I have an early Knight kit amplifier that uses push-pull 6V6s driven
by two 6SN7s, with a 6SL7 for the phono stage. In tracing out the
circuit I noticed that in addition to the global feedback from the
output-transformer secondary, there was a feedback path from the grid
of one 6V6 to a 6SN7 cathode. The feedback signal originates at the
220k-ohm 6V6 grid resistor, passes through a 1-megohm resistor, and
winds up at the 4.7k-ohm 6SN7 cathode resistor. I couldn't figure out
why this feedback path was there, particularly since the signal at the
6V6 grid would be expected to be somewhat distorted, being inside the
global feedback loop. The feedback path causes about a half-volt
offset at the 6V6 grid (the 6V6 cathodes are at about 20 volts), so
there ought to be a good reason to incur this error voltage. I
disconnected the 1-meg resistor and the overall gain decreased by 3
dB, indicating positive feedback. The gain came back up 3 dB when I
bypassed to ground the cathode where the feedback had gone.

My guess is that the purpose of this feedback path is to increase the
gain of the 6SN7 stage, basically eliminating an electrolytic bypass
at the cost of a 1-meg resistor. Is this right? I've disconnected the
feedback because I need neither the added gain nor the output bias
asymmetry.

Brian

"Brian" wrote in message
om...
I have an early Knight kit amplifier that uses push-pull 6V6s driven
by two 6SN7s, with a 6SL7 for the phono stage. In tracing out the
circuit I noticed that in addition to the global feedback from the
output-transformer secondary, there was a feedback path from the grid
of one 6V6 to a 6SN7 cathode. The feedback signal originates at the
220k-ohm 6V6 grid resistor, passes through a 1-megohm resistor, and
winds up at the 4.7k-ohm 6SN7 cathode resistor. I couldn't figure out
why this feedback path was there, particularly since the signal at the
6V6 grid would be expected to be somewhat distorted, being inside the
global feedback loop. The feedback path causes about a half-volt
offset at the 6V6 grid (the 6V6 cathodes are at about 20 volts), so
there ought to be a good reason to incur this error voltage. I
disconnected the 1-meg resistor and the overall gain decreased by 3
dB, indicating positive feedback. The gain came back up 3 dB when I
bypassed to ground the cathode where the feedback had gone.

My guess is that the purpose of this feedback path is to increase the
gain of the 6SN7 stage, basically eliminating an electrolytic bypass
at the cost of a 1-meg resistor. Is this right? I've disconnected the
feedback because I need neither the added gain nor the output bias
asymmetry.

Brian


I've seen this in done in a few '60s schematics and 'Glass Audio' articles.
How do you get the ultimate out of as few valves as possible ??
Global NFB reduces distortion but decreases gain.. Add PFB at the OP valve
drivers. You get the gain back and the global NFB keeps the distortion
low...

You balance one against the other...

Theory says it works. Reality says it's unstable and unpredictable.

kind regards
jim

"jim" wrote

I've seen this in done in a few '60s schematics and 'Glass Audio'
articles.
How do you get the ultimate out of as few valves as possible ??
Global NFB reduces distortion but decreases gain.. Add PFB at the
OP valve
drivers. You get the gain back and the global NFB keeps the
distortion
low...

You balance one against the other...

Theory says it works. Reality says it's unstable and
unpredictable.


The quad 2 uses local +ve fb to one half of the floating paraphase
splitter, and -ve to the other half, and -ve overall. It's British.
Must be a good thing.

cheers, Ian

In article , "Ian
Iveson" wrote:

The quad 2 uses local +ve fb to one half of the floating paraphase
splitter, and -ve to the other half, and -ve overall. It's British.
Must be a good thing.

As near as I can see the Quad II uses an ordinary paraphase phase
inverter, not a "floating paraphase" phase inverter? I also can't see any
large use of positive feedback in the Quad IIcircuit. I see a very small
incidental amount of positive feedback that appears to be a side effect of
the way the loop feedback is connected.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

"John Byrns" wrote

As near as I can see the Quad II uses an ordinary paraphase phase
inverter, not a "floating paraphase" phase inverter? I also can't
see any
large use of positive feedback in the Quad IIcircuit. I see a
very small
incidental amount of positive feedback that appears to be a side
effect of
the way the loop feedback is connected.


