[Alex]

Hello.

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

However, I am not sure how to correctly calculate this RC circuit.

If I know my load impedance Rl, if I know output tube internal
resistance Ra, if I know (measured) the actual leakage inductance Ls
(with the secondary short), if I know the highest frequency (Fmax) my
amplifier is intended to reproduce (15kHz), then what is the correct
way to set this RC circuit?

Who could help or refer to any article?

Is it simply R=Rl and Xc shall be so that Xc(Fmax)=XLs(Fmax)=Rl?
Or is it more tricky?

Regards,
Alex

[Phil Allison]

"Alex"

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.


** Partly for that PLUS the fact that most speakers have a sharply rising
impedance curve at high and supersonic frequencies. A nominal 8 ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.


However, I am not sure how to correctly calculate this RC circuit.


** You are not alone ....


If I know my load impedance Rl,


** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low value at
supersonic frequencies.




....... Phil

[Patrick Turner]

Phil Allison wrote:

"Alex"

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

** Partly for that PLUS the fact that most speakers have a sharply rising
impedance curve at high and supersonic frequencies. A nominal 8 ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.

However, I am not sure how to correctly calculate this RC circuit.

** You are not alone ....

If I know my load impedance Rl,

** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low value at
supersonic frequencies.

...... Phil

I don't know how many tube amps you have designed and or built or
repaired
but I've lost count. Many tube amps will oscillate at HF when connected
to
a capacitor load.

In order to gain unconditional stability in very many tube amps and some
tolerance
of capacitor loads, or having no load at all, or having an inductive
load wuth high Z
at HF, is is VERY NECESSARY to place what is called a Zobel network of
R+C in series
across one or more positions in the tube amp.

The Zobel is more likely to be needed in an Ultralinear or beam tetrode
or pentode
output stage where the gain rises as the load on the tube rises because
of the leakage inductance.

The Zobel is less likely to be of any use in a triode output stage.

Transformer series and parallel resonances between stray lumped shunt C
and leakage inductance
also confounds the designer, and the Zobel can act to damp the resonance
and the
adverse loading they cause at the resonant F.

So just what value for R&C are to be used in an output stage?

Let us assume the amp is set up with an output tranny to power 5 ohms,
but present
a load to the tubes of 5k anode to anode.

In most cases I have had to tame, generally I have tried TWO Zobel
networks,
one across each half primary of the OPT, ie, from CT to each anode
connection.
Where 5k is the a-a load, the minimum R value is 5k / 4 = 1.25k because
that's the class B load
when the amp works in class AB.
The next standard R value above is about right, say 1k5.

The value of C should be high enough to not adversely load down the amp
too much at too low a frequency. Therefore I would use C which has equal
reactance to
RLa-a / 4 at say 100kHz as a starting value. 0.001uF could be about
correct.

At 20kHz, the two x ( 0.001uF + 1k5 ) networks has Z = 16k a-a approx,
and this will not
reduce the response into a resistance load very much.

A typical amount of leakage inductance of a poor quality 5k OPT coulod
be 50mH in series with the
5k load. This is normal for very many tube amps.
At 100kHz, 50mH has reactance = 32k, a huge amount of series reactive
impedance
effectively in series with the load seen by the anodes.

If a square wave with fast rise time is used to test the amp, and the
anode
signals viewed, there can be a huge amount of overshoot and ringing at
ther anodes,
even with a resistive load at the output.
The ringing crap is transformed to the secondary, and when fed back
can cause the amp to oscillate.

The 1.5k plus 0.001uF will act to reduce the ringing at the anodes.

Zobels used across the secondary usually have R = rated RL plus a C
that makes the reactance of the C = R at 100 kHz.

So 4.7 ohms plus 0.33uF are typical values.

The other place for a Zobel are across the RL of the V1,
so that where there is 47k load, you might use 4k7 plus 470pF.

This will reduce the gain of V1 at HF, and reduce the phase shift
where OLG 1.


But unless you have designed and built a lot of amps and know the art of
trying things
and observing, and ending up with R&C values that
ONLY stop the amp oscillating, and load the amp minimally,
so that the response with a pure R load is very litle affected, then
chances are you'll get it all wrong.

The obel networks make it possible to use ESL speakers without worry of
having a huge
peak in the sine wave response between 8kHz and 32 kHz.

With any value of pure C loading from 0.05uF 5uF, the peaking caused
in the sine wave response due to having FB should not exceed 6dB.

This will indicate a good stability margin.

In fact pure C loads are rare. With most ESL, there is some C alright,
maybe 1uF to 5uF, but usually there is a series R equivalent to say 1.5
ohms
which provides enough damping to a tube amp which is in effect an active
bandpass filter
with more than 6dB attenuation beyond the LF and HF open loop poles.

See my website for numerous examples of Zobel networks
in amplifier schematics.

http://www.turneraudio.com.au

Well damped amps do not sound bogged down by being overloaded at HF by
such networks.
They usually have clearer sounding HF.

Patrick Turner.

[Alex]

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage. OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as 165mH --
the windings are not interleaved.

By briefly looking at impedances, it looks like the amplifier will not be
able to efficiently reproduce more than 8kHz anyway (at 1/2 maximum power
level, without NFB).

So if I select Zobel C=3300pF and R=7.5K will sort of "tune" the primary to
8KHz, maintaining nearly active impedance all the way to ultrasound. Load
curve will be rather more linear, than elliptical, there will be no
pronounced parallel resonance and the distortion will not be soaring at high
frequencies.

Is that correct?

Regards,
Alex

"Patrick Turner" wrote in message
...


Phil Allison wrote:

"Alex"

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

** Partly for that PLUS the fact that most speakers have a sharply
rising
impedance curve at high and supersonic frequencies. A nominal 8 ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.

