Diff drive

[bigwill]

Latest revision, swapped CCS up to the anode

[smoking-amp]

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Re-analysis of cathode Z (assuming no grid currents in the driver):

Either variant of the 2nd version using cathodes for neg. feedbacks has a linear (or constant actually) impedance at the cathodes.

The CCS in the cathodes variant has 1/gm in parallel with rp. They sum to a constant Z because of the Mu relation between their magnitudes and the -1/Mu relation between the AC signals on the grid and plate. But there is still the DC impedance here. One can also put in a cathode to cathode resistor if one needs a bigger attenuation ratio for the feedbacks.

The CCS in the driver plates makes the cathodes look like a current source, or Hi-Z. The same Mu and -1/Mu relations above canceling AC signal wise again.
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This local neg. feedback to the driver cathodes approach looks quite nice to me now. And it appears to get rid of the residual odd harmonic signature of a full diffl. pair. I'll have to use it more often now!
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future possibilities:

Looking at Bigwill's original 1st version with the dual full diffls, one might consider increasing its duties rather than chopping out parts. Since the diffl pairs are taking the difference between input and output, one might consider computing an Error Correction signal as in Hawksford Error Correction. Some additional crosscouplings required I think.

Or, one might consider trying a Harmonic Equalizer with the driver stages acting as the balanced mixers for the output tubes. Considerable thinking required on this one yet I think. Common mode feedbacks likely required.

Don

[Michael Koster]

Looks like RC-30 p.696 with CCS loads on triodes instead of pentodes
and without the direct plate to plate feedback (which might not be
great with your triode drivers anyway). Have you calculated some
real-life resistor ratios yet?

Plate feedback is very interesting with pentodes. I just read O.H. Schade
"Beam Power Tubes" and got inspired to play with pentodes (well
actually transmitting tetrodes...) and thought of plate feedback
directly to the plate of a pentode or drain of a MOSFET driver...
What didn't they think of 70 years ago???

Don
"Re-analysis (assuming no grid currents in the driver)"

The feedback from outplate to driver stage doesn't do anything
to lower the driver impedance... should be OK with receiving pentodes.

Cheers,

Michael

[smoking-amp]

"What didn't they think of 70 years ago???"

Not much apparently. But it seems that only a limited portion of the ideas ended up in commercial amplifiers.

Hawksford EC was invented by a tube guy (Hawksford references his patent) but it only showed up in commercial SS amps after Hawksford expanded on it in his subsequent journal articles.

A very hot topic lately in power supply design called ripple steering is explained in Electronic Designer's Handbook by Landee, Davis and Albrecht in 1957 and patented over and over again as it was re-discovered 4 times afterwards.

The Harmonic Equalizer from WE tube days seems like an idea that was never fully developed after NFDBK was discovered. I think there is still some interesting potential there using the driver stage as the balanced mixer. (or an external mixer a'la S. Bench)

If cathode feedbacks can get rid of the odd harmonic residual of a diffl. stage, and so it appears, this should be primo stuff for Hi-Fi P-P amp design. While current feedback in SS ampls has a somewhat dubious high distortion rep., with triodes we have firm control of the gain using a CCS. Different animal altogether.

Don

post edit:

"The feedback from outplate to driver stage doesn't do anything
to lower the driver impedance... should be OK with receiving pentodes."

I think the feedback has to be lowering the driver impedance effectively too, because it will desperately try to get the output tube to behave accurately. But one can just use a low rp triode for the driver anyway.

It may also be possible to mix the two schemes (plate feedback to driver pentode and the cathode feedback to the triode driver) by using a driver pentode with crossed plate feedbacks to the driver screens and direct plate feedbacks via resistors to the driver plates. Plus the cathode feedbacks. The driver tube now acting like a triode and a pentode at the same time.

[bigwill]

I try to avoid too much feedback in my designs, and if I do use it, the loop is very short


I think the anode load CCS is the better of the two, as it forces the triode to be more linear and work with more gain

I must admit a lot of these replies are over my head a bit, so I'm not really able to partake in most of the discussion (though the replies are very much appreciated! ), my designs are pieced together with educated guesses and intuition into the way things might work

[bigwill]

I just realised "I try to avoid too much negative feedback" is such a clichéd phrase these days

[smoking-amp]

"I just realised "I try to avoid too much negative feedback" is such a clichéd phrase these days "

I think the CCS in the plate version (post 21) should be reasonably buildable with its simplicity. The amount of actual neg. feedback of course depends on the selected resistor values for the feedback.

I myself don't like using huge amounts of feedback either usually. But since the neg. feedback equation shows that error reduces with increasing neg. feedback (ie, higher loop gain), I like to cheat and try to find some sort of anti-error signal somewhere so that the loop gain can stay low. Thats the idea behind the Hawksford EC, just compute the remaining error and subtract it out. But its theoretically equivalent to high gain, it just doesn't require actual device (tube) gains.

The WE Harmonic Equalizer also computes some anti-error stuff using a different approach (balanced mixer and common mode signals). So it gets by without high gain also.

The RCA RC-30 50 Watt design may be using the additional plate feedbacks (beside the cathode feedbacks) to take advantage of the slightly different transfer functions of plate feedback versus cathode feedback to null/tweak some distortion out. But since the driver tube and controlled output tube are in anti phase regarding current flow, this generally cannot produce wide range cancellation of distortion. (leading to odd harmonic residuals just like the diffl. stage at large signal levels)

To get tracking cancellation, the two devices need to be tracking current wise. So some extra cross coupled feedbacks (beside the cathode fdbks) are more likely to succeed. This would entail using some high value feedback resistors cross coupled between the output plates and the driver grids. Its not really an attempt to increase the overall feedback, just to partition some of it differently. This, of course, is a completely un-explored area. Maybe.., maybe..... the Citation II got it right?

Don

[Miles Prower]

Originally posted by bigwill
Came up with a simple idea today, wondered if it has ever been tried. The basic idea is having a differential driver (not a phase splitter) that can take feedback from the plate to linearise and lower the impedance of a pentode or tetrode (or further linearise a triode)

See image for the circuit I thought of

Overly complicated way to get local NFB. Some finals definitely need local NFB to sound good. Here's how I did that: Vixen Main Schemo.The feedback goes back to the grids of the drivers to avoid having the Crt of the 807s in parallel with the feedback resistors.

[gingertube]

A bit of help for those following trying to follow this thread.

The RCA RC-30 Handbook can be downloaded from Pete Millett's site.
http://www.pmillett.com/tubedata/RC30.pdf

The schematic from page 696 referenced above is this:
Apologies for the quality but best I could manage with the postscript snapshoot tool.


Cheers,
Ian

[smoking-amp]

another link to the RCA 50W schematic on Pete Milletts site (3.48 MB pdf, on pages 14 and 15):
http://www.pmillett.com/file_downloads/RCA_HiFi.pdf

a related design with just the cathode feedbacks:
http://mysite.du.edu/~etuttle/electron/elect35.htm
(also a similar design with doubled up 6V6GTs for 20 Watt is in the RCA RC-19 handbook, 1959, SP-20 amplifier)

and the AR Dynaco ST-70-C3 modification which used cathode feedbacks:
http://www.arcdb.ws/ST70C3/ARC_ST70C3_schematic.gif