Epiphone Valve Senior...scratching my head

[Styx0169]

valveseniorjventura4bjpg voltages (1).pdf

I am scratching my head with this trouble and was wondering if anyone had any ideas. I bought this amp and it had no output. I found the output transformer opened. I replaced the transformer with a MOJO768sp. I powered it up and sounded good at low volume. When I turn up the gain and master, I get an oscillation that changes frequency and amplitude as I increase the master and gain. As I increase Intensity, the oscillation goes away.

I removed output tube V4 and the problem disappears. I swapped V4 for V5 and still have the trouble. I removed V2 and still had the oscillation. I started pulling coupling capacitors to isolate. The trouble finally disappears when I remove the R7. I replaced the coupling caps, but still have the problem. The tubes were done about a year ago per the previous owner. I replaced all electrolytics in the power supply, as well as the rectifier diodes. The problem just won't go away.

Please ignore the voltage readings on the schematic. This was the only one I could find. It was from a previous post.

Any ideas? Thanks in advance.

[g1]

Try reversing the wires at the primary of the output transformer. (shown as J6 & J7 on schematic)

[Styx0169]

I swapped J6 and J7, but still have the same problem. I did make a mistake in the original post which I will correct. The oscillation is only present when V4 is installed. I did try moving the wiring around. It changes the oscillation, but any change in master and gain brings it back. Also, removal of the feedback connection at J13 does not stop the oscillation.

I do have an oscilloscope, if it helps.

[Chuck H]

Regardless of the tubes having been replaced according the previous owner I would suspect a microphonic power tube/s with the tube currently in the V4 socket being the offender. Try swapping the power tubes in their sockets and see if the problem follows the tube to the V5 socket or stays with the V4 socket.

[Styx0169]

I have tried both power tubes in V5 with no problems, Chuck. Both oscillate in V4. I've also tried three different 12AX7's in V3. Do you still think the power tubes are still microphonic?

[Jazz P Bass]

[QUOTE=Styx0169;441941]I have tried both power tubes in V5 with no problems. Both oscillate in V4. QUOTE]

It's pssible that there is something wrong with that particular socket.

The socket itself may have a carbon arc or the soldering at the socket & it's components is bad.

Then again the PI may be at fault.

[g1]

You said reversing the phase of the OT primary didn't fix it, but it should make some difference (reversing J6 & J7).
If the phase is not right it may make troubleshooting very difficult.
When J6 & J7 (blue & brown wires) are correct, there should be an increase in output level when R15 is disconnected, and it should reduce level when you reconnect R15.
If it is not making any difference either way, that is a problem.

[Chuck H]

The tube swap was to determine if it was microphonic power tube/s. Since the problem continues to manifest only with V4 plugged in I'll concede that the tubes may not be the problem and I would look elsewhere from here. Like Jazz said, possible problem with that socket or the PI triode circuit that serves V4 grid. And like g1 said, if the NFB loop has NO affect that is indicative of a problem in itself. I would leave the NFB loop disconnected for the duration of the troubleshooting since the amp will operate without it, but with it in incorrect phase. So reconnect it and correct phase if needed after the main problem/s have been solved.

At this point I suspect wiring error/s or a busted trace or pad on the board. Which, BTW is often exacerbated by attempts to affect a repair like replacing components when you don't know if they're causing the problem. It opens the door for other incidental errors that then greatly confuse troubleshooting. We've seen it here MANY times. Unfortunately, many amps that are treated that way end up as shelved projects. I don't know anyone that, if their car stopped running, would start replacing hoses, wires and thermal switches randomly. But people do that sort of thing a lot with amplifiers.

P.S. I found this thread with some gut shots of the Valve Sr. According to the thread this is a working amp.

Valve Senior is in the HOUSE! - Gibson Guitar Board - Page 3

All the plug leads and routing is clearly shown. Maybe it can help. And keep in mind that it's possible someone else already made changes (for better or worse) before you got the amp.

[Styx0169]

I know what you mean about indiscriminately replacing components Chuck, but I did have cause for most of them. I was seeing an odd DC variance with my O-scope @ R7 with no tubes installed. With a closer inspection, I noticed several of the rectifier caps were bulging on the top. Upon replacing them, I was still seeing that variance @ R7, which was passing through the coupling cap, which is why I replaced that particular cap and a few others.

I'm confident with my soldering ability since I've been doing component level troubleshooting/replacement for quite some time for my daily grind. I'm just not as familiar with tube circuits as I'd like to be. I will say that I have learned a great deal from this forum. There's a treasure trove of info here.

I did look at pics of the board and how the wiring was laid out. Everything seems to be as it should be, but I will double check just to make sure I didn't miss something. I'll check that socket, as well as the PI circuit.

I really do appreciate all the input form everyone. I usually don't have this much difficulty isolating a trouble. Thanks, again!!

[Enzo]

Just a thought, I am pretty good at soldering too. I have been doing it for 60 years. But I guarantee I do not assume I cannot botch a joint now and then, and any soldering not directly involved in troubleshooting I avoid. I note things to do later, after it is fixed.

[Chuck H]

I wasn't aware you had that sort of experience. Most posters in your situation don't. Of course bulging filters need replacing. And a lack of decoupling can cause oscillation. Seeing the same DC variance on either side of a coupling cap is also reason to change a component when hunting an oscillation.

Since you got the amp in a not working state you can't be sure someone else didn't screw it up before you got it. The OT may have blown as a result of a modification attempt.?. The Valve Jr. is a VERY popular mod platform and on the link I posted the guys are talking about getting their hands of the Sr. model and modding it (before even hearing it, of course). In other words, it's possible the circuit is altered to a non functional state, and so can't be fixed before it's corrected. Obviously this complicates troubleshooting greatly.

Look over the link I posted and be sure nothing is plugged in cattywhompus. Try to spot any areas on the board that look like they might have been worked over and check those circuits against the schem and chassis pics. If you don't find anything plainly wrong you may be looking for a cracked solder pad, lifted trace or a bad connection.

[Styx0169]

Now, I feel like an ass.LOL. When I initially replaced the OT, I found the oscillation. I took the board out of the chassis to troubleshoot. After replacing the components I thought were failed, I never completely reassembled the amp. I mounted the circuit board with all of it's hardware, but failed to attach the nuts for the gain and master pots that secure it to the front panel of the amp. Once I tightened all of the hardware, the oscillation disappeared.

[Chuck H]

You'll want to touch up the solder joints for those pots and perhaps anything else that looks at all hinky. I've done it myself and eaten plenty of crow here (yum). Good thing no one makes crow taste any better than these guys

This is just in case tightening the physical connections somehow has an iffy connection temporarily making good contact. Because as far as I can tell you shouldn't need to rely on the physical connection for ground. Then there's the lead dress issue. With the circuit half apart there certainly may have been leads running a different path than when the amp is fully assembled. Either way, it's good to be sure.

[olddawg]

Wow... none of us have ever done that before!

[Styx0169]

I've been jamming with the guy I got the amp from for years, so I knew he didn't mod the amp. I should have known better. I worked with microwave amps for cell technology for years. If you think tube amps are finicky...LOL. I'm glad I can put this one to rest after double checking those solder joints. The next project is a bit easier...ground the foot pedal on a '63 Ampeg Reverberocket that I acquired. Thank you all for you're guidance. I'll be digging through this forum to research ideas for a tube amp design.