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  #1   Report Post  
Tim Williams
 
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"Engineer" wrote in message
...
I have copied the original below, plus John's reply (also
Bill's). Hope it all makes sense!
A. How does the second output tube get it's grid-cathode
(inverted) drive if the second grid floats?


The first tube acts as a cathode follower, driving a small voltage
across the cathode resistor. The other tube picks this up (ala
grounded grid amp) and inverts it by its nature.
The pitfall comes from loss of signal - the cathode would have signal
voltage equal to input if the cathode resistor were a CCS, and since
it's a very low resistance the signal output is low. Then, there's
a similar loss going into the other cathode.

B. If it somehow still gets driven, is it balanced?


It has drive, but it is very unbalanced. Maybe if you measured the
imbalance in a test circuit, then wound a transformer with an unbalanced
primary, it would work a little better.

C. Why not parallel the 470K grid resistor with, say, a 0.1
uF cap. to ensure the cathode space charge cannot move the
grid at signal frequency and unbalance the drive?


Extra part.

D. Even if the second grid was properly decoupled, would
just a common 330 ohm resistor provide a balanced output
stage? Or would the second output tube always be
chronically under driven?

^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
Yes.

So, there it is - I thought quite fascinating. I'd really
like to hear what the RAT-ers have to say about that very


We prefer the term RATs )

poor PP "design" in several commercial radios that prompted
my first post, also John's neat idea.


There's been many bad designs. Sometimes amazing that they actually
made it to production, eh?

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms


  #2   Report Post  
Ronald
 
Posts: n/a
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Maybe a bit OT but I missed the paraphase a little in your story ....
It's also an old principle (Mullard ?) and works great with 2 x (cheap)
ECL82 .
It would make a real nice "poor-mans" PP amp .... (about 7W I think) .

"Engineer" schreef in bericht
...
Long post - apologies!

I first posted this to rec.antiques.radio+phono as that was
the focus of my question (and I didn't want to cross-post.)
However, I received a very interesting answer from John
Byrns that I thought was worth sharing with the denizens of
RAT.

I have copied the original below, plus John's reply (also
Bill's). Hope it all makes sense!

Cheers,

Roger

-------- Original Message --------
Subject: Old PP outputs stages
Date: Sun, 31 Aug 2003 15:32:14 -0700
From: Engineer
Reply-To:
Organization: Lorna & Roger at home
Newsgroups: rec.antiques.radio+phono

I've been perusing the schematics in Ed Kraushar's excellent
"Vintage Canadian Radio Schematics" CD (OK, get a life, I
know...!) and am a bit bemused by one of the phase splitter
designs employed in a couple of the sets that have push-pull
audio output stages.

The no-brainer solution is, of course, the coupling
transformer: primary in the audio amplifier plate circuit,
secondary centre tap to ground (or output tube -ve bias) and
the anti-phase secondary ends to the two output tube grids.
Apart from quibbles over transformer frequency response,
hardly a problem with AM radio, it's perfect, i.e. compact,
economical (in quantity) and provides some free gain to
boot. BTW, I would not use interstage coupling transformers
in "modern" tube Hi-Fi designs even though, IIRC, Williamson
and others did it for years, and there was at least one
solid state amplifier in the late 1960-70's that use it (The
Ravensbourne, I think it was, plus another in the same line,
the Ravensbrook - I had one!)

However, quite a few of the [old] radio receiver designs
don't use coupling transformers. They use one of the
following methods:

1. A triode phase splitter tube with equal cathode and plate
load resistors - this is the classic design still used in
some high end audio, so clearly OK for AM radio.

2. A separate phase inverter tube: the "in-phase" signal
from the directly driven output tube grid is taken through
an attenuator to the grid of a triode tube so that the
"attenuation x gain" is equal to unity (we hope), then the
output of this triode drives the second output tube grid. I
guess it works when the design is done right, the gain is
unity and the tube is new. But it's all open-loop so PP
balanced drive is dependent on component values and tube
gains remaining the same. This only gets a "B-" in my
design book, and that's generous.