Hmm, maybe, just hovering perhaps. Certainly not "ordinary", because
the two valves are coupled in a host of different ways. Just been
looking it up. I was wrong about the "local". Here's the circuit in
case anyone doesn't know it:

http://www.triodeel.com/quad2.gif

I forget which end of the floating paraphase is supposed to float. I
think it's something of a misnomer anyway.

The circuit shows unbypassed 680 + 100 ohms common to the coupled
cathodes. Feedback from the secondary winding is applied to the
junction of those two resistors, so it appears as -ve to valve 1 and
+ve to valve 2.

Looking at the input of the power stage, the grids are connected via
680k to either side of a single 2k8. There is no central ground but
rather this 2k8 floats, with one end connected to valve 2 grid, and
the other to same cathode junction as the feedback. OK, so that's
not floating much, because there is a low resistance path to ground
via the feedback winding in parallel with the 100ohms, but the
floating paraphase is never really floating is it?

When the splitter is balanced, the signal either side of that 2k8
resistor must be equal and opposite. It may seem that the +ve fb to
V2 cathode is matched by the -ve fb to its grid.

But how do you explain how come 2k8 is such a small proportion of
680k? What is the gain of V2, and how does it compare to V1?
Remember that the grid signal for V2 is developed across just half
of that 2k8, since ac ground is half-way along it.

Looking for some words from my paltry library, I find JL Hood
"...gain of V2...rather more than twice than that which would have
been expected". Expected, that is, by someone not seeing the +ve fb,
he says. Irritatingly, he fails to mention the link between the
feedback and that 2k8 resistor. It requires a fair amount of maths
to work out the whole explanation, but the gain of V2 is
indisputably much higher than it could be without +ve fb...that fact
doesn't take too much calculation.

Oh...and the screens are floating with no reference at all to
ground. Shouldn't I get half a point for that?

cheers, Ian

Ian Iveson wrote:

"John Byrns" wrote

As near as I can see the Quad II uses an ordinary paraphase phase
inverter, not a "floating paraphase" phase inverter? I also can't
see any
large use of positive feedback in the Quad IIcircuit. I see a
very small
incidental amount of positive feedback that appears to be a side
effect of
the way the loop feedback is connected.


Hmm, maybe, just hovering perhaps. Certainly not "ordinary", because
the two valves are coupled in a host of different ways. Just been
looking it up. I was wrong about the "local". Here's the circuit in
case anyone doesn't know it:

http://www.triodeel.com/quad2.gif

I forget which end of the floating paraphase is supposed to float. I
think it's something of a misnomer anyway.

The circuit shows unbypassed 680 + 100 ohms common to the coupled
cathodes. Feedback from the secondary winding is applied to the
junction of those two resistors, so it appears as -ve to valve 1 and
+ve to valve 2.

Looking at the input of the power stage, the grids are connected via
680k to either side of a single 2k8. There is no central ground but
rather this 2k8 floats, with one end connected to valve 2 grid, and
the other to same cathode junction as the feedback. OK, so that's
not floating much, because there is a low resistance path to ground
via the feedback winding in parallel with the 100ohms, but the
floating paraphase is never really floating is it?

When the splitter is balanced, the signal either side of that 2k8
resistor must be equal and opposite. It may seem that the +ve fb to
V2 cathode is matched by the -ve fb to its grid.

But how do you explain how come 2k8 is such a small proportion of
680k? What is the gain of V2, and how does it compare to V1?
Remember that the grid signal for V2 is developed across just half
of that 2k8, since ac ground is half-way along it.

Looking for some words from my paltry library, I find JL Hood
"...gain of V2...rather more than twice than that which would have
been expected". Expected, that is, by someone not seeing the +ve fb,
he says. Irritatingly, he fails to mention the link between the
feedback and that 2k8 resistor. It requires a fair amount of maths
to work out the whole explanation, but the gain of V2 is
indisputably much higher than it could be without +ve fb...that fact
doesn't take too much calculation.

