Ampeg SVT issue

[Tonegeek]

I retubed an SVT 2 pro with Tung-Sol Reissue 6550 that was working when it came in. I also replaced the 3 tubes on the driver board. It promptly went into protection mode on power up. So, I tested all the 6550 tubes and they check out fine. Even put the old ones back in and it worked. My thinking is one or more of the new tubes is not meeting its design values and arced. One of them did feel hot, while the others were only warm (from the heaters). They are rated for max plate of 650v. The SVT puts 660v on the plates. I have used the reissues in another Ampeg and they worked ok, but have to say one was bad from the onset. So this is the second time I have had issue with these tubes in an SVT. My problem now is that the distributor is hesitant to take them back without testing them first, so I shipped them and crossed my fingers. I explained to them that the problem will probably not show up on their tester due to testors using lower plate voltages. I am leaning toward swapping these for Sovteks as they have a max plate of 800v.

Has anyone else had this issue?
And if somehow the tubes are not causing the problem, what could be? I can't think of any scenario where an amp would work with one set of good tubes, and not another, unless there was a bad tube. The older tubes were worn unevenly, but still tested good.

[xtian]

You don't mention adjusting the bias at all. Did you?

[Tonegeek]

xtian:
As with any amp, I initially set the bias way cold and pay close attention as the amp warms up. The SVT pro 2 has external bias adjusters and LEDs to help you set proper bias. Long story short- I got another sextet of tubes, this time Sovteks and plugged them in. I was initially able to set a reasonable bias point but after a few minutes, the amp went into protection mode. So I waited a few minutes, fired the amp up again and this time the HV stayed on. I ran the amp for about an hour, until I felt like the bias was stable and then pushed the hell out of it into my dummy load. It works great. Did the same again this AM and it is fine. Don't know what happened that first fire up (maybe some stray metal shaving in a tube - who knows). I am gun shy now about using the Tung-Sol reissues in these amps although others seem to get away with it. The specs are max plate=650v and the SVT puts 660v on the plates. The Sovteks max at 800v. Some SVT users even run JJ 6550s and they max at 600v.

Finally, I am curious to know if others have had issues with protect mode kicking in on these amps for seemingly no reason (well there is always a REASON, right?) So is there a mod/update or maintenance procedure that might help reliability? I knock on wood and say a prayer every time I work on one of these monsters.

[tubeswell]

Protection mode can trip in for a number of reasons, including that the protection mode control circuit is powered by a secondary winding from the filament PT, and the filament winding from that same PT is connected to the output tube board with push-on spade terminal connectors that arc when the connectors have built up a layer of oxidation (oxidation = resistance) which causes voltage spike transients to be inducted back through the filament PT to the circuit powering the protection mode. These voltage spikes are a nuisance. Best cure is to solder the main filament connectors directly to the output tube board terminals. (These see just over 11A under normal operation, but much higher in the startup surge when filaments are cold, so if they are oxidised and charred, that is my bet about where the problem is)