However, I am not sure how to correctly calculate this RC circuit.

** You are not alone ....

If I know my load impedance Rl,

** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low value
at
supersonic frequencies.

...... Phil

I don't know how many tube amps you have designed and or built or
repaired
but I've lost count. Many tube amps will oscillate at HF when connected
to
a capacitor load.

In order to gain unconditional stability in very many tube amps and some
tolerance
of capacitor loads, or having no load at all, or having an inductive
load wuth high Z
at HF, is is VERY NECESSARY to place what is called a Zobel network of
R+C in series
across one or more positions in the tube amp.

The Zobel is more likely to be needed in an Ultralinear or beam tetrode
or pentode
output stage where the gain rises as the load on the tube rises because
of the leakage inductance.

The Zobel is less likely to be of any use in a triode output stage.

Transformer series and parallel resonances between stray lumped shunt C
and leakage inductance
also confounds the designer, and the Zobel can act to damp the resonance
and the
adverse loading they cause at the resonant F.

So just what value for R&C are to be used in an output stage?

Let us assume the amp is set up with an output tranny to power 5 ohms,
but present
a load to the tubes of 5k anode to anode.

In most cases I have had to tame, generally I have tried TWO Zobel
networks,
one across each half primary of the OPT, ie, from CT to each anode
connection.
Where 5k is the a-a load, the minimum R value is 5k / 4 = 1.25k because
that's the class B load
when the amp works in class AB.
The next standard R value above is about right, say 1k5.

The value of C should be high enough to not adversely load down the amp
too much at too low a frequency. Therefore I would use C which has equal
reactance to
RLa-a / 4 at say 100kHz as a starting value. 0.001uF could be about
correct.

At 20kHz, the two x ( 0.001uF + 1k5 ) networks has Z = 16k a-a approx,
and this will not
reduce the response into a resistance load very much.

A typical amount of leakage inductance of a poor quality 5k OPT coulod
be 50mH in series with the
5k load. This is normal for very many tube amps.
At 100kHz, 50mH has reactance = 32k, a huge amount of series reactive
impedance
effectively in series with the load seen by the anodes.

If a square wave with fast rise time is used to test the amp, and the
anode
signals viewed, there can be a huge amount of overshoot and ringing at
ther anodes,
even with a resistive load at the output.
The ringing crap is transformed to the secondary, and when fed back
can cause the amp to oscillate.

The 1.5k plus 0.001uF will act to reduce the ringing at the anodes.

Zobels used across the secondary usually have R = rated RL plus a C
that makes the reactance of the C = R at 100 kHz.

So 4.7 ohms plus 0.33uF are typical values.

The other place for a Zobel are across the RL of the V1,
so that where there is 47k load, you might use 4k7 plus 470pF.

This will reduce the gain of V1 at HF, and reduce the phase shift
where OLG 1.


But unless you have designed and built a lot of amps and know the art of
trying things
and observing, and ending up with R&C values that
ONLY stop the amp oscillating, and load the amp minimally,
so that the response with a pure R load is very litle affected, then
chances are you'll get it all wrong.

The obel networks make it possible to use ESL speakers without worry of
having a huge
peak in the sine wave response between 8kHz and 32 kHz.

With any value of pure C loading from 0.05uF 5uF, the peaking caused
in the sine wave response due to having FB should not exceed 6dB.

This will indicate a good stability margin.

In fact pure C loads are rare. With most ESL, there is some C alright,
maybe 1uF to 5uF, but usually there is a series R equivalent to say 1.5
ohms
which provides enough damping to a tube amp which is in effect an active
bandpass filter
with more than 6dB attenuation beyond the LF and HF open loop poles.

See my website for numerous examples of Zobel networks
in amplifier schematics.

http://www.turneraudio.com.au

Well damped amps do not sound bogged down by being overloaded at HF by
such networks.
They usually have clearer sounding HF.

Patrick Turner.

[Patrick Turner]

Alex wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

Sometimes my "other one" can be terse, and abrupt, tactless, and
inclined
to hurt everyon'e feelings, but hey, my tolerance and patience is
strained at times....

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage. OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as 165mH --
the windings are not interleaved.

Yes, the OPT is dead awful.

What I said about providing a resistance load in PP amps is also valid
for SE types.
The Zobel is to ensure the tube has a load at HF even though the tube
de-couples from the secondary
because of the increasing reactance of leakage inductance.
Its very much like using a Zobel across a speaker to neutralize
the series inductance of a voice coil.
You *want* the impedance of the speaker to be resistive if possible,
so that the crossover series L will indeed filter out HF from a bass
speaker.


By briefly looking at impedances, it looks like the amplifier will not be
able to efficiently reproduce more than 8kHz anyway (at 1/2 maximum power
level, without NFB).

That may be plenty, if its an AM radio.

So if I select Zobel C=3300pF and R=7.5K will sort of "tune" the primary to
8KHz, maintaining nearly active impedance all the way to ultrasound. Load
curve will be rather more linear, than elliptical, there will be no
pronounced parallel resonance and the distortion will not be soaring at high
frequencies.

Is that correct?

That is all about right.

You may find it difficult to apply more than 15dB of NFB.
12dB might be enough though, but you wouldn't bother trying to extend
HF response with pure resistance loads to much beyond 10kHz, -3dB point.

In the case you have, the reactance of the C can be about equal to
reactance
of leakage inductance and series R can be about 1.41 x ZR or ZC,
roughly...

12dB NFB around a pentode is usually enough to reduce the Rout
of the amp including the OPT winding resistance so that the
end result gives you an amp which has Rout lower than a triode with a
low loss OPT.

Its not uncommon to find radio OPTs which have 25% winding losses!

