SVT-2PRO Bias Problem: LEDs stay red on one side

[bob p]

This thread is about whether or not it's worth buying a "basket case" malfunctioning SVT.

I'm thinking about picking up a used Ampeg SVT-2PRO that was made during the days of SLM. The price looks good, but then the asking price is low because its been beaten up quite a bit; it's evident that the amp has lived a hard life and has seen a lot of abuse: It's got a dent in the face panel, as well as a dent in the top/cage panel and some impact damage to the back panel DI pot. In other words, it's lived a hard life, it's been whacked several times, and it's cosmetically rough enough that most people would probably pass on it. It's also got some electrical problems in the bias circuit and the owner understands that the amp should be priced at a discount. Unfortunately, the amp is located pretty far away, and checking it out in person would involve enough driving that I wouldn't want to make the trip and not purchase the amp. But then I don't want to bother even making the trip if the amp is going to turn out to be a major headache, so I'm trying to do as much homework as I can before making a decision on whether or not to take the big drive.


I've been told by the current owner that the amp "plays fine" and that the bias circuit works fine on one side of the push pull circuit, but not on the other. Specifically, when adjusting the LED idiot lights one side properly changes from the RED LED to the GREEN LED, while on the other side only the RED LED illuminates and the GREEN one never comes on. Unfortunately I can't get access to the schem as LOUD is currently closed for a long holiday weekend. Not having the schem, I have no idea what the bias LED actually mean, so I thought I'd ask about it here. As I understand it, all of the recent production SVT that have the idiot-light bias system work the same way, so I'm hoping that someone with experience can help me out.

I was able to download the Owner's Manual from the ampeg site. The link to the PDF is on the lower right corner of this page:

Ampeg: Pro Series - SVT-2PRO

The OM suggests that a persistent-RED LED problem indicates that "one ore more tubes is not properly functioning."

In researching via the web I've read some case reports where persistent-RED bias LED suggest that there's a circuit problem with the amp, and that people commonly find burned out resistors when the take they amp apart. Unfortunately, these case reports don't give enough information to make the nature of the faults very clear.

Not having a schematic it's hard for me to get a clue about what might be going on. Not having ever been inside one of these amps, I'm a little worried about whether the impact points on the front, back, and top could have caused occult damage to a PCB. The seller tells me that the amp plays well, and does not ever go into protect mode and shut things down. According to him the bias lights are the only electrical problem. He says that he's owned it for about a year and the previous owner had some tube failures and had to re-tube the amp. He didn't mention whether any resistor fixes were made when the tubes failed, but it sounds like tubes have been replaced, that the amp probably wasn't properly fixed, and that the bias LED problem probably started with the original owner and has never been properly addressed for as long as the current owner has owned the amp.

If anyone could answer a few questions for me I'd greatly appreciate it. Here they are:

0. Anyone have a copy of the service data or the schematic?
1. What do the RED and GREEN LED in the bias system actually indicate?
2. Do the late model SVT also use diodes across low value screen resistors?
3. Why does Ampeg use the low value screen resistors and parallel diodes?
4. Any ideas on what sort of failures might trigger the persistent-RED, no-GREEN behavior in the bias circuit?
5. Are there any other obvious questions that I haven't thought of that I should be asking?

In the big scheme of things, I guess the pivotal question about this amp is whether or not it's worth taking a chance on a fugly SVT on the cheap and fixing it up, and what constitutes a fair price for an amp in such condition.

Any ideas?

TIA.

[Enzo]

I'll try to post here in a second, if it will let me.

My experience with these, when the bias lights don't want to work is almost always finding a problem with the driver tubes. The power amp has one 12AX7 buffer stage followed by a 12AX7 split load phase inverter. Then there are TWO 12AU7 driver/cathode follower tubes. I find bad 12AU7s to be THE most common repair to the latter day SVT tube output stages. A new 12AU7 usually wakes it up. But also right behind that is finding either the wrong tubes in it or the tubes in the wrong holes. Sometimes I see all 12AX7s or the 12AX7 in the wrong spot, and the 12AX7 won't drive the output tube to proper bias.

There are six power tubes, each one has its cathode returned to the driver board where it finds a 10 ohm to ground. The voltage sample across each of those is sent to pairs of comparators for over and under. The push and pull are groups of three tubes, so six comparators, on each side three comparators drive the red LED and three the green. SO we adjust bias so the current is high enough to turn on one LED and the other off. One bad tube can keep a group of three tubes from balancing, but pulling the bad one lets the other two balance if they can.

That model originally had 22 ohm screens with diodes in parallel, but a service bulletin told us to remove the diodes and install 220 ohm screen resistors. I expect when a power tube fails, it often takes out the screen resistor, regardless of its value, I have surely replaced my share of 1k Marshall screen resistors. I don't really read a lot into "burnt resistors" when it comes to tube amps.

Other than a defective transformer, and of course a set of 6550s, it is just an amp like any other, and the remaining portion should be inexpensive to fix. If the amp basically functions, it sounds like the expensive stuff is OK. Just because the bias won't balance out, doesn't mean the amp can't function fairly well. And I guess the price matters, because if the transformers are OK, I imagine you could strip them out and sell them for a goodly portion of the asking price.


This browser won't let me post. I'll come back in Chrome in a bit, for some reason it seems to allow me to upload files.

[bob p]

Thanks for posting, Enzo.

