A question from a McIntosh novice

[invalidbuffa]

Wow, really great acquisition! :thmbsp: Try what Gary suggests and let us know what happens. From your description it sounds like you might have an arcing tube.

Have you looked inside the amps to see if previous work has been done?

I hope you get them up and running. They are truly magical when used with the right speakers.

Also in the photo the chassis are on backwards (the McIntosh badge should be on the same side as the preamp tubes).

 

[Phototone]

The hum control is primarily for use when you have the attached Macintosh mono preamp such as the C4 or C8. The filament voltage and B+ voltage for these preamps is provided by the power amp (your MC30), and you can minimize hum (for the combination) by adjusting the control. It has little use otherwise. Stand-alone preamps will either have their own hum adjustment (in the case of some tube preamps), or not need such, if they run their tubes on DC filament voltage. If your NOT using a c4 or c8, the hum control is not used. No tube-based higher-end power amp should have any issues with hum, if it is operating to factory specs.

 

[glmoneo]

I want to thank John Iconomou at AudioSalon.net for testing my bad tube and selling me a replacement. His showroom has so many goodies I wouldn't know where to begin.

That said, I set up the McIntosh amps again this morning. I replaced the bad tube and increased the gain to max on both, lowered my preamp volume, and

WOW! WOW! WOW! and WOW!

I played three of my favorite references. What a glorious sound. What a soundstage these threw out! Massive and natural as hell. Then, on the fourth CD, without warning, the right amp just stopped working; only a low hum emanated from the speaker. No sound from the preamp. Once I disconnected the speakers from the taps the hum went away. I just reconnected my Bryston: everything works normally.

Sigh. I guess the shop that "brought these to spec" didn't do a very good job. Any ideas, fellas? Did I blow another bad tube? Should I -- gasp -- replace all of the tubes?

 

[invalidbuffa]

Have you checked to see if they have actually been restored?

Also, when the amp stopped working, did you notice any of the tubes red plating?

 

[glmoneo]

I can only take the word of the repair shop. I'm getting a copy of their work order next week. Explain "red plating"? I'm a newbie...

 

[invalidbuffa]

You could also open the chassis and see if anything looks new, but a detailed work report is probably better. Red plating happens when an amplifier has a bias problem, and the plates of the output tube(s) glow red. The reason I asked was at one time one of my MC 30's went out (actually up in smoke) and it was the bias resistor that had failed. Had I not caught it in time and pulled the plug, the tubes would have likely red plated and the amp may have gone into meltdown.

 

[MX117.MC7150]

Can you get the kind folks at Audio Salon to test all your tubes for a fee? And, replacing with a full set of new is never a bad idea so you know what you've got. Not that expensive a proposition (check the Tube Depot, Tube Store, Doug's Tubes, etc. for pricing) and all the old ones that test good can be your backup tubes. Also, I'm guessing there is a SE FL audio society of some sort that will have a couple of knowledgeable folks sho would help out. If those were mine they would be off to Terry DeWick for proper check out and servicing. They are worthy of the best. Good luck.

 

[glmoneo]

So it sounds like bad tube(s) to you, MX117?

 

[MX117.MC7150]

No way of knowing. But, testing or replacing all tubes is certainly a good place to start. At least that is what I would do. In my tube-o-phile days would always have a complete spare set on hand. A 'learned' trait from some 40 years of tube experience. Hardly expensive unless one is driven to purchase the latest and greatest weird (cryo'd, etc.) tubes or NOS unobtanium. However, Naptown Rob's following suggestion on the tube forum is a good one, albeit, a time consuming one.

 

[Naptown Rob]

Replacing all the tubes seems to me like throwing the baby out with the bath water. You would be spending a fair amount of money for no reason.
If I were you, I would post in the Tubes forum - I'd be willing to bet that there is an AK'er in your area with a tube tester who would be happy to help you. If any of the tubes are a problem, at least you would then know which one(s). If not, then you know it;s a different issue.

 

[Stromung]

I know this is an old thread, so what happened? Bad amps or was it clipping?