Nearing end of my rope

Just back from a 10 day vacation at a nice hot southern location near the equator. Was really looking forward to listening to my M125's again. Flick the switches in wonderful anticipation, only to be greeted by huge major hum. Before I left both amps were completely silent/working perfectly. Now I'm getting this huge hum. All I did was plug the power strip back into the wall (unplugged it before I left).  Tried another power strip thinking maybe a loose connection but no change. This out of the blue crap is driving me crazy.

I'd check the address and make sure you are in the correct home. Beyond that, is the humming coming uniformly from both speakers? If that is the case then perhaps the source? If from just one of the speakers, then perhaps one the output tubes went south with you? Are you running a quad per, if that is the case you could try running only two tubes and see if the hum desists? Perhaps grab two of the tubes from the non-humming amp and try them in the suspicious amp. Beyond that, maybe some sort of massive AC interference. Tinker.

If there was no hum before you left there should be no hum now.
However I have come home from vacation only to find out something was not working as it should be and simply had to figure it out....with common sense.
I would disconnect everything and then re-enter each piece one at the time starting with the speakers and power source.
Unplugging and then re-plugging evertything will guarantee that there is nothing lose.
I had a slight hiss from my speakers after getting the m125's but it only could be heard if i was literally a foot from the tweeter. Once I purchased a manual surge line interruptor the even that mild sounding hiss disappeared.
If your amplifiers were running properly before I seem to doubt it's the 125's.
My 2€
Good luck

Sounds like a ground loop. Have 90 minutes free? Try this process:

1) Float the power strip's ground with a 3-prong to 2-prong adapter. Make sure all your gear is plugged into that strip
2) Check for hum. No hum? Done. Hum? Continue with step 3.
3) Unplug one unit from the power strip and plug it into the wall directly. All other units stay plugged into the power strip.
4) Check for hum. No hum? Done. Still hums? Go back to step 3 and isolate a different unit. All other units stay plugged into the power strip. Repeat this process, one unit at a time.

Still have hum? Disconnect your preamp outputs from your amps and install shorting plugs in both M-125 inputs. Power up the amps. No hum? Sounds like a ground loop. Remove the shorting plugs and plug your preamp back into the M-125s. Go back to step 1 and do the process again. This time really plug everything into the power strip... even your phono stage and active FM antenna. Everything. Everything. Still have hum? In both amps? Not likely--unless... do you happen to live in an apartment or condo?

One amp hums, other doesn't?

Mark your tubes with a Sharpie L1, L2, ... and R1, R2, ... (Sharpie wipes off with lighter fluid.) NOTE: If you are running vintage Genalex, diamond base Telefunkens, Western Electric 350B, or similar tubes skip the Sharpie and lighter fluid steps.

a) Swap rectifiers between the two amps, hum doesn't move? Put them back where they came from.
b) Swap input tube sets between the two amps, hum doesn't move? Put them back where they came from.
c) Swap output tube sets between the two amps, hum doesn't move? Put them back where they came from.

If the hum moves you have a suspect. If the hum doesn't move it would be reasonable to begin a genuine debugging process.

Check all cables.

I recently had a bad hum magically appear with my VTA ST120 system. Turned out one of the RCA cables had come loose from the preamp to the amp. An easy fix for me. Good luck with yours.

Mice.

One of the reasons I'm so glad I selected these particular amps is the wonderfully helpful community that's out there! Thanks for the suggestions!

I've managed to isolate it down to one of the two amps and have eliminated tubes as the issue. Next step will be to go inside and have a look around. Thinking maybe a dodgy ground somewhere. Darn gremlins!

Tracked down the issue, loose RCA jack. I could rotate it with my fingers. Is it good practice to locktite these nuts to prevent them from backing off? And others where grounds are attached?

LeGrace wrote:Tracked down the issue, loose RCA jack.  I could rotate it with my fingers. Is it good practice to locktite these nuts to prevent them from backing off? And others where grounds are attached?

Loctite® is a non-conductor. So, used sparingly or in locations where it is not in the conducting path is a good thing. Note that it comes in a number of grades, from pretty soft to "holds forever". Go for the light-hold material.



https://tds.us.henkel.com/NA/UT/HNAUTTDS.nsf/web/8BCC6C1313D95F87882571870000D818/$File/564-EN.pdf

Just gently crimp them from time to time a little deoxit works wonders

Also careful use of some pliers can tighten up the outer ground connection on the RCA plug. I think I would try this before using Loctite.

I'm glad it turned out to be a simple thing and we seemed to be on the right track with the cable suggestions.

Just gently crimp the cable ends and rub a little deoxit on the jacks.

CrO2 is quite conductive.

http://www.tandfonline.com/doi/abs/10.1080/10408437708243431

Certainly enough to be troublesome - with potential from hum-loops or rF rectification if contacts are not very tight - ask me how I know this!

That aside, KEP nuts, star washers,knurled lock washers and plain lock washers are all good (but in decreasing order) for maintaining good contact and preventing of nuts from loosening.

I think the OP was referring to an RCA jack that had worked its way loose over the years. Most VTA amps use a swaged device on a holder that is either riveted or bolted to the chassis. And in this case, KEP nuts or star washers would/should be used. These less-costly-per-each jacks are stamped and assembled without solder, relying on deformation and friction to hold together, with the tabs integral to the contact body. This is the sort that *appears* to be used on the VTA 70 & 120, but at least 4+ steps up from OEM Dynaco.

But on the M125, I believe that the one below is what is used.

https://www.parts-express.com/Data/Default/Images/Catalog/Original/091-1120_HR_0.jpg A good one. As you can see, the ground tab needs to be kept tight - so a little bit of Loctite is a good thing. Note also the nut is not available from the outside. Lockwasher/star washers are absolutely suggested for this application, and yet they rarely come with the jack itself.

Back in the day, they were like this:

https://www.parts-express.com/Data/Default/Images/Catalog/Original/091-1120_HR_0.jpg and came in singles, pairs and banks. Each piece was stamped in place, but the tab was integral, not a separate piece.

A few years ago when I had the PH-10 and the PH-12 with hum problems it was never suggested by Roy to try shorting plugs on the unused RCA inputs and at the time I had never heard of them. It may have solved a lot of problems if they were suggested and tested. But it's kind of odd that the Akido AE-2 Tube Buffer that I use as a preamp when I'm not using the Cary PREAMP went together without a hitch and there has never been a hum problem ever. With or without shorting plugs. That being said as a safeguard I'm using shorting plugs on every unused RCA input.

For fasteners such as the RCA nuts, XLR nuts etc. I like a dollop of hot glue. Stays sort of flexible but holds with enough force to prevent loosening. Easily removed, unlike Lactate type products. with a fingernail or dental probe.