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In Reply to: RE: MQ-70 driver for 4 pairs 300B PP?? posted by hifipaul on April 23, 2016 at 13:45:04
Hi!
8 x 300b in push pull per monoblock. Whether the Luxman MQ70 driver (12AX7 / 6AQ8) able to drive 8 of them??
Thx!!
Follow Ups:
You'll need a bit of drive for this. Sets of cathode followers will work, but come with a fair bit of PS/parts count complexity. Consider a custom IT. Multi-filar' one each for the primary and each 300B with its own secondary. Split the secondary CT so you can bias them separately.
Now you just need a good PP driver( for this job, it will be a small power amp). A pair of 6V6's, with plate-to-grid local FB will do, and so will EL84. Consider a two-stage amp here, pentode LTP, E-Linear connected to the primary of the IT from taps about 20%( what would normally be U-L locations for these tubes. You can connect them U-L if you wish, or leave them rigged as pentodes.
With a 2-2k5 a-a load on the 300B's you should not need any FB on the power finals. The primary will need to tolerate a third of an amp of idle current.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
A LONG time ago, mid 1980s, I ( with the help of Bob Goodman and Robert Fulton ) designed and built an eight chassis 450 pound P-P-P 6B4G amp, that could run one to three pairs of 6B4Gs. The " Triode Music Amp ". Front end was all 6SN7GTBs, from a 612 VDC double regulated B+ supply.
I found that for the P-P driver stage, EIGHT 6SN7s in parallel, ( 16 sections total ) sounded best to my ear. The amp was fantastic sounding to me. There was plenty of " moxie " to drive the Finals, and as I recall, each 6SN7 section did 12 mA. of standing current.
It is 2016, and today I'd just use highly efficient speakers and build Directly Coupled, two-stage SET Type 45 amps, making it KISS in design and really thought-out in its execution. After all my years of searching and DIY building, its MY conclusion. High mu driver tube, 70 to 100, 100 mu preferred.
Have fun.
Jeff Medwin
Thats 8 6sn7s to drive the PPP 6b4s. Reasonable. Tubenstien wants to drive 8 300bs with a single 6aq8; it ain't gonna work. Although I agree with you about the low power/high efficiency stuff, it does not answer Tubensteins question.
Perhaps a schematic of your PPP 6b4 amp would help. PP4P 300b is what he really wants to build. We should help him with that. I suggested 8 sections of cathode followers, to no avail. Maybe some of your old schematics will help.
Paul,
It was from 1983 and I've looked all over my computer's hard drive, for a schematic, to no avail !! Sorry .
Jeff
That circuit can barely drive a pair of 300bs. 4 pair is out of the question.
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