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Old 4th May 2013, 04:59 PM   #11
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Not really necessary to multiply a non-linear current with such accuracy.
Its better the paralleling device leave the triode at fixed current, which
can easily be done with one PNP or P-CH device, and one current sensing
resistor above the plate.
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Old 4th May 2013, 07:08 PM   #12
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
Not really necessary to multiply a non-linear current with such accuracy.
Go and explain that to the tube guys, but just leave me enough time to find a safe place to hide...
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Old 5th May 2013, 03:22 AM   #13
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The most linear loadline for a triode is a flat line of constant current.
Gm is not linear, Mu is...

Well actually, the most linear voltage gain is a little beyond constant
into CCS + negative loadline land. Go there, and I will be impressed.
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Old 5th May 2013, 07:23 AM   #14
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Quote:
Originally Posted by kenpeter View Post
The most linear loadline for a triode is a flat line of constant current.
Gm is not linear, Mu is...

Well actually, the most linear voltage gain is a little beyond constant
into CCS + negative loadline land. Go there, and I will be impressed.
The loads of the SolidGlass gain stages are bootstrapped, which is a close approximation (and yields a relatively low THD with little FB).

There are other applications for C-Vac: you can boost accurately any type of device, including semiconductors, a J-Fet for example. There are amplifiers using many small devices in parallel, with C-Vac you only need one pair.
Or if you want something really exotic, germanium transistors, a PA composed of 1,000 pairs of 2N45's for example.

You could also discard completely the silly input device, and just keep the C-Vac circuit alone: you'd be left with an ideal textbook Bjt, having constant, linear and accurately defined parameters like h21, and h11, zero Vbe or offset etc.
Associate one N and one P, and you have an ideal follower, without problems of Xover distortion, gm doubling and the kind....
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Old 5th May 2013, 06:55 PM   #15
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To get good emulation of a triode, you need to keep the voltage feedback on it's plate accurate. So a constant current thru the triode will just give you a good V follower, not a triode response.

Splitting the supplies for tube and SS will help the efficiency, but not the tube sound faithfulness, since the output's voltage swing becomes less a factor for the triode's plate feedback.

I suggest some type of voltage multiplier for the tube plate feedback when splitting these two supplies.
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Old 5th May 2013, 07:17 PM   #16
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Originally Posted by smoking-amp View Post
Splitting the supplies for tube and SS will help the efficiency, but not the tube sound faithfulness, since the output's voltage swing becomes less a factor for the triode's plate feedback.
By applying 300V to the anode of the triode, you will simulate a triode amplifier with a 300V supply. It will have an output limited to +/- 20V perhaps, but is it really a problem? It will work in small signal conditions, almost at its best.

Using a supply of 30V for the same purpose will have the opposite effect, and what you propose is a sort of middle term.

OK, why not for the sake of truthfulness, but then resorting to an impedance-changing trick is probably overcomplicated and not necessary: pushing the "HV" supply to 60V (for example) to respect the relative excursion levels might be sufficient.
I am not expert enough in the tube field to decide, but I think that there is some sort of self-similarity at different voltage levels, provided you don't exceed the tube limits, which is not really a problem here, since you can arbitrarily decide at which current level the tube operates
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Old 5th May 2013, 07:35 PM   #17
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Well, depends on what you want to accomplish I guess. Going for a small delta Vout from a 300 V supplied tube will be fairly linear. Maybe too linear to sound much different from an Op Amp. I was thinking of emulation of a tube output stage, where the output plates will generally be pushed within 60V of the cathode, and nearly 2X the B+ (using a P-P xfmr). As you suggested, (at least for a 3/2 power current tube model) self similarity would allow for operation of the "pilot" tube at reduced voltage and current. Real tubes may run into some linearity problems eventually at low enough current levels, due to leakage currents and grid structure end currents.

Not too hard to make a low current voltage multiplier with a HV Mosfet and some divider resistors, with some feedback driver. Another approach would be to use a small P-P xfmr for direct voltage transform from the output back to the "pilot" plates or screen grids.

A clever approach which has been previously used for tube emulation is to just take a good SS amp and use a tube in "reverse" in the global neg. feedback path. (CCS on its cathode, grounded grid, amplifier output cap. drives the plate, neg. amp feedback taken from the cathode, 1/Mu attenuation feedback) This gives a SE (single ended) tube amplifier emulation instead of a PP tube amp. emulation.

Another approach is to series a SS amp and a small tube amp at their outputs (with a very low output Z winding on the tube amp, like 0.1 Ohm). The SS amp runs normally with its internal resistive neg. global feedback. But the tube amp uses the summed output for its global feedback signal. Sort of a tube "veneer" added amp this way. Total sum looks like what the tube amp wants this way. Can also be looked at as a class G amp with the SS amp providing the tracking supply (thru the tube's OT).
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Last edited by smoking-amp; 5th May 2013 at 08:05 PM.
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Old 5th May 2013, 08:23 PM   #18
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A plate voltage feedback xfmr (from the output back to the "pilot" tube) could be just a 1:1 bifilar xfmr to get 2:1 overall voltage multiplication between the SS and tube supplies. This would provide phenomenal OT bandwidth and still provide a respectible voltage on the tube(s). Maybe just use an off the shelf 600:600 Ohm xfmr.

Edcor makes some 1:1 xfmrs (at least they seem to be bifilar when 1:1 ratio'd) XSM---- up to 15K:15K Ohm. Around $13 each.
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Last edited by smoking-amp; 5th May 2013 at 08:29 PM.
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Old 6th May 2013, 03:40 AM   #19
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Smoking, the one transistor, one current sense resistor
(Sziklai pair method) *IS* using triode Mu for feedback.

Runaway of the sand is another matter to consider. With
a SE amp, and passive resistive load, controlled voltage
gain makes runaway of a simple Sziklai pair unlikely.

But you push pull against another boosted triode (Elvee
does this in another thread), you need at least one to set
a limit for current. Gm multiplier works well as any other
scheme. I'm not convinced its the simplest or best way...

--

If you want to go rail to rail, then you need something
more complicated. As you have to scale the feedback
across the plate-cathode to a lesser swing that keeps
reasonable voltage across the triode at all times. This
feedback cut is gonna boost the simulated Mu by the
same ratio, but should still behave triodishy...

Last edited by kenpeter; 6th May 2013 at 03:46 AM.
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Old 6th May 2013, 04:00 AM   #20
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Apologies for old schematic with clueless stabilty (wouldn't build)...
I'm just illustrating the concept of cutting the plate feedback: both
to multiply Mu, and more importantly, to keep plate voltage swing
within reason.
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