Reply
 
Thread Tools Display Modes
  #1   Report Post  
Posted to alt.ham-radio,rec.antiques.radio+phono,rec.audio.tubes,rec.ham-radio
HiTech RedNeck HiTech RedNeck is offline
external usenet poster
 
Posts: 2
Default Single ended tube audio output stages

Here's a "philosophical" question that I could answer at least part of in a
minute with an empirical test, but I want to see how people think about it.

In the conventional single ended tube audio output stage, having a
conventional audio output transformer the ends of whose primary is wired to
B+ and the output tube plate, and the ends of whose secondary is wired to
the speaker (don't throw in any complications like additional circuitry on
either side of the transformer; keep the tube in conduction at all times,
i.e. class A operation; and provide an input such that the output stays
within the rated frequency and power range of the speaker and transformer),
WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+? Or will it remain
in the range somewhere between B+ and zero volts, with a midpoint
approximately halfway between the extremes, a midpoint that depends on the
amplitude of the output signal?

But if it rises above B+, why? And then why don't tube manuals reflect the
maximum instantaneous permissible plate voltage, but rather what looks like
maximum quiescent voltage, when they give "absolute maximum" figures for
plate voltage? (Yes, I know that the data for television horizontal output
tubes generally give such instantaneous maximum plate voltage information,
but these tubes cut off conduction in normal operation and they normally
operate into an underdamped circuit which rings, i.e. flyback, when the tube
cuts off.)



  #2   Report Post  
Posted to alt.ham-radio,rec.antiques.radio+phono,rec.audio.tubes,rec.ham-radio
Phil Allison Phil Allison is offline
external usenet poster
 
Posts: 1,444
Default Single ended tube audio output stages


"HiTech RedNeck"

i.e. class A operation; and provide an input such that the output stays
within the rated frequency and power range of the speaker and
transformer),
WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+? Or will it
remain
in the range somewhere between B+ and zero volts, with a midpoint
approximately halfway between the extremes, a midpoint that depends on the
amplitude of the output signal?



** How dopey - of course the plate voltage swings around the B+ voltage that
is present at idle. The actual plate voltage may fall to around 50 volts and
rise to nearly (say 50 volts short) double the B+ voltage during a cycle at
full output.

Same goes for push-pull operation too.


But if it rises above B+, why?



** Variations in current flow in the primary result in voltage changes, up
and down from the B+ value. This is simply how inductors all behave.


And then why don't tube manuals reflect the
maximum instantaneous permissible plate voltage, but rather what looks
like
maximum quiescent voltage, when they give "absolute maximum" figures for
plate voltage?



** In tube data sheets "plate voltage" = average DC voltage present at
the plate terminal.

Got nothing to do with the maximum, instantaneous plate voltage which is
determined by possible insulation failure and the chance of arc over between
terminals. See the data for the EL34:

http://www.triodeel.com/6ca7ap2.gif

" Plate voltage without plate current = 2000 volts "

However for single tube class A:

" Supply voltage = 265 volts "

Which, with 100mA plate current is on the plate dissipation limit.



....... Phil




  #3   Report Post  
Posted to rec.audio.tubes,rec.antiques.radio+phono
Patrick Turner Patrick Turner is offline
external usenet poster
 
Posts: 3,964
Default Single ended tube audio output stages



HiTech RedNeck wrote:

Here's a "philosophical" question that I could answer at least part of in a
minute with an empirical test, but I want to see how people think about it.

In the conventional single ended tube audio output stage, having a
conventional audio output transformer the ends of whose primary is wired to
B+ and the output tube plate, and the ends of whose secondary is wired to
the speaker (don't throw in any complications like additional circuitry on
either side of the transformer; keep the tube in conduction at all times,
i.e. class A operation; and provide an input such that the output stays
within the rated frequency and power range of the speaker and transformer),
WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+? Or will it remain
in the range somewhere between B+ and zero volts, with a midpoint
approximately halfway between the extremes, a midpoint that depends on the
amplitude of the output signal?

But if it rises above B+, why? And then why don't tube manuals reflect the
maximum instantaneous permissible plate voltage, but rather what looks like
maximum quiescent voltage, when they give "absolute maximum" figures for
plate voltage? (Yes, I know that the data for television horizontal output
tubes generally give such instantaneous maximum plate voltage information,
but these tubes cut off conduction in normal operation and they normally
operate into an underdamped circuit which rings, i.e. flyback, when the tube
cuts off.)


In SE amps the anode voltage always rises abobe the B+.
The B+ is the reference viltage and anode signal swings above and below
reference.
Try to study situations where the ac current in an inductance is being
reduced,
and you will find an emf is released under control of the tube.

Try a Google search.

You need to understand fundamental LCR behaviours.

In PP amps, the anode voltage without a load can rise
several times above the B+. The OPT has primary inductance,
and leakage inductance, and both store magnetic energy.

Patrick Turner.
  #4   Report Post  
Posted to alt.ham-radio,rec.antiques.radio+phono,rec.audio.tubes,rec.ham-radio
Ian Iveson Ian Iveson is offline
external usenet poster
 
Posts: 960
Default Single ended tube audio output stages

HiTech RedNeck:

In the conventional single ended tube audio output stage,
having a
conventional audio output transformer the ends of whose
primary is wired to
B+ and the output tube plate, and the ends of whose
secondary is wired to
the speaker (don't throw in any complications like
additional circuitry on
either side of the transformer; keep the tube in
conduction at all times,
i.e. class A operation; and provide an input such that the
output stays
within the rated frequency and power range of the speaker
and transformer),
WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+?
Or will it remain
in the range somewhere between B+ and zero volts, with a
midpoint
approximately halfway between the extremes, a midpoint
that depends on the
amplitude of the output signal?