If the speaker is in the radio cabinet with the radio chassis it'll be
unlikely
any other type of load will be used, so you won't have to fiddle
endlessly around
to "critically damp" the circuit after FB is applied to ensure it is
stable with any value of pure C loading
because such a load won't ever be connected.

Sometimes the S winding is wound onto a core first with P winding on
after.
If there is a gap between core and P winding available, say 1.5mm, maybe
you can sneak in
another secondary with the same turns as the existing, doesn't matter if
the wire is thinner,
as long as the added winding has the same voltage as the existing S
winding.
This will reduce LL from 165mH to maybe 60mH, and reduce winding
resistance losses, which are usually
highest in the S windings on radio OPT.

Its a pain to wind maybe 120 turns on by threading wire around and
around, but I did it once
with a pair of Rola OPT meant for 6BQ5, and then 20dB global NFB was
fine.

The sound was glorious with sensitive full range "ceiling" speakers.

I had to be very dedicated.....

Patrick Turner.




Regards,
Alex

"Patrick Turner" wrote in message
...


Phil Allison wrote:

"Alex"

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

** Partly for that PLUS the fact that most speakers have a sharply
rising
impedance curve at high and supersonic frequencies. A nominal 8 ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.

However, I am not sure how to correctly calculate this RC circuit.

** You are not alone ....

If I know my load impedance Rl,

** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low value
at
supersonic frequencies.

...... Phil

I don't know how many tube amps you have designed and or built or
repaired
but I've lost count. Many tube amps will oscillate at HF when connected
to
a capacitor load.

In order to gain unconditional stability in very many tube amps and some
tolerance
of capacitor loads, or having no load at all, or having an inductive
load wuth high Z
at HF, is is VERY NECESSARY to place what is called a Zobel network of
R+C in series
across one or more positions in the tube amp.

The Zobel is more likely to be needed in an Ultralinear or beam tetrode
or pentode
output stage where the gain rises as the load on the tube rises because
of the leakage inductance.

The Zobel is less likely to be of any use in a triode output stage.

Transformer series and parallel resonances between stray lumped shunt C
and leakage inductance
also confounds the designer, and the Zobel can act to damp the resonance
and the
adverse loading they cause at the resonant F.

So just what value for R&C are to be used in an output stage?

Let us assume the amp is set up with an output tranny to power 5 ohms,
but present
a load to the tubes of 5k anode to anode.

In most cases I have had to tame, generally I have tried TWO Zobel
networks,
one across each half primary of the OPT, ie, from CT to each anode
connection.
Where 5k is the a-a load, the minimum R value is 5k / 4 = 1.25k because
that's the class B load
when the amp works in class AB.
The next standard R value above is about right, say 1k5.

The value of C should be high enough to not adversely load down the amp
too much at too low a frequency. Therefore I would use C which has equal
reactance to
RLa-a / 4 at say 100kHz as a starting value. 0.001uF could be about
correct.

At 20kHz, the two x ( 0.001uF + 1k5 ) networks has Z = 16k a-a approx,
and this will not
reduce the response into a resistance load very much.

A typical amount of leakage inductance of a poor quality 5k OPT coulod
be 50mH in series with the
5k load. This is normal for very many tube amps.
At 100kHz, 50mH has reactance = 32k, a huge amount of series reactive
impedance
effectively in series with the load seen by the anodes.

If a square wave with fast rise time is used to test the amp, and the
anode
signals viewed, there can be a huge amount of overshoot and ringing at
ther anodes,
even with a resistive load at the output.
The ringing crap is transformed to the secondary, and when fed back
can cause the amp to oscillate.

The 1.5k plus 0.001uF will act to reduce the ringing at the anodes.

Zobels used across the secondary usually have R = rated RL plus a C
that makes the reactance of the C = R at 100 kHz.

So 4.7 ohms plus 0.33uF are typical values.

The other place for a Zobel are across the RL of the V1,
so that where there is 47k load, you might use 4k7 plus 470pF.

This will reduce the gain of V1 at HF, and reduce the phase shift
where OLG 1.


But unless you have designed and built a lot of amps and know the art of
trying things
and observing, and ending up with R&C values that
ONLY stop the amp oscillating, and load the amp minimally,
so that the response with a pure R load is very litle affected, then
chances are you'll get it all wrong.

The obel networks make it possible to use ESL speakers without worry of
having a huge
peak in the sine wave response between 8kHz and 32 kHz.

With any value of pure C loading from 0.05uF 5uF, the peaking caused
in the sine wave response due to having FB should not exceed 6dB.

This will indicate a good stability margin.

In fact pure C loads are rare. With most ESL, there is some C alright,
maybe 1uF to 5uF, but usually there is a series R equivalent to say 1.5
ohms
which provides enough damping to a tube amp which is in effect an active
bandpass filter
with more than 6dB attenuation beyond the LF and HF open loop poles.

See my website for numerous examples of Zobel networks
in amplifier schematics.

http://www.turneraudio.com.au

Well damped amps do not sound bogged down by being overloaded at HF by
such networks.
They usually have clearer sounding HF.

Patrick Turner.

[Alex]

I took your advice, Partick, and empirically selected the best RC network,
observing transient pulse response on the oscilloscope (with the NFB
temporarily disabled).

The best damping (of the OPT leakage + speaker inductance) took place with
R=15K and C=4700pF -- no spikes, no humps, just nice nearly exponential
drooping of the tops of the pulses. Of course, the Rola speaker will not be
changed, so no more fiddling.

Your assessment of the transformer was on the spot. It is crap: 0.7ohm
secondary DC resistance, 330ohms primary resistance. Total losses referred
to primary 1,5Kohm (series). That was measured on the RLC meter @ 1KHz with
the secondary shorted.

Overall NFB will be about 12dB. It is applied to the cathode of the first
12AX7 stage.