Oh...and the screens are floating with no reference at all to
ground. Shouldn't I get half a point for that?

cheers, Ian

The Quad has a paraphase PI with each EF86 with a gain of around 200.

The instataneous AV at various points around the circuit are as
follows:-

V3 anode, +160v, grid -40 = anode signal of V1,
V4 anode, -160v, grid +40 = anode signal of V2,
Feedback signal at top of R10 = +1.2v
Signal at top of 2.7 k = +1.04 approx.
Vg-k both V1&2 = approx 0.2v,
so input voltage g-g = 0.4 v,
so total input voltage = v at R10 + 0.4 = approx 1.6 v.

Only about 0.16v is across the 2.7k.

Global FB is applied to the second grid of the PI,
and fully to the bottom of the common Rk of R4, 680 ohms,
thus bootstrapping the input stage, or allowing
whati is the normal grounding point of the of whole of the input stage
including the
bias R for the output tubes to be in series with the AV input signal.
In effect, the FB is applied in common mode,
but the differential signal between input and fed back signal is all
that is amplified.
I think the cuicuit would work OK without the 2.7k, but
then you have a balanced short tail pair, which would be slightly
unbalanced, if driven more at one side than the other, so
to compensate, there is that 2.7k.

Once there, the 2.7 k helps the selfbalancing of these sorts of stages.

The pentode inputs are in class A, and the 2H screen currents cancel,
and so no need to reference the screens to the common cathode, although
this could be done.

A more modern "pure" way of doing this circuit is to have a CCS
tail for the EF86, and take the FB
straight to the second input grid of V2, and have the bias R for the
output tubes taken to ground.

I think the modern way would be better, and easy to implement,
with a single fet CCS, but in 1953, there were no fets.
And when there were fets, people were throwing out tubes.

Patrick Turner.

In article , "Ian
Iveson" wrote:

"John Byrns" wrote

As near as I can see the Quad II uses an ordinary paraphase phase
inverter, not a "floating paraphase" phase inverter? I also can't
see any
large use of positive feedback in the Quad IIcircuit. I see a
very small
incidental amount of positive feedback that appears to be a side
effect of
the way the loop feedback is connected.


Hmm, maybe, just hovering perhaps. Certainly not "ordinary", because
the two valves are coupled in a host of different ways. Just been
looking it up. I was wrong about the "local". Here's the circuit in
case anyone doesn't know it:

http://www.triodeel.com/quad2.gif

I forget which end of the floating paraphase is supposed to float. I
think it's something of a misnomer anyway.

I may not have the precise definition of "floating paraphase" vs. ordinary
"paraphase" phase inverter, but I consider the ordinary "paraphase" phase
inverter to be one where the phase inverter valve runs open loop, with a
divider at its input to knock the gain down to -1, while I consider the
"floating paraphase" to be one that uses the phase inverter valve in a
op-amp type configuration with a closed loop gain of -1. The Quad II
circuit topology would fit the latter description, except that the 100 Ohm
resistor at the common point is so small that it effectively becomes an
ordinary "paraphase" phase inverter, if this resistor were 1000 times
larger, then it would be a "floating paraphase" in my book.

The circuit shows unbypassed 680 + 100 ohms common to the coupled
cathodes. Feedback from the secondary winding is applied to the
junction of those two resistors, so it appears as -ve to valve 1 and
+ve to valve 2.

It appears as -ve to the upper valve, but there is effectively no
feedback, either -ve or +ve to the lower valve because the feedback signal
is also applied to the grid virtually undiminished by passing backwards
through grid voltage divider for the "paraphase" phase inverter.

There is some positive feedback to the grid of the lower output valve, and
negative feedback to the grid of the upper output valve that I didn't
notice before, don't know how much feedback occurs around these loops from
the output transformer to the grids of the output valves, whether it is
significant, or is relatively insignificant? These feedback paths look
like a side effect of the main negative feedback scheme that couldn't be
easily eliminated like the positive feedback to the lower phase inverter
valve was.