3. This is the odd one that prompted this posting. There's
no interstage transformer, phase splitter or inverter tube,
as such. Here, the audio amplifier output goes to only one
output grid - via a coupling capacitor, of course. The two
output tubes share an undecoupled cathode resistor
(typically 330 ohms in the case of a pair of 6V6's.) So far
so good - we presume the second O/P tube is cathode driven,
providing the grid is grounded to signal by a large
capacitor. But it's not! In at least two designs (Astra
1946-47 Model DR103-47 and Brunswick 1931-32 Model B-H) the
second output tube grid goes to ground via 470K but without
any decoupling capacitor.

My questions on method (3), above, for the vintage radio
experts a

A. How does the second output tube get it's grid-cathode
(inverted) drive if the second grid floats?
B. If it somehow still gets driven, is it balanced?
C. Why not parallel the 470K grid resistor with, say, a 0.1
uF cap. to ensure the cathode space charge cannot move the
grid at signal frequency and unbalance the drive?
D. Even if the second grid was properly decoupled, would
just a common 330 ohm resistor provide a balanced output
stage? Or would the second output tube always be
chronically under driven?

Cheers,

Roger

PS. Anyone [who] wants to see "state of the art" phase
splitter design, see the Mullard "5-10" (also "5-20")
schematics. Here's one source, the "5-10":
http://www.bonavolta.ch/hobby/en/audio/el84_7.htm
Note C3 to ground. But we're well away from vintage radio
now! [BTW, I built two of these a long time ago, wish I
still had then!]

Bill Sheppard replied:

In article
,
(Bill Sheppard) wrote:

"John Byrns gave an excellent synopsis on the common-cathode, grounded
grid scheme 3 or 4 years ago. Turns out the thing will always be
lopsided because the grounded-grid tube will never 'see' as high a
signal level between its G1 and cathode as the driven tube sees. This is
with the grid directly grounded. Maybe John can elaborate further.
John...?"


John Byrn then replied as follows:

"I had forgotten about posting on that circuit. Without
looking it up, I
think my main complaint was about the construction articles
that were
popular for a while, and featured this circuit. They
basically just used
an unbypassed common cathode resistor, whose value was
essentially the
same as normally used with a push pull pair of the same
tubes. This
doesn't really work at all well because the resistor is way
too low in
value, and the second tube receives very little drive. The
circuit should
work very well for class A operation if the cathode resistor
is replaced
with a current source though. Since a current source would
be more
complex than the phase inverter we were trying to eliminate,
I did some
simulations and found that the circuit worked fairly well if
the cathode
resistor were increased in value, to maybe 3 or 4 times the
normal value,
I forget exactly what value seemed to be a good compromise.
The grid
resistors of the two output tubes must then be connected to
a tap on the
cathode resistor to provide the correct bias, basically the
upper part of
the cathode resistor would be equal to the normal cathode
resistor.

"Someone in this group recently posted about a radio, I
forget what it was,
that had what I thought was the cleverest phase inverter for
a radio that
I have ever seen. I forget what tubes the actual radio
used, but
basically the detector and audio amplifier tube was
something like a
12SQ7, with the normal plate load resistor split between the
cathode and
plate circuits the same way as in a split load phase
inverter, with the
two output stage grids connected to the plate and cathode as
with the
split load inverter. A normal split load phase inverter
would provide no
voltage gain, so an additional tube would normally be
required to provide
the needed gain. In this circuit the detector circuit and
volume control
are floated up from ground, and referenced to the cathode of
the 12SQ7.
This effectively drives the 12SQ7 between grid and cathode,
providing
normal gain, which is cut in half to each output grid by the
effect of the
split load.

"The most obvious problem with this scheme is that the
detector can no
longer be used to provide the AGC voltage for the radio
because the diode
detector circuit is flapping around with the amplified audio
at some
positive potential. This problem is easily overcome by
using a tube like
the 12SF7, which includes a diode, as the IF amplifier. The
diode can
then serve as a shunt AGC rectifier feed from the primary of
the IF
transformer, as is often done with a second diode anyway.

"This circuit is so clever I wonder why it wasn't commonly
used? Does it
have some subtle problem that is not obvious? Actually now
that I think
about it, I have also seen it used in some old British
construction
articles too.