To provide stability at high frequencies I use the following "trick", which
I can share.
I drop a small capacitor from the plate of the output tube to its grid.
About 10pF, but it depends. It works exactly like a classic compensation
capacitor in old op-amps, like LM301, creating one dominant pole from two
staggered ones: grid and plate.

Regards,
Alex

"Patrick Turner" wrote in message
...


Alex wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

Sometimes my "other one" can be terse, and abrupt, tactless, and
inclined
to hurt everyon'e feelings, but hey, my tolerance and patience is
strained at times....

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.
OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as
165mH --
the windings are not interleaved.

Yes, the OPT is dead awful.

What I said about providing a resistance load in PP amps is also valid
for SE types.
The Zobel is to ensure the tube has a load at HF even though the tube
de-couples from the secondary
because of the increasing reactance of leakage inductance.
Its very much like using a Zobel across a speaker to neutralize
the series inductance of a voice coil.
You *want* the impedance of the speaker to be resistive if possible,
so that the crossover series L will indeed filter out HF from a bass
speaker.


By briefly looking at impedances, it looks like the amplifier will not
be
able to efficiently reproduce more than 8kHz anyway (at 1/2 maximum
power
level, without NFB).

That may be plenty, if its an AM radio.

So if I select Zobel C=3300pF and R=7.5K will sort of "tune" the primary
to
8KHz, maintaining nearly active impedance all the way to ultrasound.
Load
curve will be rather more linear, than elliptical, there will be no
pronounced parallel resonance and the distortion will not be soaring at
high
frequencies.

Is that correct?

That is all about right.

You may find it difficult to apply more than 15dB of NFB.
12dB might be enough though, but you wouldn't bother trying to extend
HF response with pure resistance loads to much beyond 10kHz, -3dB point.

In the case you have, the reactance of the C can be about equal to
reactance
of leakage inductance and series R can be about 1.41 x ZR or ZC,
roughly...

12dB NFB around a pentode is usually enough to reduce the Rout
of the amp including the OPT winding resistance so that the
end result gives you an amp which has Rout lower than a triode with a
low loss OPT.

Its not uncommon to find radio OPTs which have 25% winding losses!

If the speaker is in the radio cabinet with the radio chassis it'll be
unlikely
any other type of load will be used, so you won't have to fiddle
endlessly around
to "critically damp" the circuit after FB is applied to ensure it is
stable with any value of pure C loading
because such a load won't ever be connected.

Sometimes the S winding is wound onto a core first with P winding on
after.
If there is a gap between core and P winding available, say 1.5mm, maybe
you can sneak in
another secondary with the same turns as the existing, doesn't matter if
the wire is thinner,
as long as the added winding has the same voltage as the existing S
winding.
This will reduce LL from 165mH to maybe 60mH, and reduce winding
resistance losses, which are usually
highest in the S windings on radio OPT.

Its a pain to wind maybe 120 turns on by threading wire around and
around, but I did it once
with a pair of Rola OPT meant for 6BQ5, and then 20dB global NFB was
fine.

The sound was glorious with sensitive full range "ceiling" speakers.

I had to be very dedicated.....

Patrick Turner.




Regards,
Alex

"Patrick Turner" wrote in message
...


Phil Allison wrote:

"Alex"

I noticed that in most cases a capacitor or a RC circuit is
connected
across the primary of a tube OPT. It is apparently to compensate
for
the leakage inductance of the transformer.

** Partly for that PLUS the fact that most speakers have a sharply
rising
impedance curve at high and supersonic frequencies. A nominal 8
ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.

However, I am not sure how to correctly calculate this RC circuit.

** You are not alone ....

If I know my load impedance Rl,

** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR
when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low
value
at
supersonic frequencies.

...... Phil

I don't know how many tube amps you have designed and or built or
repaired
but I've lost count. Many tube amps will oscillate at HF when
connected
to
a capacitor load.

In order to gain unconditional stability in very many tube amps and
some
tolerance
of capacitor loads, or having no load at all, or having an inductive
load wuth high Z
at HF, is is VERY NECESSARY to place what is called a Zobel network of
R+C in series
across one or more positions in the tube amp.

The Zobel is more likely to be needed in an Ultralinear or beam
tetrode
or pentode
output stage where the gain rises as the load on the tube rises
because
of the leakage inductance.

The Zobel is less likely to be of any use in a triode output stage.

Transformer series and parallel resonances between stray lumped shunt
C
and leakage inductance
also confounds the designer, and the Zobel can act to damp the
resonance
and the
adverse loading they cause at the resonant F.

So just what value for R&C are to be used in an output stage?

Let us assume the amp is set up with an output tranny to power 5 ohms,
but present
a load to the tubes of 5k anode to anode.

In most cases I have had to tame, generally I have tried TWO Zobel
networks,
one across each half primary of the OPT, ie, from CT to each anode
connection.
Where 5k is the a-a load, the minimum R value is 5k / 4 = 1.25k
because
that's the class B load
when the amp works in class AB.
The next standard R value above is about right, say 1k5.

The value of C should be high enough to not adversely load down the
amp
too much at too low a frequency. Therefore I would use C which has
equal
reactance to
RLa-a / 4 at say 100kHz as a starting value. 0.001uF could be about
correct.

At 20kHz, the two x ( 0.001uF + 1k5 ) networks has Z = 16k a-a approx,
and this will not
reduce the response into a resistance load very much.

A typical amount of leakage inductance of a poor quality 5k OPT coulod
be 50mH in series with the
5k load. This is normal for very many tube amps.
At 100kHz, 50mH has reactance = 32k, a huge amount of series reactive
impedance
effectively in series with the load seen by the anodes.