Looking at the input of the power stage, the grids are connected via
680k to either side of a single 2k8. There is no central ground but
rather this 2k8 floats, with one end connected to valve 2 grid, and
the other to same cathode junction as the feedback. OK, so that's
not floating much, because there is a low resistance path to ground
via the feedback winding in parallel with the 100ohms, but the
floating paraphase is never really floating is it?

It is rarely completely floating, there is one rarely used circuit
configuration where it is completely floating, but in most circuits it is
floating for all practical purposes. In the Quad circuit the junction is
effectively grounded by the 100 Ohm resistor the feedback is applied
across, effectively making it an ordinary "paraphase" not a "floating
paraphase".

When the splitter is balanced, the signal either side of that 2k8
resistor must be equal and opposite. It may seem that the +ve fb to
V2 cathode is matched by the -ve fb to its grid.

But how do you explain how come 2k8 is such a small proportion of
680k? What is the gain of V2, and how does it compare to V1?
Remember that the grid signal for V2 is developed across just half
of that 2k8, since ac ground is half-way along it.

I don't see any AC ground half way along the 2.7 k resistor, all I see is
the bottom end of the 2.7 k resistor grounded by a 100 Ohm resistor. That
makes the loss for the feedback signal around the inverter valve something
like a factor of 0.00015, while the gain of the valve is 360 at the very
most, hardly a combination that would make a very good unity gain
inverter, especially when you consider that the input signal to the
inverter valve is scaled by about 0.004 at the input, about right for the
likely actual gain of the inverter valve operating as an ordinary
"paraphase" phase inverter.

Looking for some words from my paltry library, I find JL Hood
"...gain of V2...rather more than twice than that which would have
been expected". Expected, that is, by someone not seeing the +ve fb,
he says. Irritatingly, he fails to mention the link between the
feedback and that 2k8 resistor. It requires a fair amount of maths
to work out the whole explanation, but the gain of V2 is
indisputably much higher than it could be without +ve fb...that fact
doesn't take too much calculation.

I don't follow any of that reasoning, it doesn't ring true either.

Oh...and the screens are floating with no reference at all to
ground. Shouldn't I get half a point for that?

Yes, certainly, that's an interesting detail.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

"John Byrns" wrote in message
...
In article , "Ian
Iveson" wrote:

The quad 2 uses local +ve fb to one half of the floating paraphase
splitter, and -ve to the other half, and -ve overall. It's British.
Must be a good thing.

As near as I can see the Quad II uses an ordinary paraphase phase
inverter, not a "floating paraphase" phase inverter? I also can't see any
large use of positive feedback in the Quad IIcircuit. I see a very small
incidental amount of positive feedback that appears to be a side effect of
the way the loop feedback is connected.


Regards,

John Byrns

Its not quite ordinary, The cathodes share a common resistor as in the
cathode coupled variety but the second tube s grid is also coupled to the
plate of the first tube in the same manner as an ordinary paraphase unit. I
think this is called an enhanced para phase. Dont think you would call the
extra connect positive feedback either. Guess it could be though ,just one
of those perspective things.

Jimmy

"Ian Iveson" wrote in message
...
"jim" wrote

I've seen this in done in a few '60s schematics and 'Glass Audio'
articles.
How do you get the ultimate out of as few valves as possible ??
Global NFB reduces distortion but decreases gain.. Add PFB at the
OP valve
drivers. You get the gain back and the global NFB keeps the
distortion
low...

You balance one against the other...

Theory says it works. Reality says it's unstable and
unpredictable.


The quad 2 uses local +ve fb to one half of the floating paraphase
splitter, and -ve to the other half, and -ve overall. It's British.
Must be a good thing.

