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Fixed it! I might've been the one that broke it though

Discussion in 'Amp Tech Center' started by tubegeek, May 11, 2020.

  1. tubegeek

    tubegeek Friend of Leo's

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    20200510_201022.jpg I got lucky when I bought my first amp back in the late '70's: early 60's Ampeg Rocket R-12A, push-pull 7591's and octal (6SL7) preamp trem and phase splitter. 5Y3 rectifier.

    Before I ever started messing with amps I had brought it to the Magic Shop, it needed... something... I forget.

    Anyway I didn't use it for a pretty long time and when I brought it out the tremolo was dead. Clicking the footswitch would give one dip and then nothing after.

    Tried the usual stuff: switched the 6SL7s for each other, put in a new one, poked at it a bit, nothing. So I got the schematic and decided to replace the red molded .05 caps in the tremolo oscillator. Nothing. Checked the resistors, found one resistor with a different value from the schematic and also found a 470K grid stopper on the trem tube. I bypassed that and put the stock value in for the wrong resistor and replaced the cathode bypass cap. By now I'm getting annoyed. It's a trem oscillator and really not that cranky a circuit.

    So I traced out the layout of every part in that area. Aha! One of my replacement caps was soldered with one end in the wrong eyelet! That must be it! I messed up!

    Nope. I mean, yes, I messed up, but nope, that didn't fix it. WTF?

    OK, something must be making incorrect contact unseen behind the board: everything I can see looks correct. I beep checked around and BEEP WE HAVE A WINNER. I had left too much extra lead on one of the new caps and it had grounded out to the chassis below when I soldered it into the eyelet.

    Now I undid the grid stopper bypass I had done (I figured the Magic Shop had put it there for a reason plus it makes good sense.) And I took one of the stock .05s out and upped it to .1 to slow down the trem a little. I pulled the new tube and put back the original.

    I still don't know what the problem was before I started - I suspect the .05s I pulled but I haven't tested yet.

    What a great little amp this is. And the tremolo sounds great when it's working!
     
    Last edited: May 11, 2020
    Jake Nelson and ce24 like this.
  2. Jon Snell

    Jon Snell Tele-Holic

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    Screenshot 2020-05-12 at 09.50.49.png It is usually those old capacitors that go leaky causing issues.
    I have repaired many ReverbeRockets over the years.
     
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  3. tubegeek

    tubegeek Friend of Leo's

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    This is a non-Reverb Rocket, but that's not important. R-12A with 6SL7s and 7591s, 5Y3, trem (speed & intensity) tone, volume, 3 jacks Accordion, Mic & Guitar.

    So the grid stopper is on a different version of the schematic, huh? It's not on the print in the amp but I guess at some point it got added as a revision then.

    Glad I left it! Thanks for that info.

    I wish I knew what Harry Kolbe (Magic Shop) did - I think the trem was the problem when I brought it to him. The 3 .05's looked original when I took them out, but who knows?

    Jon: do you know what speaker would have been in it originally? By the time I bought it, I think it had a Celestion, then I put something else in but I've forgotten what it was. I'll check and post it.
     
  4. tubegeek

    tubegeek Friend of Leo's

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    .
     
  5. Jon Snell

    Jon Snell Tele-Holic

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    One of mine had a Jensen from new and the other Electro Voice.
    The Jensen had 7591A (R12A) output valves and two 6SL7 as it was from 1961 whereas the EV had 6V6GT as output valves and reverb employing two 6SL7 and two 6SN7.
    Both had 5Y3 rectifiers and both were 110volt only. So I fitted 230 - 110 autotransformers in the cases for them.
    I sold both on as I had no use for them after refurbishment but left the weathered exterior as it was. One is used a a harmonica amplifier using a Shure Green Bullet from 1968, (High impedance and matches well), the other with a guitar for blues. Both in professional bands that toured with them throughout the UK.
     
  6. Jon Snell

    Jon Snell Tele-Holic

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    Watch out for the 5M6 grid resistor, they do go very high value and cause all sorts of issues with noises and distortion. Screenshot 2020-05-14 at 09.04.59.png
     
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