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    VTA ST-70 bias problem following catastrophic rectifier failure(s)

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    Dougbot

    Posts : 6
    Join date : 2015-05-19

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    Post by Dougbot Sun May 02, 2021 8:57 pm

    Hi all,
    I'm the proud owner of a VTA ST-70 (with stepped attenuator and cap upgrades) that I built from the kit about 8 years ago. I did the "yellow sheet" diode mod a couple of years ago, as I did have a few rectifier tubes die up until then. With the new new 1N4007 diodes in place, I tried a JJ GZ34 rectifier, which was cheaper than the more well-regarded Gold Lions I had been running. This tube lasted about 2 weeks, and then failed with quite a light show that also took out the fuse on the amp. I switched back to the Gold Lions and everything was ok for a year or two until recently, when one of these failed in similar fashion to the JJ (light show and blown fuse). Since then, I haven't been able to get my tubes (Gold Lion KT77) up to the proper bias reading. With the trim pots turned full clockwise, I'm only seeing a max of ~0.42 volts on my multimeter at all of the test points. I suspected one of the rectifier tube failures might have damaged something, so I checked the 1N4007 diodes (yellow sheet mod). These were indeed both dead (failed closed), so I swapped in new diodes. This didn't fix the bias issue though. I thought the KT77 tubes might just be old (work and taking care of kids has caused me to be less attentive to this sort of thing lately, and I haven't been keeping up on checking bias and remembering how long tubes have been in place in the amp), so I got a fresh quad of Gold Lion KT77s. Upon installing these, the max bias reading I am seeing is now even lower than it was with the old tubes in the amp.

    I've tested all the 10 ohm resistors at the bias test points, and these all read ~9.8 ohms on my DMM. Does anyone have suggestions of other things I should test in the circuit to try and track down the problem? The failure of the JJ rectifier did involve a loud buzzing before the fuse blew, so I'm a little worried that a transformer might have been damaged. I think the faint sound of transformer vibration when the amp is running became more noticeable after this failure, but hopefully I'm imagining this...

    Thanks in advance for any advice anyone can offer!!

    -Doug
    Tom Pickett
    Tom Pickett

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    Join date : 2020-11-22
    Age : 55
    Location : Texas

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    Post by Tom Pickett Sun May 02, 2021 9:50 pm

    Dougbot wrote:Hi all,
    I'm the proud owner of a VTA ST-70 (with stepped attenuator and cap upgrades) that I built from the kit about 8 years ago. I did the "yellow sheet" diode mod a couple of years ago, as I did have a few rectifier tubes die up until then. With the new new 1N4007 diodes in place, I tried a JJ GZ34 rectifier, which was cheaper than the more well-regarded Gold Lions I had been running. This tube lasted about 2 weeks, and then failed with quite a light show that also took out the fuse on the amp. I switched back to the Gold Lions and everything was ok for a year or two until recently, when one of these failed in similar fashion to the JJ (light show and blown fuse). Since then, I haven't been able to get my tubes (Gold Lion KT77) up to the proper bias reading. With the trim pots turned full clockwise, I'm only seeing a max of ~0.42 volts on my multimeter at all of the test points. I suspected one of the rectifier tube failures might have damaged something, so I checked the 1N4007 diodes (yellow sheet mod). These were indeed both dead (failed closed), so I swapped in new diodes. This didn't fix the bias issue though. I thought the KT77 tubes might just be old (work and taking care of kids has caused me to be less attentive to this sort of thing lately, and I haven't been keeping up on checking bias and remembering how long tubes have been in place in the amp), so I got a fresh quad of Gold Lion KT77s. Upon installing these, the max bias reading I am seeing is now even lower than it was with the old tubes in the amp.

    I've tested all the 10 ohm resistors at the bias test points, and these all read ~9.8 ohms on my DMM. Does anyone have suggestions of other things I should test in the circuit to try and track down the problem? The failure of the JJ rectifier did involve a loud buzzing before the fuse blew, so I'm a little worried that a transformer might have been damaged. I think the faint sound of transformer vibration when the amp is running became more noticeable after this failure, but hopefully I'm imagining this...

    Thanks in advance for any advice anyone can offer!!

    -Doug

    Hi Doug.... I hope you're having a good day despite the problems with your amplifier.

    I have very rarely seen big transformers go bad because most of the time they can handle a lot of abuse.
    I believe that it may be something eles wrong other than your transformers and I hope that I am correct about that.