cheers, Ian



The Austin Allegro was British too ! Hi, Ian....
I have a reprint of a Glass Audio article somewhere headed 'A balanced
feedback amplifier'
PP EL84s if I remember right. A Williamson .The author achieved high
sensitivity by making the PFB adjustable using a preset
pot, winding it up until the amp oscillated and then backing it off. Some
impressive claims were made for the circuit but no details of the test
procedures or the effect of different output loads were mentioned
Analysis of the paraphase is somewhat difficult (particularly for me ! ) It
seems to get drawn in a number of different ways, as well. Taking a
proportion of the output of V1 to drive V2 with an opposite phase signal
using a resistance divider proportioned to be equal to the gain of V1 is
pretty obvious but when it becomes floating and self balancing, these
unequal resistances disappear. Using equal resistances, V2 grid is driven
by the difference between the two outputs, which appears across the common
resistor which ties V2 grid down to earth. Balance should therefore be as
accurate as the matching of the two resistors. Differences in the
characteristics of the two triodes should theoretically be cancelled out
too. HT is not wasted, as it is across the tail resistor of an LTP, and the
paraphase should therefore be capable of producing larger output signals.
Remember, I know bugger all, so such an analysis is probably completely
wrong
kind regards
jim

Brian wrote:

I have an early Knight kit amplifier that uses push-pull 6V6s driven
by two 6SN7s, with a 6SL7 for the phono stage. In tracing out the
circuit I noticed that in addition to the global feedback from the
output-transformer secondary, there was a feedback path from the grid
of one 6V6 to a 6SN7 cathode. The feedback signal originates at the
220k-ohm 6V6 grid resistor, passes through a 1-megohm resistor, and
winds up at the 4.7k-ohm 6SN7 cathode resistor. I couldn't figure out
why this feedback path was there, particularly since the signal at the
6V6 grid would be expected to be somewhat distorted, being inside the
global feedback loop. The feedback path causes about a half-volt
offset at the 6V6 grid (the 6V6 cathodes are at about 20 volts), so
there ought to be a good reason to incur this error voltage. I
disconnected the 1-meg resistor and the overall gain decreased by 3
dB, indicating positive feedback. The gain came back up 3 dB when I
bypassed to ground the cathode where the feedback had gone.

My guess is that the purpose of this feedback path is to increase the
gain of the 6SN7 stage, basically eliminating an electrolytic bypass
at the cost of a 1-meg resistor. Is this right? I've disconnected the
feedback because I need neither the added gain nor the output bias
asymmetry.

Brian

There is an example of a positive voltage feedback loop
in a 6V6 PP amp in RDH4, where the gain of the 6SN7 driver stage is
boosted
4 times, or 12 dB, so that the amount of globally applied FB is
effectively increased 12 dB, and thus thd/imd reduced well
below what would be predicted by global FB use alone.
Gain after FB is applied, in dB = 20 x log [ A / ( 1 + {A x B } ) .
where A is the open loop gain without FB, and B is the fraction of the
output
voltage fed back to the input.
If A = 100, and B = 0.1 the FB = 19.17 dB.
The gain reduction is from 100 to 9.09, and distortion is also reduced
by around this amount.
If A = 400, and B = 0.1, the FB = 19.78 dB,
but the gain reduction
is from 400 to 9.75, a larger gain reduction,
and so you get less thd.

Its not all roses though, as the gain boost from internally boosting the
voltage gain of the drive amp makes it less stable, and increases its
distortions.
But the main thd in most amps is due to non linearity in the the output
stage,
by a factor of 1 : 3, so the extra driver thd is insignificant, and the
thd at tha amp output is reduced more by PVFB than it is increased.

I prefer to make the amp substantially linear before any FB is applied,
and then only NFB is required, and none of the side effects of positive FB

emerge, so I have never used PFB.

There would be those who would argue about the subjective ruination
of the sound with the use of all these loopy loops of FB,
back and forth and I leave them to argue with each other.

Patrick Turner.

Let me clarify the circuit: The negative feedback from the
output-transformer secondary goes to the stage that precedes the phase
splitter, in the usual way. The positive feedback that originates at a
6V6 grid goes to the stage that precedes the stage where the negative
feedback is applied. The loops are not nested. This is why I mentioned
that the positive feedback applies a somewhat distorted signal, since
that signal is taken from within the negative-feedback loop but
applied outside it.

Weird, huh?

Brian

Brian,

This is a long shot . . . the positive feedback may be there to increase
gain (doubtful?) but maybe it's a form of "pre-distortion" if it occurs 180
degrees out of phase from the output. The purpose of "pre-distortion" is to
lower overall distortion, but the audiophool in me shudders from the very
concept.

