Ampeg SVT VR need tech advice

[r0ddan_swe]

Greetings
My first post. English is a second language.
Regarding an Ampeg SVT VR.
Got the preamp out. Found a resistor that didn't look ok. It's a wire wound 5W resistor at 8.5 kohm. R24 in the schematic for the preamp. I'm not a tech myself, but to me I seems as if the resistor has something to do with lowering voltage from 480V to 315V on the anode/plate side of the tubes. And/or some kind of protection.
The preamp is at a tech/amp-builder now and he will change it out. It might come back with a 10 kohm resistor instead.
The question is if the value of the resistor is critical?
If so, or if not. Please explain.

Best regards
// Jonas

 

[r0ddan_swe]

 

[Bobbadfinger]

Since no one chimed in...

This is a dropping resistor feeding the preamp node. No protection duty at all.
Tell the tech to order the correct part. In a guitar amp I doubt it would impact tone. This is no guitar amp.
A 10K would drop about 30V more, maybe it would lower amp's headroom a bit.
Why take the chance ? Do it right, do it once.

More important is to figure out why it cracked open...At idle, dissipates more than 3W, so it'll get hot.
But it can be a short down the line. It's good insurance to check the electrolitic caps around R24, specially C7.

 

[r0ddan_swe]

Thank you!
After a bit of thinking that was my aproach aswell. Why take a chance. As it turns out thou, the resistor was not broke. A bit cracked but not broke. Measured just fine. Put a tiny bit of superglue on it. Gave the tubeholders and the potentiometers some love and attention and put it together again. Plays fine. There is just one thing I can’t figure out. The ultra low switch on ch 1 behaves just like a midscoop, as it should. On ch 2 it behaves more like a mid boost. Super strange… I might have put a connector backwards when I put it together again, but from memory there was only one way to put them… Maybe that will be my thread number two
Thanks again.

 

[byacey]

Superglue won't stand up to much heat. A 10K will run a little warmer because it's dropping more voltage across it.

 

[Bobbadfinger]

These amps are a royal PITA to desassemble; if you had it done already, I'd have changed the resistor anyway. Not the type of part to hack on it.

 

[r0ddan_swe]

Superglue won't stand up to much heat. A 10K will run a little warmer because it's dropping more voltage across it.

Thank you. The glue is just there so it won’t crack even more, or fall apart. One sunny day I will service that nuclear reactor of an amp, but as for now it will be fine. The original dropping resistor is in there still as it measured to spec. Other than that it looks fine. Solderings still shiny and no strange noises or anything… And regarding the strange behaviour of ultra low on ch 2, I hardly ever use anything but ch 1 so… maybe it has been wired wrong and I haven’t noticed. Only ever played this vr so I don’t even know if it is strange or not. Maybe it is as it should be, who knows…

 

[r0ddan_swe]

These amps are a royal PITA to desassemble; if you had it done already, I'd have changed the resistor anyway. Not the type of part to hack on it.

Totally agree. But there is no ”for real” Ampeg tech around here. I could get it to one, but I doubt he would be fine with just getting the board. He would like to have the whole amp so he can measure everything while it’s running. And it’s the taking apart and putting together that is time consuming. Not difficult, but as you said, a pain where the sun do not shine… the part is cheap and the soldering of and on takes a few minutes… It’s working and I’m happy and life is good

 

[r0ddan_swe]

I might add. This amp have been serviced once as it went silent on me at a gig. There was this extra encore we didn’t expect, so I had shut it down. When I put it back on it went silent so it had some service on it. Been working since with new tubes in v1 v2 and v4 and the two drivers in the poweramp. Maybe it was at the same time the dropping resistor cracked. But as I said, it measures to spec still. Just looks a little funky, but who doesn’t.

 

[agedhorse]

No amp tech would want to see only the board.

If he replaces the resistor, it needs to be replaced with the correct part.

The resistor serves 3 purposes, as an RC filter element, as a dropping resistor and also for protection of the transformer in the event of a short circuit on that supply. I believe it's a safety related component with regards to the amp's safety approval, so it must be replaced with the same value and type.