If a square wave with fast rise time is used to test the amp, and the
anode
signals viewed, there can be a huge amount of overshoot and ringing at
ther anodes,
even with a resistive load at the output.
The ringing crap is transformed to the secondary, and when fed back
can cause the amp to oscillate.

The 1.5k plus 0.001uF will act to reduce the ringing at the anodes.

Zobels used across the secondary usually have R = rated RL plus a C
that makes the reactance of the C = R at 100 kHz.

So 4.7 ohms plus 0.33uF are typical values.

The other place for a Zobel are across the RL of the V1,
so that where there is 47k load, you might use 4k7 plus 470pF.

This will reduce the gain of V1 at HF, and reduce the phase shift
where OLG 1.


But unless you have designed and built a lot of amps and know the art
of
trying things
and observing, and ending up with R&C values that
ONLY stop the amp oscillating, and load the amp minimally,
so that the response with a pure R load is very litle affected, then
chances are you'll get it all wrong.

The obel networks make it possible to use ESL speakers without worry
of
having a huge
peak in the sine wave response between 8kHz and 32 kHz.

With any value of pure C loading from 0.05uF 5uF, the peaking caused
in the sine wave response due to having FB should not exceed 6dB.

This will indicate a good stability margin.

In fact pure C loads are rare. With most ESL, there is some C alright,
maybe 1uF to 5uF, but usually there is a series R equivalent to say
1.5
ohms
which provides enough damping to a tube amp which is in effect an
active
bandpass filter
with more than 6dB attenuation beyond the LF and HF open loop poles.

See my website for numerous examples of Zobel networks
in amplifier schematics.

http://www.turneraudio.com.au

Well damped amps do not sound bogged down by being overloaded at HF by
such networks.
They usually have clearer sounding HF.

Patrick Turner.

[Phil Allison]

"Alex The **** Head"


Your reply is constructive and informative, unlike the other one.


** How ****ing wrong YOU are - dickwad.



My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.


** What pathetic TROLLING **** you are for NOT saying so before !!

This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!


Go shove the rusty pile of **** UP YOUR ARSE !!




...... Phil

[Phil Allison]

The **** Head"


Your reply is constructive and informative, unlike the other one.


** How ****ing wrong YOU are - dickwad.


My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.


** What pathetic TROLLING **** you are for NOT saying so before !!

This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!

Go shove that rusty pile of **** UP YOUR ARSE !!





...... Phil

[Phil Allison]

"Alex the Top Posting **** "


** DO NOT TOP POST


YOU ****ING PILE OF **** !!




...... Phil

[mick]

On Sat, 03 May 2008 00:16:56 +1000, Phil Allison wrote:

snip

This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!

Go shove that rusty pile of **** UP YOUR ARSE !!

snip


Oh, the temptation to get Phil wound up on this one... :-)

Detector - AF amp - SE output stage - speaker
Does it become on-topic if the "wireless" has a "PU" or "Gram" input?

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net
Filtering everything posted from googlegroups to kill spam.

[Ian Thompson-Bell]

mick wrote:
On Sat, 03 May 2008 00:16:56 +1000, Phil Allison wrote:

snip
This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!

Go shove that rusty pile of **** UP YOUR ARSE !!

snip


Oh, the temptation to get Phil wound up on this one... :-)

Detector - AF amp - SE output stage - speaker
Does it become on-topic if the "wireless" has a "PU" or "Gram" input?


IMV if it has tubes in it making audio come out it's OT.

Cheers

Ian

[Patrick Turner]

mick wrote:

On Sat, 03 May 2008 00:16:56 +1000, Phil Allison wrote:

snip

This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!

Go shove that rusty pile of **** UP YOUR ARSE !!

snip

Oh, the temptation to get Phil wound up on this one... :-)

Our dear Phil does get into a tiz over a trifle sometimes.

I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from 1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

AM radios are vey tubey things, and discussing them here is fine by me.

But we rarely get tubed TV sets being discussed or tubed FM radios.

Patrick Turner.



Detector - AF amp - SE output stage - speaker
Does it become on-topic if the "wireless" has a "PU" or "Gram" input?

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net
Filtering everything posted from googlegroups to kill spam.

[John Byrns]

Phil, you are dumber than dirt. Radios are part of what you call
"recreational audio". I suggest you read the charter for the group
rec.audio.tubes, here is a copy to help you since you seem otherwise
helpless.

===========

CHARTER

Rec.audio.tubes is dedicated to the discussion of vacuum tube
audio equipment, including the following topics:

* hi-fi applications
* guitar (and other instrument) amplifiers
* radio circuits
* theory and design
* vintage and modern equipment
* repair/modification/restoration
* purchase recommendations

Participants from all backgrounds are welcome. Polite expressions of
opinion are welcome. Personal attacks and flame-wars are not. Let's
keep things civil, informative, and fun.

===========

Notice the bit about "radio circuits", that would seem to cover Alex's
radio pretty well. Also you seem to lack respect for the final section
of the charter.


Regards,

John Byrns


In article ,
"Phil Allison" wrote:

The **** Head"


Your reply is constructive and informative, unlike the other one.


** How ****ing wrong YOU are - dickwad.


My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.


** What pathetic TROLLING **** you are for NOT saying so before !!

This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!

Go shove that rusty pile of **** UP YOUR ARSE !!





..... Phil

--
Surf my web pages at, http://fmamradios.com/

[Jon Yaeger]

Hmmm. . . . let's see . . . what's ruder . . . top posting or you
unleashing a scatalogical temper tantrum???





in article , Phil Allison at
wrote on 5/2/08 10:38 AM:


"Alex the Top Posting **** "


** DO NOT TOP POST


YOU ****ING PILE OF **** !!




