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I was thinking an output stage like this (lower schematic) but with 6L6's. But use a low mu triode 2C50 (mu 10) instead of the pentode Gary uses. I like the direct coupled from plate to grid with the local feedback, he says the output z of the driver effects the feedback here, low is better which he gets with the ccs in the cathode of the drivers using the current from the feedback resistors. Not sure how this would be best done with a dual triode Rp guessing 3.5K ohms.
The top schematic shows an early version using a dual triode, but he later found that the plate resistors were effecting the feedback.
I would be nice to use a dual triode and keep the output z low if possible while hopefully making it a little more simple.
Better yet would be to not use an input transformer, any thoughts or experiences helpful.
Follow Ups:
I did some work with the 811A transmitter triode... This is a WW-2 era
tube. What made the project exciting is that the grid is ALWAYS biased
above the cathode, and for the most part - conducts serious current.
It was an inexpensive project too... There are also plenty of other good
and inexpensive transmitter triodes eligible for this treatment. The 805 and if you are bold, the Eimac tubes used by Ham Radio folks. The 811A
is inexpensive, plentiful and bulletproof. It's also pretty as it lights up brightly.
Also keep in mind that you 2A3 can also be handed off to unusual modes
of operation other than lowly Class A1. For instance, the Brook 12A
really stands out above the crowd as far as using innovative ideas to get
more power than most ever can out of 2A3s.
I think some of the most exciting stuff out there is Jack Elliano's stuff at Electra-print. He uses special interstage transformer and very low-Z drivers to get 30W out of 300B's..... By driving the grid positive. :)
You still need something to split the phase if you lose the input transformer. The lower schematic expects a balanced input with the phase "pre-split".A pro preramp with balanced outputs would suffice but any HiFi preamp is going to either need a transformer or inverting buffer to split the phase.
Note that even professional amps with balanced inputs still have unbalanced input stages. They either use an internal transformer or differential amp (typically an OPAMP) either of which can be forced into an unbalanced mode yet the amp will still work. In the lower schematic you posted, forcing the input to unbalanced with a jumper wire in the XLR connector would mean half the audio waveform would not be present. It would no longer be an amplifier but rather a half wave rectifier with gain depending on where the bias is!
Edits: 04/19/12
"The diagram below illustrates how this CCS might be employed in the tail of a long tailed pair."This would work just fine. Gary Pimms CCS feed by a negative supply (as shown in GSH's post) will work even better.
"In the lower schematic you posted, forcing the input to unbalanced with a jumper wire in the XLR connector would mean half the audio waveform would not be present. It would no longer be an amplifier but rather a half wave rectifier with gain depending on where the bias is!"
All he has to do is connect the undriven grid to ground through a cap and turn it into a long tailed pair phase inverter.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/19/12
a
GSH,
You might want to look at the Fi Primer 6CK4 PP for a really simple schematic along these lines,
http://tinpan.fortunecity.com/saints/668/primer/6ck4-pp.gif
You can forgo the phase splitting input transformer by just accepting the phase splitting at the input jack. The splitting input transformer is a better idea on many fronts, but you don’t need it.
To further simplify you can substitute a choke under the driver cathodes in place of a CCS to avoid the negative supply.
Using a 2C50 is possible if you really have enough gain upstream. But you will need to pay attention to what you need for grid swing in your design. Adding a bit more swing than you need can be a good thing.
As you experiment I don’t think I would sweat driver output impedance, just play around till you have something you like, then start fine tuning.
Matt
FWIW, I built this about 7 years ago with EL34's but I did not include the servo part (thought it was too complicated). I did use the 6AU6 as driver. I tried it with a LL1676 (IIRC) input transformer but my preamp didn't have low enough Z to drive it well. Switched at one time to TVC and that worked great.
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