 

[Wasnex]

The VR is a functional re-creation of the original SVTs, but it does not use the original tube types or signal path.

The original amps used 12DW7 in channel 1, and the signal flowed sequentially through both sections of each tube.

12DW7 has to triodes; one is like 12AX7 and the other is like 12AU7. The original amps also used a 6C4 which is pretty much half a 12AU7.

The VR achieves the same gain as the original SVTs, but using 12AX7s and 12AU7s in place of the sections of each 12DW7. This is done by routing the signal in and out of different tubes. I believe the order is: V1a, V2b, V2a, V3b, V3a, V4b. So V1 and V3 are 12AX7s and V2 and V4 are 12AU7s.

It's a lot easier to understand how the circuit works if you look at the schematic for an early amp.

For the 12DW7 tubes, pins 1,2,3 are the 12AU7 section and 6,7,8 are the 12AX7 section.

Notice the Ultra-Low switch on channel 2 only has two positions. Left is off and right is ultra low. I don't how it works on the VR but with many vintage amps, you need to crank up the Gain and Bass controls more than with channel 1. In the off position, the tone has pretty strong mids.

I have read that the connectors on some modern SVTs are not polarized. I don't know if this applies to the VR. If you come across non-polarized connectors, I suggest you mark them with a permanent marker. In other words create a visual key so you know how they are supposed to mate.

 

[beans-on-toast]

If you change the value of R24, the preamp voltages will be off. Use the proper part. The coating helps the resistor dissipate the heat. With the cracks, it could run hotter and not be able to dissipate the rated wattage. In normal operation, the resistor dissipates just over 3W. At power up it will be higher, thus 5W rating provides a safety margin.

 

[r0ddan_swe]

The VR is a functional re-creation of the original SVTs, but it does not use the original tube types or signal path.

The original amps used 12DW7 in channel 1, and the signal flowed sequentially through both sections of each tube.

12DW7 has to triodes; one is like 12AX7 and the other is like 12AU7. The original amps also used a 6C4 which is pretty much half a 12AU7.

The VR achieves the same gain as the original SVTs, but using 12AX7s and 12AU7s in place of the sections of each 12DW7. This is done by routing the signal in and out of different tubes. I believe the order is: V1a, V2b, V2a, V3b, V3a, V4b. So V1 and V3 are 12AX7s and V2 and V4 are 12AU7s.

It's a lot easier to understand how the circuit works if you look at the schematic for an early amp.

View attachment 4328199
For the 12DW7 tubes, pins 1,2,3 are the 12AU7 section and 6,7,8 are the 12AX7 section.

Notice the Ultra-Low switch on channel 2 only has two positions. Left is off and right is ultra low. I don't how it works on the VR but with many vintage amps, you need to crank up the Gain and Bass controls more than with channel 1. In the off position, the tone has pretty strong mids.

I have read that the connectors on some modern SVTs are not polarized. I don't know if this applies to the VR. If you come across non-polarized connectors, I suggest you mark them with a permanent marker. In other words create a visual key so you know how they are supposed to mate.

Very informative, great post! I wish I had your knowledge about circuitry, I don't. The first part I can read and understand. Bear in mind that English is a second language to me. I understand that the VR is a modern version of the original SVT with 12ax7:s and 12au7:s instead of the original tubes. I have read that in numerous posts before. I can't read schematics as you can. I can recognize parts, I have a vague idea what they do and I can google all night ;-).To me it's an amp that sounds great. When I bought it, it was about half the price of a brand new but only tested and still in the original package. Plastic wrap and everything.

