by bordonbert Sun Mar 31, 2024 8:21 pm
Aha! A second Fx Loop issue today. That's a record.
Your most likely problem with a faulty Fx loop in an amp which has seen little use is simply dirty contacts on the socket switch wafers. That has to be your starting point. Just spray the Bejeesus out of them with a proper contact cleaner spray. Definitely NOT WD40 or any equivalent, I mean something like Servisol Super 10. WD = "W"ater "D"isplacement. You could try a patch cable just bridging the Send and Return sockets to see if it is the switch contacts before even cleaning them.
Other than that I can tell you that the Fx Loop is fed directly from the main signal line where one of the opamps acts as a driver for the Loop Send. The signal from the Return has its own opamp buffer which is an area of possibility. It is then blended back into the main signal line immediately after. So if the amp is working then the chances are you should have a signal at the Send socket. The only other thing in that path is the -10dB switch and its resistors. Does the problem occur with that both in and out?
Are you able to take the Loop Send signal and feed it into another amp. That would confirm that everything is working fine up to the Send socket. If you can then feed a signal back into the Return socket from a different source, guitar into another amp to use its Fx Loop Send signal or using just a boosting pedal for example, you can check the return side to see if that is the issue. Then bridge the Loop sockets with a patch cable to see if the issue is in the switching contacts of the sockets themselves, bridging with a cable takes them out of the picture, and your testing is complete. You should know where the problem lies then.
It can't be much I can tell you, and the chances are that it is down to the sockets being corroded after standing for a long time unused. Here is a link to a useful site for you:
Tube 50 S-Manual You should find the Tube 100 there too. It will help to be able to have a look at what you are working on.
Do those simple tests and get back with the results and we can see where it leads us.
And if you hang around here you will find I'm not a huge advocate of things like valve rolling. There is a lot less in it than people make out and it is a distraction from things which really do make a difference. The engineering definition and categorisation of valves is not understood at all outside of the engineering profession. Design engineers know the lengths taken in GOOD circuitry to make differences in components like valves irrelevant. For example, the opamp was invented as a means to do away with the active gain element parameters. In a decent circuit at audio frequencies and preamp levels there is absolutely no difference whatsoever between the major opamp types and this can be clearly displayed in microscopically accurate test equipment on the lab bench. But people still believe there are "magical" types which just sound so much better. Too many myths creep in and eventually a new industry is born, just like the after-market hype around getting the perfect valves for "your sound"! Beware of becoming wrapped up in that spider's web. Does your amp sound good with its current valves? If that's a "yes" then don't waste your time trying to "gild the lily" and chase some aural perfection which you don't even know exists. There is an old saying "perfection is the enemy of good". It certainly applies in this case.
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