What the easiest ways to measure the output impedance of a push pull
tube amplifier?
Thanks
Hank

On Nov 1, 9:24*pm, "Henry Kolesnik"
wrote:
What the easiest ways to measure the output impedance of a push pull
tube amplifier?
Thanks
Hank

Henry, do you mean the true Thevenin O/P impedance of the unit as a
"near voltage" source, i.e. very low indeed, less than, say, 0.5 ohm?
Or do you mean what is the nominal speaker impedance that it is set up
for, e.g. 4, 8 or 16 ohms?
Cheers,
Roger

normal speaker impedance
"Engineer" wrote in message
...
On Nov 1, 9:24 pm, "Henry Kolesnik"
wrote:
What the easiest ways to measure the output impedance of a push pull
tube amplifier?
Thanks
Hank

Henry, do you mean the true Thevenin O/P impedance of the unit as a
"near voltage" source, i.e. very low indeed, less than, say, 0.5 ohm?
Or do you mean what is the nominal speaker impedance that it is set up
for, e.g. 4, 8 or 16 ohms?
Cheers,
Roger

Henry Kolesnik wrote:

normal speaker impedance


A low, known AC voltage is applied to the secondary winding, and the
voltage is measured across the other. Be careful, because the voltage on
the primary winding will be high enough to shock you.

The voltage on the primary over the applied voltage gives you the
turns ratio. Square that number and you have the impedance ratio from
primary to secondary. Since the transformer is in a push-pull
application, the primary impedance is 'Plate to plate'.


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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

This is in an old military radio which I suspect is not 3.2 ohms but
somewhat higher like maybe 600. It's kind of hard to get to the
primary. I guess I should have stated that at the beginning. I kind of
recall some way of measuring the open circuit voltage from a tone and
then with a pot connected across the output, set the pot to where the
voltage is 1/2 and then measure the pot. Do I have that correct?
Hank
"Michael A. Terrell" wrote in message
m...

Henry Kolesnik wrote:

normal speaker impedance


A low, known AC voltage is applied to the secondary winding, and the
voltage is measured across the other. Be careful, because the voltage
on
the primary winding will be high enough to shock you.

The voltage on the primary over the applied voltage gives you the
turns ratio. Square that number and you have the impedance ratio from
primary to secondary. Since the transformer is in a push-pull
application, the primary impedance is 'Plate to plate'.


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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

Henry Kolesnik wrote in message
...
This is in an old military radio which I suspect is not 3.2 ohms but
somewhat higher like maybe 600. It's kind of hard to get to the
primary. I guess I should have stated that at the beginning. I kind of
recall some way of measuring the open circuit voltage from a tone and
then with a pot connected across the output, set the pot to where the
voltage is 1/2 and then measure the pot. Do I have that correct?
Hank
"Michael A. Terrell" wrote in message
m...

Henry Kolesnik wrote:

normal speaker impedance


A low, known AC voltage is applied to the secondary winding, and the
voltage is measured across the other. Be careful, because the voltage
on
the primary winding will be high enough to shock you.

The voltage on the primary over the applied voltage gives you the
turns ratio. Square that number and you have the impedance ratio from
primary to secondary. Since the transformer is in a push-pull
application, the primary impedance is 'Plate to plate'.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

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in
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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.


I would go for a set of watty resistors in series, tone source and a DVM on
AC volts, with amp set at low output.
If on some set of resistors of total value of R , the o/p voltage is greater
than with value R+d and R-d , where d is about R/4 then R is the output
impedance. If highest at R-d then go down, in steps, till it peaks, if R+d
is highest then go higher for peak value


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"Henry Kolesnik" wrote in message
...
What the easiest ways to measure the output impedance of a push pull tube
amplifier?

Load the amplifier with an adjustable resistor. When the loaded output
voltage drops to 1/2 of the no-load voltage, the resistor can be removed and
measured with an ohmmeter.

In message , Charles
writes

"Henry Kolesnik" wrote in message
...
What the easiest ways to measure the output impedance of a push pull tube
amplifier?