..... Phil

[John Byrns]

Phil, you are dumber than dirt. Radios are part of what you call
"recreational audio". I suggest you read the charter for the group
rec.audio.tubes, here is a copy to help you since you seem otherwise
helpless.

===========

CHARTER

Rec.audio.tubes is dedicated to the discussion of vacuum tube
audio equipment, including the following topics:

* hi-fi applications
* guitar (and other instrument) amplifiers
* radio circuits
* theory and design
* vintage and modern equipment
* repair/modification/restoration
* purchase recommendations

Participants from all backgrounds are welcome. Polite expressions of
opinion are welcome. Personal attacks and flame-wars are not. Let's
keep things civil, informative, and fun.

===========

Notice the bit about "radio circuits", that would seem to cover Alex's
radio pretty well. Also you seem to lack respect for the final section
of the charter.


Regards,

John Byrns


In article ,
"Phil Allison" wrote:

"Alex The **** Head"


Your reply is constructive and informative, unlike the other one.


** How ****ing wrong YOU are - dickwad.



My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.


** What pathetic TROLLING **** you are for NOT saying so before !!

This newsgroup is all about " recreational audio " - OK

NOT ****ING STUPID BLOODY RADIO SETS !!!!!!!


Go shove the rusty pile of **** UP YOUR ARSE !!




..... Phil

--
Surf my web pages at, http://fmamradios.com/

[Phil Allison]

"Patrick Turneroid Raving Nut Case "
Phil Allison wrote:

"Alex"

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

** Partly for that PLUS the fact that most speakers have a sharply
rising
impedance curve at high and supersonic frequencies. A nominal 8 ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.

However, I am not sure how to correctly calculate this RC circuit.

** You are not alone ....

If I know my load impedance Rl,

** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low value
at
supersonic frequencies.



I don't know how many tube amps you have designed and or built or
repaired count. Many tube amps will oscillate at HF when connected
to a capacitor load.


** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

The OP did not enquire about any such thing.


In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.

Snip rest of the Turneroid's hobby horse ride & public masturbation.

GOD it is BORING !!!!




...... Phil

[Phil Allison]

"John Byrns = Insane Top Posting **** Head"



** Go drop dead you asinine pile of pedantic, autistic ****.





....... Phil

[Phil Allison]

"Alex"


** STOP TOP POSTING !!

YOU PREPOSTEROUSLY ARROGANT ****HEAD !!




...... Phil

[Phil Allison]

"Alex"


** STOP TOP POSTING !!

YOU PREPOSTEROUSLY ARROGANT ****HEAD !!




...... Phil

[Phil Allison]

"Alex"


Overall NFB will be about 12dB. It is applied to the cathode of the first
12AX7 stage.


** Absurd thing to do. It will completely ruin the " tone " of the radio
which in part depends on the audio power stage having a high source
impedance.

Time you discovered just how the Z of loudspeakers varies with drive
frequency and that this variation is commonly used to equalise the sound of
valve radios and guitar amps.



....... Phil

[Phil Allison]

"flipper the ****wit"


Try measuring the capacitance of a loudspeaker.


** Try shoving your fat head down the dunny -

ASD ****ED IMBECILE.




...... Phil

[Alex]

"Patrick Turner" wrote in message
...

I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from 1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

[Phil Allison]

"flipper the ****wit"


** Try shoving your fat head down the dunny -

you ASD ****ED IMBECILE !!




....... Phil

[John Byrns]

In article ,
flipper wrote:

On Sat, 3 May 2008 10:08:10 +1000, "Phil Allison"
wrote:

** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Try measuring the capacitance of a loudspeaker.

What frequency should I measure at, or do you want a complete sweep?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Phil Allison]

"flipper the ****wit"


** Try shoving your fat head down the dunny -

you ASD ****ED IMBECILE !!





...... Phil

[Patrick Turner]

Phil Allison wrote:

"Patrick Turneroid Raving Nut Case "
Phil Allison wrote:

"Alex"

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

** Partly for that PLUS the fact that most speakers have a sharply
rising
impedance curve at high and supersonic frequencies. A nominal 8 ohm
speaker at 250 Hz can easily be 100 ohms at 50 kHz.

However, I am not sure how to correctly calculate this RC circuit.

** You are not alone ....

If I know my load impedance Rl,

** No you don't.

You just know the " nominal " impedance = useless.

What you need to make sure of is the amp does not break into HF
oscillations ( parasitic or continuous) when there is no load OR when
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation
etwork - as the OT has little leakage and the NFB reduces to low value
at
supersonic frequencies.



I don't know how many tube amps you have designed and or built or
repaired count. Many tube amps will oscillate at HF when connected
to a capacitor load.

** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

The OP did not enquire about any such thing.

In order to gain unconditional stability ....

** An unnecessary, pedantic, pseudo technical wank.

Snip rest of the Turneroid's hobby horse ride & public masturbation.

GOD it is BORING !!!!

Well yes, Phil, I'm ****ing boring alright.
I have lived a long life so far encountering lots of boring people
who complain I am so boring.
I have usually opened a door for them to allow them the pleasure
of finding a less boring set of circumstances far away from me.


But if you wanna make an amp stable, you test with capacitors across the
output
secondary of the OPT.

Once the amp can withstand any value of C across the OPT sec when NFB is
used
then the amp is unconditionally stable because even with the extra phase
shift
caused by the cap, the amp won't oscillate, ever.


But NEVER did I say you should test with C across the tubes.

C + R Zobel networks, sure, but never a pure C across any tube in the
whole amp line up. When NFB is used, pure C just about always makes the
amp MORE unstable.

Have you taken your medications today Phil?

Patrick Turner.





..... Phil

[Patrick Turner]

Phil Allison wrote:

"Alex"

Overall NFB will be about 12dB. It is applied to the cathode of the first
12AX7 stage.