"Regards,

"John Byrns"

Me again...

So, there it is - I thought quite fascinating. I'd really
like to hear what the RAT-ers have to say about that very
poor PP "design" in several commercial radios that prompted
my first post, also John's neat idea.

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"



  #3   Report Post  
Engineer
 
Posts: n/a
Default [Fwd: Old PP outputs stages]

Long post - apologies!

I first posted this to rec.antiques.radio+phono as that was
the focus of my question (and I didn't want to cross-post.)
However, I received a very interesting answer from John
Byrns that I thought was worth sharing with the denizens of
RAT.

I have copied the original below, plus John's reply (also
Bill's). Hope it all makes sense!

Cheers,

Roger

-------- Original Message --------
Subject: Old PP outputs stages
Date: Sun, 31 Aug 2003 15:32:14 -0700
From: Engineer
Reply-To:
Organization: Lorna & Roger at home
Newsgroups: rec.antiques.radio+phono

I've been perusing the schematics in Ed Kraushar's excellent
"Vintage Canadian Radio Schematics" CD (OK, get a life, I
know...!) and am a bit bemused by one of the phase splitter
designs employed in a couple of the sets that have push-pull
audio output stages.

The no-brainer solution is, of course, the coupling
transformer: primary in the audio amplifier plate circuit,
secondary centre tap to ground (or output tube -ve bias) and
the anti-phase secondary ends to the two output tube grids.
Apart from quibbles over transformer frequency response,
hardly a problem with AM radio, it's perfect, i.e. compact,
economical (in quantity) and provides some free gain to
boot. BTW, I would not use interstage coupling transformers
in "modern" tube Hi-Fi designs even though, IIRC, Williamson
and others did it for years, and there was at least one
solid state amplifier in the late 1960-70's that use it (The
Ravensbourne, I think it was, plus another in the same line,
the Ravensbrook - I had one!)

However, quite a few of the [old] radio receiver designs
don't use coupling transformers. They use one of the
following methods:

1. A triode phase splitter tube with equal cathode and plate
load resistors - this is the classic design still used in
some high end audio, so clearly OK for AM radio.

2. A separate phase inverter tube: the "in-phase" signal
from the directly driven output tube grid is taken through
an attenuator to the grid of a triode tube so that the
"attenuation x gain" is equal to unity (we hope), then the
output of this triode drives the second output tube grid. I
guess it works when the design is done right, the gain is
unity and the tube is new. But it's all open-loop so PP
balanced drive is dependent on component values and tube
gains remaining the same. This only gets a "B-" in my
design book, and that's generous.

3. This is the odd one that prompted this posting. There's
no interstage transformer, phase splitter or inverter tube,
as such. Here, the audio amplifier output goes to only one
output grid - via a coupling capacitor, of course. The two
output tubes share an undecoupled cathode resistor
(typically 330 ohms in the case of a pair of 6V6's.) So far
so good - we presume the second O/P tube is cathode driven,
providing the grid is grounded to signal by a large
capacitor. But it's not! In at least two designs (Astra
1946-47 Model DR103-47 and Brunswick 1931-32 Model B-H) the
second output tube grid goes to ground via 470K but without
any decoupling capacitor.

My questions on method (3), above, for the vintage radio
experts a

A. How does the second output tube get it's grid-cathode
(inverted) drive if the second grid floats?
B. If it somehow still gets driven, is it balanced?
C. Why not parallel the 470K grid resistor with, say, a 0.1
uF cap. to ensure the cathode space charge cannot move the
grid at signal frequency and unbalance the drive?
D. Even if the second grid was properly decoupled, would
just a common 330 ohm resistor provide a balanced output
stage? Or would the second output tube always be
chronically under driven?

Cheers,

Roger

PS. Anyone [who] wants to see "state of the art" phase
splitter design, see the Mullard "5-10" (also "5-20")
schematics. Here's one source, the "5-10":
http://www.bonavolta.ch/hobby/en/audio/el84_7.htm
Note C3 to ground. But we're well away from vintage radio
now! [BTW, I built two of these a long time ago, wish I
still had then!]