Load the amplifier with an adjustable resistor. When the loaded output
voltage drops to 1/2 of the no-load voltage, the resistor can be removed and
measured with an ohmmeter.

Just in case the amplifier is 'unhappy' with such a low load, it might
be better to load the output with a resistor low enough to produce a
less severe - but measurable - drop in output voltage (say by 10 or
20%). Then do a simple 'pot-down' calculation to get the output
impedance.
--
Ian

Henry Kolesnik wrote:

This is in an old military radio which I suspect is not 3.2 ohms but
somewhat higher like maybe 600. It's kind of hard to get to the
primary. I guess I should have stated that at the beginning. I kind of
recall some way of measuring the open circuit voltage from a tone and
then with a pot connected across the output, set the pot to where the
voltage is 1/2 and then measure the pot. Do I have that correct?


That is how we measured the input impedance on the PRC77 The input
was fed a 1 kHz tone, and a series resistance was added, till the
modulation dropped to half.

If it is an old military radio it is likely 600 ohms. What is the
brand and model, or the military ID number?

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

It's a BC-794 but I think it's been modified and if not something else
is kaput.
"Michael A. Terrell" wrote in message
m...

Henry Kolesnik wrote:

This is in an old military radio which I suspect is not 3.2 ohms but
somewhat higher like maybe 600. It's kind of hard to get to the
primary. I guess I should have stated that at the beginning. I kind
of
recall some way of measuring the open circuit voltage from a tone and
then with a pot connected across the output, set the pot to where the
voltage is 1/2 and then measure the pot. Do I have that correct?


That is how we measured the input impedance on the PRC77 The input
was fed a 1 kHz tone, and a series resistance was added, till the
modulation dropped to half.

If it is an old military radio it is likely 600 ohms. What is the
brand and model, or the military ID number?

--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included
in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

On Nov 2, 5:47*pm, "Charles" wrote:
"Henry Kolesnik" wrote in message

...

What the easiest ways to measure the output impedance of a push pull tube
amplifier?

Load the amplifier with an adjustable resistor. *When the loaded output
voltage drops to 1/2 of the no-load voltage, the resistor can be removed and
measured with an ohmmeter.

Sorry, not so. This will only give you what I originally mentioned,
i.e. equivalent generator source impedance (see Thevenin), and it will
be very low - no use in selecting speakers. You need the O/P
transformer (OPT) ratio. Make sure the receiver/amplifier is OFF. Use
a filament transformer run off a variac to energise the OPT secondary
(use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
the OPT primary and secondary voltages at a few levels. Calculate
each ratio and average them. Figure out the correct plate load for
the O/P tube (not covered here) - it will likely be in the range 4
Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
So use a 4 ohm speaker.
Cheers,
Roger

In message
,
Engineer writes
On Nov 2, 5:47*pm, "Charles" wrote:
"Henry Kolesnik" wrote in message

...

What the easiest ways to measure the output impedance of a push pull tube
amplifier?

Load the amplifier with an adjustable resistor. *When the loaded output
voltage drops to 1/2 of the no-load voltage, the resistor can be removed and
measured with an ohmmeter.

Sorry, not so. This will only give you what I originally mentioned,
i.e. equivalent generator source impedance (see Thevenin),

Surely this IS the output impedance?

and it will
be very low - no use in selecting speakers.

You need the O/P
transformer (OPT) ratio. Make sure the receiver/amplifier is OFF. Use
a filament transformer run off a variac to energise the OPT secondary
(use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
the OPT primary and secondary voltages at a few levels. Calculate
each ratio and average them. Figure out the correct plate load for
the O/P tube (not covered here) - it will likely be in the range 4
Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
So use a 4 ohm speaker.
Cheers,
Roger

But what does the invariable negative feedback (from the OPT secondary
to an earlier amplifier stage) do to the output impedance?
--
Ian

Roger
50 years ago I wound and rewound many a transformer with great success.
But today I'm older, more patient and lazy and looking for an easy way
out that may not be totally accurate.
But I kind of recall reading an excellent article with a trick that I
thought was pretty ingenious but I never saved it. It may have been a
ham mag, or Howard Sams booklet or maybe Rufus Turner. Maybe someone
will remember.
Hank
"Engineer" wrote in message
...
On Nov 2, 5:47 pm, "Charles" wrote:
"Henry Kolesnik" wrote in message

...

