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    Potential damage (to Bob's kit ST70)?

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    SnugDynaco

    Posts : 19
    Join date : 2011-09-11

    Potential damage (to Bob's kit ST70)?

    Post by SnugDynaco on Thu Oct 08, 2015 1:44 pm

    Hello again, all.

    A few weeks ago, I discovered that the left speaker leads became disconnected while the system was running. The time duration that this occurred was at least thirty seconds, maybe as much as one minute. The system was running at low volume, and upon discovery was shut down.

    Since then I have begun to notice some low frequency noise. I do not think that it was there before, or maybe not as pronounced, as the amp was very quiet when I first completed. The noise, or hum, presents at soft passages, and is hidden completely by nearly all music played at low to moderate volume. It is not a 60-cycle hum.

    Did I do any damage, if so, what?

    Speakers are Heresy II's, with recently replaced Crites caps. The completed kit ST70 has been running on the supplied power tubes since completion 18 months ago +/-. I use it often, so the tubes may be aging (?)

    Thanks to all, Paul

    wildiowa

    Posts : 120
    Join date : 2012-03-19

    Re: Potential damage (to Bob's kit ST70)?

    Post by wildiowa on Thu Oct 08, 2015 4:55 pm

    Man I don't know about this one...a 30 sec speaker disconnect is not necessarily a death sentence, but I do realize that is a no-no with almost all tube amps. Too bad it isn't a noticeable, certain change in the amp or other symptoms as you could go to work on it but this sorta-maybe-kinda thing is hard to tell. Maybe not to get worked up about it unless something more happens or develops? Kinda like the placebo effect that has been discussed here...if we think something is different, we think it....good luck

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Potential damage (to Bob's kit ST70)?

    Post by PeterCapo on Thu Oct 08, 2015 5:33 pm

    First impression is not inclined to think you damaged anything. I remember reading something about this subject recently, and the article said it isn't as much of a problem as it is often thought to be. If I can find the article, I'll post the link.

    This low frequency noise, which channel is it coming out of, left, right or both?

    wildiowa

    Posts : 120
    Join date : 2012-03-19

    Re: Potential damage (to Bob's kit ST70)?

    Post by wildiowa on Thu Oct 08, 2015 7:08 pm

    I come from the guitar amp world and I know if you did this to a Marshall it would be all over, very soon. You absolutely have to have some kind of load on there or kabloooeeee. Other amps are more tolerant.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Potential damage (to Bob's kit ST70)?

    Post by PeterCapo on Thu Oct 08, 2015 7:12 pm

    http://tronola.com/html/unloaded_vt_amps.html

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Potential damage (to Bob's kit ST70)?

    Post by GP49 on Thu Oct 08, 2015 8:30 pm

    One minute at low volume...I doubt that there is any damage at all. The article linked by PeterCapo is as good a guide as any. It explains that guitar amp circuits, which have little negative feedback, can suffer damage when operated unloaded, while hi-fi amps with negative feedback, as are all Dynaco designs and the upgraded circuits sold by Bob, are far more tolerant; and that a hi-fi amp operated without a load, but below clipping, is likely to have no problems.

    SnugDynaco

    Posts : 19
    Join date : 2011-09-11

    Re: Potential damage (to Bob's kit ST70)?

    Post by SnugDynaco on Thu Oct 08, 2015 9:03 pm

    Thanks, everyone, for comments and a relative peace of mind. The Lafferty article was an interesting read. The amp is not sparking and the house is still standing.

    I am getting ready to order a backup set of EL34s, and rectifier. When replacing tubes, should they come out after letting the powered-down system discharge for a period of time? Should the rectifier come out first, or last?

    The speakers are on a wood floor, and, with two cats in the family, we Swiffer fairly often. I think that is what happened to cause the disconnect.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Potential damage (to Bob's kit ST70)?

    Post by PeterCapo on Fri Oct 09, 2015 9:30 am

    .


    Last edited by PeterCapo on Fri Oct 09, 2015 5:00 pm; edited 1 time in total

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Potential damage (to Bob's kit ST70)?

    Post by deepee99 on Fri Oct 09, 2015 3:17 pm

    A dead short across live speaker connections could be an amp killer but it doesn't sound like that happened in your case.
    This is over-kill, but I'd let the tubes cool completely down before pulling them. (Save you from 2nd-degree burns, anyway.) Before installing the replacement tubes and rectifier, zero-out the bias pots per the assembly manual, then re-tension the tube sockets, and give them a little squirt of de-oxit.

    SnugDynaco

    Posts : 19
    Join date : 2011-09-11

    Re: Potential damage (to Bob's kit ST70)?

    Post by SnugDynaco on Tue Oct 13, 2015 1:44 pm

    I changed out the EL34s power, with Mullard reproductions, and the GZ34 rectifier, with a Gold Lion. All is quiet now - no hum or noise. One must place his ear right in front of the idling speakers, at the high, mid, and low positions, to pick up the slight speaker hiss.

    These required very little biasing compared to the pre-existing Valve Art tubes, which were original to the kit and now having approx. 18 months of almost constant use. It was time for replacement.

    Thank you again for your comments and assistance.


    erlingt

    Posts : 15
    Join date : 2010-06-23
    Location : Denmark

    Re: Potential damage (to Bob's kit ST70)?

    Post by erlingt on Tue Oct 13, 2015 2:58 pm

    SnugDynaco wrote:... 18 months of almost constant use.  It was time for replacement. ...


    Is about 13.000 hours, yes it was time for replacement Very Happy


    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Potential damage (to Bob's kit ST70)?

    Post by kaner on Tue Oct 13, 2015 10:44 pm

    Save those Valve Arts as backups and enjoy your Mullard's. How does the sound compare? Were you able to bias the Valve Arts & still have noise?

    BTW I've run my amp with a speaker plug accidentally pulled out for more than a minute. Scared the $##% out of me once i noticed. The VTA amp is pretty tough.

    Kaner

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