Bill Sheppard replied:

In article
,
(Bill Sheppard) wrote:

"John Byrns gave an excellent synopsis on the common-cathode, grounded
grid scheme 3 or 4 years ago. Turns out the thing will always be
lopsided because the grounded-grid tube will never 'see' as high a
signal level between its G1 and cathode as the driven tube sees. This is
with the grid directly grounded. Maybe John can elaborate further.
John...?"


John Byrn then replied as follows:

"I had forgotten about posting on that circuit. Without
looking it up, I
think my main complaint was about the construction articles
that were
popular for a while, and featured this circuit. They
basically just used
an unbypassed common cathode resistor, whose value was
essentially the
same as normally used with a push pull pair of the same
tubes. This
doesn't really work at all well because the resistor is way
too low in
value, and the second tube receives very little drive. The
circuit should
work very well for class A operation if the cathode resistor
is replaced
with a current source though. Since a current source would
be more
complex than the phase inverter we were trying to eliminate,
I did some
simulations and found that the circuit worked fairly well if
the cathode
resistor were increased in value, to maybe 3 or 4 times the
normal value,
I forget exactly what value seemed to be a good compromise.
The grid
resistors of the two output tubes must then be connected to
a tap on the
cathode resistor to provide the correct bias, basically the
upper part of
the cathode resistor would be equal to the normal cathode
resistor.

"Someone in this group recently posted about a radio, I
forget what it was,
that had what I thought was the cleverest phase inverter for
a radio that
I have ever seen. I forget what tubes the actual radio
used, but
basically the detector and audio amplifier tube was
something like a
12SQ7, with the normal plate load resistor split between the
cathode and
plate circuits the same way as in a split load phase
inverter, with the
two output stage grids connected to the plate and cathode as
with the
split load inverter. A normal split load phase inverter
would provide no
voltage gain, so an additional tube would normally be
required to provide
the needed gain. In this circuit the detector circuit and
volume control
are floated up from ground, and referenced to the cathode of
the 12SQ7.
This effectively drives the 12SQ7 between grid and cathode,
providing
normal gain, which is cut in half to each output grid by the
effect of the
split load.

"The most obvious problem with this scheme is that the
detector can no
longer be used to provide the AGC voltage for the radio
because the diode
detector circuit is flapping around with the amplified audio
at some
positive potential. This problem is easily overcome by
using a tube like
the 12SF7, which includes a diode, as the IF amplifier. The
diode can
then serve as a shunt AGC rectifier feed from the primary of
the IF
transformer, as is often done with a second diode anyway.

"This circuit is so clever I wonder why it wasn't commonly
used? Does it
have some subtle problem that is not obvious? Actually now
that I think
about it, I have also seen it used in some old British
construction
articles too.

"Regards,

"John Byrns"

Me again...

So, there it is - I thought quite fascinating. I'd really
like to hear what the RAT-ers have to say about that very
poor PP "design" in several commercial radios that prompted
my first post, also John's neat idea.

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"
  #4   Report Post  
Fred Nachbaur
 
Posts: n/a
Default



Engineer wrote:
john stewart wrote:

(snip)


Hi Roger- I read your query along with John B's very good
description of some of these phase splitters. Here are two more
for you to ponder, as they were used in some 30's & 40's radios.

In this scheme there is no seperate phase inverter tube or transformer.
A resister of the proper value (about 2K) is inserted into the G2 lead
of the upper pentode of a PP pair. The resulting signal on that G2 is
capacitively coupled to the G1 of the lower member of the PP pair.
Of course there is some imbalance, but that is partially corrected by
the PP output transformer. This circuit was often seen using PP 6F6's
but could be applied to other pentodes or beam tetrodes as well.



John, the screen idea is what I came up with while trying to
figure out how to phase split without an extra tube - the
purpose of the original "kluge" - but I didn't calculate any
values. Done properly, it just might just work better than a
common cathode resistor - a good experiment for someone with
an old PP 6V6 amp to play with (I don't have one or I'd try
it!)


Another notion I've intended to try out (and indeed will, one of these
days) is to use the cathode coupling (short-tail pair) and augment the
signal to the "off" phase with a voltage divider from plate of the "on"
phase to grid. This would seem to give a less distorted signal than
taking it from the screen grid.

