Mm 210-65

[tommy d]

Hello All,

My MM 210-65 "sags" on power once in a while - maybe for 3 seconds and then comes roaring back to life and then works fine for the next hour or so. I've noticed it more often when I'm playing hard, stop for a beat and then start again.

In all other ways the amp works and sounds great...just less power for a few seconds occasionally.

It is the 12AX7 model and has been recently retube but not biased to my knowledge - I bought it recently with the new tubes.

Recomendations of where to start trouble shooting would be greatly appreciated!

Thanks in advance Valve Men!

[Alex R]

Could be a lot of things - but try something quick and easy first. These amps had quite a few heavy components that stand off the board a little way on their leads - to help dissipate heat. Big white rectangular resistors especially. They waggle around and the leads fatigue and break, or solder joints crack (sometimes invisibly). See if you can move any of them. Or even simpler, give the amp a few whacks and see if the issue is motion-related (in which case the broken lead/bad solderjoint diagnosis is very likely the cause).

Alex

[tommy d]

Sounds good to me, should I also check the caps too? I read in another thread that the old ones are silver mallory and should be replaced.

Thanks for helping a noob

[Alex R]

I'm pretty noo myself & I'm sure other guys will have useful suggestions, it's just that I've had in a couple of MM amps with this fault recently - strikes me as an unreliable design feature.

As to the caps, I'd say if it doesn't hum then that's not an immediate priority, though there will come a time when the electrolytic caps should be replaced as they are understood to have a limited lifespan of about 25 yrs. The silver Mallory can caps that sit above the chassis are not known for being particularly long-lived, hence the comment, but sometimes electro caps will last 40yrs or more and still keep the hum down.

Alex

[Amp Kat]

It's not so much the hum that's causing it but the low ESR that the caps are discharging to fast not being able to supply the voltage that's being taken out of it. Think of it as a big tank of hot water and when several people are taking a shower the hot water runs out and doesn't come back until it heats up again. Same thing voltage runs out faster than it can be replenished. Those caps are 40 years old and need to come out. Are there white or yellow build-up like lumps on the end of the positive side. That's a sure sign that they are toast. While your at it replace as many of the blue E-caps on the main board and check your rail voltage of the op-amps for +/- 16 volts.

[tommy d]

Ok, the amp doesn't hum at all. I like the cold/broken solder idea and it is an easy fix and leaves a small footprint in terms of introducing new variables.

I'll run the soldering iron around any suspect joints and see what happens. I am in academic hell until the 15th... So I wont be able to report any progress or failure until then.

Thanks again.

[this post should be above the other post]

[tommy d]

Thanks AmpKat! There is no substitute for experience...

Change of plan: I'll run the soldering iron around to correct for physical aging and replace a bunch of caps to correct for chemical aging.

Your analogy of insufficient voltage makes a lot of sense and was what I was imagining when I asked about the caps, but I was just guessing.

Do you have a parts source recommendation?

[tommy d]

update!

I found a cold solder joint on the tube socket and repaired it. I haven't had a chance to torture test yet but the weekend is here and volume will reign.


Thanks for the advice Alex R. - I knew you were on to something when I smacked the hell out of it and it returned to full power.

Still gathering caps...

[tommy d]

Enzo! Cap job complete (all eCaps replaced with Atoms). Sounds really good , has much greater bandpass and is dead quiet up to about 7 on the master with the volume fixed at 5.

I put in a pair of Eminence RamRods (buy USA!) and got just what I was looking for in terms of sound: bright but not harsh with a "funky" mid high that sounds classic '70's with a bridge or bridge/middle pu on a strat, bass to mid is strong and flat. These speakers can take finger-popping EAD strings no problem and are excellent overall for the twang/surf styles. Kind of stiff for blues but a variac break-in and a little acetone should fix that. I haven't done extended "ear-plug" torture testing with the new speakers because I want the caps and speakers to settle in first.

BTW Mouser electronics out of Texas is excellent and a joy to deal with. I would put Mouser on the same level as Newegg.com for excellent shipping, fair prices and superfast turnaround: the best! Texas --> Michigan three days UPS Ground: quality baby!

Thanks guys for your great advice and insight, I now have a thirty year old tube amp looking inside and out that I really love for less than $550.00 with new caps, new tubes, a tuneup, (new reverb tank soon) and new speakers.

[Amp Kat]

You did the right thing and it will last a long time now that the new caps are in it. It's a no brainer but sometimes people say well it's not humming and don't change them only to come back a month later saying they have serious problems. Nice job dude !

[tommy d]

Thank you AmpKat for your kind words and encouragement, sometimes a little help is all you need to succeed and do the right thing

I always read the senior guys posting "change all the ecaps", so like a good tone soldier, I followed orders!