Yes, you have to crank up the volume on ch 2 a bit more. I doesn't bother me as I never use ch 2 other than when I'm alone with the amp to test how it sounds. Most of the times I plug in, power up and play happily. On ch 1... Right now the amp is my basement with my pedalboard and a 212. I changed the tubes in the power amp, all of them. When I then tested the amp there was some strange noises in it (when bashed upon). As if something wasn't seated properly or as if a potentiometer needed a workout. So I took it home to reseat the tubes and give some attention to the pots. That's when I saw the resistor (R24). I took the board to an amp builder who lives nearby. My first post was before he called back and told me the resistor measured fine. Also before I told him not to put a 10k to replace it if it was broken, but rather wait for the correct part (or at least a wire wound 8.2k as that is easily available).
Now when it's all together again and I play it I notice this strange behavior of the ultra low switch on ch 2... There is a chord between the board that holds the switch and the preamp board. It is as if two connectors have been placed wrong. In the off position it sounds normal to me, but in the on position it has strong mids. The ultra high sounds as it should as far as I can tell. Totally unusable that is, makes the amp scream with treble.
I will try and post a pic of ch 2 from the schematics for the VR. Maybe you can tell if my theory about the mixup of chords on J9 to J12 can be right.

 

[beans-on-toast]

When it matters, connectors can normally mate in only one orientation. This is the case with the connector below the power resistor in the image above. They click together.

Orientation is also important for the 4-pin connector to the ultra high and ultra low switch board.

 

[r0ddan_swe]

When it matters, connectors can normally mate in only one orientation. This is the case with the connector below the power resistor in the image above. They click together.

Orientation is also important for the 4-pin connector to the ultra high and ultra low switch board.

The connector goes only one way, I know. But when it was put together, is it possible the two chords were put in the wrong holes in the connector? There are four cords in that connector... My English limits my ability to communicate here. Do you understand what I mean? two cords may have switched places. And in that case, will the switch for ultra low just work the opposite way around?

 

[beans-on-toast]

The connector goes only one way, I know. But when it was put together, is it possible the two chords were put in the wrong holes in the connector? There are four cords in that connector... My English limits my ability to communicate here. Do you understand what I mean? two cords may have switched places. And in that case, will the switch for ultra low just work the opposite way around?

Anything is possible. The only way to know if the connector is wired correctly is to trace the wires and compare them to the schematic.

 

[Wasnex]

The connector goes only one way, I know. But when it was put together, is it possible the two chords were put in the wrong holes in the connector? There are four cords in that connector... My English limits my ability to communicate here. Do you understand what I mean? two cords may have switched places. And in that case, will the switch for ultra low just work the opposite way around?

The connector would not mate properly if the pins were cross threaded. It would be noticeable because it would not be possible to completely close the connector without damaging pins.

The only way I could imagine what you are inferring happening would be if the pins were removed from the connector and then reinserted in the wrong order. Typically there is no reason to do this, and if there is a reason, it's not easy to remove replace the pins.

 

[JimmyM]

Thank you!
After a bit of thinking that was my aproach aswell. Why take a chance. As it turns out thou, the resistor was not broke. A bit cracked but not broke. Measured just fine. Put a tiny bit of superglue on it. Gave the tubeholders and the potentiometers some love and attention and put it together again. Plays fine. There is just one thing I can’t figure out. The ultra low switch on ch 1 behaves just like a midscoop, as it should. On ch 2 it behaves more like a mid boost. Super strange… I might have put a connector backwards when I put it together again, but from memory there was only one way to put them… Maybe that will be my thread number two
Thanks again.

The two channels are more different than alike.

 

[r0ddan_swe]

The connector would not mate properly if the pins were cross threaded. It would be noticeable because it would not be possible to completely close the connector without damaging pins.

The only way I could imagine what you are inferring happening would be if the pins were removed from the connector and then reinserted in the wrong order. Typically there is no reason to do this, and if there is a reason, it's not easy to remove replace the pins.

Not sure we are talking about the same thing here. Or if there is a language barrier I can’t cross when typing in english. I believe those connectors are called molex. Used in many applications. What I am talking about is when the cords were put inside the female connector. J12. Someone might have put the cord that goes in spot 1 in the spot 2 ande vice versa. A mistake, human error. You can read schematic. Your understanding in that area exceeds mine by far I am sure. What would happen soundwise if that happened? Whould the switch behave backwards so to speak? I can’t tell…

 

[agedhorse]

It’s possible that it’s a manufacturing error, but not very likely.