ampeg svt cl (korean) burning cathode resistor on 12ax7

[dangin]

Got one of these that came in for just a biasing...turn on and get a shorted tube and burned out 220 ohm on the output tube board. visually looked like a shorting 6550. ...replace the 220 ohm and turn on without the 6550's and looks good to go.. put in new tubes (EH kt88's all matched) and poof, burns out the 220 ohm (R14) cathode grounding for the 12ax7 driver. without the kt's in I'm getting -45v at each grid and 402v at every screen grid....This is my first SVT and it is not behaving well. Any help is greatly appreciated.

[Jazz P Bass]

I cannot seem to find a schematic for the output tube board or the speaker board.
The resistor (R14 220 ohm) is part of the feedback path.
And it comes from the "speaker board".
I would look at the path that R14 takes on said board.
(I would think it would go to the output transformer secondary)
It sounds like something is shorted or miswired on that board.
Or
Note: You must have a load hooked up to the output transformer when testing.

[dangin]

Thankyou for the speedy reply, still in wake-up mode here in CA but I will keep you posted. Thanks

[dangin]

Wow..problem solved I believe. Thankyou Jazz P Bass for the fresh perspective. I made a mistake! Embarrassed to say being my first thread and all...You made me look in the right area where low and behold clip connector to speaker board flipped...rookie mistake but I confess I'm not really a rookie.Yes, I sharpie all the connectors but this one had just slipped off .So, in reflection of this, a note to self: this particular jumper connector slides off very easily as I noticed did others. FYI.
I'm working on a lot of amps and old synths at the moment so I promise a more interesting thread next time! Thanks again

[dangin]

Sweet, amp is running, all voltages appear good but biasing lights issue now. 12au7 swapped out, no fix. One side is fine; goes to a single green led, other side will show red and green or just red (will not reduce to just the green)...hmm
Voltages on 12au7's are identical.

[Jazz P Bass]

Is there not a note on the schematic about setting bias?
If the tubes are not exactly the same & you cannot get "green, green" then set both for "red".

[dangin]

I read that but I am a little weary about that idea...seems a little bandaid-ish. I will try. The tubes came as a matched set and they're EH ....
ok it seems to have settled to 2 green lights but sounds like ass! going to let it sit a little longer to see what it wants...I don't like being the "ass-monkey" to any amp!

[dangin]

ok, thanks Jazz P Bass. I really appreciate your insight. Amp is all good now...playing nicely for the last 40 minutes or so. I'm going to lock it down and give it back to the owner heading for New York this afternoon. Now I have 3 tranny ampegs, a Vox Super Continental and a Yamaha Symphonic Ensemble oh and a Silvertone 1470 to plough through..ouch and a Peavy Transtube...plenty of good wholesome fun!

[Gingertube]

I had one of these (2 years old) turn up at my place for a repair on the weekend. 2 of the 6 output tubes no good, 1 does'nt light up, the other tests Ok except it has a heater cathode short.
I have a new set of 6550 for it.
Question:
I note that the original SVT screen circuit with 22 Ohms bypassed by a diode has been changed to just 220 Ohm screen resistors (Factory change). That is certainly a step in right direction but has anyone modded one of these by fitting 1K screen resistors to protect the output tubes a bit?
If so - does it change the sound? Is this recommended?
Thanks,
Ian

[dangin]

This maybe total balogni but I thought the less resistance you have there, the better tonally (dynamically) the sound and I think the diode is supposed to save the tube..? Saying that, I can vouch for these amps being very sensitive, if it's not broke....
One more question....I am finding this amp quite awkward to work on and now I hear a faint click and the bias status LEDs on one side change. Is there any problem with manually biasing these? I think a sensitivity IC maybe compromised...but of coarse no TP figures on the schematic...

[Jazz P Bass]

The clicking could only be the "Fault" relay, no?
Monitor the output pin of IC21 (Fault Relay IC)
If it is triggering, then you have to find out why.
There are a number of changes on that board.
I have included the schematic that indicates the changes.

[Prairie Dawg]

These amps run the tubes damned hard and good tubes are hard to find. I have found with these that the only way of getting a proper resolution of these kinds of problems is to start with a known good set of power tubes. Not a set you think are good or a set somebody tells you are good, but a set that you personally have put through the wringer on a test rig built for the purpose. A lot of tube sellers use a maxi matcher, which is fine but it only tests at 400v which is 260 volts shy of working voltage. I am a bit of a coward so I only run them up to 600. Then, and only then, can you start working your way back from the tubes through the bias set mechanism and the fault detection system. In fact what I did was I went and got myself a dedicated set of 6550s that I wrung out on my test rig and they stay in the shop for test purposes only. A guy here helped me out with one a while ago and although the experience was a little painful, working my way through it taught me plenty-about a lot of things including people. Thanks Enzo.

[RogerT]

This may, or may not play a part. I recently had an Ampeg SVT AV, that needed, "also, just tubes, and a bias." Did the tubes, and could and found many broken solder connections, on the main board. And then a bubbled up radial 47uf cap, in the power supply. I replaced it, and the two next to it. They are glued in with silicone type stuff. Just cut it away, carefully, and replace, appropriately. Got it all, put back together, after the solder work, and the caps, in the supply, and the bias lights came up, dark, at first, and then I slowly turned them up, and they did just like they were designed to do. Brought them into the green, and all was well. Put it through the documented burn in time, and all, continued to behave. Good luck. It's a fine piece of equipment, for sure. It's a heavy beast, but, guess that's why, they are the queen of the castle.

[RogerT]

This may, or may not play a part. I recently had an Ampeg SVT AV, that needed, "also, just tubes, and a bias." Did the tubes, and found many broken solder connections, on the main board. And then a bubbled up radial 47uf cap, in the power supply. I replaced it, and the two next to it, as they were heading the same direction, but definitely, a different part of the circuit. They are glued in with silicone type stuff. Just cut it away, carefully, and replace, appropriately. Got it all, put back together, after the solder work, and the caps, in the supply, and the bias lights came up, dark, at first, and then I slowly turned them up, and they did just like they were designed to do. Brought them into the green, and all was well. Put it through the documented burn in time, and all, continued to behave. Good luck. It's a fine piece of equipment, for sure. It's a heavy beast, but, guess that's why, they are the queen of the castle.

[Enzo]

Hi Roger, welcome.

Just want to point out this thread has been dead for five years now