MC240 - 12BH7's - should they have balanced sections or is it not as important in it's application?

[Paul K]

Hi!

Just wondering if I can sub in an old RCA black plate that has differing GM readings (perhaps 20% difference) without it affecting the rest of the amp's performance.

Also suggestions about tolerances for all the other tubes with respect to each other and their own individual sections would be helpful!

Thank you!

 

[Pio1980]

You probably can, but best performance would come with the tubes fully balanced.
The 12AX7 in the front end would only affect relative channel to channel balance if badly mismatched, and the affected channel if one side flat.

 

[62caddy]

Not certain which tube model you own but the following is from the MC 240 manual:

The excellent performance that is inherent
in all Mclntosh amplifiers does not
depend on the critical adjustment of bias or
balance controls in the output circuit. The
patented Mclntosh circuit delivers its advertised
specifications without any need for
these controls and is not dependent on
carefully balanced tubes for its performance.

 

[Paul K]

Not certain which tube model you own but the following is from the MC 240 manual:

The excellent performance that is inherent
in all Mclntosh amplifiers does not
depend on the critical adjustment of bias or
balance controls in the output circuit. The
patented Mclntosh circuit delivers its advertised
specifications without any need for
these controls and is not dependent on
carefully balanced tubes for its performance.

Mine is the early model and yes I have read that in the manual. However, I believe that refers to the output tubes and not the small signal ones.

 

[62caddy]

Mine is the early model and yes I have read that in the manual. However, I believe that refers to the output tubes and not the small signal ones.

Correct.

I understood part of your question concerned other tubes sections as well.

 

[Paul K]

Correct.

I understood part of your question concerned other tubes sections as well.

Ha! Sorry about that - I always have so many questions about this unit that I am so in need of sorting out it just becomes overwhelming!

 

[Thocom]

Not certain which tube model you own but the following is from the MC 240 manual:

The excellent performance that is inherent
in all Mclntosh amplifiers does not
depend on the critical adjustment of bias or
balance controls in the output circuit. The
patented Mclntosh circuit delivers its advertised
specifications without any need for
these controls and is not dependent on
carefully balanced tubes for its performance.

I can attest to that. I have 4 old Genelex England KT66 tubes and one is borderline (scumbag Ebayer, ok vent over) the other three are very strong. The amp still sounds pretty awesome.

I dont run it with these tubes for any length, I do have a matched quad, but I suppose I could. Right?

 

[dcgillespie]

With the exception of the 12AX7 input stage, all the succeeding amplification stages in the 240 are push-pull by design, and are either inherently self balancing, or operate with enough local NFB so as to make any mismatch within the sections of a given tube insignificant relative to achieving published performance standards. The qualities of the Unity Coupled Output stage are such that it would take a significant drive signal mismatch to cause any notable performance loss. Assuming that all tube sections are good, it would typically take component failure to produce a drive signal mismatch great enough to cause such a performance loss.

Dave

 

[Pio1980]

Dave is correct on this, as always.
I matched output tubes in a pair of 75 monoblocks by driving the amps to clipping into a dummy load, they looked ok up to assymmetry before matching. Yes, the circuit is self balancing for the most part, until stressed. I also recall swapping out some of the other tubes to restore normal operation in these old tired amps.
For the large part, tho complex, they impress as a bulletproof stable industrial lab bench design.