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Updates on 81 Pro Reverb problems

Discussion in 'Amp Tech Center' started by Axer57, Sep 28, 2017.

  1. Axer57

    Axer57 Tele-Meister

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    I posted here a while ago and received a great amount of help with my pro reverb. Unfortunately I had to take it to a tech anyway. The original problem was , the vibrato channel was weak and distorted. I took it to a tech who has his own line of hand built amps. He had my amp for 3 months , then said I should blackface it , which he was sure would solve my problem. I agreed . He replaced the transformers , added a rectifier tube , installed a bias pot in the chassis , replaced all the electrolytic caps ....ect. he said it sounded good at first but then it got worse. Each time I called he said he was still working on it. Finally at 9 months he said he gave up on it. I picked it up . When I got it home I plugged in my biasrite probe and a new set of 6L6GC's. The bias instantly rose out of control and red plated the tube not in the probe. I quickly shut it down and tried another set of tubes. Same reaction only it was the tube in the probe this time. I tried adjusting the bias as the current kept rising. This had no affect at all. I realize it's impossible to diagnose this one without knowing verything the guy did , but I just need some help finding a starting point. He gave me a printed layout of the scheme he was using. It's an AB763 Super... Any help would be greatly appreciated.
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I suspect the bias voltage supply. Your tech may have wired it wrong.

    Monitor the negative bias voltage on the power tube grids, pin 5, at startup and see if you can turn the bias adjust to get the highest negative voltage possible (-60 is better than -40) for the coolest bias. Let us know what voltage you can set.
     
  3. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    Was this a real tech with a shop? If so 1) I hope you didn't give him a dime, and 2) you got ALL your original parts back.

    Honestly, I'm dumbfounded. "Blackfacing" an amp does not involve replacing transformers - he was essentially trying to convert your amp into an earlier model, which is silly. It would have been more logical to sell it and buy a different amp.

    But his "needing" to do that work makes no sense. Is this really the entire story? How much did he quote you to do the work? Just based on what you posted he sounds like an amateur hack - especially having "given up". NO professional would do that.

    If you paid him it sounds like a solid small claims case - and you need a new tech, not DIY advice. Not with the extensive changes he *may* have made. You'
    d need to post the transformer codes and a list of the "as built" changes he made, as I would not trust that he actually did NAY of the work he claimed.
     
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  4. Axer57

    Axer57 Tele-Meister

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    I have had my probe on V1 each time I have turned it on , hooked to my DMM , set to ma So I monitor the bias voltage from the start. I left the amp on standby for 5 minutes before turning it on full. The bias voltage just takes off. At about -45ma I tried to adjust it to get it to stop rising. Turning the bias pot had no affect. Before I knew it it was at -80ma , so I kicked the amp back on standby. I tried it twice with 2 different good sets on 6L6GC's but got the same result both times. The bias just keeps rising. As far as the guy who had my amp , he has his own line of amps that he builds. I think he got in way over his head with mine. My Pro is an Ultralinear model. That's why you have to put different transformers in to Blackface it. He did the work mechanically speaking . It has new Fender transformers in it , he cut the chassis and added a rectifier tube, I see quite a bit of new wiring inside and the filter caps and fiber board are all new. He quoted me $400.00 for everything upon completion. When he gave up he said no charge. So I got alot of good new parts for free.
     
  5. Wally

    Wally Telefied Ad Free Member

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    you need a tech. There was no need to do what he did to the amp. I also would not buy one of his amps.
     
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  6. D'tar

    D'tar Friend of Leo's

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    o_O:confused::(:cry::eek::mad:


     
  7. Axer57

    Axer57 Tele-Meister

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    I agree with everyone , I would never take anything to this guy again. Any Ideas on why my bias voltage is running away? If the amp worked when he first got it done , then there must be something that's gone bad since then.
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    In this case I unfortunately think you're going to need to find a new tech to diagnose the issues and make sure of what was/wasn't done.