** Absurd thing to do. It will completely ruin the " tone " of the radio
which in part depends on the audio power stage having a high source
impedance.

Time you discovered just how the Z of loudspeakers varies with drive
frequency and that this variation is commonly used to equalise the sound of
valve radios and guitar amps.

I know exactly what you are saying, and sure, some radios need to have a
current source pentode output tube to extend the bass and treble.

There have been plenty that were deliberately engineered
to have a response from say 100Hz to 5kHz, and the actual
audio response of the set due to RC audio couplings and very poor IF
bandwidth
is only 200Hz to 2.5kHz, and the rising Z of the speaker at each end of
the band means you end up
with 100Hz to 5kHz.

Some were a lot better than others to achieve these design aims but most
were
appalling junk that is only good for speach and the news.

But usually there is also 10% of acumulated other distortions in the
radio, and above a whisper
they are really horrid things to listen to.

On the other hand, many radios including reflex sets where the IF amp
also
amplified the audio as well had a shunt FB network between OPT sec and
the audio source
including the volume control. This saved the cost of an extra tube.
I have one which I repaired which wasn't picked up after I worked two
days on the critter.
Its nowhere nearly as good sounding as my own design of AM radio
which is also in my kitchen.

Time YOU woke up to what the possibilities are with re-engineering
old radio sets. One day, all the analog free to air broadcast radio will
be turned off and it'll only be digital, but until then analog AM radio
has a
big part in my house to bring me Radio National; here its 846kHz, and
obviously
has 30Hz to 9kHz bandwidth, AND has low distortion, and and my radio
with its special capabilities
IS BETTER than all the others I have ever heard.

I found most old AM tube radios are bloody old junk.

Then after everyone suffered from 1930 to about 1960,
we got transistor AM radios which were worse.

Finally we got FM and AM was left as is, and makers of receivers and
tuners
mainly treated AM like ****, ie SNAFU.

Patrick Turner.





...... Phil

[Patrick Turner]

Alex wrote:

"Patrick Turner" wrote in message
...

I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from 1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

Try

http://www.turneraudio.com.au/am-fm-...x-decoder.html

Patrick Turner.

[Patrick Turner]

Alex wrote:

"Patrick Turner" wrote in message
...

I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from 1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

The kitchen radio I mentioned from 1999 above
has a different circuit to one I just gave you at
http://www.turneraudio.com.au/am-fm-...x-decoder.html

I don't have the kitchen radio schematic at the website.

But what my kitchen radio does have is..

3 gang tuning cap.

Two RF coils with ferrite cores for high Q and two C gangs are coupled
so that there is no sideband cutting of AF
right across the AM band, which means each coil is tuned to a slightly
different F.

There is no ferrite antenna, just loose coupling from a wire antenna to
one input coil.

There is a variable µ twin triode used as a cascode RF amp stage and RC
coupled to the 6AN7 F converter
to make an IF signal.
There is AVC voltage applied only to the cascode input triodes.

The IF amp is 6BX6 with unbypassed Rk and no AVC applied, so its fairly
linear.
There is a cathode follower and diode detector cabable of detecting
about 10 times the actual voltage detected with very good linearity and
bandwidth.

Two standard IF trannies are used, but the first one after the F
converter
has one IF coil monted on a slide and a knob on the front of the radio
rotates to move the coil to coilo distance about 10mm.
This allows the radio to be tuned normally to a single
a single peak when tuning and then after closing the coils closer, the
IF BW is much increased to allow 9Khz of audio to be detected, but all
while
retaining very good skirt selectivity and rejection of
stations only 45kHz away even if they have 20 times the transmitting
power.

There are in fact 6 tuned circuits in all, so the attenutation
of signals away from the wanted stations is excellent,
yet what is wanted sounds very nearly as good as FM.

The 3rd gang of the cap is for the oscillator.

I paid very careful attention to tracking.

TRF sets are quite poor performers today because the stations are now
close together,
unlike in 1935, when the band wasn't crowded.

TRF usually used only 3 tuned circuits which needed careful tuning,
and they gave a peaked response, not a flat topped band band pass
character of a critically coupled IFT.

The Superhet with IFTs is definately The Best hi-fi AM tuner you can
have if its built right,
and 99% were atrocious, and all mainly conformed to the lowest common
denominator of utter crap.

When Radio was king before TV, there were very many makers all vying for
a market share.
They all agreed with each other to ensure the standards were lousy.

Patrick Turner.

[John Byrns]

In article ,
flipper wrote:

On Sat, 03 May 2008 09:56:16 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sat, 3 May 2008 10:08:10 +1000, "Phil Allison"
wrote:

** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Try measuring the capacitance of a loudspeaker.

What frequency should I measure at, or do you want a complete sweep?

Doesn't really matter. The purpose was just to inform Phil there's
capacitance there.

I would think it matters a lot, I thought whether there is capacitance
or inductance there depends on the frequency? There are a lot of
resonance effects involved in the operation of loudspeakers.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

John Byrns wrote:

snip,


Try measuring the capacitance of a loudspeaker.

What frequency should I measure at, or do you want a complete sweep?

Doesn't really matter. The purpose was just to inform Phil there's
capacitance there.

I would think it matters a lot, I thought whether there is capacitance
or inductance there depends on the frequency? There are a lot of
resonance effects involved in the operation of loudspeakers.

Regards,

John Byrns

How much the equivalent capacitance in a dynamic comes&dome
speaker with crossover filters affect an amp depends on the speaker
and the model for it in its enclosure.
If you went looking for a pure 0.22uF in most speakers, you would never
find it.
This is just the right C needed to make a tube amp oscillate at HF if
the amp has loop FB and has not been set up for stable operation.