By making this variable, the amount of asymmetry can be varied to either
side. This might actually be useful for blues harp or acoustic guitar
players, since it appears that acoustic feedback can be reduced this way.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

  #5   Report Post  
Engineer
 
Posts: n/a
Default

john stewart wrote:

(snip)

Hi Roger- I read your query along with John B's very good
description of some of these phase splitters. Here are two more
for you to ponder, as they were used in some 30's & 40's radios.

In this scheme there is no seperate phase inverter tube or transformer.
A resister of the proper value (about 2K) is inserted into the G2 lead
of the upper pentode of a PP pair. The resulting signal on that G2 is
capacitively coupled to the G1 of the lower member of the PP pair.
Of course there is some imbalance, but that is partially corrected by
the PP output transformer. This circuit was often seen using PP 6F6's
but could be applied to other pentodes or beam tetrodes as well.


John, the screen idea is what I came up with while trying to
figure out how to phase split without an extra tube - the
purpose of the original "kluge" - but I didn't calculate any
values. Done properly, it just might just work better than a
common cathode resistor - a good experiment for someone with
an old PP 6V6 amp to play with (I don't have one or I'd try
it!)

(snip)

Cheers, John Stewart


Cheers,

Roger

PS. Thanks to all for the great comments.

--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"


  #6   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Fred Nachbaur" wrote

...
Another notion I've intended to try out (and indeed will, one of

these
days) is to use the cathode coupling (short-tail pair) and augment

the
signal to the "off" phase with a voltage divider from plate of the

"on"
phase to grid. This would seem to give a less distorted signal

than
taking it from the screen grid.

By making this variable, the amount of asymmetry can be varied to

either
side. This might actually be useful for blues harp or acoustic

guitar
players, since it appears that acoustic feedback can be reduced

this way.

Confused Ian again...

you only identify two legs of the divider so your scheme seems
ambiguous to me.

Do you mean top to anode, bottom to same grid, and middle to
opposite grid. Or top to anode, bottom to ground, and middle to
opposite grid? Or maybe even top to anode, bottom to opposite
anode, and middle to opposite grid?

Interesting experiments, partly because of the problem getting a low
enough source impedance for that opposite grid without causing
distortion and/or wasting too much current through the divider or
voltage across the current-sensing resistor. More interestingly
because of the difference between the series and parallel-derived
alternatives, when it comes to maintaining correct phase
relationship across the whole bandwidth.

Also wondering which sources of distortion will tend to cancel, and
which will augment. For example with the series derivation from
opposite screen, will the nonlinear blip in screen current at low Va
result in a signal that will counter or reinforce the simultaneous
drop in Ia?

Dunno if that made sense...I know what I mean...

cheers, Ian



  #7   Report Post  
Fabio Berutti
 
Posts: n/a
Default

I'm listening to the radio with such an amp right now. My problem was this
VERY hot summer: I just couldn't run my usual 6L6GC PP poweramp the whole
day without sweating just passing in front of it, therefore I put together a
real poor-man's amp aimed at 2Wpower with less than 10W dissipation. As I
said, it's truly for ambience listening. I took the schematics from
Triode (http://www.triodeel.com/compact.html) but I replaced the EL84s with
the two halves of a JJ/Tesla ECC99 (let's say roughly a 12BH7, a 3+3W double
triode with a Ri=3000 Ohms). Instead of a 6C4 the driver is (obviously) an
ECC82 used for both channels. The OPTs are readily available, "universal"
Hammond mod. 125H, connected in order to get 10K A-A. The common cathode
resistor is a 390R, 4W, metal oxide. Anode voltage is about 260V, with the
cathodes sitting at about 10V above ground (A-K=250V). Power supply is
guaranteed by a diodes bridge and a resistive Pi filter using 2x 47uF/450V
electrolytics.
I'm pretty satisfied with it, because with just 3 tubes I get the "musical
wallpaper" I need all day long (I'm a professional engineer working mostly
at home) without resorting to the sand state of matter. Now it works
connected to a couple of self-made small loudpeakers using a single Fostex
FE83 cone (2WRMS): it doesn't even sound so bad.
I never measured any parameter because I didn't care, but if the fellow
rodents are interested I can take some 'scope pictures.

