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Old 15th November 2013, 09:38 PM   #11
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Have tried direct coupling with 6Bl7's gets very complicated with 4 tubes, Best to stick to two tubes. never tried it with Mosfets.
Perhaps I wasn't clear enough. DC couple enhancement mode MOSFETs to the tubed small signal circuitry and use individual caps. to connect the buffers to each of the O/P tube grids. If you run an IRFBC20 source follower with a drain current of 20 mA., expect the O/P impedance to be well under 100 ohms. Multiple smallish grid to ground resistances will not be a problem.
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Old 15th November 2013, 10:20 PM   #12
mskl99 is offline mskl99  United States
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Originally Posted by multi View Post
even made a 12 x 807.
Holy Cow, what primary impedance did you choose for that one?
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Old 15th November 2013, 10:22 PM   #13
Matt BH is offline Matt BH  United Kingdom
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Hey Eli, This exactly the way I built a multiple 6l6oid amp a while back. It used Tubelab's and Chrish's design. Seperate FETs for each tube, seperate bias pot each and seperate coupling cap to the previous long tail pairs anodes for each side. I used an anode choke in mine to get enough swing to drive the outputs in triode with my supply. I got 100W reliably from four 5b25x types in triode.

I would say that seperate bias is a must in push pull parallel output stages.

6L6GC AB2 Amp

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Old 15th November 2013, 10:30 PM   #14
Matt BH is offline Matt BH  United Kingdom
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Sorry, I see you meant the other way round. Yes a good approach to drive the increased miller capacitance of multiple pairs. I think sand is cheap enough to use seperate FETs for each tube and enjoy DC coupling there.

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Old 16th November 2013, 12:47 AM   #15
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Originally Posted by Matt BH View Post
Sorry, I see you meant the other way round. Yes a good approach to drive the increased miller capacitance of multiple pairs. I think sand is cheap enough to use seperate FETs for each tube and enjoy DC coupling there.

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Matt.
There are pluses and minuses to both approaches. IMO, the key thing is that FETs can be terrific "spear carriers" for tubes. Think heaterless pentode.
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Old 16th November 2013, 06:22 AM   #16
tinitus is offline tinitus  Europe
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Originally Posted by Eli Duttman View Post
DC couple enhancement mode MOSFETs to the tubed small signal circuitry and use individual caps. to connect the buffers to each of the O/P tube grids.
a small PCB with multiple taps
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Old 16th November 2013, 10:56 AM   #17
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by multi View Post
Changed the commercial amps to separate caps and no more trouble.
good to know


Quote:
Originally Posted by Eli Duttman View Post
Perhaps I wasn't clear enough. DC couple enhancement mode MOSFETs to the tubed small signal circuitry and use individual caps. to connect the buffers to each of the O/P tube grids. If you run an IRFBC20 source follower with a drain current of 20 mA., expect the O/P impedance to be well under 100 ohms. Multiple smallish grid to ground resistances will not be a problem.
sorry that I'm a bit confused (probably my english)
but direct coupling is not just like a walk in the park

are you really suggesting a DC coupled buffer for each tube ?
or I have misunderstood, and you meant 'just' one for each 'side', and still use multiple/seperate coupling cap to each tube
(and so basicly the mosfets buffers are mostly to ease the harder load/drive)
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Old 16th November 2013, 06:55 PM   #18
tinitus is offline tinitus  Europe
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stumbled over this old thread
may be worth a look

IRFBC20 as source follower drivers for EL34s in AB1 PP

still searching for that schematic

Quote:
Originally Posted by Eli Duttman View Post
If you run an IRFBC20 source follower ...
ah, ok, got it ... like in tube 'cathode' buffer

configured like a differential tube driver
and a balanced preamp

sorry, I seem to be on my manic tube ride now
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Old 16th November 2013, 09:03 PM   #19
tinitus is offline tinitus  Europe
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still searching for that schematic
he, found something ... balanced/symmetric cathode follower

but still no source follower mosfet in there
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Old 16th November 2013, 09:22 PM   #20
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DC coupling "N" channel enhancement mode MOSFET source followers to tube plates IS a walk in the park.

The phono preamp schematic I've uploaded illustrates how it's done. Obviously, the gate is at the same potential as the plate it's tied to. The source sets up a few V. below the gate, but it's OK to ignore that small differential. Select a rational drain current value and use Ohm's Law to compute the source load resistor.
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