Basket case Ampeg SVT2 pro

[Climberkalen]

So, a while back I took in a SVT2 pro on a trade. Completely disassembled in a plastic tote box. Got it together and fault light was stuck on. So I did the following; Added the cap on IC2, did the power tube board update, touched up iffy looking solder joints, replaced the burned 16pin ribbon cable from the tube board to power-amp board. Go to fire it up, get the thunk sound and we got a green light but no output. Both bias lights are on, adjusting the pots do nothing. The pair of 12au7s filimants are ever so faintly glowing, So im assuming their toast. Cheapo but tested NOS pair of 12au7s are on the way. So I'm hearing a very faint buzz out of the speaker. So out of curiosity, I check the speaker Jack's for continuity. I check between tip and sleeve. Meter beeps, on both jacks. Pulled out the 6 pin plug from the OTX and the meter stops beeping. The pessimistic half of my brain thinks shorted OTX, the optimistic half is thinking it wouldn't make any noise if it was a bad OTX. I had a pop trying to take readings on the power tubes. Double and triple checked schematics and connections. So im reasonably confident its wired correctly . I know how to safely work on high voltages/discharge caps. This is just a super complex circuit and my brains turn into pudding staring into the abyss of the schematics. Im hoping good 12au7s with fix the bias light issue. Caps on the power board look OK. They check within tolerance on the meter. I haven't checked dc leakage yet, I figured it would trip the fault circuit if they were. I'm also trying to avoid shotgunning components unless they are obviously bad like the ribbon cable. Any thoughts or advice are greatly appreciated. I would like to fix this monster and send it to a new home.

[Jon Snell]

"I'm also trying to avoid shotgunning components unless they are obviously bad like the ribbon cable."

The valves are OK as they do not have to look like a light bulb.
I would reasemble the amplifier after cold checking anything that looks iffy.
Then fault find in the normal way.

[glebert]

By OTX do you mean output transformer? Never heard it called that, I always use OT. A beep test on an output transformer isn't going to tell you anything, the secondary side of this monster should look almost like a dead short. My go-to test for OTs is feeding them a small-ish AC wave with a variac and dim bulb tester with no load on the other side of the OT. If you get anything other than a very dim glow of the bulb there is some sort of short.

[Climberkalen]

Thanks for the replies. I'm fairly certain the OT is fine. Not knowing any history of this amp, it was just a lurking fear in my mind. I've seen it used both ways, OTx or OT on different forums.

I didn't do a contiuity test on the actual transformer. Nor was I expecting to test it that way. I was just checking the speaker out jacks. Hearing the faint hum, I was thinking maybe corroded or loose jacks. Meter showed continuity between between tip and sleeve of both jacks. So I pull the board out and do another continuity check. There is no continuity when the 6pin molex from the OT is removed. I haven't saw that before so it was just throwing me off. For shits and giggles I took the probe and stuck it on blue/white lead going to the OT from the power tube board. I get a nice loud pop from the speaker hahaha. So im reasonably confident the transformer is fine. Just scratching my head, wondering why it's showing continuity.

Regarding the 12au7s. I know they aren't supposed to glow like a lightbulb, but they are super dim. Like half as bright as the filimants on the ax7 next to them. I Stuck em in the preamp of my laney and they sounded like a busted fuzz pedal. That laney isn't picky and usually sounds good with any tubes. I know that's not really a valid testing method. But it's a decent indicator in my mind that their on their way out. But for 20$ shipped a pair of 12au7s is an easy, cheap possible fix. It's just an assumption, and if it's not the fix, I have a spare set of tester 12au7s.

As well as its probably all original tubes, svetlana 6550s, sovtek 12ax7s and the three12au7s are ancient looking Jan Phillips. I think they are also an issue because I'm getting nothing on the 6 cathode sense resistors. Like .1mv, reading around here on the forums should be higher than than right?
I need to break down and order a cheapo variac, I know. I also need a tube tester and oscilloscope. I'll openly admit I've got real lucky with most of my repairs. It's usually a glaring issue, burned traces or a cap that blows its guts. If it's anything beyond what I know or understand, I'll send em elsewhere.

In the meantime while tubes ship, is there anything else I should look at.

[The Dude]

If the amp won't bias, it's likely you have an output/amp problem. Please give us voltages on the output tube sockets. This amp has an effects loop, so you can run the send to another amp to check the preamp and insert a signal into the return to bypass the preamp and see if the amp is working. This might help narrow down the problem to amp or preamp.