Ciao to all

Fabio

"Fred Nachbaur" ha scritto nel messaggio
news:JZT4b.128124$K44.120827@edtnps84...


Engineer wrote:
john stewart wrote:

(snip)


Hi Roger- I read your query along with John B's very good
description of some of these phase splitters. Here are two more
for you to ponder, as they were used in some 30's & 40's radios.

In this scheme there is no seperate phase inverter tube or transformer.
A resister of the proper value (about 2K) is inserted into the G2 lead
of the upper pentode of a PP pair. The resulting signal on that G2 is
capacitively coupled to the G1 of the lower member of the PP pair.
Of course there is some imbalance, but that is partially corrected by
the PP output transformer. This circuit was often seen using PP 6F6's
but could be applied to other pentodes or beam tetrodes as well.



John, the screen idea is what I came up with while trying to
figure out how to phase split without an extra tube - the
purpose of the original "kluge" - but I didn't calculate any
values. Done properly, it just might just work better than a
common cathode resistor - a good experiment for someone with
an old PP 6V6 amp to play with (I don't have one or I'd try
it!)


Another notion I've intended to try out (and indeed will, one of these
days) is to use the cathode coupling (short-tail pair) and augment the
signal to the "off" phase with a voltage divider from plate of the "on"
phase to grid. This would seem to give a less distorted signal than
taking it from the screen grid.

By making this variable, the amount of asymmetry can be varied to either
side. This might actually be useful for blues harp or acoustic guitar
players, since it appears that acoustic feedback can be reduced this way.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+



  #8   Report Post  
Fred Nachbaur
 
Posts: n/a
Default



Ian Iveson wrote:
"Fred Nachbaur" wrote


...
Another notion I've intended to try out (and indeed will, one of
these days) is to use the cathode coupling (short-tail pair) and augment
the signal to the "off" phase with a voltage divider from plate of the
"on" phase to grid. This would seem to give a less distorted signal
than taking it from the screen grid.

By making this variable, the amount of asymmetry can be varied to
either side. This might actually be useful for blues harp or acoustic
guitar players, since it appears that acoustic feedback can be reduced
this way.

Confused Ian again...

you only identify two legs of the divider so your scheme seems
ambiguous to me.

Do you mean top to anode, bottom to same grid, and middle to
opposite grid. Or top to anode, bottom to ground, and middle to
opposite grid? Or maybe even top to anode, bottom to opposite
anode, and middle to opposite grid?


None of the above. Top to "on" phase anode, middle to "off" phase grid,
and bottom to ground.

Interesting experiments, partly because of the problem getting a low
enough source impedance for that opposite grid without causing
distortion and/or wasting too much current through the divider or
voltage across the current-sensing resistor.


That's why it's nice (I think) to take it from the anode rather than the
screen. The absolute value of the divider resistors can be made quite
high, since the grid impedance will be very high in class A1. (Such
approaches would probably bomb spectacularly if grid current is drawn.)

The idea is merely to "grid-enhance" the cathode-coupled signal, much
like the experimental "grid-enhanced cathode-coupled paraphase" did.
http://www.dogstar.dantimax.dk/tubestuf/paraph.htm#tt42

More interestingly
because of the difference between the series and parallel-derived
alternatives, when it comes to maintaining correct phase
relationship across the whole bandwidth.

Also wondering which sources of distortion will tend to cancel, and
which will augment. For example with the series derivation from
opposite screen, will the nonlinear blip in screen current at low Va
result in a signal that will counter or reinforce the simultaneous
drop in Ia?


