MC 240 ReTube or Replace

I doubt that amp technology has changed very much. There is better parts available today. But you're already replaced some parts. A set of tubes is much cheaper than an amp.  BTW I really like the 7581 tubes.

Mcintosh MC240 is one of the best sounding amps around.

You rebuilt the power supply? Did you replace the filter caps and resistors to bias the unit correctly. Loosing a valve is easy to check. Swap left to right on the power tubes after giving them a GOOD visual. Same with the small valves, swap left to right.

3-5000 hours on power valves, 7-10,000 for the small valves. BUT valves do fail.. No matter who makes the gear.. No matter who makes the valves.

Regards

That McIntosh amp is a classic.  If it were mine, I wouldn’t sell it.  They go for big bucks at AC. 

Don't sell it retube it and if necessary do additional work on it, it's worth it.

Fire sale is good, I need another to match my 1961.
Heavy gives good advice on swapping tubes.
you might consider having it gone thru by competent tech: Audio Classics, Terry Dewick, Mike Samea in Michigan can do a bang up modification w HK style regulation, Finally Randy Vikan at Elite Audio Tech on Key Penninsula in WA is fantastic.
Tube amps are dominated by sound of the transformer, find a better output than the 240 ???? Doubt it.
best to you
jim

Yup the power section was replaced with the capacitors.My older than dirt back will NOT allow me to lug around a 60 lb amp these days....
There are internal and external fuses which have randomly blown and to do a lot of further testing I'm going to need to get a lot more of each fuse.........
This is causing my pore ole back to complain!

https://www.amazon.com/Extreme-Max-5001-5083-Hydraulic-Motorcycle/dp/B06XNZ5GV6/ref=asc_df_B06XNZ5GV...

:-)

PM me to offload it cheap, and I help you out.
Alternatively it sounds like it could be a capacitor. Any decent tube person will likely be able to sort it out.
It could also be something with the speaker (like an XO cap), which is presenting a difficult load, or oscillation…
You could switch the RHS and LHS channels to ensure it is related to the amplifier channel.And also which the driving source like a CD player with different interconnects into the unit with LHS/RHS swapped.
I am not even 100% certain it is the amplifier. My amp was also bad until I found it was a bad set of interconnects… Not the ones that I had just soldered up, but the 5 year old store bought ones.

Thanks for the suggestion of lifts....I've used them over the years for several motorcycle builds :-)BUT you still have to get the amp onto the lift And THEY DON'T DO STAIRS WELL :-)

OK I swapped the output tubes side to side, reseated all the other tubes and replaced the internal b+ fuse.I powered it up and listened to it for 2 3/4 hours with no problems....
That has been it's history...  After 3 or so power ups .. a burst of static and a blown fuse.

We'll see what happens next :-(

It sure sounds like a valve or a bad connection at a valve. Clean and tighten the pin pockets. Then take the valves and plunge them into a cotton ball with WD40 a dozen times. Wipe everything off and insert the valves back in the sockets.. 101 valve maintenance..

On that PS rebuild, did you re-bias the the PS, did it come with a little pot? or did it have a fixed resistor?

7581 vs 6L6GC 23watts vs 30watts, your better off with 6L6WGC (military aircraft) or just GC 30 watt valve, tough too.

Regards

As to the biasing I have resistors on hand and used such to achieve proper biasing.The amp has over 1000 hours on it with no other problems.

For anyone following this it's been 2 weeks now with no problems.
So I'm thinking that I may have an intermittent loose connection on a tube socket.
I've also been thinking about the Quick Silver 60 watt mono amps which would give me less than 3dB, and the Mc Gary Audio SA 1 which has less power, of which neither will make an improvement in my system dynamically speaking.

They get dirty, they get loose. They are 55+ years old. I've fixed at least a dozen amps and preamp with cleaning pin pockets and tightening them..

Sold for JUNK and fixed for pennies.. :-)

Regards