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Thread: SVT classic default

  1. #1
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    SVT classic default

    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?

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    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).

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    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.

  4. #4
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    How about this schematic?
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    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Quote Originally Posted by catstrat View Post
    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

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    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    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.

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    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.

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    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.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Quote Originally Posted by catstrat View Post
    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.
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    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    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.

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

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    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.

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    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    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?

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    IME the Orange tester finds almost every tube that will fail when used in an amp. And it's not good for power tube matching if your main criteria is bias current - two of the same number can be a poor bias match, two that are a number apart can be a great bias match.
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    I fixed one a little while back using a custom-built bias-probe (I made myself) that has multiple terminals for multiple meters that permit simultaneous measurement of Va, Vg2, Ik and Vg1 - one tube at a time. I ran this on all six tubes to work out which ones should go on which side. Got the sides balanced to within 1mA! The comparator window LEDs work properly'n'all.

    But it had multiple failures prior to this success story that took me days to hunt down and figure out. Blown Rg2 was easy to spot, but the blown PS for the bias protection circuit comparator window wasn't immediately obvious. But the tell-tale signs were there - scorched filament spade terminal contacts for the primary filament leads to the output tube board. These needed clipping off, cleaning up, and soldering the leads directly to the cleaned spade connector tabs on the board. These connection points see 11.1A under idle conditions (leaving aside startup spikes). As far as I can make out, the build-up of oxidised crud between the connectors and the spades had resulted in increasing resistance - and high voltage spikes and arcing, which left a scorched black mess everywhere around the terminal connectors. The voltage spikes from these fireworks had gone back through the filament PT winding and thence across into the winding for the PS for the bias protection circuit and taken out the PS filter caps! Anyway after I found and fixed all the dud parts and got it going again, it has held up so far (touch wood).
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    I'm going to start tomorrow troubleshooting the fault circuit and check IC2B per Marcus bass.I'll study the schematic tonight. I tell you Tubeswell, as many cracked solder joints this amp had when it came to me i should know to check the fault circuit for cracked joints to. It had bad solder joints on all the power tubes. Ampeg sent out a service notice about it.. I'm willing to bet a cracked solder joint is the problem here and i just have find it. It's just had multiple problems. But, it goes with the territory. It's my job, no matter how aggravating it is sometimes, it's what we all get paid for. I think the customer,who is a friend of mine, bought this amp for about 200.00 dollars. I think he did alright. SVTs are great amps and when they work right,they are hard to beat if you want that tube sound. But you better buy a forklift to move it for you.
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    Senior Member Enzo's Avatar
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    I am not going to rewrite it now, but a couple years ago we did an in depth circuit analysis of the bias and protect setup in these. What each part does, how the compartors work, etc. Search it out. It may look complex, but the principle behind it is straightforward.

    Just don't start guessing. "Maybe a stuck relay?" Fine, if you think that, don't replace the relay, go look. Is the coil getting energized or not. If energy flows through the coil but the contacts don't transfer, then yes, the relay is bad. But if the voltage at the coil never changes, then neither will the contacts. And it isn't the fault of the relay. To me a shorted shunting diode across the coil is more likely than a bad coil. Everything is a circuit. The relay is controlled by a transistor, which is in turn controlled by other circuits, so trace it back.
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    Thanks Enzo for the info. I'll see if I can find the piece you're talking about. I'll start tracing it down in the morning and hopefully find it.

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    Quote Originally Posted by catstrat View Post
    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.
    Another reason I hate those Orange VT1000 testers.!!

