1962 Ampeg Reverberocket R-12 Low, Distorted Output

Discussion in 'Amp Tech Center' started by Faceman, Aug 20, 2019.

  1. Faceman

    Faceman TDPRI Member

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    Hello All,
    I have a 1962 Ampeg Reverberocket on the bench. It came in with no power, no output. The customer bought it from an estate sale and plugged it in as soon as they got home. Sure enough, the supply filter cap can had a few open caps in it. I changed all the caps and serviced the sockets and pots. Changed the power cord to a 3 prong as well. The tubes are original 5Y3, 6V6 (2), 6SN7 (4). My tube tester is on the fritz and can only test for shorts.

    I now have output but it is low and a bit distorted. When I swap the pre tubes around the volume varies in loudness but never reaches full volume. Although the sound changes a bit when swapping tubes around, I don't believe it to be a tube issue. I cannot find a schematic with pin-out voltages anywhere on this amp.

    Any thoughts or experiences?
     
  2. radiocaster

    radiocaster Poster Extraordinaire

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    The tubes are not original, there are supposed to be 2 6SL7s.
     
  3. Faceman

    Faceman TDPRI Member

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    You are correct. My mistake. There are 2 6SL7s.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Faceman, regardless of whether or not you have a schematic with voltages, you need to make a voltage chart. There are certain ‘truths’ that have to be in the circuit, and an aberrant voltage will stick out like a sore thumb in many cases. Imho, switching those tubes around does not prove whether or not any tube is bad or good. One needs at least one known good tube of each type when working on an amp. If the voltages look correct when considering the source voltage and the circuit, then the tube should produce signal properly. If not! Then the tube or something in that part of the circuit is bad.
    During the process of making this voltage chart, one can use one of if not the oldest testing procedure in tube amplifier history. Starting at the power tube plates, one should hear a pop when making contact to take voltage measurement. As one moves from the power tubes to the PI and the rest of the signal processing gain stages, that pop should increase with the pop at the input stages being the loudest. If you encounter an area where the pop does NOT exhibit this behavior, there is a problem in that area. The reverb area is not part of this pop test....but take the voltages there for sure.
     
    Last edited: Aug 20, 2019
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  5. Faceman

    Faceman TDPRI Member

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    Sound advice Wally. Thanks


    Sent from my iPhone using Tapatalk
     
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  6. slider313

    slider313 Tele-Afflicted Silver Supporter

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    I've serviced more than a few Reverberockets over the years. Did you replace the can itself, or did you use individual caps? Did you observe and follow the grounding scheme? Did you check the voltages off of each section of the can, to the schematic and on the plates of the power tubes? Did you check the value of all the resistors? Low and distorted signal could be anything from an open resistor in the circuit to a bad output transformer.

    FYI, three or four of the Reverberockets I've serviced needed output transformers.
     
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  7. Faceman

    Faceman TDPRI Member

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    Slider313,
    Thanks for the response,

    1) I did replace the cap can with an exact replacement.
    2) I did not test voltage at the can, however, the readings at the rectifier look to be correct. There are no voltages called out on the schematic nor are there on any schematics I have found online. So, I have no reference point
    3) I am going to go back through the resistors again. I didn’t find any open resistors the first time but will see if I missed something.
    4) The output transformer seemed to test okay as far as resistance on the winding but I will take a closer look at that as well.

    Thanks again!
     
  8. Wally

    Wally Telefied Ad Free Member

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    nothing wrong with checking resistors. I would check voltages first since that tells you what a lot of the resistors are doing...or not doing. If the voltages look somewhat appropriate, then one starts to wonder about things that aren’t directly connected to those voltage readings.
    I once resurrected a 5B5 I bought as a wreck...but all original except for the covering. New tweed, reconed the F15N field coil, replaced all caps...e-caps and the paper in oils. I finally got it back together and got to hear it. Very disappointing Sonics. All voltages were understandable...and The schematic had no voltages. But....the sound was strong enough and correct enough to hear that the amp just wasn’t right. So...I checked all resistors. The only ones that stayed in the circuit were the input stage resistors. That amp had been played! 250K plate resistors were all reading 470-480K. After all of the board resistors were done, the amp came to life...magnificent. That was close to hearing one back when it was new, right? Everything was fresh..and the original trannies were pumping along like they were new, too!
     
