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Patrick Turner Patrick Turner is offline
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Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 17, 12:36*am, John L Stewart 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



But 27k is the RL only when the 27k is terminated by a B+ which has no
ac, or put another way, when there is no signal voltage in the OPT and
no Vac bootstrapping tap.

Beginners may have failed to understand you.

BTW, Ra at 7k7 is a bit optimistic unless Ia is fairly high; Ra varies
considerably with Ia.

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.


OK, this point becomes more obvious the more you think about it, but
there's more to consider.

One could connect the 27k load to a tap on the OPT where twice the
6SN7 anode signal exists, ie, cross coupled to the opposite side
output tube anode, and then you have a case where the 6SN7 would rise
above µ, but not much, because the 6SN7 Ra is much less than 27k, and
to make a large amount of gain occur one would need to have a feed
point for bootstrapping several times the anode signal at the opposite
output tue anode.
But usually when one feeds more Vac to a bootstrapped RL one has real
PFB, not just something which leads the tube towards being loaded by a
dummy CCS, ie a load with no current change.
When I tried doing this the results were dissapointing, increased Rout
and THD


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


Indeed what you are saying does make sense, but the the effect of
bootstrapping where it is a form of mild PFB on THD is yet to be
formularized.

In RDH4, PFB is mentioned and a schematic is given where a PFB loop
between two cascaded small signal voltage triodes is boosted 12dB and
the same amount of global NFB is used. The effect of PB when the
signal is below say 10Vrms is to increase open loop sensitivty without
much increase in THD or reduction of bandwidth so that global NFB may
still be applied while retaining stability, at least with resistance
loads.
RDH4 goes on to say what a marvel PFB can be. Its as if PFB offers
"free voltage gain" without using expensive hardware to get it, just
resistors are needed.

But I have never ever seen any commercial amp with PFB used
deliberately to increase OL Gain except of course in samples of the
parafeed phase inverter where V1 anode output is divided down to drive
V2, to make another phase of drive signal "for free". Anything for
free in electronics usually has a price, and THD increase with any
form of PFB is one of the costs. Even in Quad-II there is 6dB of PFB
so that the two EF86 may generate just enough gain between them to
drive the KT66 AND allow about 10dB of global FB.
Some other type of input-plus-driver stage with the same tube gain and
no PFB would have input sensitivity needing 2.8Vrms input for clipping
instead of only 1.4Vrms as Quad-II requires, if the same amount of
global FB is maintained.

In earier times amp makers wished there amps to be sensitive to 0.2V
for clipping; Leak amps with GNFB needed only 0.1Vrms for clipping.
Mullard 520 had EF86 in pentode followed by a 12AX7 LTP.
But no PFB, and no bootstrapping.
Williamson had triode output stage and 6SN7 low µ triode drive stages
and the concertina phase inverter with lots of local NFB as a buffer
stage between SE input and balanced amp driver. Sensitivity was 2Vrms
for clipping.
Again, no bootstraping or PFB.

As I have mentioned, boostrapping RL of driver anode loads to OPT
taps is mainly used to achieve a high voltage swing without the driver
going near cut off or grid current.

Bootstrapping from an anode winding on an OPT does require a well
smoothed B+ rail for the OPT CT. Any hum at the CT also exists ant any
tapping points on the anode primary winding and for bootstrapping to
work one needs to have any PFB feed of noise to be fed back while
remaining common mode and it would be all too easy for some imbalance
of noise and the noise becomes differentially applied to OP tube grids
resulting in its amplification as any other wanted differential
signal. Many old amps like Quad-II had quite high hum at the OPT CT,
17Vrms in the Quad case because there was no beans to pay for a plate
choke and additional filter cap, and there must have been a bean
counter at Quad's head office. They had a guy there who passionately
believed in CMMR and preached its virtues so suppressing any murmers
about yet another bit more engineering to make a toy amp heavy.
So bootstrapping anything from a conveniently phased OPT tap somewhere
can be a problem unless is noise is suppressed by a good PSU.

In Quad-II, it would be possible to have say series 82k plus 100k
anode supply RL to each EF86 anode. The total of 182k is virtually the
same as what is the standard original 180k. Then you could have 22uF
electro caps from junction of the two R to the CFB winding which has a
CT that is virtually grounded. This would make the load seen by the
EF86 to become higher than the 180k, maybe double because pentodes
have high Ra not reduced by the internal NFB in a triode, and hence
the EF86 gain would nearly double, and GNFB reduced by 1/2 to maintain
the same amount of GNFB. But what's the use? I think ppl would find
the 0.7Vrms input sensitivity welcome but THD and Rout would not be
less.

The CFB winding in the Acoustical Connection does not have a common
mode hum signal to upset the apple cart. But I doubt much would be
really achieved in overall THD reduction or anyother betterments.

What works better in Quad-II is to make an LTP with EF80 with 3 times
the anode current in EF86 and therefore raise OLG by nearly 2X and
construct the input pair as a true LTP with CCS, and with input sig to
one side and NFB to the other, and no need to bootstrap or worry about
PFB and its THD increasing propensities.

6EJ7 is also a nice driver pentode. Radford used a pair in the LTP for
the ST100 PP amp with 2x KT88.

50 ways to leave a lover, slip out the back Jack, find a new plan
Stan,

and 50 ways to make an amplifier.

Patrick Turner.




--
John L Stewart