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Hello!
Can we adopt MQ-70 driver stage for 4 pairs 300B in push pull (B+ 400v @80ma for each tube)?
Follow Ups:
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.
Hi!
8 x 300b in push pull per monoblock. Whether the Luxman MQ70 driver (12AX7 / 6AQ8) able to drive 8 of them??
Thx!!
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.
I'VE DONE WHAT YOU WANT TO DO.
Why don't you simply just get yourself real efficient speakers, so you can build minimally compromised LOW POWER tube amplifier designs?
You can eliminate two cap couples, phase splitting, paralleled outputs, and the questionable sonics of 300Bs !!
Life is too short to be implementing with lesser paths.
Jeff Medwin
obviously jeff if he's building push pull parallel he's not looking for 1 watt. :)
I think the choice of speakers is a great matter of taste. I don't care for high efficiency speakers and whenever I mention it, the response is often that I haven't heard the good ones. I've heard horns, Altecs, Lowthers, and more. I been to the Serious Stereo room at RMAF & VSAC several times over the years. They don't sound natural to me. Sorry to be divergent. No offense intended. Just my opinion and it drives my amplifier building choices.
John
De gustibus non est disputandum
There is also a limit to achievable speaker efficiency. I have yet to see anything within an order of magnitude of what is required for delivering adequate SPL's with these wee SE amps( or even not-so-wee)...:)
So far, nothing has sounded so good to me as a Class A, PP amp operating with adequate headroom. The current system runs to nearly 100W/side and is tri-amped around a Rane AC23B crossover. Keeping the amps out of clipping is worth quite a bit to me...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
No problem. I have actually heard what you heard.But I can tell you this, which you seem to have missed - you do NOT always get to hear things at a show like RMAF as best as it would be when set up leisurely at home !! I, just like you, have often been disappointed at RMAF shows, but that is seldom the case with similar equipment set up leisurely at my home.
Its a major job for a very small outfit to do a display room, and "strut their stuff", after transporting their gear over several states, getting little sleep, and having FAR TOO LITTLE time to adjust the speakers, and set up the components to the hotel room and a new environment !!
Good audio decisions take time and experience. It has taken me a decade ( ten years !! ) to figure-out how nicely a two stage DC SET amp I DIY construct can play.
I can tell you, I am generally pleased with what I am getting, but - not fully sold yet on A7 VOTTs, because my system is not quite done. I am still working on multiple fronts, such as speaker mass loading, crossover tweaking, and amplifier film cap tweaking. Within two months I will have a better handle. Yeah, its tough. Also fun, when it approaches perfection.
I so far have concluded "ALTEC tells the truth", and as such, it is incredibly intolerant of even the most minor mistakes in set up. Its taking " everything I know ", truly all my audio experience, to get it to sound the way I would want it to play. A challenge - for sure, but that is what makes it fun also.
'Have a nice journey for yourself !!
Jeff Medwin
Edits: 04/23/16
Jeff, I agree with almost all that you write, but bump on the fact that its always about 'you'.
Subjectivity is after all no more than subjective.
Shane
Each phase of the driver stage has to be able to drive the Miller capacitance of 4 300Bs.
That will take a stage with low output impedance and a lot of current delivery capability.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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