McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions...

Posted by Audible Nectar (A) on September 26, 2006 at 06:15:26

Questions from the POV of a "non-circuit genius", but first, a little background....

I own two pairs of McIntosh MC-30s and have been collecting NOS brands of tubes to run these long into the future.....RCA and GE 6L6GC of identical construction. I have had all of these tubes tested on a "maxi-matcher" (-48V grid and 400V on plate) and have grouped them into pairs based on current draw readings (in ma). Most of my tubes test between 19 and 35 ma at -48v/400V plate on the current draw reading.

I understand that the MC30 is self biasing and can work around output tube matching to a degree...but I also wonder if I am selecting tubes out of a group (or am ordering matched quads from a dealer) what should be used?? Like with the Groove Tubes outputs for example......they rank tested tubes on a 1-10 scale (higher numbers are higher current - right??).....so what does this amp prefer to see?

I also understand that this can be "tweaked at the bench", but even in that case wonder what tubes I should be ordering/using in terms of readings.

Also: Can the MC-30 use 12BH7 as well as 12BH7A? I have a nice stash of 12BH7 early RCA blackplates (single wing) and wonder if those are OK to use vs. 12BH7A.

Finally: I have read here and in other places that for the last 12AX7 (next to the rectifier....runs the bias?), it is ideal to simply use a 12AX7 that is effective for "current", instead of using/wasting the good Tele/Amperex tubes for that last slot. Any suggestions here? It seems to be a slot that runs that tube harder - thought it would be a good idea to seek out a better suited tube rather than wasting my best 12AX7's in that slot.

If it matters, these are the later MC-30s (serial numbers 22XXX and 5Exxx - as well as a pair of 6Exxx).

Thanks in advance for reading - and I apologize if "tubes" was the wrong place to put this......thought it might get good look/exposure from the tube inmates here.

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Topic - McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions... - Audible Nectar 06:15:26 09/26/06 (16)

Re: McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions... - jdiamantis 12:56:54 09/28/06 (0)

seriously, think about a CCS on each splitter-driver - Timbo in Oz 15:33:57 09/27/06 (4)

Re: seriously, think about a CCS on each splitter-driver - Zung 11:49:25 09/28/06 (3)

yeah, you can get away with a good deal less NFB with A CCS on small signal tubes in diff'l - more so with - Timbo in Oz 19:56:07 09/30/06 (2)

Should I bother to do the calculations, on the triode connected amp? - Timbo in Oz 14:54:35 10/04/06 (0)

Triode-connecting a Mac? - Zung 08:37:57 10/01/06 (0)

Re: McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions... - Zung 14:40:27 09/26/06 (4)

Re: Fixed bias - Jim Womble 18:00:24 09/26/06 (1)

Re: Fixed bias - Zung 12:06:36 09/27/06 (0)

Re: McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions... - Audible Nectar 15:37:50 09/26/06 (1)

Re: McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions... - Zung 12:14:04 09/27/06 (0)

Re: McIntosh MC-30: Detailed 6L6GC, 12BH7, and 12AX7 questions... - dubai2000 11:52:13 09/26/06 (0)

There's not much to these gems.... - doodlebug 08:40:33 09/26/06 (3)

Re: There's not much to these gems.... - Audible Nectar 12:36:32 09/26/06 (2)

Finally!!! - Sector-7G 13:48:00 09/26/06 (0)

Re: There's not much to these gems.... - Jim McShane 12:44:30 09/26/06 (0)

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