Most dynamic speakers present an amp with inductance and resistance in
series.
As F rises, the reactance of the inductance rises above the series
resistance value
so the impedance of the L&R becomes increasingly inductive.
This means that at say above 50kHz, the speaker has such high impedance
that its as if there is no speaker connected at all, and some tube amps
will oscillate at HF if no load is connected.
This is especially so in pentode amps because the gain of the output
tubes rises
with load value increase; gain = gm x RL approximately, and so
effectively there is more NFB applied when gain increases, and
the margin of stability reduces to allow oscillation because open loop
gain 1 where
phase shift has reached 180 degrees.

The speaker inductance does not cause phase shift at HF.
That's because its reactance goes higher as F increases, but a
capacitor's
reactance becomes lower, and there IS a 90 degree phase lag at HF and
perhaps resonant
effects with leakage inductance which at HF is the dominant reactance of
the
amp's output impedance. Amps have inductiove output Z.
NFB is taken to the input *after* this inductance and when just the
right C value id used as a load,
you have a second order LC filter which has a lot of phase shift before
much attenuation has occurred.

If the speaker has impedance equalisation Zobel C&R networks across
drivers, then
the voice coil inductance is neutralized and as F rises the speaker
appears to the amp
as inceasingly resistive, which loads the amp and stops the gain rise
and prevents
the instability due to the gain rise.

But in a radio, there is rarely any such fancy thing as impedance eq,
Zobels or any NFB.

They just have a 6V6 connected to the speaker via OPT with no NFB
and its all mainly crap what you hear.

OK to get the news each other hour or the weather reports.

Or the football scores. Fidelity isn't required for really important
stuff.

And if Russian missles are on their way, the bloody radio won't save
your arse.

Patrick Turner.





--
Surf my web pages at, http://fmamradios.com/

[Phil Allison]

"flipper"



** Go shove your fat head down the dunny -

you pathetic ASD ****ED IMBECILE.




..... Phil

[The Phantom]

On Sat, 03 May 2008 13:07:51 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sat, 03 May 2008 09:56:16 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sat, 3 May 2008 10:08:10 +1000, "Phil Allison"
wrote:

** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Try measuring the capacitance of a loudspeaker.

What frequency should I measure at, or do you want a complete sweep?

Doesn't really matter. The purpose was just to inform Phil there's
capacitance there.

I would think it matters a lot, I thought whether there is capacitance
or inductance there depends on the frequency? There are a lot of
resonance effects involved in the operation of loudspeakers.

This sounds like something that should be cataloged on the web. Frequency
sweeps of raw drivers, complete systems with and without passive
crossovers, etc. It would be nice to be able to get the characteristic of
a given system.



Regards,

John Byrns

[Iain Churches[_2_]]

"Phil Allison" wrote in message
...

Patrick T wrote:

I don't know how many tube amps you have designed and or built or
repaired count. Many tube amps will oscillate at HF when connected
to a capacitor load.


** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Quite wrong Phil. 0.22µF is a standard (albeit tough) test
for tube amp stability. You will find it mentioned in articles
by Tremaine and Crowhurst.

In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.

Unconditional stability is more important now that is was
in the hey-day of tube amps when many speakers were 15
Ohms, and formed benevolent loads.

Unconditional stability is a "must"

Iain

[Iain Churches[_2_]]

"Phil Allison" wrote in message
...

Patrick T wrote:

I don't know how many tube amps you have designed and or built or
repaired count. Many tube amps will oscillate at HF when connected
to a capacitor load.


** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Quite wrong Phil. 0.22µF connected in place of a speaker across
the secondary of the OPT is a standard (albeit tough) test
for tube amp stability. You will find it mentioned in articles
by Tremaine and Crowhurst.

In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.

Unconditional stability is more important now that is was
in the hey-day of tube amps when many speakers were 15
Ohms, and formed benevolent loads.

Unconditional stability is a "must"

Iain

[robert casey]

Phil Allison wrote:




** Go shove your fat head down the dunny -


We used to do that to deserving people in college. Then flush that
toilet. We called it a "Ka-Boom"...

What a "dirty kaboom" is is left as an exercise for the student...

[BretLudwig]

Phil is a phucking ****.

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Message posted using http://www.talkaboutaudio.com/group/rec.audio.tubes/
More information at http://www.talkaboutaudio.com/faq.html

[Phil Allison]

"Iain Churches MASSIVE ****ING IDIOT "


** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Quite wrong Phil.


** As if a know nothing, rote learning moron like Chercus would know.


0.22µF is a standard (albeit tough) test
for tube amp stability.


** It is a completely ABSURD test that does not represent any kind of
real loudspeaker.


In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.


Unconditional stability is a "must"


** Utter ******** !!

**** off - you vile

CRIMINAL ****WIT !!




...... Phil

[Phil Allison]

"Iain Churches = MASSIVE ****ING IDIOT "

** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Quite wrong Phil.


** As if a know nothing, rote learning moron like Cherchus would know.


0.22µF is a standard (albeit tough) test
for tube amp stability.


** It is a completely ABSURD test that does not represent any kind of
real loudspeaker.


In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.


Unconditional stability is a "must"


** Utter ******** !!

**** off - you vile

CRIMINAL ****WIT !!




...... Phil

[John Byrns]

In article ,
"Alex" wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage. OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as 165mH --
the windings are not interleaved.

There is an interesting variation on this capacitor thing that you will
see in some old radios. Rather than using a single capacitor which
slowly rolls off the high frequency response, two capacitors are used,
one across the primary, and another across the secondary of the output
transformer. The two capacitors, along with the leakage inductance of
the transformer, form a third order low pass filter. This provides a
sharp cutoff to attenuate noise energy above the desired audio band
while providing flatter frequency response below cutoff in the desired
audio band, than a single capacitor can.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/