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    EL34 getting way too hot..

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    pstpierre

    Posts : 2
    Join date : 2010-10-08

    EL34 getting way too hot..

    Post by pstpierre on Tue Dec 21, 2010 8:02 pm

    Hi all,

    I just finished rebuilding an ST70. I purchased it a couple of years ago for around $225. It was not working but looked original with a pretty clean chassis. Well the can cap was dead, the power cord cracked and all the original Dynaco output tubes failed on my tube tester. I cleaned up the chassis with brasso and it shines really nice with only some minor pitting. I replace the can cap with an sds capacitor replacement board with some high value caps on it. Section 2 is 190 UF but only 500V (pairs of 250V). The others are lower I don't remember the values maybe 50uf but the voltages are better 800V (pairs of 400v). I also used the solid state bias on the board bypassing the selenium rectifier. I replaced all the output tubes with electroharmonic EL34s. Amazingly when I powered it up it work fine. It continued to worked great for about two months. But, just last week the RCA rectifier tube glowed like a nuclear furnace an popped the fuse. Oh well I figured is was expected as this is an old tube. I replace it with a solid state plug in adapter and adjusted the bias to 1.4v a litte lower than the recommended 1.53. Turned it on and everything seemed good. But after about 20 minutes the EL34 stated crackling and they were very hot. I shut down the amp and now I'm pondering what to check. I check some of the voltages right after I added the solid state rectifier, pins 4 and 8 of the rectifier are 359V AC. That seemed OK to me. On the EL34s pins 2 and 8 are 464V and pin 3 is 446V. It seemed a little high but I think too much of it. Eyelet 20 was 386V and eyelet 19 was 303V. The biasing pot was -41v on the left and -27 on the right. I'm not sure what to check next. I would most appreciated some advice on what to check next.

    Thanks from quiet in Seattle, Paul

    pstpierre

    Posts : 2
    Join date : 2010-10-08

    Re: EL34 getting way too hot..

    Post by pstpierre on Tue Dec 21, 2010 8:23 pm

    I meant to say that on the rectifier pins 2 and 8 are 464V not the EL34s

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: EL34 getting way too hot..

    Post by GP49 on Tue Dec 21, 2010 11:47 pm

    If by "biasing pot" you mean the voltage on the grid of the output tubes, "-27V" is very likely improper.

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