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Amp died- just go through regular startup process right?

Discussion in 'Amp Central Station' started by TelZilla, Mar 18, 2020.

  1. TelZilla

    TelZilla Friend of Leo's

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    Details:
    -Homebrew 5F6A, used without incident for a year or so
    -Playing it the other night, hitting it pretty hard with a Klon style pedal, a boost, and a fuzz
    -It just died
    -Fuse was blown so I replaced it
    -Amp still dead

    I assume I should just go through the normal startup process (I printed something out years ago and I just use that) to troubleshoot it.

    Does everyone agree?
     
  2. radiocaster

    radiocaster Poster Extraordinaire

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    My first hunch would be the power supply resistors. Tubes are also suspect, but maybe not quite as much because it is completely dead. However, these are guesses, only a thorough check can determine the actual problem. Isn't there a second fuse?
     
  3. Wally

    Wally Telefied Ad Free Member

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    Does the pilot lamp light up? Did the new fuse hold? If you built it, did you make up a voltage chart? If so, what are your voltages now? If the pilot lamp lights up and you have heater voltage in the 12A-7s and the power tubes, do you have heater voltage to the rectifier? If the voltages are good coming to the rectifier but there is no B+ coming out, you have a problem with the rectifier, I would think. make up a voltage chart....that is the start, imho....
     
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  4. TelZilla

    TelZilla Friend of Leo's

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    I definitely had voltages at one point, but don't any more.

    Here's the current situation:
    -With a new fuse in, power on, but still on standby, pilot light lights up and stays on for about 12 seconds. After that, the pilot light clicks off.
    -At that point, fuse is blown, but the actual pilot light bulb is still good.

    So I took all the tubes out and tried again with a fresh fuse. 16 minutes and pilot light is still bright.
    -Measured 349.7 V AC from pin 4 of V6 to ground- same with pin 6 of V6 (again, no tubes in the amp).
    -Measured about 7 V AC on the heaters and pilot light. -
    -Measured 5.5 V AC on pins 8 & 2 of V6

    So I put in the recto tube (using a JJ GZ34)- still no other tubes. With the amp on standby, I switched the power on. This time the pilot light stayed on for 29 seconds, then the fuse blew again. The HV secondary measured pretty much the same thing as without the tube in there. Recto tube was definitely glowing, don;t think it was redplating (but maybe it was on its way- it was only 29 seconds...

    And now I've burnt up all my fuses, so I'll have to quit.

    So bad recto tube? I have another GZ34 and a 5U4GB around somewhere...
     
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  5. muchxs

    muchxs Doctor of Teleocity

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    Bet you toasted a 5881.

    Look for burnt screen grid resistors.
     
  6. Shuster

    Shuster Poster Extraordinaire

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    If I were you, I'd call the crazy who built it, and give him a piece of your mind!! And don't hold back:twisted:;)
     
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  7. TelZilla

    TelZilla Friend of Leo's

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    You know, I would, but the guy’s a [email protected]#&ing hack. ZERO quality control!
     
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  8. Wally

    Wally Telefied Ad Free Member

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    Well, the amp worked for a year; so I don’t think the builder made some gross error. a fuse blown with only the rectifier in is a big clue..... lift the B+ feed at the rectifier. If the fuse blows again, you have a bad rectifier or a bad PT. See if another GZ34 keeps the fuse from blowing. If and when the fuse holds, re-establish the B+...fire up...does the fuse blow? If so, you may have a shorted filter cap. If the fuse holds, one starts suspecting a bad tube or something in a circuit associated with a tube. Plug the power tubes bad in. Does the fuse hold? If not, there is a problem area. If it holds, install the 12A-7s one at a time...V3,V2,V1 in that order. If an when the fuse blows, there is your area of concern...tube or circuit. Early in the morning...but that would be my approach as I am thinking right now.
     
  9. TelZilla

    TelZilla Friend of Leo's

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    Thanks. Ill try it.

    ‘Twas a joke. The builder is me.
     
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  10. Wally

    Wally Telefied Ad Free Member

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    I thought that might be the case.....
     
  11. TelZilla

    TelZilla Friend of Leo's

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    And i AM a hack.

    I hope its not the PT. For this one I sprung for some expensive iron.
     
  12. muchxs

    muchxs Doctor of Teleocity

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    With few exceptions...

    Transformers don't just die especially the PT. The usual PT failure mode is "Fuse blew, use a bigger fuse."

    Don't do that.

    Throwing fuses at an amp that eats fuses will zorch a transformer.

    If you have a shorted 5881 you're feeding the short through the output transformer.


    I don't participate in troubleshooting kits and homebrews as much as I used to. When you say,

    "I'm a hack."

    I think,

    "How bad can it be?"

    Having seen a few kits built by hacks between now and then brings troubleshooting to another level. Troubleshooting is usually chasing one problem or a set of problems that can be traced to one root cause. I've seen kit amps where there isn't a single good solder joint out of all of 'em. I've seen kit amps with ridiculous loop dee loos of wire and my personal favorite, excess wire everywhere with soldering iron burns on everything.

    You're running out of options. When the amp is on standby high voltage is disconnected.


    I have an aircraft 2 amp circuit breaker salvaged out of an old bomber. Handiest "tool" I have. That saved me a stack of cash on fuses.

    I will never touch a JJ 5AR4 rectifier again after experiencing a 100% failure rate with a large batch.

    Got a spare GZ34? Cue chorus of angels sound effect if it turns out to be a zorched rectifier.

    Smoke 'em if you got 'em.
     
  13. TelZilla

    TelZilla Friend of Leo's

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    1. I say I'm a hack just cuz I'm no pro and I'm not the kinda guy who likes tooting his own horn. But I've built 10ish amps now and never had one fail like this. My lead dress is not Dr. Z quality, but it's a hell of a lot better than it was in 2005 when I turned a 90's solid state Harvard into a 5F1 (still cranking, it's my bench amp). It's not that I don't do dumb stuff, but usually the dumb stuff happens at first start up, and then once I get it worked out, things work. This one's different, and when it first went kaput, I was pushing it hard (loud, with a boost, a Klonnish OD and a Keeley Fuzz bender all going).

    2. The other recto is also a JJ (labelled online as GZ34/5AR4). What do you prefer?

    Thanks again all.
     
  14. muchxs

    muchxs Doctor of Teleocity

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    It's a public forum. My comment is a shout- out to the hobbyists who build amps quick 'n' dirty primarily to save a buck.

    Ted Senior said,

    "Hey guys, these kits aren't Lego™ sets. They require some thought, experience, and the correct tools. If you do not think you have the time, ability, nor tools to do this, maybe this isn't for you right now. If you still want to build one of our kits, but are a little uncertain, please enlist the aid of a qualified tech." -- Ted Weber.

    Despite my misfortune with JJ 5AR4s, they can't be all bad. ("Famous Last Words")

    If your other 5AR4 is "known good", pop it in there as well as a fresh fuse.


    I used to suggest a break- in procedure on fresh engine rebuilds. "Drive it to Burlington. Then bring it back. We'll dump the break- in oil, check the valve lash and see where we stand." If it makes it 100 miles it should go 1000 miles. If it makes it 1000 miles it's probably all set.
     
  15. TelZilla

    TelZilla Friend of Leo's

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    Well, mystery solved. I did look at the screen grid resistors and tgey looked fine.
    So I stuck the other GZ34 in and it worked. Nice.

    So whats your favorite 5AR4?
     
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