Power Tubes biased on Ampeg SVT-CL, Chassis orientation yields problem

[nevetslab]

One of my regular customers sent me their Ampeg SVT-CL amp that had shut down several times on stage.

I powered it up on the bench, speaker connected, and found both upper and lower power tubes Bias LED's were RED and GRN. AC Mains current/wattage was nominal (2A/220W @ 120VAC. less with the fan unplugged). I was unable to get the RED led to go out on both upper/lower tube halves without getting GRN to go out. RED came on first. So, pulled the two chassis out of the cabinet, and removed the hold-down clamps, re-connected the preamp to the power amp chassis and stood both vertical, as has been my practice to go thru sorting tubes for getting a balanced matched set of tubes.

I found two of the tubes in this particular set (which hadn't been replaced since I set it up back in 2013) weren't matching up with the rest of the set. I had just replaced a full set of J/J KT88's on a different amp of theirs and did have several in that set that might be suitable to work with this set (ARS supplier, JJ/Tesla KT88's). After a little effort, I had cobbled a working set using a pair of the J/J KT88's. Ran the amp under Burst Pink Noise driving speaker for a couple hours, finding current levels of the six tubes remaining steady.

I powered down, cooled the tubes off, removed them, then put the two chassis back into the cabinet, after installing the hold-down clamps and the revised set of tubes in place. When I had the amp reassembled, I powered back up, and found, to my amazement, it was now drawing an additional 20W (fan disconnected) than it had when the chassis's were vertical during tube matching. AND, like before, I was unable to get the RED LED's off without having the GRN LED's also go out.

I started over this morning, verifying in Vertical orientation, all worked fine. Laying the two chassis down on the bench horizontal, but not installed into the chassis, with the tube clamps installed, I was back to this RED LED condition. I removed the clamps, put the tubes back in, and still had this RED LED condition. Stand the chassis' vertical, problem is gone.

This I've never seen before.

I'm about to install a new matched Sextet of J/J 6550 tubes to see if that exhibits this same behavior. I've been using the Chassis' standing up vertical to have access to the cathode resistors as test points for years and have ALWAYS ended up with a working matched set of tubes that doesn't exhibit this behavior. Guessing the age of these tubes, being over ten years old may be involved.

Anyone ever seen this before?

[The Dude]

I haven't seen that EXACT problem, but have seen tubes exhibit problems depending on their orientation in the past. I chalked it up to a possible physically loose element(s) in the tube, but that is a bit of speculation. New tubes solved the problem, so I didn't do much more research.

[nevetslab]

I just got the 6550 tubes installed, biased up for a close match, amp still vertical. I'll give it half an hour and check again, tweak as needed for best match, then lay it down horizontal as it would be installed in the cabinet (but not installing it). I too think this has to do with the age of the tubes, but don't yet know.

I also was pondering if the presence of the grounded hold-down clamps were involved. Having removed them earlier today, I found that wasn't a factor. Orientation with this old tubes so far appears to be the issue. I also discovered I couldn't find ANY Base Clamps available as an alternative to the traditional spring-loaded hold-down clamps.

[nevetslab]

After letting the amp run at idle for half an hour, I checked and recorded the readings across the cathode resistors, finding after tweaking the AC Mains voltage back to where it was, I had nearly the same readings. Switched to S/B, then carefully laid the power and preamp chassis down horizontal, reconnected the dummy load and switched out of S/B. The Bias LED's remained GRN, so fresh tubes do NOT have this problem that I discovered yesterday. AND, regrettably for my client, I can't continue using these 10-year-old tubes that matched up fine when the amp chassis were vertical.

Learn something new every day.

[The Dude]

10 years is a pretty good run for output tubes!

[nevetslab]

YUP. I found my service notes from 8/20/13 when I had installed the new Matched Sextet of ARS KT88's (JJ/Tesla tubes, which looking at them next to current J/J KT88's, nearly identical, though the new J/J KT88's show a relief at the top and bottom sides of the plates that I don't see on the older ARS KT88's. Very minor difference, while all else they look identical mechanically. I'll post a photo of that later, after my iPhone 6 gets charged back up.

The new set of Matched J/J KT88's now cost me $380. Prior to Putin invading Ukraine, they were $245 for a matched Sextet.

[Helmholtz]

Prior to Putin invading Ukraine, they were $245 for a matched Sextet.

Do you suspect a connection?

[nevetslab]

Do you suspect a connection?

With regards to the increase in selling price over here in the US? Well, during the initial few months at the start of the invasion, getting ANY tubes out of the Baltic states as well as out of Russia, it was like herding cats. KT88 tubes, IF you could even find any, a sextet of matched tubes was selling for over $1200. Since the invasion, ALL of the tubes selling prices are now considerably higher than they were prior to the war. I still find shortages occurring. Just placed an order for that sextet of KT88's, six ECC83S J/J preamp tubes, 6V6 J/J matched pair and found no ECC82/12AU7 J/J Driver tubes, so bought a pair of EH tubes.