Need for grid stoppers on JTM45 ValveStorm build

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Alan66
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Need for grid stoppers on JTM45 ValveStorm build

Post by Alan66 » Tue Mar 09, 2021 5:09 pm

I completed a ValveStorm JTM45 build in January. I had all sorts of very loud noises the first time I fired it up with output tubes and a speaker load. I read somewhere (I think on this forum) that adding 5.6K grid stoppers would fix the problem, and it did. Then I read that this was simply using a band-aid approach and didn’t really fix the underlying issue.

I’m not new to tube amps, I’ve built several single-ended amps and was always able to diagnose and fix problems. I’m not nearly as familiar with push-pull amps with a B+ of 400VDC and up. What exactly is the underlying issue here? I assumed it was oscillation, and why is using grid stoppers only putting a band-aid on the problem?

The reason I’m asking this is because I have had two KT66‘s fail in the same manner. Both tubes were arcing between the copper leads that go to the pins, either where they are molded into the glass or where they exit the glass going to the tube pins. The arc looked like a small sun and quickly burned through the glass allowing air to get into the tube. Since this happened outside the actual vacuum envelope, I tend to think that there is a problem with this batch of tubes and not the amplifier. I’ve triple checked my bias circuit and monitored the plate and screen voltage and all appears to be working fine.

The vendor I bought the KT66 tubes from apparently bought a lot of closeout Groove Tubes KT66C’s when they went out of production, and says that the Groove Tubes warranty does not apply. He replaced the first tube that failed, but told me that it was caused by a problem with my DIY amp. After the second tube failed, he condescendingly said that this confirms that there is a problem with my amp and that any other tubes I put in it will also “fail”.

The failures occurred in different sockets, and each time when the amp was dimed. I will add that I am running a pair of NOS RCA 5881 tubes in the amp now and haven’t had any problems whatsoever. I want to know, before I buy a more expensive pair of Gold Lion KT66 reissues, whether I am right or if the vendor is right and I do indeed have a strange condition in my amp that is causing tube failure? I’m hoping that someone here will have a definitive answer either way. I still have the tube and can post a photo of where the failure occurred.

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