Both MK-III monoblocks started to hum

I am running out of ideas...

Pair of rebuilt MK-III monoblocks - both were rebuilt about two years ago - new stacked capacitors, electrolytic capacitors, switches, RCA connectors, binding posts, output tubes, etc. Both have 5U4 rectifiers. Tubes are Gold Lion KT88 and RCA 6AN8A. I tried different tubes (6AN8 and 6550) - no change. Worked great for a couple of years. Dead silent background.

A couple of months ago both started buzzing loudly when cold (from the speakers), then would suddenly stop after 20 seconds and go silent. Would play music for hours with no problems. Bias is rock solid in both amps.

Today is the first day when they don't seem to stop buzzing... Both of them!

I disconnected preamp (PAS-3), still the same problem. Moved one of them to my bench - same problem on the bench - connected to a source or not - no change.

The only thing that I found a couple of weeks ago is that one had a cracked negative lead on repro RCA connector. I replaced both RCA connectors with modern connectors.


Can someone suggest what could have caused this? I mean two independent monoblocks failing at the same time - how is that possible?!

I shorted the input for monoblock on the bench, powered it up, and it is silent.

Same thing with the other monoblock - shorting the input makes it very quiet in background.

Yiper, seems to me something UPsteam; check all your grd leads on the interconnects. I would
ohm out ALL the cords going to the amp; hope this helps, Dale

I recently put my old plate amp subwoofer into my system. Speaker Level from VTA-120 into Subwoofer. Speaker Out of Subwoofer into
Speakers. Adding the Subwoofer created audible hum. Hum goes away if I unplug the subwoofer. Ended up turning down the subwoofer to the point the hum doesn't bother me. Not much subwoofer output, but will have to do until I replace the amp on the subwoofer. Currently short of time and money.

My question is - how do I fix my monoblocks or do they need fixing? How come connecting preamp does not quiet them down?

I just opened my PAS-3 and will replace the RCA connector backplane with the brand new one, but I doubt that it will help in this situation.

Hello!


Would you describe the noise as truly buzz? Like an 'angry insect' buzz?

I have been struggling with both 120 Hz hum and (angry insect) buzz in my system. When using my tube preamp (home-built) with my Dyna ST-70, I was only getting hum and it was tolerable. The preamp uses a grounded IEC cable (grounded to the chassis). However when I tried using a solid state amp (ungrounded power cable) I would get a terrible buzz. I put my PAS 3 into service with the same solid state amp and the PAS 3 has a switch that will float the ground wire from the wall socket (or not). I can use this switch to eliminate the buzz.

So my advice is to check the grounding of the PAS 3. If possible float the ground in your preamp and see if that helps. Be careful though, stay safe. If you do float a ground dont touch the affected chassis while conducting the test.

Best of luck!

David

In my last house I had hum with grounded power cord gear. I started using 3-2 wire adapters as 'ground lifters' (safe or not).

One year we had our electrical service upgraded from 60A to 150A and the electrician said we needed new 'drop' wiring from the electric pole outside (I don't remember if we had a transformer immediately behind the house, but no matter re: the end result).

When all was done, he told me the ground (earth for some people) conductor was totally corroded and not connected.

My audio gear was quiet after that and I could use grounded cords with no difference in hum (gone/gone).

Murray

Hops wrote:I recently put my old plate amp subwoofer into my system.  Speaker Level from VTA-120 into Subwoofer.   Speaker Out of Subwoofer into
Speakers.   Adding the Subwoofer created audible hum.   Hum goes away if I unplug the subwoofer.    Ended up turning down the subwoofer to the point the hum doesn't bother me.   Not much subwoofer output, but will have to do until I replace the amp on the subwoofer.  Currently short of time and money.

The best way to deal with this is NOT a new plate amp for your sub. Nope. Get yourself an active crossover. You can find a used Behringer crossover on EBay for about $125 or so. $200 new at Musician's Friend. That would be vastly better than what you're doing now.

But if you REALLY want to do it right, you'd drop about $500 on a miniDSP DDRC24 and UMK-1 measurement microphone. The DDRC24 comes with a DIRAC DSP EQ processor and a license to run the DIRAC room analysis software. You measure the room and then create a filter to specifically tune your whole system and room, to get the precise response you want. Makes a HUGE difference. It is more money than the Behringer solution, but is very much worth it if you can afford it.

Getting a new plate amp isn't going to begin to compete with either of these fixes.

ViperZ wrote:I shorted the input for monoblock on the bench, powered it up, and it is silent.

Same thing with the other monoblock - shorting the input makes it very quiet in background.