What the easiest ways to measure the output impedance of a push pull
tube
amplifier?

Load the amplifier with an adjustable resistor. When the loaded output
voltage drops to 1/2 of the no-load voltage, the resistor can be
removed and
measured with an ohmmeter.

Sorry, not so. This will only give you what I originally mentioned,
i.e. equivalent generator source impedance (see Thevenin), and it will
be very low - no use in selecting speakers. You need the O/P
transformer (OPT) ratio. Make sure the receiver/amplifier is OFF. Use
a filament transformer run off a variac to energise the OPT secondary
(use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
the OPT primary and secondary voltages at a few levels. Calculate
each ratio and average them. Figure out the correct plate load for
the O/P tube (not covered here) - it will likely be in the range 4
Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
So use a 4 ohm speaker.
Cheers,
Roger

I just recalled I have an old GR 583-A Output Power Meter and if it
still works it'll tell me. It's been wasting gravity for years, Google
it and you'll see one on Sphere. Thanks to all for the comments, and
I'd like to know any tricks?
Hank
"Henry Kolesnik" wrote in message
news
Roger
50 years ago I wound and rewound many a transformer with great
success. But today I'm older, more patient and lazy and looking for an
easy way out that may not be totally accurate.
But I kind of recall reading an excellent article with a trick that I
thought was pretty ingenious but I never saved it. It may have been a
ham mag, or Howard Sams booklet or maybe Rufus Turner. Maybe someone
will remember.
Hank
"Engineer" wrote in message
...
On Nov 2, 5:47 pm, "Charles" wrote:
"Henry Kolesnik" wrote in message

...

What the easiest ways to measure the output impedance of a push
pull tube
amplifier?

Load the amplifier with an adjustable resistor. When the loaded
output
voltage drops to 1/2 of the no-load voltage, the resistor can be
removed and
measured with an ohmmeter.

Sorry, not so. This will only give you what I originally mentioned,
i.e. equivalent generator source impedance (see Thevenin), and it will
be very low - no use in selecting speakers. You need the O/P
transformer (OPT) ratio. Make sure the receiver/amplifier is OFF. Use
a filament transformer run off a variac to energise the OPT secondary
(use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
the OPT primary and secondary voltages at a few levels. Calculate
each ratio and average them. Figure out the correct plate load for
the O/P tube (not covered here) - it will likely be in the range 4
Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
So use a 4 ohm speaker.
Cheers,
Roger

On Nov 3, 6:12*pm, Ian Jackson
wrote:
In message
,
Engineer writes

(snip)


But what does the invariable negative feedback (from the OPT secondary
to an earlier amplifier stage) do to the output impedance?
--
Ian

Ian, it reduces the source output impedance of the amplifier as a
voltage generator (increasing the damping factor) but does not affect
the speaker impedance to be used. The speaker impedance determines
the O/P tube plate (anode) load via the OPT ratio.
Cheers,
Roger

In message
,
Engineer writes
On Nov 3, 6:12*pm, Ian Jackson
wrote:
In message
,
Engineer writes

(snip)


But what does the invariable negative feedback (from the OPT secondary
to an earlier amplifier stage) do to the output impedance?
--
Ian

Ian, it reduces the source output impedance of the amplifier as a
voltage generator (increasing the damping factor) but does not affect
the speaker impedance to be used. The speaker impedance determines
the O/P tube plate (anode) load via the OPT ratio.
Cheers,
Roger

But the original question was "What the easiest ways to measure the
output impedance of a push pull tube amplifier?" It didn't mention
loudspeakers.
--
Ian

On Nov 4, 4:49*am, Ian Jackson
wrote:
In message
,
Engineer writes





On Nov 3, 6:12 pm, Ian Jackson
wrote:
In message
,
Engineer writes

(snip)

