SVT classic default

[catstrat]

I've worked on several of these in the past. I can't even find the right pre amp schematic for this one. It was stuck in default so I replaced some bad screen resistors and removed the diodes in parralel with the screens.Anyway,I've heard of some people bypassing the fault mode circuit. I'm not crazy about doing this but do you wire across the relay? I'm about ready to do it on this one. If I do this can I just wire a fuse inline with high voltage like Marshalls?

[TimmyP1955]

Pull the 6550s. If it stays in protection, there's a protection circuit problem most likely. If it powers up green, measure the voltages on all of the power tube sockets - if they are good, you likely have a bad 6550.

The left three sockets are one side of the wave, the right three sockets are the other side. Put one tube in each side and power it up - it might go into protection if one of them is the duffer (I can't actually recall).

[catstrat]

Thanks Timmy for replying. It stays in fault mode even with the 6550s removed. Those were tested using an Orange tester and tested good. The other SVTs I've worked on are fixed by replacing a bad screen resistor or fixing solder joints or something like that. On this one, I took care of all that. The thing was coming on and staying on last night but wasn't getting signal from the preamp. That's the reason I need a preamp schematic for it. As I was testing the preamp circuit that's when it tripped into fault mode. Which leads to the question about bypassing the fault mode circuit altogether. I don't think the early SVTs had the default circuit at all. I hear people complaining all the time about the amp not coming out of fault mode. Thanks and I'll keep looking.

[g1]

How about this schematic?

[MarkusBass]

Which leads to the question about bypassing the fault mode circuit altogether. .... I hear people complaining all the time about the amp not coming out of fault mode.

This is a very bad idea. Whenever I had this amp with such a problem, it was always a failed circuit and not general problem that "the amp does not come out of fault mode". And the circuit should be fixed. Of course, the protection circuit may fail itself but this is very rare. In most cases real reason was either open 10 Ohms resistors or missing ground. If this amp sometimes works correctly and sometimes not, I would check the ground of the power amp.
BTW, if the amp is in the fault mode, it means that IC2B has negative voltage on pin #7. It means that either the opamp has failed, or it has positive voltage on the inverting input (pin #6). Aren't you just curious where does this positive voltage come from? At least I would check it .

Mark

[Enzo]

Please do yourself a favor, make sure the right tubes are in the right sockets. SO many times I got SVTs in with all 12AX7s instead of 12AU7s or the 12AX7 and 12AU7s were in the wrong sockets. Get those wrong and it won;t bias right.

[catstrat]

Hold on everyone. I pulled the 6550s again this morning, turned the amp on and it came out of the fault mode and into play mode.Timmy suggested this yesterday. Even though I tested the tubes it's possible for a tube to have a problem the tester just misses, I guess. An intermittent problem. I'm going to retest them and during the test I'm going jar each tube to see if something is loose inside the tube. Everyone on here has always been so helpful when I get stumped. Let me do this and I'll get back with you.

[catstrat]

Thanks G1 but it's not the right schematic. This amp only has two pre amp tubes, one 12ax7 pre driver, two 12au7 drivers and 6 6550 output tubes. That's it. The older svt pre amps have like 3 or 4 tubes in the pre amp. This preamp is nothing like the pre amps I've seen on other SVTs. I suspect on this model the pre amp could be from some other Ampeg design. Seems like I ran into that a few years ago.Thanks anyway.

[g1]

Can't remember who (nevetslab?), but someone had described a routine of installing pairs at a time to match the power tubes and weed out the bad ones.

[g1]

This amp only has two pre amp tubes, one 12ax7 pre driver, two 12au7 drivers and 6 6550 output tubes. That's it.

Not any of these? Same tube line-up you described.

[catstrat]

Thanks Mark. I wasn't too crazy about the idea of bypassing the fault circuit anyway but I heard some suggesting it. You just shed some light on how the fault circuit works that I didn't know. I've already changed the screen resistors to 220 ohm per Ampeg service bulletin. It also said to remove the diodes. I've done all that. That's why I was a little baffled. All the other SVTs I've worked on in the past that has this problem is a screen resistor burned open. 98 per cent of the time that's the problem usually caused by a shorted tube. If i can't find the culprit tube the owner may have to spring for a set of 6550s. I'm convinced now it's a faulty tube which means my Orange tube tester, my wife bought me, let me down. I already tested them once. I'll let you know. Thanks.

[catstrat]

Thanks G1. Those schematics look like what I'm looking for. I still have to find why the preamp isn't passing any signal.

[catstrat]

I retested the 6550s using the Orange valve tester and they all tested good. Then I tested them using my B&K model 700 tester and it found the bad one. A tube has a loose connection in it. I installed 5 into the amp and I'm going to see if it stays on. If it does then I'll never trust a tubetester again. I think the best tube tester you're going to have is the amp itself. I use tube testers because it does tell me if a tube is shorted or worn out. Hopefully now I can find out why there is no signal from the preamp.

[Enzo]

We've said it many times, a tube tester can tell you a tube is bad, but it can;t tell you a tube is good. If the tester says bad, believe it. And stop there. yes indeed, a working amp is the best tube tester.


Look at the base of your octal tube, the pins are hollow. The glass tube itself has wire legs sticking out, like a 12AX7. Those wire legs poke into the hollow pins on the base, and are then soldered into the pins. Once in a while that solder fails, and SOMETIMES you can save the tube by reflowing solder into the pins.

[catstrat]

Hi Enzo. Well I jumped the gun again. I know that tube was faulty but as I was testing the pre amp circuit, the thing went back into fault mode again. I was testing it with only 5 output tubes in it. Will that cause problems on SVTs? I 've never had an svt that acted like this one. I removed the 5 tubes and it still won't come out of fault mode. By the way, the 12au7s are in there proper places along with the 12ax7. So I guess there is a problem with fault circuit. Maybe a sticking relay?