Erno Borbely employed it in an article in Glass Audio he wrote about an SE
design (1996, Vol.8, No. 5).

Just a guess . . . .

Jon

From: (Brian)
Organization: http://groups.google.com
Newsgroups: rec.audio.tubes
Date: 24 Nov 2003 21:44:18 -0800
Subject: Positive Feedback in P-P 6V6 Amp

Let me clarify the circuit: The negative feedback from the
output-transformer secondary goes to the stage that precedes the phase
splitter, in the usual way. The positive feedback that originates at a
6V6 grid goes to the stage that precedes the stage where the negative
feedback is applied. The loops are not nested. This is why I mentioned
that the positive feedback applies a somewhat distorted signal, since
that signal is taken from within the negative-feedback loop but
applied outside it.

Weird, huh?

Brian

Jon Yaeger wrote:

Brian,

This is a long shot . . . the positive feedback may be there to increase
gain (doubtful?) but maybe it's a form of "pre-distortion" if it occurs 180
degrees out of phase from the output. The purpose of "pre-distortion" is to
lower overall distortion, but the audiophool in me shudders from the very
concept.

Erno Borbely employed it in an article in Glass Audio he wrote about an SE
design (1996, Vol.8, No. 5).

Just a guess . . . .

I'd like to see a schematic of this amp posted at ABSE or somewhere,
so we could see it.

The error signal contains the inverse of the distortion in the output.
So it could be applied in some way to a preceeding gain stage,
and thus cancel the natural open loop distortions of the amp.
Perhaps it ain't as wacky as it seems.
But not an audiophile solution, more that of a pure engineer,
ie, a mean and tricky solution to a tecnical problem.
I wonder how stable this amp is with no R load at the output,
and just a pure capacitance, say 0.22 uF, which can make many
FB amps oscillate strongly at some F above the AF band..

Patrick Turner.




Jon

From: (Brian)
Organization: http://groups.google.com
Newsgroups: rec.audio.tubes
Date: 24 Nov 2003 21:44:18 -0800
Subject: Positive Feedback in P-P 6V6 Amp

Let me clarify the circuit: The negative feedback from the
output-transformer secondary goes to the stage that precedes the phase
splitter, in the usual way. The positive feedback that originates at a
6V6 grid goes to the stage that precedes the stage where the negative
feedback is applied. The loops are not nested. This is why I mentioned
that the positive feedback applies a somewhat distorted signal, since
that signal is taken from within the negative-feedback loop but
applied outside it.

Weird, huh?

Brian

I'd like to see a schematic of this amp posted at ABSE or somewhere,
so we could see it.


I wish I had a diagram. This is an early Knight kit, deco styling, no
model number, just says "KNIGHT HI-FI 10 WATT AMPLIFIER." Works fine,
with or without the funny feedback.

Since the gain effect of the funny feedback is only 3 dB, I'm still
guessing that it is simply intended as a cheap way to bypass a cathode
resistor. Bypassing that stage with an actual capacitor yields the
same gain increase.

Next time I have the amp out, I'll run it through my spectrum analyzer
and see whether the feedback affects distortion. I couldn't see any
time-domain effects on the sine wave I was using to determine the
feedback gain effect.

Brian

Brian wrote:

Let me clarify the circuit: The negative feedback from the
output-transformer secondary goes to the stage that precedes the phase
splitter, in the usual way. The positive feedback that originates at a
6V6 grid goes to the stage that precedes the stage where the negative
feedback is applied. The loops are not nested. This is why I mentioned
that the positive feedback applies a somewhat distorted signal, since
that signal is taken from within the negative-feedback loop but
applied outside it.

Weird, huh?

Sounds like "accountant inspired design synergy" , ( AIDS ) where
he was loathe to use another tube, or he got a bargain lot price on low
gain tubes.

Accountants prefer the the sound of money, not music.

Patrick Turner.



Brian

I hooked up my HP 141T/8552B/8556A spectrum analyzer to the push-pull
6V6 amp today and measured the following harmonic levels at 10 volts
P-P output (1.5 watts) into 8 ohms:

2nd 3rd
feedback -64 -52 dB
no feedback -55 -48

So not only does the unusual feedback path increase the overall gain 3
dB, it lowers the distortion.