But what does the invariable negative feedback (from the OPT secondary
to an earlier amplifier stage) do to the output impedance?
--
Ian

Ian, it reduces the source output impedance of the amplifier as a
voltage generator (increasing the damping factor) but does not affect
the speaker impedance to be used. *The speaker impedance determines
the O/P tube plate (anode) load via the OPT ratio.
Cheers,
Roger

But the original question was "What the easiest ways to measure the
output impedance of a push pull tube amplifier?" It didn't mention
loudspeakers.
--
Ian- Hide quoted text -

- Show quoted text -

Henry mentioned a miltary radio with a high output impedance (but I
don't know any with P-P output...) I have a BC348 and the OPT (from a
single 6K6 O/P tube) is designed for 'phones only, both "high" and
"low" impedance. The "low" impedance 'phone O/P is 300 ohms so using
a regular LS is not possible. I put a typical radio OPT in this radio
(I left the original 'phones OPT in place so it could be restored to
original sometime in the next 100 years!) The new OPT and the nominal
8 ohms LS used puts a reasonably correct plate load on the 6K6, about
7K. Volume is quite good - I'd guess about 2 watts output (but not
measured.)
There is no NFB in the original radio but there is a little bit now as
I routed the Rk decoupling cap to the top of the OPT secondary as a
token NFB (properly phased.) Of course, invariably there is a lot of
NFB on P-P audio amplifiers and, as noted above, it reduces the true
output impedance of the amplifier as a voltage source, but it does NOT
change the correct LS impedance as the latter defines the O/P tubes'
plate-to-plate load.
I don't know why Henry first asked about a "PP amplifier then "morphed
to a "military radio" but it doesn't matter - it's all the same
physics!
Cheers,
Roger

Neil Sutcliffe was kind enough to send me a manual on his GR 783A which
is a more recent version of my GR 583A Output Impedance Meter. The o/p
xfrmr on my Hmmarlund Super Pro 210X Type O is 10 ohms and I looked
inside and saw no evidence that it was changed. I confirmed that the
meter works by the same technique on my Ten Tec SP 325 which is 600 ohms
and it measured 600 ohms. The GR 583A is a neat piece of gear, easy as
pie to use.

--
Thanks & 73
Hank WD5JFR
"Henry Kolesnik" wrote in message
...
I just recalled I have an old GR 583-A Output Power Meter and if it
still works it'll tell me. It's been wasting gravity for years, Google
it and you'll see one on Sphere. Thanks to all for the comments, and
I'd like to know any tricks?
Hank
"Henry Kolesnik" wrote in message
news
Roger
50 years ago I wound and rewound many a transformer with great
success. But today I'm older, more patient and lazy and looking for
an easy way out that may not be totally accurate.
But I kind of recall reading an excellent article with a trick that I
thought was pretty ingenious but I never saved it. It may have been a
ham mag, or Howard Sams booklet or maybe Rufus Turner. Maybe someone
will remember.
Hank
"Engineer" wrote in message
...
On Nov 2, 5:47 pm, "Charles" wrote:
"Henry Kolesnik" wrote in message

...

What the easiest ways to measure the output impedance of a push
pull tube
amplifier?

Load the amplifier with an adjustable resistor. When the loaded
output
voltage drops to 1/2 of the no-load voltage, the resistor can be
removed and
measured with an ohmmeter.

Sorry, not so. This will only give you what I originally mentioned,
i.e. equivalent generator source impedance (see Thevenin), and it
will
be very low - no use in selecting speakers. You need the O/P
transformer (OPT) ratio. Make sure the receiver/amplifier is OFF.
Use
a filament transformer run off a variac to energise the OPT secondary
(use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
the OPT primary and secondary voltages at a few levels. Calculate
each ratio and average them. Figure out the correct plate load for
the O/P tube (not covered here) - it will likely be in the range 4
Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
So use a 4 ohm speaker.
Cheers,
Roger

In article ,
Henry Kolesnik wrote:
Neil Sutcliffe was kind enough to send me a manual on his GR 783A which
is a more recent version of my GR 583A Output Impedance Meter. The o/p
xfrmr on my Hmmarlund Super Pro 210X Type O is 10 ohms and I looked
inside and saw no evidence that it was changed. I confirmed that the
meter works by the same technique on my Ten Tec SP 325 which is 600 ohms
and it measured 600 ohms. The GR 583A is a neat piece of gear, easy as
pie to use.

