Incredible high plate voltage in vintage Plexi clone

Discussion in 'The Workbench' started by KBBerger, Dec 7, 2019.

  1. KBBerger

    KBBerger Member

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    Hey there! I just got a `73 Sound City Super Amplifier, which is basically a Plexi clone.
    It has a half Power/full power switch on the back. I cleaned the amp up and before I inserted the tubes I measured the voltage between pin 3 and 8 of each socket.

    At full power mode it shows about 918V (yes, 918!).

    I live in Germany where we have 230V and bought the amp from an older German guy. Could it be a 110V version?? because switched to half power it measures 450V

    What should I do now? In very confused ...
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    First of all STOP.
    Do not operate the amplifier.

    Second, it appears that the amp may be a 120V version.

    The voltage change should be performed by a qualified technician.
    There is a risk that the power transformer or other will be severely damaged.

    Normally what a technician could do
    could slowly raise the voltage with a variac and monitor the power supplies...
    This would indicate a problem long before full voltage was applied.
    And this can be done even without knowing what voltage the amp was built for.
    Therefore avoid smoking the power transformer ?

    The transformer may have multiple taps or windings; so that it can be changed over to a different mains voltage.
    But these taps should be connected and tested by a qualified technician, to avoid very expensive damage.
     
    Last edited: Dec 7, 2019
  3. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Sensible reply ampmad man, dont be tempted to try anything your self, so get it to a good tech to get it checked out.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    I only learned the correct tone from you, Ken. Have a nice weekend.
     
  5. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    You too my friend.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    "At full power mode it shows about 918V (yes, 918!)"

    I don't think I would push it that far. :lol:
    It sounds twice as good true, but I think it would last about 14 seconds before the smoke comes out...
    But for that brief moment it's going to sound killer.
     
  7. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Dont forget that if this is the case that it has the incorrect mains tap connected then the heater voltage will be far to high and that will kill all the tubes if left on.

    Dont take any chances, get it to a Tech pronto.
     
  8. KBBerger

    KBBerger Member

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    Ok guys, I’ll take it to a tech on Monday. Biasing and so own is something i do on my own. But this goes beyond my knowledge. Thanks guys. I’ll keep you updated:)
     
  9. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Wise man KB.
     
  10. KBBerger

    KBBerger Member

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    I think the suspect is the voltage selector on the back. It can be set to 110V or 220V. It is set to 220V but the plate Voltage shows 918 as I said above.

    i did one more test without the power tubes installed. I switched the voltage selector to 110V, inserted the Pins of my DMM into Pin 3 & 8 and switched the amp on for three seconds.

    Strangely the DMM showed no Voltage. Switched off the amp, set the Voltage selector back to 220V and checked again. And again it showed 918V. don’t worry, I just checked for 2-3 seconds. And because of the fact that the amp has been running this way for many years I doubt that it will die right away.

    At least the voltage selector indicates, that the Transformers and so on were made to work here in Germany too.

    It seems that something is wrong with the selector, it seems to me, like it is stuck at 110V inside. Maybe the selector is broken.

    i will bring it to my tech on Monday with the things I already found out. If the selector is broken I’ll let my tech wire the amp for 220V. I don’t think that I’ll ever need to select 110V in my life.
     
    ibmorjamn likes this.
  11. ibmorjamn

    ibmorjamn Well-Known Member

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    I saw 73 SC on eBay for quite some time . I had it saved but just never pulled the trigger on it . I hope it turns out good , would like to hear a clip .
     
  12. KBBerger

    KBBerger Member

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    I tested it before I bought it, sounded really awesome. Luckily it was switched to half power mode all the time, so 450V for the tubes, not 918V...

    Thr best thing is the cab I got with it. It’s the original oversized Sound City 4x12 from 1973. All around it’s about 5cm bigger than a standard 1960 cab.
    I took a look inside today and what I saw made me smile from ear to ear. Brand new looking G12H 30W Greenbacks, Date Code says they were manufactured on June 11. in 1973.

    I traded my 6100 for this Halfstack. And as I saw what speakers were inside, I knew that it was worth the trade. Don’t get me wrong, I really loved my 6100. But I wanted to give it away as long as it’s still working perfectly. I’ve read so many horror stories about repairing the 6100. A horror for any tech.

    I’ve owned a 1971 London City Plexi clone a few years ago and I still regret selling it, so I didn’t think long as I got the trade-offer for the sound city amp.

    So i left a 6100 there and brought home a Plexi clone and an oversized 4x12 with 46 years old G12H's :)
     
    WellBurnTheSky and ibmorjamn like this.
  13. ibmorjamn

    ibmorjamn Well-Known Member

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  14. ampmadscientist

    ampmadscientist Well-Known Member

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    I had one up to about 718 once, sounded awesome.:lol:
     
  15. KBBerger

    KBBerger Member

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    You all make me wanna try it on 918V :D

    But I’ll stay strong, the tech will fix it, I’m sure :)
     
  16. neikeel

    neikeel Well-Known Member

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    When you switched on (with tubes out) what was the heater voltage?
    How is it deriving half the output voltage?
    Presume filter caps are 500v in series etc?
    Some pics would be really interesting if you get chance.
     
  17. MickeyJ

    MickeyJ Active Member

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    if you're taking the reading from 3 to 8, do a resistance reading from 8 to ground, it may have an odd connection to ground maybe via a resistor ( larger than 1 ohm).
    I'd first make sure your multimeter works properly, maybe put the leads into the wall socket itself, or test some batteries or something like that, then take a reading from 3 to the chassis instead.
     
  18. MickeyJ

    MickeyJ Active Member

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    Following that test, if it still reads over 900v ( i somehow doubt it has that much voltage on the plates) then I'd follow Mr Underwoods good advice.
     
  19. Scumback Speakers

    Scumback Speakers Well-Known Member Sponsor

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    This happened once to one of my builds when I used a different Mercury Magnetics transformer than what was standard in a Metro kit build. The solid state rectifier wiring was different on the standard kit build, and when I used the MM PT it produced over 900v on the plates. Had to rewire that section and it came down to around 490v. No I don't know what was done cuz I took it to an amp tech who fixed it. Sorry!
     
  20. EndGame00

    EndGame00 Well-Known Member

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    I bet the tone is tighter than a chick’s ass doing kegel exercises 24/7:agreed:
     

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