    Did *you* ever hear it working properly after you picked it up or are you taking his word for it?

    Without knowing what exactly was done or what modifications were made or what circuit you really have it's gonna be really tricky to help over a forum since there isn't a reference point of how things should be for this amp any longer. With how sketchy the whole thing is there isn't a reason to trust that the layout or schematic he provided you is correct either.
     
  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    Can you post pictures of the insides? Definitely sounds like a mistake in the bias supply so possibly someone here will spot it.

    Building amps is easier than troubleshooting amps. All you have to do is follow a known "recipe" to the letter.
     
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  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    "When he gave up he said no charge. So I got a lot of good new parts for free."

    He got your original Fender parts and you got replacement stuff. He probably came out on top.

    It is tech time for you and your amp.
     
  11. Axer57

    Axer57 Tele-Meister

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    He gave me back everything he took out. Transformers and such.
     
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  12. aerhed

    aerhed Friend of Leo's

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    Find someone to put them back in.
     
  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Good. Your tech seems to be incompetent but at least he's honest.
     
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  14. jazzguitar

    jazzguitar Tele-Afflicted

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    I wonder. These amp circuits are pretty straightforward, not rocket science. There is a limited number of things that can go wrong inside the amp, and while it is true, some problems like hiss or intermittent static noise can be hard to track down, I can't be that complicated to make the amp work fine if it is more or less dead now.

    And suggesting blackfacing when the problem is not known is insane IMO. A well serviced Silverface sounds great in its own right even if you prefer the model's blackface predecessor.
     
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  15. clintj

    clintj Friend of Leo's

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    A good tech, someone good with Fenders, needs to sit down with that chassis. It's going to take a layout or two, a highlighter, a multimeter, and a nice tall drink to trace the new circuit out and see where the issues are.

    What tech looks at a weak channel and says the whole amp needs to be blackfaced to fix it? (No, don't answer that in public. That's not nice to do.)
     
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  16. Axer57

    Axer57 Tele-Meister

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    I understood all this , but $400.00 didn't seem that high for all the stuff he promised to replace. I am pretty limited on techs where I live. The next closest one is about a 3 hour drive. What I can see that he replaced are. New output and power transformers , added a new choke , new GZ34 tube and socket , replaced about 4 tube sockets , replaced fiber board for filter caps , new filter caps , and alot of new wiring. The layout he gave me , that he was using says Super Reverb AB763. It has red retraced lines on it where I would guess , are the lines he changed. at this point it looks easier to me to just buy a new fiber main board and build it from scratch I will prolly take it to another tech , just gotta find the time to drive it that far. Just thought I would throw it in here and see if anyone had any general ideas on where to start. I can't even turn it on long enough to check voltages without it going thermal due to the runaway bias.
     
  17. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Map shows you're halfway between Chicago and Nashville, so you have some good options if you're willing to drive. What he replaced or didn't really doesn't matter at this point, unfortunately.
     
  18. Axer57

    Axer57 Tele-Meister

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    DSCN0233.JPG DSCN0240.JPG DSCN0230.JPG DSCN0231.JPG DSCN0234.JPG DSCN0235.JPG DSCN0236.JPG DSCN0237.JPG DSCN0238.JPG here are the pics
     
    Last edited: Oct 2, 2017
  19. GigsbyBoyUK

    GigsbyBoyUK Friend of Leo's

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    Not the first time I've heard of (and experienced) someone 'who builds amps' being terrible at repairing other people's amps.
     
  20. Wally

    Wally Telefied Ad Free Member

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    Axer, there is no reason to gut the amp and build a new circuit. There is every reason to put it in the hands
    of a good tech. The first place to start is to check the output of that PTP bias circuit and see what is underneath...and maybe then strip that shrink wrap off of the resistor and diode and see what is going on there.
    We have to admit that he did a lot of work for free.....and to no avail, it seems.
     
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