Can you connect a source directly to eliminate it being the PAS

You do not mention if the "rebuilds" included the installation of grounded 3-wire AC line cords. Also, what happens if you unplug the 6AN8? One possibility is that both amps may have been connected to something with a "hot" chassis and the grounded line cords did thier job and burned up some ground connection/PC board lands on the MK-IIIs, thus preventing you from being shocked.

Otherwise, it sounds like your problem is in the front end somewhere. This whole thing smells like a bad ground somewhere, but it's interesting that it happens with both units at the same time. That would suggest either two problems exactly the same with both units, or perhaps a bad ground in your house wiring as previously noted.

But is the can cap soldering of the chassis actually good? It's amazing how many techs to NOT have large soldering irons that can not put out enough actual HEAT/BTUs to do the job right.

If the can soldering is good, then the next thing I would do, as a test, is to ground one of the amps to a cold water pipe by literally connecting a piece of wire from your bathroom or kitchen sink's cold water pipe to one of the amp chassis. Connect to the cold water pipe FIRST. Be careful; you might even want to wear pair of gloves when you do this; I don't want to get sued for trying to help you If the hum goes away, then it's time to stop right there and call a licensed Master Electrician and have your house wiring checked, starting with the ground rod.

If proper grounding is not the issue, I would be looking over ALL the other connections on the amp for starters. Then I'd be looking at the "new" filter cap cans. I say this because IF it's component failure in both amps at the same time, THOSE would be my third, though less likely, suspect. Even IF the plate supply filter for the preamp stage was bad, I don't think shorting the input would eliminate the hum as you stated. I mention this because I've had a LOT of trouble with the cans supplied by CE Distro, so much so that I was getting them from a different source, Dynakit Parts. However, Kevin at DKP recently told me he too was obtaining some of his can caps from CE. So I'd call him and confirm his source before ordering any if needed. Finding good reliable can caps is SO difficult now that my last few McIntosh customers elected to have me disconnect the old cans, leaving them in place, and replace them with good old (new) Sprague axials under the chassis.

All that being said, you should also, of course, measure the resistance values of every resistor in there. But again, it is very unlikely that the same two resistors would drift up in value on two different units at the same time. But confirming resistor values is always a good thing to do, especially with old carbon composition resistors. I believe there's less than 20 or so of them in there.

Though I rarely post here, this has really sparked my curiosity. So please DO let me know how you make out.

Good luck and best wishes.

Jack Donahue
Donahue Audio

Hi Viper, a few years ago I rebuilt a Dynaco ST-35. I had a loud hum when all was finished.I even sent my amp to Frank Van AlStine to see what was up.I put in nice new gold input jacks and I forgot to ground them to the chassis. The old Dynaco kind were grounded to the chassis by the way they were built.The new ones have nylon or plastic washers to isolate them from the chassis and must have a separate wire which the manual does not show to ground. A stupid mistake by me which cost a lot. My amps been singing fine ever since....Mark Korda

Mark Korda wrote:Hi Viper, a few years ago I rebuilt a Dynaco ST-35. I had a loud hum when all was finished.I even sent my amp to Frank Van AlStine to see what was up.I put in nice new gold input jacks and I forgot to ground them to the chassis. The old Dynaco kind were grounded to the chassis by the way they were built.The new ones have nylon or plastic washers to isolate them from the chassis and must have a separate wire which the manual does not show to ground. A stupid mistake by me which cost a lot. My amps been singing fine ever since....Mark Korda

I'm pretty sure these were ground screw related problems. Such a pain!

make sure the input jacks on the MK3 amps are isolated from chassis ground.
I know it seems it wouldn't make any difference, but it does, I had that same exact problem a few days ago.
Replaced the cheap RCA jacks with isolated RCAs, the ground connections go to ground on the driver board,
not to chassis ground (even though they are eventually connected somewhere). Made a HUGE difference in the hum problem - gone !

Hi tubes4hifi,, that was good to know.Iwas told that I did not ground my RCA jacks but I didn't notice or forgot where the ground was connected.Is this a good example of a ground loop problem? I

No, this was not a ground loop problem, but rather a lack of ground. As described in posts above. It was poor contact between board ground plane and the chassis ground that is done via a screw. My chassis were painted by previous owner, which screwed up that ground connection. After I installed modern RCA connectors, they completely eliminated negative connector connection to chassis, so it was relying purely on that ground screw. I cleaned off the paint, added contact cleaner, and tightly closed the screws. No more problems.

Ahhh, once again a case of an electronics design Engineer (at Dynaco) relying on a screw for grounding. Is there at least some kind of lock washer under it? Looks like there might be an internal tooth #4 LW under there in that pic. Once again, me trying to suggest the high tech solution to a simple physical/mechanical problem.

Yes, there is a lock washer under that screw. I have no idea why they wouldn't run a short wire from that board to a nearby ground lug on the chassis. It works now, so I'm not worried.