Firing up an old tube amp.

Discussion in 'Amps and Cabs [BG]' started by JimChjones, Mar 1, 2018.

  1. JimChjones


    Aug 6, 2017
    SE England
    So, I have in front of me a 1974ish Sound City valve amplifier. To my certain knowledge it hasn't been powered up for ten years, and maybe quite a lot longer. No real idea of provenance or whether it was operational when last used. Take it apart, vacuum out all the dust, look for obvious failed components sure, but what other precautions should be taken with something this old that hasn't been used for so long? Are there any tests that are advisable for an amateur equipped with just a multimeter before electricity is allowed anywhere near it?
  2. bklee

    bklee Supporting Member

    I would make sure that the (master) volume knob and gain/preamp/channel volume knob are all the way down. Also, have the speaker (load) hooked up and the instrument lead connected. If the amp has significant issues with shorts or other catastrophic failures, the speaker could be at risk, so don'r use your best cabinet. I had the same situation a few years ago and when I "fired" it up, it began to smoke. So, make sure you can safely cut the power if that happens. Unfortunately, there is no completely safe way to do what you are proposing.
  3. beans-on-toast


    Aug 7, 2008
    A variac allows you to slowly bring it up in stages to full operating voltage. This will reform the capacitors.

    An ammeter will monitor current. Without an ammeter, google light bulb limiter. This will help you determine if there is a short and allow you to shut it down before damage is done.

    Take it to a tech and let them power it up properly.
    Last edited: Mar 2, 2018
  4. blubass


    Aug 3, 2007
    Modesto Ca
    Current: Blackstar, DR strings, Nady. Previous endorsements with: GK, Rotosound, Ernie Ball, Cleartone, EMG, Dean, Dava Picks, Rebel Straps, Dickies
    If you're an amateur, how would you know what to look for if you did fire it up? A multimeter probably isn't going to help much at all without power. Continuity is one thing you could check for with the amp off, but unless you had a schematic and knew what should be grounded and where, it wouldn't do you any good. Based on your knowledge, I think this is one of those things that you should have a repair shop look into. They'd be better suited to safely power on your amp.
  5. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    I have to admit that it's most likely that I would just plug it in and power it up :D but these guys telling you to have a tech power it up with a Variac are correct.
    dukeorock, Artman, pacojas and 20 others like this.
  6. lfmn16

    lfmn16 Inactive

    Sep 21, 2011
    charles town, wv
    It's the difference between what you should do and what you would do. I'm with you, I'd just plug and play.
    City, pacojas, PillO and 5 others like this.
  7. BassmanPaul

    BassmanPaul Inactive

    That could produce a big bang. It’s not such a good idea. A lamp limiting box just places a light bulb in the line wire and limits the current to the maximum that the lamp will pass. By using different wattage lamps in the socket you can control the inrush current on power up to a small value.

    The lamp should flash brightly and then drop down to a glow. If it stays bright there is a short that has to be found and eliminated before anything else is done. Don’t put yer finger into the empty socket! :)
  8. seamonkey


    Aug 6, 2004
    There's less work for the tech to repair afterwards if you let them power it on in the first place.
  9. BassmanPaul

    BassmanPaul Inactive

    I have a 20A Variac that came from a lighting console. It works very well but you should monitor the current flow as well as the voltage to detect a possible short.
  10. seamonkey


    Aug 6, 2004
    Variac: $100
    Bench charge for a good tech who owns a variac: $100
  11. lfmn16

    lfmn16 Inactive

    Sep 21, 2011
    charles town, wv
    If you put your finger in, it's not empty.

    homer thinking.jpg
  12. 2BitHack


    Nov 11, 2014
    this thing sounds dangerous, best to call the bomb squad and a hazmat team right away.
    1bassleft and Old Garage-Bander like this.
  13. I like your style JimmyM.
    The smoke will reveal the real deal. :woot:

    ps Almost BOOKMARKed your style...
    Planespotter and JimmyM like this.
  14. What Beans said. ^
    beans-on-toast likes this.
  15. Just do as Marty mcfly did. Plug it in turn all knobs to all and hit your best power cord. Lol. Nah I agree with the other guys about letting a tech do it
    Artman likes this.
  16. Johnny Crab

    Johnny Crab HELIX user & BOSE Abuser

    Feb 11, 2004
    Got lucky and my late Dad left one(variable transformer was what his was called) to me decades ago when he graduated from "electronic tech" to "Software & Systems Analyst". Used it from early '70's to early '80's for SVT maintenance and hot-rodding/repairing guitarist's amps when they brought them to me. Power up was mostly done with the chassis on a bench, a clear piece of plexiglass to look through at the main components facing up(in case a capacitor popped or worse), and then voltage was brought up. If something smoked or heated up, I could see it through the plexi. Back then I did not have an amp meter....but do now(get an AC/DC true RMS one, $150 or less).

    @JimChjones if it was my amp, I'd start by getting a schematic and replacing all of the power section capacitors before anything. About 95% of "repairs" I've done on older tube amps are power supply caps, a few cooked transformers, power supply diodes, and the easy ones(bad cables, microphonic tubes, bad tubes).
    Last edited: Mar 1, 2018
    DrummerwStrings likes this.
  17. popcat


    Jan 7, 2016
    Sign at repair shop:
    Repairs $50/hr.
    $100/hr. if you worked on it first.
  18. beans-on-toast


    Aug 7, 2008
    Because chances are, the amp will need some additional TLC. At the very least, deoxitization.
    jumbodbassman and JimmyM like this.
  19. somebrains


    Feb 7, 2017
    Man, this isn't like firing up a 390FE 67 Galaxie 500 for the first time in 40 years.
    Granted last time I did that kerosene and a breaker bar were used after we stripped the top end.

    Variac and proper shop methods are fine.
    Unknown "barn find" tube amps have been bought and sold without a Darwin Award mention.

    Visually inspect it, power it low and slow.
    If you are going to treat it as a hand grenade I would go pay someone.

    Otherwise look at your guitar player's gear, it's probably in not the best repair either but he isn't immolated by flaky rehearsal space power and run it until it's broken ownership.
    Aqualung60 and JimmyM like this.
  20. mpdd

    mpdd neoconceptualist

    Mar 24, 2010
    All the radio shacks are closed so I hope you don't blow a fuse.
    Downunderwonder likes this.