fender reverb pot acting like drive pot? Solved! Fender installed incorrect capacitor!

Discussion in 'Amp Tech Center' started by texwest, Sep 4, 2020.

  1. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    Hi Guys! Just got done recapping a vintage 65 princeton reverb. Turned it on for the first time and it sounds great except the reverb pot is acting like a drive knob when you turn it up. It's like it creating more distortion as it goes up. Put it at about 3 and it sounds ok. All the way down the amp sounds great.

    Already tried different tubes in reverb driver and reverb return.

    Any ideas what is going wrong. Thinking about replacing all the caps in the reverb circuit. But wondering if it might be something like the reverb transformer.

    I'm sure you can help me. Thanks
     
    Last edited: Sep 4, 2020
  2. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,614
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    What all did you change / fix / touch / look at in your recap? Pics help, too.

    It's weird, but I think this is the third thread in a week where a PR reverb knob is adding volume or gain.
     
    texwest, hepular and D'tar like this.
  3. D'tar

    D'tar Friend of Leo's

    Posts:
    3,033
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Throw a meter on the outer lugs of the reverb pot. Make sure its a 100k not a 1M.
     
    texwest likes this.
  4. JonFirstCoast

    JonFirstCoast TDPRI Member

    Age:
    40
    Posts:
    6
    Joined:
    Sep 2, 2020
    Location:
    Jacksonville, FL
    +1
    Well its at least the second for sure...and if you can learn from my experience start at the reverb mix and reverb attenuator resistors
     
    hepular and texwest like this.
  5. 24 track

    24 track Doctor of Teleocity Silver Supporter

    Posts:
    16,286
    Joined:
    Nov 6, 2014
    Location:
    kamloops bc
    if the reverb driver transformer is a fender 037099 they are rare for some reason , lots of people carry them but none in stock anywhere , I need one for Supersonic 22 right now .
     
  6. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    I put it new filter caps and cathode bypass caps and bias cap. The only parts near the reverb circuit that I changed were the cathode bypass caps on the send and return tubes. The guy who brought it in reported certain symptoms of extreme noise and hum and I thought it prudent to do some basic maintenance before I turned it on. After turning it on I quickly realized it was having a total power tube failure. So replaced those and now everything is perfect except this gain when you turn up the reverb.

    Could I have a bad brand new 22uf cap causing this? The voltage on the send tube cathode is 8 v. The voltages on both cathodes of the return tube are 1.46v.

    I already checked the pot. It's reading at 90k. The pot's connection to ground looks good.

    Haven't gotten around to checking it with a different reverb tank and cables yet. Need to do that.
     
    Last edited: Sep 5, 2020
  7. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    Since it's the reverb pot I figured I should maybe change the .003 cap and 470k resistor. The 470k is reading about 560k right now. The 3.3meg is at 3.6meg. The 220k at the reverb plugs is at 227k. The dwell resistor is really close to 1meg.

    Maybe getting that 470k and 3.3meg closer to value will do the trick.
     
    BigDaddy23 likes this.
  8. Jon Snell

    Jon Snell Tele-Holic

    Posts:
    536
    Joined:
    Aug 31, 2015
    Location:
    Jurassic Coast. UK.
    One basic lesson learnt here. Always check, test then 'basic maintenance'.

    At least you will know whether the issue was there before or after the component changes.
    Don't just look at the ground connections, remove the possibility by resoldering and use a link wire to shunt signal paths.
    Work from the output/drive section backwards. Follow the signal flow with your oscilloscope, then you will see where the problem is quite easily. It takes any guess work out of the equation.
    The resistors you describe as high value are not totally way out. You are clutching at straws I think.
     
    texwest likes this.
  9. radiocaster

    radiocaster Poster Extraordinaire

    Posts:
    7,369
    Joined:
    Aug 18, 2015
    Location:
    europe
    Did you accidentally substitute a 12AX7 for the 12AT7?
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,936
    Joined:
    Dec 29, 2012
    Location:
    United States
    I see this symptom fairly often. It is usually caused by a bad ground, sometimes at the pan. Clean all four RCA connectors and swap the cable (correct connection but using other cable) to see if it's the cable and see if anything changes.
     
    jondanger, King Fan and texwest like this.
  11. hepular

    hepular Tele-Meister

    Posts:
    453
    Joined:
    Feb 17, 2019
    Location:
    abilene, tx
    iirc, i got a similar result when i completely bypassed the preamp stage when i didn't get the reverb mix installed correctly in my superchamp: modded the values after @Bendyha and neglected to 'twin' the film cap & resistor--which is an unorthodox way of getting reverb with your lead channel on an SC, BUT . . . . it was much better after fixing.

    but that doesn't seem any way close to this . . .
     