    The first thing that I would do since you have already checked all of the resistors and you have replaced the yellow sheet diode's I would check the voltages on the main transformer on the output side to see if the voltages are where they are supposed to be and then we can go from there because if the voltages are correct then that will illuminate your transformer and the problem will be somewhere else up the line in the circuit.
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    JXBJXB

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    Join date : 2021-03-14

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    Post by JXBJXB Mon May 03, 2021 1:20 pm

    @Dougbot some of this sounds like symptoms I saw with one of my two M-125s last week with a solid-state rectifier failure. Check your quad cap? Specifically, one of Bob's techs (Joe) told me to take all tubes out, and with the amp unplugged, set your DMM to measure ohms and measure between the "square" terminal on it (should have a jumper going over to pin 8 on the rectifier socket) and ground. Or, measure from pin 8 on the rectifier socket to ground. If you get a high resistance value that steadily climbs? The cap is likely good. If you get no/low resistance reading or a steady reading? You may have taken out at least one section of the quad cap.

    In my case, a swap of the quad cap got my amp back up and running.

    You also don't say that you have a known-good rectifier tube at this point - I'd make triple-sure you have a good one. I'm using some 60's-vintage American-made 5U4GB until I get some fresh 5AR4 rectifiers delivered.

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    Dougbot

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    Post by Dougbot Mon May 03, 2021 2:34 pm

    Thanks so much for the suggestions Tom and JXBJXB! I can confirm that I tried setting bias both with the current Gold Lion reissue GZ34 in the amp, and with a brand new Mullard reissue GZ34 that I received with the new KT77s, suggesting the issue does not lie in the rectifier tube. When I replaced the failed 1N4007 "yellow sheet" diodes, I used higher rated 1N5408s, but I don't think this could be the source of the problem because I saw the same issue before and after diode replacement.

    I'll check voltages when I get a chance, hopefully in the next couple of days!

    -Doug

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    Dougbot

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    Post by Dougbot Fri May 07, 2021 2:39 am

    I finally got a chance to check the quad cap with all tubes removed and the amp unplugged. Measuring resistance to ground showed an open circuit on all 4 terminals of the can cap. I also tried the capacitance setting on my DMM, and got nothing (open circuit) between ground and all 4 terminals. Is it possible that all 4 sections of the cap are bad? Anything else I should try, or just order a new can?
    peterh
    peterh

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    Post by peterh Fri May 07, 2021 2:46 am

    Dougbot wrote:I finally got a chance to check the quad cap with all tubes removed and the amp unplugged. Measuring resistance to ground showed an open circuit on all 4 terminals of the can cap. I also tried the capacitance setting on my DMM, and got nothing (open circuit) between ground and all 4 terminals. Is it possible that all 4 sections of the cap are bad? Anything else I should try, or just order a new can?

    It should be "open circuit" when it has been charged !
    A DMM would see a low resistance that increases to very large after a few seconds.
    The important is that there is no shorts.

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    Dougbot

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    Post by Dougbot Fri May 07, 2021 8:49 pm

    I did discharge the cap before taking the measurement by shorting each terminal of the capacitor to ground with a 5 watt resistor alligator clipped between a couple of jumper wires. Maybe my DMM is too slow to register the increasing resistance as the cap was charging?

    Is there reason not to trust the capacitance function of my DMM, which couldn't register any reading on the can cap? I verified that it was working on other caps in the amp while I had it open.
    peterh
    peterh

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    Post by peterh Sat May 08, 2021 1:40 am

    Dougbot wrote:I did discharge the cap before taking the measurement by shorting each terminal of the capacitor to ground with a 5 watt resistor alligator clipped between a couple of jumper wires. Maybe my DMM is too slow to register the increasing resistance as the cap was charging?

    Is there reason not to trust the capacitance function of my DMM, which couldn't register any reading on the can cap? I verified that it was working on other caps in the amp while I had it open.
    No if you have double-checked your DMM with other capacitors in similar capacity, then it seems that your can cap is bad.
    First check that the can caps outer metal has good connection to ground ( and one side of your DMM probes)
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    Dougbot

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    Post by Dougbot Mon May 24, 2021 2:29 am

    I finally got a chance to work on the amp and replaced the quad cap today. With the old one out of the amp I was more patient with the DMM, and confirmed that two of the sections show no capacitance value, but the other two seem intact. The first time I checked it I probably didn't wait long enough with the probes on the terminals (my meter seems slow to report capacitance). Anyway, with the new cap in there it biases just fine and sounds dramatically better. The weird transient hum when powering on is gone as well. Huge thanks to JXBJXB for this suggestion!

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    JXBJXB

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    Post by JXBJXB Tue Jun 01, 2021 5:25 pm

    Dougbot wrote:Huge thanks to JXBJXB for this suggestion!

    glad it worked out!

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