If the output really is 600 ohms it sounds like it uses a cathode follower
and was never designed to feed a speaker. 600 ohms is the old line driving
impedance and headphones are available that will work fine from this at
line level.

--
*Why do we say something is out of whack? What is a whack?

Dave Plowman London SW
To e-mail, change noise into sound.

Dave
The SP 310X has a pair of 6F6 triode connected driving a pushpull
transformer with output of 15 watts. It was mainly a military Rx BC 794
etc butmine is a civilian model.
No cathode follower.
Any headphones work fine at 10 ohms.

--
Thanks & 73
Hank WD5JFR
"Dave Plowman (News)" wrote in message
...
In article ,
Henry Kolesnik wrote:
Neil Sutcliffe was kind enough to send me a manual on his GR 783A
which
is a more recent version of my GR 583A Output Impedance Meter. The
o/p
xfrmr on my Hmmarlund Super Pro 210X Type O is 10 ohms and I looked
inside and saw no evidence that it was changed. I confirmed that the
meter works by the same technique on my Ten Tec SP 325 which is 600
ohms
and it measured 600 ohms. The GR 583A is a neat piece of gear, easy
as
pie to use.

If the output really is 600 ohms it sounds like it uses a cathode
follower
and was never designed to feed a speaker. 600 ohms is the old line
driving
impedance and headphones are available that will work fine from this
at
line level.

--
*Why do we say something is out of whack? What is a whack?

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Henry Kolesnik wrote:
The SP 310X has a pair of 6F6 triode connected driving a pushpull
transformer with output of 15 watts. It was mainly a military Rx BC 794
etc butmine is a civilian model.

I can't see rhyme or reason for such a high power output at that impedance.

No cathode follower.
Any headphones work fine at 10 ohms.

If 10 ohm headphones work fine I doubt the output is 600 ohms.

--
*Filthy stinking rich -- well, two out of three ain't bad

Dave Plowman London SW
To e-mail, change noise into sound.

Dave
I was wrong it's only 10 watts. I know of no reason, perhaps someone
here will tell us. They made tens of thousands of these so it must have
had a need...
"Dave Plowman (News)" wrote in message
...
In article ,
Henry Kolesnik wrote:
The SP 310X has a pair of 6F6 triode connected driving a pushpull
transformer with output of 15 watts. It was mainly a military Rx BC
794
etc butmine is a civilian model.

I can't see rhyme or reason for such a high power output at that
impedance.

No cathode follower.
Any headphones work fine at 10 ohms.

If 10 ohm headphones work fine I doubt the output is 600 ohms.

--
*Filthy stinking rich -- well, two out of three ain't bad

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Henry Kolesnik wrote:
I was wrong it's only 10 watts. I know of no reason, perhaps someone
here will tell us. They made tens of thousands of these so it must have
had a need...

Usual reason would be to drive several small speakers or to do so over a
long run - like 100v line. Where a matching transformer is situated with
the speaker.

--
*I believe five out of four people have trouble with fractions. *

Dave Plowman London SW
To e-mail, change noise into sound.

I'll see you on the third half of the show
"Dave Plowman (News)" wrote in message
...
In article ,
Henry Kolesnik wrote:
I was wrong it's only 10 watts. I know of no reason, perhaps someone
here will tell us. They made tens of thousands of these so it must
have
had a need...

Usual reason would be to drive several small speakers or to do so over
a
long run - like 100v line. Where a matching transformer is situated
with
the speaker.

--
*I believe five out of four people have trouble with fractions. *

Dave Plowman London SW
To e-mail, change noise into sound.