[Climberkalen]

The Dude, I haven't done that before. So to clarify the procedure; "preamp out/fx loop send" of the ampeg into the "power amp in" of another amp right? Also i assume one or more of the power tubes are toast. I tested a pair on the laney, even tho it's nowhere near the same plate voltage. I'll pull the tubes and stick the meter straight in the sockets and post voltages from them later this evening. I have a rush job on h2o blues driver mod and pro junior bias mod. Soon as im done with the pedal I'll check voltages and the preamp

[g1]

I didn't do a contiuity test on the actual transformer. Nor was I expecting to test it that way. I was just checking the speaker out jacks.

The speaker jacks connect to the OT, so when you measure there, you are measuring across the OT secondary, which is usually very low ohms. When you disconnect the molex, the speaker jack is no longer connected to the OT, so you no longer measure the low resistance there.

It sounds like the 12AU7s are bad, but I think the filament thing is just a coincidence. You can't tell anything by filament brightness, they are either dead or alive. Some you can't see at all except maybe in the dark. If you can feel the tube warm up, the filament is good.

[The Dude]

The Dude, I haven't done that before. So to clarify the procedure; "preamp out/fx loop send" of the ampeg into the "power amp in" of another amp right?......

Yes, that's correct.

[Climberkalen]

Thanks for that clarification there g1. I was thinking the same thing regarding the contiuity on the speaker jack board. I was up late messing with that thing and just double checking I didn't have somthing backwards causing that.

I know the filimants are not an indication of a proper functioning tube. That just stood out as odd. I know that pair is critical for the power tubes so I want them to be solid. I'll follow up this post with meter readings on the power tube sockets. There is most definitely more than one issue going on. Just backtracking my way back from the power section.

Again, thanks for all the help! The info on this forum got me from a box of parts to a 90 percent functional box of parts hahaha.

[Climberkalen]

alrighty then folks, good news. Preamp seems like its working. Ran it into the effects return of my dsl 20. Now the the thing im unfamiliar with. I dont use external preamps, so was the ampeg supposed to be sending signal with just the power on? As well as that, none of the front pannel controls did anything. It was like 11pm so I didn't mess with it too much, but I got a clean sounding signal running through it.

I also took readings on the tube sockets, no power tubes installed, no 12au7s or ax7 in the power board. Here's what I got

V1 pin1 is 0.1mV, pin2 is -3.0mV, pin 3 is fluctuating all over the place. Pin 4 is -209.4V pin 5 is 52.9 but climbing slowly pin 6 is over 600v ( it overloads my fluke) fluke only goes to 600v so im assuming it's 650v like it's supposed to be. Pin 7 is -3.0 mV and pin8 is -.3mV but drops to 0.

V2 is the same readings except pin 5 is fluctuating up and down so I can't get an accurate reading on it.

V3 is similar readings as well except pin 3 drops to nothing and pin 5 is 112v.

V4 is similar to v1 except pin 5 is 129v

V5 is similar readings but pin 3 is fluctuating

V6 is similar except pin 5 is 140v slowly falling

I'm assuming that that not having the 3 tubes not in the power board are throwing the numbers off a little.

[The Dude]

Please recheck pin 4 and pin 5. You say screen voltage is negative 209 and grid is positive 53. Screen voltage should be positive and grid voltage negative. Also, pin 3 is plate voltage, which should be around 660V. And, check from pin 2 to pin 7 for about 6.3VAC filament voltage. You need a meter than is capable of measuring these voltages. Also, here is the schematic:

ampeg_svt-2_pro.pdf

[Climberkalen]

The Dude did my goofy ass read the pinout backwards? I went clockwise with the keyway facing me. That's the same schem I have but much better resolution, Mine is ultra blurry?

[The Dude]

Not sure what you did, but voltages don't make sense even if you did read backwards. There should not be +53V or -209 on any tube pin.

[Climberkalen]

Have I got a molex on backwards that goes to the power tube board? I'll retake readings and double check im on the right pins. I'll also try to get some decent photos of all the quick connections. I may very well have a quick connection going to the wrong place. I used photos online and the schematic to hook everything together.

[g1]

Where do you have your black probe when checking voltages? Perhaps some kind of grounding issue.