Friend of Leo's

    Aug 6, 2003
    Cochise, AZ
    I had a similar problem with an old Magnatone. I ended up replacing a couple tube sockets and the problem went away.
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