Sound fades and dies

Discussion in 'Amp Tech Center' started by R. Stilskin, Oct 2, 2019.

  1. R. Stilskin

    R. Stilskin TDPRI Member

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    Hi, new guy here. I took up the guitar at an older age and am having alot of fun. I bought a Randall RVC, a store specific version of the RD5 but having an issue. I've downloaded a book and want to learn about tube amps but I'm hoping someone can point me in the right direction.

    The RVC is a 5 watt, 6v6 tube, one channel with toggle switchable eq. Gain, tone and volume controls along with the toggle switch mentioned previously. The volume control continues to work (like a master) when using the effects loop. I mention that because I have a Bugera V22 (which has channel and master volumes) and all volume is defeated when using the loop.

    Got the amp a year ago used. After a month it ran for a short time, volume faded and went out. Couple of days later I tried it, did same thing only faster. Then it didn't generate sound at all. Put it away for months, broke it out yesterday and it did same thing only just barely, couple of seconds and fade. I'm able to use it as a power amp, the loop and volume work fine.

    Tubes checked, that's about it. Sprayed connections with Deoxit. I'm going to use this as a chance to learn what I should about the stuff I own. I'm aware of the dangers involved. What do you folks with experience in these things think is the most likely culprit? I appreciate any help I can get.

    *Edit - I'm not an experienced forum user. I just realized I can post photos. On the way.
     
    Last edited: Oct 2, 2019
  2. corliss1

    corliss1 Friend of Leo's

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    If I'm reading that right, you're saying plugging into the effects loop still works, right? So that means the issue is in the preamp. Try swapping hte 12ax7 tubes? Do those tubes heater's glow?

    A voltage chart while the amp is in the not-working state would likely give you a major clue. Also make sure to measure the heater voltages at the 12ax7 pins as well, which are usually in AC volts, although I don't have a schematic handy for this amp to confirm, but that would be my guess.
     
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  3. Wally

    Wally Telefied Ad Free Member

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    A schematic can be seen here...
    https://music-electronics-forum.com/attachment.php?attachmentid=47359&d=1519907644

    The 12AX7 tubes are heated by a rectified DC voltage. I would check all heater voltages. If the tubes are being heated, then I would suspect almost anything...but would not rule out a bad power supply filter cap failure. Of course, simple things first...tubes are to be checked first if the heater voltage is present.
    Of course, working on live circuits is dangerous for all but especially for a novice. Welcome, and take care. This would not be the simplest amp one could choose to learn on.
     
  4. muchxs

    muchxs Doctor of Teleocity

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    Run a patch cord from the "send" to the "return" jacks. See if it works properly.

    Byrne Electronics in Saint Johnsbury will take care of you. He's "by appointment only" since the TV repair biz went the way of the buffalo.
     
  5. wabashslim

    wabashslim Tele-Holic

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    This can happen with a leaky coupling capacitor, or if a grid-leak resistor opens up or gets disconnected,. The grids are supposed to be referenced to ground through a resistor(s) and without that resistor the grid's DC level will "drift" positive until it reaches cutoff and all sound stops...OR if a coupling cap is leaking DC plate voltage from the previous stage to the grid, same thing happens. With the amp OFF and all caps discharged fully, measure the resistance from the 12AX7 grid pins 2 & 7 to ground, should be close to the schematic values. The next thing, if you're up to it, is run the amp and measure voltage on those same pins to ground, should be 0 or very low.
    Of course this isn't the only possible cause of your problem but it drove me nuts once.
     
  6. R. Stilskin

    R. Stilskin TDPRI Member

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    Thank you for all of the replies. I've changed tubes and sprayed sockets with Deoxit. The "glow" question is a good one, changed out tubes but will watch them closer today. The next I'll try is the patch cord suggestion from "muchxs". Then I'm going to read and proceed slowly. I build furniture and cabinets for a living so I'm pretty used to being careful. Thanks again for the suggestions. Once I feel comfortable, I'll dig in and check the things you guys suggest.
     
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