All good questions. I can answer them all easily: "Don't know." However,
I'm tempted to say that if such considerations are important, then it's
unlikely that a "Muntzed" circuit would be a good starting point in the
first place.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

  #9   Report Post  
John Byrns
 
Posts: n/a
Default

In article ,
"Steve O'Neill" wrote:

"There’s been a lot of discussion and speculation regarding the compact amp
and it’s true mode of operation. I decided to build it so I could take some
measurements. I built only the output stage using a decent vintage PP OPT
measured at ~ 7800 ohm plate to plate with 4,8 and 16 ohm output taps. This
was originally designed for a 6V6 amp. 10 ohm resistors were placed in
series with the plate leads so DC and AC current could be measured. The
cathode resistor was 125 ohm and a regulated PS was used at 260VDC. Output
tubes were used/test good Sylvania 6BQ5s within a few mA at design
conditions and within about 10% on gm. V1 is the top or “driven” tube and
V2 is the bottom tube. Load =8ohm NI resistor.

At idle V1Ip =37mA,V2Ip =38mA, V across cathode R = 10.1VDC .

Max power out @ 1KHz (obvious clipping)
V across 8 ohm = 7.4 VAC = 6.9W: Waveform was distorted, unsymmetrical but
rounded
Input to V1 betw grid and ground = 10.0 VAC, betw grid and cathode =
6.5VAC, input to V2 betw grid and cathode = 3.3VAC (also voltage across
cathode resistor).
AC plate current of V1 = 44mA: of V2 = 22mA.

At 5VAC across 8 ohms @ 1KHz (3.1 watts or about –3dB below design output)
Waveform was fairly clean with a little evidence of flattening on one side
(2H?).
Input to V1 betw grid and ground = 5.6VAC, betw grid and cathode = 3.7VAC,
input to V2 betw grid and cathode = 1.95VAC (also voltage across cathode
resistor).
AC plate current of V1 = 30mA: of V2 = 16mA

It’s been speculated that the lower tube was actually a CCS to counter the
DC current of the upper tube thus allowing use of a PP OPT. A suggested
test for this was to bypass the cathode resistor with a cap. The expected
result if the lower tube was a CCS would be little change in output. Under
the 3W output conditions when the cathode resistor was bypassed the RMS
value of the output actually went up from 5.0 VAC to 5.1VAC. The output
became very distorted with one side becoming quite flat as if one of the
output tubes had been removed. Under this condition the AC plate current of
V1 = 46mA: of V2 = 4mA.

I made no distortion or frequency response measurements nor did I listen to
the amp.

Based on these measurements I conclude that the amp is truly a PP design
with but with very poor balance. I also conclude that V2 isn’t a CCS at
least as traditionally defined since V2 is driven and contributes to the
output. I also observed that this design is very sensitive to output tube
balance if anywhere near the “rated” 6W is going to be achieved.

Any other interpretations??? "



I would comment that the idea that the second tube is a constant current
source would seem to be wrong based on the output transformer primary
impedance used. If the second tube were only a CCS, then half the
transformer primary should look like approximately 5,000 Ohms to the upper
tube, this would imply a 20,000 Ohm center tapped primary. In reality
with the commonly used cathode resistor values, the second tube does not
contribute nearly as much power as it should, which makes me wonder if a
transformer primary higher than 8,000 Ohms, but less than 20,000 Ohms
might not give slightly more power output? The optimum value would need
to be calculated. This would be like connecting a load somewhat higher
than 8 Ohms to the 8 Ohm tap on the secondary of your transformer with the
7,800 Ohm primary.

Would it be fair to use a transistor in the cathode tail circuit of this
amplifier? A transistor hooked up as a constant current source would
probably bump the power output up a notch.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
  #10   Report Post  
Engineer
 
Posts: n/a
Default

Fred Nachbaur wrote:

(snip)

Interesting experiments, partly because of the problem getting a low
enough source impedance for that opposite grid without causing
distortion and/or wasting too much current through the divider or
voltage across the current-sensing resistor.


My guess is that it would be just 2 Kohms, quite low enough
to drive a properly biased G1.

That's why it's nice (I think) to take it from the anode rather than the
screen. The absolute value of the divider resistors can be made quite
high, since the grid impedance will be very high in class A1. (Such
approaches would probably bomb spectacularly if grid current is drawn.)


Should work OK - I would not expect any G1 current on the
second O/P tube. But I think the B+ would have to be
decoupled to a "totally zero" impedance down to the lowest
frequency of the O/P stage, even below or you might have an
oscillator - I would not care to speculate on the frequency,
somewhere between "motor-boat" and stuck-pig squeal
perhaps!