  12. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    The only new components in the reverb circuit are the 3 new cathode caps. Could a bad new cathode cap cause this?

    I tested the amp with a different cable and tank and I have the same problem.
     
  13. D'tar

    D'tar Friend of Leo's

    Posts:
    3,033
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Does reverb work? Can you ground the grid of the recovery triode. Still misbehaving? Measure V3p7 to chassis with the reverb pot maxed. Again at zero. Whats your ohm reading.
     
  14. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    I found the problem! This amp has always been wrong since 1965!! I'm surprised no one else figured it out. But the problem wasn't quite as noticable with single coil pickups. I decided to play the amp with my strat and it wasn't as bad.

    This made me realize the problem might be in the stage right before the reverb tubes and specifically in the.02, the 500 pf and 1meg resistor.

    I was right. Fender put a 5000pf cap in the reverb feed instead of a 500pf.

    Well that amp builder went dyslexic and thought .005 looked like 500. LOL

    IMG_20200905_122939576[1].jpg
     
    Last edited: Sep 5, 2020
    BobbyZ, King Fan and hepular like this.
  15. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    I did not turn the amp on until I had replaced the filter caps, cathode caps and bias cap. The owner reported very loud hum and terrible noises and I didn't want to turn it on and risk any damage without first replacing those electrolytics.

    When I turned the amp on with the bias meters connected to both power tubes, I immediately knew it needed new power tubes. One meter spiked up to 100ma and the other did nothing at all and there was no sound, but terrible crackling and hum. Replaced tubes and it worked great except the reverb.

    Even if I had turned it on just to see what happened, I still would have done the same thing. I would have turned it on and listened to it and immediately shut it down and never known about the reverb problem.

    I'm sincerely curious, you still think I did it wrong?
     
    King Fan likes this.
  16. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    12,894
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    Just had a similar issue with a 64 PR, reverb knob added gain not in a good way. Thanks to the folks here I found a bad connection on a reverb cable. Never would've thought it would be that simple.
    A wrong cap like that would be easy to miss!
    Good catch!
     
    texwest and King Fan like this.
  17. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,614
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Yep. Things come in threes.

    Except when they come in nines.

    Or twenty-sevens.... :D
     
    Fretting out and texwest like this.
  18. Jon Snell

    Jon Snell Tele-Holic

    Posts:
    536
    Joined:
    Aug 31, 2015
    Location:
    Jurassic Coast. UK.
    I didn't mean you did anything wrong.

    What I meant is we all have our own methods and over the years I have found that if the faulty amplifier is visually inspected for swollen capacitors and burn ups, then powered up, before any work is carried out, a basic idea of the issue/s can be found out at an early stage.
    For instance, I would have noticed a faulty valve, replaced the pair and then it would have worked well enough to examine for other faults.
    It just saves the thought in the back of ones mind, "Was the fault there before or after I replaced the capacitors" etc. Anything for an easy, logical, life :)
    Sorry if I caused any bad feeling but being dyslexic is good for repairs, as I can concentrate my direction but I am cr@p at explaining what I mean, without messing it up.
    Pleased you found the problem.
     
    BobbyZ and texwest like this.
  19. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    Actually I didn't take it too badly. I'm a self taught tube amp tech and have a lot to learn. I've been slowly learning since about 2006 when I started working on my own amps. I always manage to figure out what's wrong - by the seat of my pants. I know there is so much I don't know and really wanted to know about my process.

    The output tubes didn't look bad at all. They had the black painted glass. Honestly, I need to buy a tube tester, but I've always managed without one.

    Is there a good way to inspect an old can capacitor? Will it show evidence if one or more sections are bad?
     
  20. texwest

    texwest TDPRI Member

    Age:
    55
    Posts:
    33
    Joined:
    Sep 1, 2008
    Location:
    Urbana
    Yeah it's such a cool feeling when you figure it out the mystery!

    It wasn't until I pulled it and had it in my hand that I saw .005 and it made total sense why the reverb was adding some overdrive.

    Still surprised the amp lived this long and no one ever figured that out. The owner told me later that he never had the reverb above 3 and another amp tech tried to fix the reverb by changing the tank.

    Lesson - look more carefully at the markings on the components and don't trust the factory did things right!
     
    BobbyZ and Jon Snell like this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.