My experience with these, when the bias lights don't want to work is almost always finding a problem with the driver tubes. The power amp has one 12AX7 buffer stage followed by a 12AX7 split load phase inverter. Then there are TWO 12AU7 driver/cathode follower tubes. I find bad 12AU7s to be THE most common repair to the latter day SVT tube output stages. A new 12AU7 usually wakes it up. But also right behind that is finding either the wrong tubes in it or the tubes in the wrong holes. Sometimes I see all 12AX7s or the 12AX7 in the wrong spot, and the 12AX7 won't drive the output tube to proper bias.

On the subject of driver tubes: the amp reportedly has one half of the push-pull circuit biasing properly, while the other side isn't. that has me thinking that it can't be something simple like a 12AX7 being stuck in the wrong spot, as I would expect that to make both sides fail to reach the proper bias point. If it's a tube problem, then I'm thinking that it would have to be one triode in a 12AU7 going bad.

I've read a bit about ampeg using 10R current sensing resistors and comparators to verify power tube bias. My understanding about this is that if one of those resistors should fail, then the comparator would activate the amp's protection circuit and shut everything down, activating the fault light on the front panel. From what I've been told that is not happening, so the problem can't be that bad. I guess this means that current flow is less than critically aberrant, which would cause a simple RED LED error. Now it just comes down to finding the problem and fixing it.

One bad tube can keep a group of three tubes from balancing, but pulling the bad one lets the other two balance if they can.

for diagnostic purposes then, you'd just try pulling successively, each one of the three tubes on the troublesome side, one at a time, and seeing if that side of the push pull circuit would bias to a GREEN LED with only 2 tubes present on that side? so for the test you'd try biasing the amp with 3 tubes on the good side and 2 on the bad side? I guess there really would be nothing wrong with running the amp that way. i was thinking that the comparator circuit might notice zero current flowing through one cathode resistor and send the amp into protection, but not having a schematic it's not possible to know how the protection circuit actually works.

That model originally had 22 ohm screens with diodes in parallel, but a service bulletin told us to remove the diodes and install 220 ohm screen resistors. I expect when a power tube fails, it often takes out the screen resistor, regardless of its value, I have surely replaced my share of 1k Marshall screen resistors. I don't really read a lot into "burnt resistors" when it comes to tube amps.

I know what you mean about screen resistors. It's key to size them small enough that they remain a consumable item, and that they burn up when the power tubes go, so that you don't take the OT out at the same time. My understanding is that with the SVT the low resistance plate and screen resistors in the output stage serve a role like fuses. I'm still not quite sure why they put the diodes in parallel with the screen resistors though. Can you explain that?

Other than a defective transformer, and of course a set of 6550s, it is just an amp like any other, and the remaining portion should be inexpensive to fix. If the amp basically functions, it sounds like the expensive stuff is OK. Just because the bias won't balance out, doesn't mean the amp can't function fairly well. And I guess the price matters, because if the transformers are OK, I imagine you could strip them out and sell them for a goodly portion of the asking price.

that's what i was thinking -- that as long as the amp plays but won't bias, then all of the expensive stuff is probably OK. that would mean that the price for admission to the SVT club is to be willing to troubleshoot and repair a defective amp, and that I'd be willing to keep it in spite of it's cosmetic battle scars. Knowing that those protection circuits are in there, I'd really like to get hold of some service documentation before taking on this sort of project.

Knowing where the boards are in the bottom of the amp, do you see any potential for a PCB to get damaged if the amp is dropped on it's top? I'm thinking that the graphic EQ board might be at risk, but that the main board on the bottom isn't as likely to get damaged if the amp were to fall on it's top, or if something heavy were to put a large crease in the top of the amp. from the preliminary pictures i've seen, it looks like a tree trunk fell on top of the amp and put a crease right down the middle, or perhaps the amp fell head-first onto a concrete curb.

thanks for your insights.

[Enzo]

There is a 12AX7 input, followed by the concertina PI. A problem there will affect both sides. There are two 12AU7s, one for push and one for pull. They are not split between sides as one might expect. Thus one bad 12AU7 (or wrong tube in a socket) will affect only one half of the power tubes - the push or the pull.

But since the 12AU7 is running signal from one triode through the other, one bad triode in a 12AU7 would kill/degrade one side of the power tubes.

[bob p]

by any chance would you have a schematic or the service manual? i tried contacting LOUD but they were closed for a long weekend.

[Enzo]

I have them, I just had to get out of where I was and over here to Chrome.

[Enzo]

That seemed to work, so here is the preamp and EQ

[Enzo]

Layouts

[bob p]

Thanks Enzo. those 4 files open just fine. I see a couple of board layouts, and parts lists, and a schem of the power tubes, but I can't seem to find the preamp schematic, or the protection circuits that include the logic for the cathode current sensors, the comparators, or the bias circuit. Perhaps that stuff is located in some other files?

[Enzo]

Click on the thumbnail in post #6. The first page of that is the drive circuit and the whole lower right quadrant of the page is the comparators and balancing circuits.

In post #7, the preamp is the second file, the layout is the second file in post #8.

The driver layout is top in post #8.


I just opened all those from the posts, so I think they went up OK. I posted seven files in three posts.

[bob p]

Somehow I missed one of the posts when I was scrolling, and I only saw 2 posts instead of 3. Sorry. I'm working from a laptop with a small screen.

One of the files still isn't working. the EQ file. It has a view count of zero because nobody's been able to open it. Could you repost it? thx.

[Enzo]

OK, trying again

[Enzo]

No, not working, i tmust be the same as a previous upload, even though I changed the name.

[Enzo]

OK, I cannot get it to upload. I have been having great trouble uploading attachments lately, not only this forum.

[Enzo]

I give up, I cannot upload that particular file for some reason.

Wait...