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    The Orange tester has been pretty reliable up to this point but everything has it's quirks i guess. There is a reason i held onto the B&K. I get amps through, like most techs, that have 7 pin tubes. All of those are vintage amps so hopefully i got it covered. The Orange tester i like because it can quickly test most tube used in today's amps. At least i don't have to do much to set it up.
    I found a cracked solder joint on the power in module board where the fault relay is at.i plugged in the amp and turned it on and started probing the wires around the relay and i could make the relay make or break at will.I resoldered it and tested the amp and it worked until a few minutes ago and it's stuck in fault mode again. But now i'm sure the problem is still either a corroded push on connector, which someone on here suggested, or another cracked solder joint i just haven't found yet. As many cracked joints this amp had i should know to look more carefully. Well i think i'm at least close to getting to the problem. The customer is real cool about it. He's a long time customer.Stay tuned for the next episode.

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    A new problem happened this morning. I was testing the amp this morning and I smelled something get hot so I stopped the test. I heard the relay rattle , that's what I call the noise, and the amp meter on the variac swung to about 13 amps. I shut everything down. I pulled the 6550s and tested the amp. There was no more excessive amperage draw but now the relay opens and then closes in a cyclic manner about every 3 to 4 seconds. So does anyone have the schematic for the relay circuit or can shed some light on it. I found the relay control wire to the main board. I removed all the boards and didn't find any evidence of burned or overheated tracings or anything else. Someone on here gave me a few things to check. IC2 I believe was one. If pin 7 has a negative voltage then it means either the op amp is bad or pin 6 has positive Voltage on it. Having the schematic would sure help things. Sometimes Ampeg uses the preamp from one amp matched to the output from another amp. At least the SVTs. I'll see if I can't find it online. By the way, why do some relay circuits have a diode across the relay coil?

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    Never mind the schematic. I found an SVT 2 which I think is the same fault circuit as the SVT classic. I remember it as being something like this.

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    SVT-CL schematic(s)

    All the ones I have anyhow
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    Thanks tubeswell. If I would have thought about the SVT 2 having the same fault circuit, or it looks the same, I could have saved you the trouble. Thanks anyway.

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    Actually I just realized G1 posted the fault circuit from an SVT 2 a few days ago. If I would have looked closer I could have saved the both of us the trouble.

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    Back to this SVT. I checked R3,R4 and R2. Out of the circuit they all tested good. I removed the MPS A13 from circuit and it tested in a way that i didn't trust but since i stock those i just replaced it. I think better safe than sorry. I found the article on here where Enzo explained how the fault circuit works. It helped a lot. I checked the voltages at IC2B per Timmy and i had negative volts at pin 7. Question. I thought i was supposed to have 15 volts plus,minus at pins 4 and 8 on a TL072. That voltage is missing on IC2. When i check the voltage with my meter at R3 or R4 on the ends closest to the MPSA13 that trips the relay. But i'm going to recheck the voltages again tomorrow to verify. I measured 115 volts at R3 and that doesn't look right. I'll try another grounding point for my meter.

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    These are the exact voltages i just measured.
    IC2 pins 1,2,3 0 volts
    pin4 ( -13 volts)
    pin 5 +15 volts
    pin 6 +15 volts
    pin 7 0 volts
    pin 8 + 4 volts
    At J 35 , + 11 volts constant. The relay cycles about every 3 seconds but the time varies on the cycling.
    R 2 7 volts
    R 3 2 volts
    R 4 2 Volts

    Having an actual schematic on the main board circuit would sure help. I've looked all over. Enzo explained how it works really well and i can troubleshoot from there but i guess i just want to see the whole circuit. Where does IC2 get its voltages from? The other thing,some SVTs i've worked on have the fault relay mounted on the main/driver tube P C board. This amp has the fault relay on the AC power in module. Just to make sure we're talking about the same amp. Thanks.

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    Gosh. I just realized the schematic I was looking for is right in front of me.

  31. #31
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    Which one is correct for your unit?
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    The relay is on the AC power in module. Seems with some SVTs I've worked on in the past the schematic doesn't always match the circuit I'm troubleshooting. But I'm pretty sure the schematics are right on this amp.. I was just curious about the fault relay location on the classic. Some of them a silver relay is on the main p c board. That's all.

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