Brian

very cool stuff
nice to have the tools to measure it with too huh?

Doug

"Brian" wrote in message
om...
I hooked up my HP 141T/8552B/8556A spectrum analyzer to the push-pull
6V6 amp today and measured the following harmonic levels at 10 volts
P-P output (1.5 watts) into 8 ohms:

2nd 3rd
feedback -64 -52 dB
no feedback -55 -48

So not only does the unusual feedback path increase the overall gain 3
dB, it lowers the distortion.

Brian

"doug" wrote in message
news:UFLxb.513594$6C4.282925@pd7tw1no...
: very cool stuff
: nice to have the tools to measure it with too huh?
:
: Doug

Yep, Doug, all true
it's da tools puttin' da smile
on the scientists' face
even on
a rany day
Rudy
~~~~
: "Brian" wrote in message
: om...
: I hooked up my HP 141T/8552B/8556A spectrum analyzer to the push-pull
: 6V6 amp today and measured the following harmonic levels at 10 volts
: P-P output (1.5 watts) into 8 ohms:
:
: 2nd 3rd
: feedback -64 -52 dB
: no feedback -55 -48
:
: So not only does the unusual feedback path increase the overall gain 3
: dB, it lowers the distortion.
:
: Brian
:
:

Ruud Broens wrote:

"doug" wrote in message
news:UFLxb.513594$6C4.282925@pd7tw1no...
: very cool stuff
: nice to have the tools to measure it with too huh?
:
: Doug

Yep, Doug, all true
it's da tools puttin' da smile
on the scientists' face
even on
a rany day
Rudy

And some would wonder what change would the
positive feedback make on the sound.

Patrick Turner.


~~~~
: "Brian" wrote in message
: om...
: I hooked up my HP 141T/8552B/8556A spectrum analyzer to the push-pull
: 6V6 amp today and measured the following harmonic levels at 10 volts
: P-P output (1.5 watts) into 8 ohms:
:
: 2nd 3rd
: feedback -64 -52 dB
: no feedback -55 -48
:
: So not only does the unusual feedback path increase the overall gain 3
: dB, it lowers the distortion.
:
: Brian
:
:

"Patrick Turner" wrote in message
...
:
:
: Ruud Broens wrote:
:
: "doug" wrote in message
: news:UFLxb.513594$6C4.282925@pd7tw1no...
: : very cool stuff
: : nice to have the tools to measure it with too huh?
: :
: : Doug
:
: Yep, Doug, all true
: it's da tools puttin' da smile
: on the scientists' face
: even on
: a rany day
: Rudy
:
: And some would wonder what change would the
: positive feedback make on the sound.
:
: Patrick Turner.
:
:
Now, Patrick, i could say something very interesting
actually about that. but no, waaayy OT, they say,
how sad
Rudy

some people havent got ears that are trained very well
and while mine are from years of live sound reinforcement and PA system
finalizing, as well as recording and mixing engineer in some interesting
projects I still like to be able to look at a plot to see whether I am
hearing 2nd harmonic distortion or 3rd harmonic or IMD or what.


Doug


"Patrick Turner" wrote in message
...


Ruud Broens wrote:

"doug" wrote in message
news:UFLxb.513594$6C4.282925@pd7tw1no...
: very cool stuff
: nice to have the tools to measure it with too huh?
:
: Doug

Yep, Doug, all true
it's da tools puttin' da smile
on the scientists' face
even on
a rany day
Rudy

And some would wonder what change would the
positive feedback make on the sound.

Patrick Turner.


~~~~
: "Brian" wrote in message
: om...
: I hooked up my HP 141T/8552B/8556A spectrum analyzer to the
push-pull
: 6V6 amp today and measured the following harmonic levels at 10 volts
: P-P output (1.5 watts) into 8 ohms:
:
: 2nd 3rd
: feedback -64 -52 dB
: no feedback -55 -48
:
: So not only does the unusual feedback path increase the overall gain
3
: dB, it lowers the distortion.
:
: Brian
:
: