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RE: MQ-70 driver for 4 pairs 300B PP??

Posted by hifipaul (A) on April 23, 2016 at 13:45:04

In Reply to: MQ-70 driver for 4 pairs 300B PP?? posted by Tubenstein on April 23, 2016 at 08:03:03:

You will need a DC coupled cathode follower to drive each 300b. That means 1 extra dual triode/PP 300b pair, + all of the associated caps, resistors, and PS oomph.

If you really want to do this circuit, it can be done; but the parts count will go way up. The output xfmr will be huge.

Is it 8 300bs/channel or 4. Either way that's a lot of 300Bs for a stereo amp. If it's 8; than the output xfmr will be ~ 1.2kohm at ~ 1/2 amp.

Power xfmrs that can deliver 400v at 1/2 amp (3/4 amp if you want to derate it) are big and expensive. You will need 12 amps of filament power at 5v, spec it at 18 amps.

The feedback loop will need to be redesigned for the new output xfmr.

You will need at least 2 chassis / channel, maybe 3.

8 300Bs will dissipate 200 watts (bias) + 60 watts (filament) = 260 watts. Add on ~ 40 watts for the rest of the circuitry and you have a nice 300 watt space heater. If you meant 8 300Bs / channel, then you'll have a nice 600 watt space heater.

The driver circuit is not the issue, that's been designed. The rest of the amp is a bit more complicated.

Should you build this monster, please invite me over for an audition.

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Schiit Audio

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