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    buzzzzzzz + high bias voltage

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    insaneboy

    Posts : 3
    Join date : 2011-01-14
    Location : Maine

    buzzzzzzz + high bias voltage

    Post by insaneboy on Fri Jan 14, 2011 11:27 pm

    Ok, so....

    Had this ST70 for a while now. it was completely rebuilt in '96, only thing original left is the chassis and transformers, (I bought it 1/2 done and completed the rest between classes in college)

    In any case it worked great, (well one 7199 died after 5 years or so, which radio shack replaced for free!) couple years ago bought new output tubes figured the ones I got back in '96 might be getting tired. Had it all set up in my office at work, all was good for a week or so. Than it started humming a bit. nothing out of the speakers, the main transformer was making the noise. found low bias voltage, turned it up.... after a short while hum became more of a buzz and could be felt on the table. shut it down, left it for a day. found it would work fine for a few minutes when cold, than start humming again.

    Office got moved(downsizing YAY, now I share a tiny office) soon after so I just shelved the dynaco at home (I have small children, hot delicate tube amps are not a good mix.

    Now I'm considering selling, and I don't do broken. What I read: low bias voltage, transformer noise = rectifier tube going bad. SO a brand new JJ GZ34/5AR4 arrived today, plugged it in and BAM instant LOUD buzz. shut it down affraid pull out the meter (Note: bias pots were turned all the way down before I tried the new rectifier) turned the amp back on bias voltage starts to rise, doesn't stop at just a volt or so it keeps going, already got over 4v by the time I got to the off switch.

    DOH

    So after all that, Question is, is my current assumption that the 50uf bias caps gone correct? (in my case they're 100uf) or is something else pooched (note: selenium rectifier is long gone, diode in place and it checked out)

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: buzzzzzzz + high bias voltage

    Post by Bob Latino on Sat Jan 15, 2011 10:10 am

    Try this ....

    With all the tubes out turn the amp on and measure the negative DC voltage on pin 6 of each output tube. While you are measuring turn the bias pot for that channel. You *should* get a range of about -23 to about -48 VDC while turning the pot. If you get no voltage reading at all then ...
    1. Check the AC voltage where the RED/BLACK wire connects to the diode. If you get about 50 to 55 VAC then the transformer bias supply is OK. If not the transformer bias supply is bad.
    2. Next measure the DC voltage on the other side of the diode - see if you get about -70 - 75 VDC. If you don't the diode is bad - If you do get about -70 to -75 VDC then either one or both resistors on the 7 lug terminal strip are bad OR the two capacitors that you mention are bad. Replace the caps and/or the two 10 K resistors.

    Bob

    insaneboy

    Posts : 3
    Join date : 2011-01-14
    Location : Maine

    Re: buzzzzzzz + high bias voltage

    Post by insaneboy on Sat Jan 15, 2011 10:08 pm

    cool, thanks Bob.


    On pin 6 on all sockets I got, at most, 200mV

    1) AC voltage was 55.3V
    2) DC volatage was -77.8V (is that too high?)

    I guess I'll be picking up resistors and caps on my lunch break tuesday, I'll post the results when I have a chance to install them.

    insaneboy

    Posts : 3
    Join date : 2011-01-14
    Location : Maine

    Re: buzzzzzzz + high bias voltage

    Post by insaneboy on Sat Mar 05, 2011 2:33 pm

    Finally got some time. replaced the caps, nothing. replaced the resistors, got some voltage at pin 6. noticed a itty bitty spark at the joint between the diode and red/black wire re-soldered the joint and everything was happy!

    tubes biased nicely, ran it hard for about an hour and the main transformer only showed 83degrees, sweet! Still buzzing, but it's not nearly as loud and seems to be more of a case vibration buzz (if I push on the transformer in one direction buzz goes away and becomes just a light hum)

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