2. amps-preamps
  3. 169899-137306

MC 240 ReTube or Replace

About a year and a half ago I acquired an old McIntosh MC 240.

I replaced the capacitors and retubed it using:

Genalex Gold Lion 12AU7 gol

Genalex Gold Lion 12AX7 gold

Tube Amp Doctor TAD 12BH7

TUNG-SOL 7581A

 

Now after some 1,000 hours of operation, driving a pair of Salk SS 6m speakers, one channel will intermittently give out a blast of static within minutes of power up and blows the fuse. This leads me to believe I have a tube which may be intermittently shorting.

 

With the MC 240 being almost 60 years old I’m wondering if modern technology has surpassed it.

Thus, the question retube or replace it.

 

Currently my choices are:

Quick Silver 60 watt mono amps

Mc Gary Audio SA 1

Van Alstine Vision SET 400

 

I would appreciate any comments.

TNX



pelletfan
pelletfan OP
14 posts
 
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 :-(
oldhvymec
5,101 posts
 
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
pelletfan OP
14 posts
 
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.
pelletfan OP
14 posts
 
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.
oldhvymec
5,101 posts
 
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
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