(snip)

Cheers,

Roger

--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"


  #12   Report Post  
john stewart
 
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Steve O'Neill wrote:

Hi

You stated at the end of your post:

"Luckily for those radios that escaped the factory
the bottom tube will not get in the way since it's plate reflects
a CC load to the output transformer"

Curious how you arrive at the conclusion that the bottom tube plate acts as
a CC load to the OPT. I built the TRIODEEL compact amp output stage using
recommended circuit values and measured the thing. By the measurements I
made, the bottom tube did not behave as a CC load. The whole thing acted
like a poorly balanced phase inverter: nothing more, nothing less. Did I
measure something wrongly or are we speaking of different circuits here?

--
Steve


I finally got a look at that circuit by way of a link posted by Fabio Berutti.
It is simple enough alright but doomed to poor performance. It may appeal
to the SE crowd since it is bound to generate lots of even order harmonics
with its very shot tail.

I managed to find a drawing of the amp I built using this circuit. It is dated
Feb 3, 1958. At the time I was trying to find a way of building a very low
cost amp for hifi. I had come across a good supply of cheap power
transformers with 170-0-170 HT, so bought several. The basic amps
were various combos of PP pairs of 25L6, 35L6 or 50L6's, so the heaters
could be run straight off the power line. I used a 6SJ7 to drive the PP
amp with the short tail.

I quickly came to the same conclusion that you did, although with less
sophisticated
test equipment. It looked terrible on the scope trace & did not sound much
better.
Why anyone would think this circuit would deliver hifi I'm not sure. We are
told time & again that in this type of phase inverter that for balance the tail

needs to be at least as large as the plate loads & preferable several times
the plate load. This amp finished up with a 6SF5 split load phase inverter.

Pentodes & Beam Tetrodes are inherentely CC devices when operated
in their linear mode. If the short tail were bypassed then you would have
a CC load applied to one end of the output transformer. However, in
this instance, the lower of the PP pair is partially driven, so perhaps not
as CC as when not driven. It would be interesting to get into the circuit
to get some idea of what impedance we are talking about. However,
who wants or needs to spend time it what amounts to trying
to push a rope?

BTW, I have posted both John B' s phase inverter & some other
inverters (one referred too by Fred N), all from RDH4 on ABSE
for everyones enjoyment. See Phase Inverters. John Stewart




  #13   Report Post  
john stewart
 
Posts: n/a
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Curious how you arrive at the conclusion that the bottom tube plate acts as
a CC load to the OPT. I built the TRIODEEL compact amp output stage using
recommended circuit values and measured the thing. By the measurements I
made, the bottom tube did not behave as a CC load. The whole thing acted
like a poorly balanced phase inverter: nothing more, nothing less. Did I
measure something wrongly or are we speaking of different circuits here?
Steve


I forgot to mention in the previous post that it would seem the performance


of the subject circuit is further compromised by the fact that the G2 current


also runs thru the short tail. That would give rise to an even more interesting
set of harmonics & some unbelievable IM products.

Perhaps Fred N could sell that to one of his guitar pickin' buds!!!

Cheers, John Stewart

  #14   Report Post  
Choky
 
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"john stewart" wrote in message
...
snippp


Perhaps Fred N could sell that to one of his guitar pickin' buds!!!

Cheers, John Stewart

FredN can't sell anything;
he needs any bloody toob gadget only for his self
he even can't sell any S**** gadget-he need that also for converting to tube
gadget
hehe

--
..
Choky
Prodanovic Aleksandar
YU

"don't use force,
use a larger hammer"
- ZM
..


  #15   Report Post  
Fred Nachbaur
 
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john stewart wrote:

[...]



I forgot to mention in the previous post that it would seem the performance



of the subject circuit is further compromised by the fact that the G2 current



also runs thru the short tail. That would give rise to an even more interesting
set of harmonics & some unbelievable IM products.

Perhaps Fred N could sell that to one of his guitar pickin' buds!!!

Cheers, John Stewart


Yes, that sort of thing is just the ticket for music producing
amplifiers. ;-)

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

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