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    ST70 issue- 12AU7 tube blown

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    friarmac207

    Posts : 3
    Join date : 2012-03-26

    ST70 issue- 12AU7 tube blown

    Post by friarmac207 on Wed Mar 28, 2012 4:57 pm

    HI folks, I'm new to the ST70s , and tube amps in general. I just picked up a rebuilt ST70 with VTA board upgrade. I'm using Sovtek 5881 output tubes, 12BH7 phase splitters, and a 12AU7 for the preamp tube. Upon first power up of the amp, the 12AU7 began "blue arcing" for about 30 seconds-inside the tube- and then appears to have failed. I am able to get stable bias on all output tubes (42.5mA), and no apparent component issues on the VTA board.
    The 12AU7 was a JJ tube, and I suspect that it is simply a dud. Has anyone seen a similar symptom? While I'm taking in and learning a LOT from this forum, I'm a self admitted noob, and could use some experienced input.
    Much thanks!!
    David

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: ST70 issue- 12AU7 tube blown

    Post by mantha3 on Wed Mar 28, 2012 7:17 pm

    I'd get another tube. Call some guitar shops and they may have some on hand.

    You may want to pick up some lower cost spare tubes to have on hand.

    friarmac207

    Posts : 3
    Join date : 2012-03-26

    Re: ST70 issue- 12AU7 tube blown

    Post by friarmac207 on Thu Mar 29, 2012 4:33 pm

    Thank you, Mantha3. I picked up an EH tube today and the problem WAS a bad JJ tube. THe amp is running fine now. Time for some Jason Isbell....

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: ST70 issue- 12AU7 tube blown

    Post by mantha3 on Thu Mar 29, 2012 6:58 pm

    Nice!

    You may want to have a spare on hand... Or get some NOS 12AU7 and do some tube rolling and have the set you have now as extra... An extra GZ34 is not a bad idea either.

    Take some time to read up on this forum. Lots of great material on the exact amp you now own.

    Take note of what was replaced on the rebuild... Quad cap in other capacitors etc. Google "ST70 rebuild" and read up. Nice amp when all the rebuild stuff that should be replaced is done. Nice amp regardless I should say

    friarmac207

    Posts : 3
    Join date : 2012-03-26

    Re: ST70 issue- 12AU7 tube blown

    Post by friarmac207 on Thu Mar 29, 2012 8:13 pm

    I also picked up a Gold Lion ECC82, but wanted to let the EH "take the heat" before rolling the GL into it.
    I've been studying the forum like a grad student, and learning a LOT along the way.
    This amp was a complete frame up rebuild with a VTA board, and cap upgrades. I am stunned by the sound, and I've only got 5 hours on these new tubes. It's holding it bias well, and sounding very sweet indeed.
    Thanks so much for the assist.
    David

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