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  9. slider313

    slider313 Tele-Afflicted Silver Supporter

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    I had an Ampeg Jet here, a few years back, that had no sound at all. B+ was correct, plate voltage checked out, bias voltage was good, speaker good, etc. It had an open 470k resistor feeding the phase inverter. Replaced it and the amp sounded great.
     
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  10. Faceman

    Faceman TDPRI Member

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    I checked all resistors and found none open or out of spec. Got to be one I missed maybe...

    Again, very low output.

    Here is the voltages. They look a little strange to me but again, no reference point. Maybe someone can help out..

    AC in: I set the variac to 120VAC
    DC output from rectifier 360VDC

    Note, the AC heater voltage is not measured on the list but is present. I only listed the DC voltages.

    IMG_0025.JPG
     
  11. Wally

    Wally Telefied Ad Free Member

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    Are you sure about your heater voltage measurements....pins 7/8
    for the 6SL7 and 6SN7 and pins 2/7 for the power tubes? Has the heater voltage been elevated?
    The drawn schematic By J. Piazza has only that voltage...6VAC. V2 is a problem. No plate voltage or cathode voltage????
    When you were doing the measurements, did you pay attention to the Pop test? If not, do that....start in the power tubes and work back toward the input...V3 6SN7 is the reverb and will not play a part in this pop assessment. I am going to think that you will have a drastic drop in the POP! there.
     
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  12. D'tar

    D'tar Friend of Leo's

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    Quick look at the ole schematic V2 p2 plate and V3 p5 plate are both fed by node "F" of the B+ string. Check the 22K dropping resistor feeding that node.

    upload_2019-8-22_9-38-34.png
     
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  13. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I also never trust a Piazza schematic. Been burned too many times.
     
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  14. D'tar

    D'tar Friend of Leo's

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    Ha. Good to know Corliss
     
  15. Faceman

    Faceman TDPRI Member

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    Great info guys.

    Wally, oddly enough, the only POP I got was on pin 5 V2. The rest were very minor pops. I will check again tonight.

    D’tar, thanks. You beat me to it. While day dreaming about amp repair at work lol, my first thought was tracing the dead cathodes and plates back to the power source to see if I had a resistor out of spec. I did test all resistors and caps (tho all new) for open but didn’t measure every one yet.

    My other thought was retesting voltages with the reverb and tremolo circuits engaged. I don’t think they were switched on when I tested the voltages. Not sure if that will make a difference but I am a Fender guy and know that is the case on those.

    Anyhow...great stuff. I appreciate all the suggestions.
     
  16. D'tar

    D'tar Friend of Leo's

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    The F node should supply B+ at all times. I just re read that you checked all resistors etc. The 22k may be just fine. If you have B+ on one side and 0vdc on the other the resistor has either failed open, you have a poor connection or you have a short to ground at the F node. The filter there may be suspect as well. Recheck vdc across the resistor then ohm F to chassis. If low resistance you are looking for a short to ground through either a component,(filter cap etc) or a connection, (socket, wire, solder joint, component lead etc.) Good luck.
     
    Last edited: Aug 22, 2019
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  17. Wally

    Wally Telefied Ad Free Member

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    That Pop! Was on pin 5, correct?

    Corliss1, so have you corrected those schematics where you have found errors?
     
  18. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Nope, probably should have, but didn't think about it at the time.
     
  19. Faceman

    Faceman TDPRI Member

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    Yes, pin 5
     
    Last edited: Aug 22, 2019
  20. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    FWIW if your tube tester tests for shorts that's all it would be good for anyway.

    Older tube testers don't supply sufficient plate voltages to test tubes for guitar amp use. They max out at 185 plate volts on power tubes and hit preamp tubes with around 50-60 volts. Which makes them useless *except* for testing for shorts.

    Plus they need to be calibrated every 5 years or so anyway, which runs $150-300.

    The only ones that are worth having are new ones like the Orange Tube Tester, a couple of boutique units or calibrated rare/vintage early-1960's military units that DO test using high voltages - but will run you about $3,000 and STILL need to be calibrated.
     
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