40 or 50's PA conversion guidence needed?

Discussion in 'Shock Brother's DIY Amps' started by wrathfuldeity, Jun 30, 2020.

  1. wrathfuldeity

    wrathfuldeity Tele-Afflicted

    Apr 25, 2011
    Turdcaster, WA
    Posted a thread “What did I find?” in the Glowing Tube section. https://www.tdpri.com/threads/what-did-i-find.1030071/

    Now with a couple weeks to think about things, I need your assessment of my thoughts so far. Please be candid and let me know if I’m in crazy town. Thanks!

    It’s a no name, no model octal pp6v6…duh. I’d don’t know if it works but it is darn clean, seemingly untouched, apparently some quality transformers and have not found any burn, spark burns, the cord is still supple and etc., nothing really indicating that it was not working when last used…whatever decade that was...lol.

    Muchxs noted in the above thread…

    If that old amp could talk, what would it say?
    "Brothers and sisters..."
    It's super simple inside. Could use a re- cap. Just plug into a mic input. They were all high impedance inputs back then so it should be usable as a guitar amp, as- is.
    If not it could be easily converted to a guitar amp. I'd use a 6SL7 or a 6SC7 on the front end, make it a 5C3 Deluxe. It's pretty close already.
    Don't power it up without a speaker connected. That's the quickest way to blow up a tube amp. Tubes lighting up tell you nothing. It's not worth the risk.

    Also had a brief phone call with Skip Simmons who did a cap job and voice mod for an old se6L6 recordplayer conversion for me about 10-12 years ago. After looking at the pics, Skip notes that since its an unknown amp, all bets are off and a gamble…if it even works let alone sounds good. He noted that it could be a challenging endeavor given its an unknown circuit. Subsequently found out that Skip does podcast with the Fret Board Journal and listened to a few.

    My skills: I don’t have any skills nor knowledge. I've changed out ¼” input and spkr jacks, a grounded 3-prong cord, added an inline fuse and done some very minor pedal mods. Also have a digital multi-meter, soldering iron and have done some simple soldering but would gladly practice more.

    Ideas and options:

    First option, I don’t even know if it works, besides turning it on for 5 seconds and all the tubes glowed. Perhaps a first place to start; is to clean it, reset the tubes, spray pots, sockets, amphenol connectors; get an amphenol adaptor (swithcraft 332ax); jerry rig a speaker to it and bring it up slowly on a variac in an attempt to hear if there is any potential. If there is potential, then send it to a reputable tech and let them do their thing. I don’t mind spending the time and $.

    Second option, is to get a copy of the Jack Darr “Electric Guitar Amplifier Handbook” which Skip noted in a podcast, as a straightforward reference. Try to figure out the filter, electrolytic caps and cap can. And then do a Mouser order. And try to replace them by clipping the old caps, but leaving the leads and just soldering to the old leads. Then fire it up and see what it sounds like. However, I’m rather clueless about the cap values, +/-orientation and whatnot. Wondering if this is a workable option?

    Last option is to see if I can find a good schematic/layout, turret board and chassis for a 5c3 build and then harvest the transformers and glass and have a slooow go at it…or at least find someone to build it for me.

    Utc transformers

    Power Transformer

    UTC R 12 (no information found, suspect it is a common utility transformer)

    UTC s-15

    Output transformer


    UTC s-28



    I’d appreciate your thoughts and guidance. Thank you!

    Today, had a bit of time to do some more photo documentation.





    Last edited: Jun 30, 2020
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    I wouldn't do option 1 - caps are old, could explode violently, and take out the nice transformers with it.

    All that old stuff used super common part numbers. I keep 22uf/500V caps in stock, so I'd have it knocked out in an hour or so :D Replace electrolytics, bring up on variac, see how it sounds without other tweaking. Make sure resistors are relatively close in value. Go from there. That's pretty much how amp restores go.

    *usual disclaimer about tube amps and dying and stuff*
  3. archetype

    archetype Fiend of Leo's

    Jun 4, 2005
    Williamsville NY
    No matter what its eventual fate, making it work or turning it into something else, you'll replace the electrolytic caps and any component that's fried. Why not go ahead and do that now and see what it sounds like with a guitar? You can get an Amphenol mic female to 1/4" phone female adapter so you don't need to put a jack in it to test it.

    It might sound awesome.
  4. tubegeek

    tubegeek Tele-Afflicted

    Jan 31, 2020
    Brooklyn, NY
    +1: re-cap, provide for an input connection, attach a speaker, fire it up.

    Then you'll know if it's worth a try.

    Old potted UTC iron is GOOD if in working order (no shorts/opens.) You may just have something there.
  5. elpico

    elpico Tele-Holic

    Sep 14, 2011
    Vancouver BC
    Probably a good idea to update to a 3 prong plug at the same time as replacing the electrolytic caps. I'd be sad to see it torn apart. Just get it running safely then you can mod the circuit in it if you're not happy with the sound. The only thing I can imagine you wanting to change after that might be the value of the caps in the EQ or coupling caps, but get it running safe first and try it out.
    tubegeek likes this.
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