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John L Stewart John L Stewart is offline
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Location: Toronto
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Quote:
Originally Posted by flipper View Post
On Wed, 16 Mar 2011 13:36:45 +0000, John L Stewart
wrote:


First of all, calculate the driver gain in grounded cathode mode from
published specs-

For the 6SN7 family at 250 volts supply
Mu = 20
Rp = 7.7 K while grid volts is –8

Let A1 be the gain with these conditions,
Then A1 = ( mu*Rl ) / ( rp + Rl )
= ( 20* 27 ) / ( 7.7 + 27 )
= 15.56

Then find the maximum gain possible for any non-feedback triode stage-

Then A2 = mu
= 20

Gain change is A2/A1
= 20 /15.56
= 1.29

In DB becomes 20 log A2/A1
= 2.18 db

The feed back in the case of the bootstrapped driver is to the triodes
anode, so no gain above mu is possible. There is no gain thru the triode
being plate driven.

NTL, some experimenters have actually built working amplifiers this way
using many triodes such as the 6S4 & 12B4. Starting digging, you will
find them.

Hope this all mkes sense. Cheers to all, John


Maybe I'm not thinking straight but I don't see that answering the
question because it depends on what appears to be an arbitrary claim
that gain cannot exceed mu. I mean, it begs the question by declaring
a limit, which is the essence of the question.

Are you presuming that the PFB is designed to produce a unity voltage
feedback (relative to the anode signal swing) so that Rl 'appears'
infinite?

If that's the case then it's trivial to show from the gain equation
that gain approaches, and does not exceed, mu but it's not intuitively
obvious from the schematic that's the amount of feedback being
applied.

If the amount of voltage feedback exceeds plate swing then gain can,
indeed, be above mu.

To illustrate I spiced a simple circuit with a 250V source having
100Vrms on it (simulating an arbitrary transformer feedback signal)
applied to a 6SN7 plate through a 27k and cathode to ground through an
unbypassed 470 ohm. Opposite phase 1Vrms (for visual 'instant math'
convenience) was then applied to the grid.

Plate swing was 52Vrms for a gain of 52.
------------------------------------------------------------------

Hi Flipper- Sounds intriging & I am curious as usual. Could you post a JPG of your simulation so I can try it here?

Far as I know the only way to get gain more tham mu is thru a transformer as the ancients did it before pentodes (or tetrodes) or some kind of PFB applied to the grid or cathode.

And the kind you don't want by way of parasitics.

I did some quick tests on the first of the Twin Coupled Amps I had built a few years back. The final in this thing runs 6LU8s in PP, the triode sections cathode coupled to the OP grids. At clipping it is good for 37 watts. IMD is down more than 50 db at one db below max OP.

It was nice that the caps didn't blow since it has not been run in a few years. Tests are done using a HV Diff Probe into a PICO Scope ADC216. Does scope, SA & RMS volts with 16 bit resolution.

With the 6SN7 27K plate resistors returned to the 400 volt regulated supply rather than thru the PFB circuit the gain was 11.4.

Then reconnected to the PFB circuit the SN7 gain was 16.

If you do the math, turns out to be 3 db, somewhat higher than I recall. I may have done that kind of test on one of the other bootstrapped amps I mentioned in a previous post. Oddly I found I had used a tail resistor of 33K to the -150 volt supply on this 6SN7. Usually I've put in more like 18K. That would account for some of the differance since the SN7 is working at a higher plate resistance here.

If you can't get that JPG on here then email to me direct at

Cheers, John