But if it rises above B+, why? And then why don't tube
manuals reflect the
maximum instantaneous permissible plate voltage, but
rather what looks like
maximum quiescent voltage, when they give "absolute
maximum" figures for
plate voltage? (Yes, I know that the data for television
horizontal output
tubes generally give such instantaneous maximum plate
voltage information,
but these tubes cut off conduction in normal operation and
they normally
operate into an underdamped circuit which rings, i.e.
flyback, when the tube
cuts off.)


No experiment is necessary, because you know transformer
coupled SE amps work, and they wouldn't if Va couldn't rise
above HT.

Presumably you are aware that the voltage across a perfect
choke is proportional to the rate of change of current?
That's the same principle that allows a flyback circuit to
generate high voltages. So, a choke-loaded valve stage will
tend to generate whatever voltage is necessary to maintain
constant current through the choke, which consequently
appears to the anode to be a high impedance at audio
frequencies.

I think the common point for logic to fail the unwary is
that a transformer doesn't appear to behave like a choke. As
long as you have an appropriate load on the secondary, then
the primary presents that load, multiplied by the square of
the turns ratio, to the anode. So, if you connect an 8 ohm
resistor to the secondary, and the transformer impedance
ratio is given as 5,000:8, then the anode sees a 5 kohm
resistance. So now what generates the anode voltage when it
rises above HT?

Fortunately the answer is simple. Consider the load
presented by the primary to be that reflected resistance
**in parallel with** the transformer's primary inductance,
which is lots of Henries, usually. At frequencies above low
bass, the inductance passes very little current, and leads
to almost no power loss, but it's still there, doing what
chokes do.

That's why SE amplifiers typically have either no feedback
and sod all low bass, or plenty feedback and distorted bass,
or somewhere in between, because of their relatively low
primary inductance which shunts low F. Of course there is
more to it than that, but it's still a common trait.

Ian


  #5   Report Post  
Posted to rec.audio.tubes
Patrick Turner Patrick Turner is offline
external usenet poster
 
Posts: 3,964
Default Single ended tube audio output stages



Ian Iveson wrote:

HiTech RedNeck:

In the conventional single ended tube audio output stage,
having a
conventional audio output transformer the ends of whose
primary is wired to
B+ and the output tube plate, and the ends of whose
secondary is wired to
the speaker (don't throw in any complications like
additional circuitry on
either side of the transformer; keep the tube in
conduction at all times,
i.e. class A operation; and provide an input such that the
output stays
within the rated frequency and power range of the speaker
and transformer),
WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+?
Or will it remain
in the range somewhere between B+ and zero volts, with a
midpoint
approximately halfway between the extremes, a midpoint
that depends on the
amplitude of the output signal?

But if it rises above B+, why? And then why don't tube
manuals reflect the
maximum instantaneous permissible plate voltage, but
rather what looks like
maximum quiescent voltage, when they give "absolute
maximum" figures for
plate voltage? (Yes, I know that the data for television
horizontal output
tubes generally give such instantaneous maximum plate
voltage information,
but these tubes cut off conduction in normal operation and
they normally
operate into an underdamped circuit which rings, i.e.
flyback, when the tube
cuts off.)


No experiment is necessary, because you know transformer
coupled SE amps work, and they wouldn't if Va couldn't rise
above HT.

Presumably you are aware that the voltage across a perfect
choke is proportional to the rate of change of current?
That's the same principle that allows a flyback circuit to
generate high voltages. So, a choke-loaded valve stage will
tend to generate whatever voltage is necessary to maintain
constant current through the choke, which consequently
appears to the anode to be a high impedance at audio
frequencies.

I think the common point for logic to fail the unwary is
that a transformer doesn't appear to behave like a choke. As
long as you have an appropriate load on the secondary, then
the primary presents that load, multiplied by the square of
the turns ratio, to the anode. So, if you connect an 8 ohm
resistor to the secondary, and the transformer impedance
ratio is given as 5,000:8, then the anode sees a 5 kohm
resistance. So now what generates the anode voltage when it
rises above HT?

Fortunately the answer is simple. Consider the load
presented by the primary to be that reflected resistance
**in parallel with** the transformer's primary inductance,
which is lots of Henries, usually. At frequencies above low
bass, the inductance passes very little current, and leads
to almost no power loss, but it's still there, doing what
chokes do.

That's why SE amplifiers typically have either no feedback
and sod all low bass, or plenty feedback and distorted bass,
or somewhere in between, because of their relatively low
primary inductance which shunts low F. Of course there is
more to it than that, but it's still a common trait.


I have built a large number of SE amps ranging from 4 watts to 60 watts.

The bass performance is fine with all of them, even where a poor
OPT with not quite enough inductance is present.

The latest 845SE60 monoblocks have THUNDEROUS bass if you want it.
With or without GNFB.

In fact the bass is mainly quite terrific, and better integrated with
the rest of the music than any other types of amp.
Any SE amp which has a subjective absence of bass simply has not been
designed with
enough power ceiling or bass extension.

So, one doesn't need a huge amount of inductance. Just enough is fine.
Usually, in a SE amp it means the reactance of primary inductance =
nominal
rated anode load value at 20Hz or lower F.
The other condition is that saturation and full mid-frequency maximum
anode swing voltage ( at just before ) does not occur at F above at
least 20Hz.
If these conditions are met, the negative grid swings will control Ia
and the rise in anode voltage and liberate the magnetc energy
into the load just as precisely as if there was a resistance load
wuth dc flow taken to a B+ of twice Ea.

It is positively queer how it exactly occurs, but it does,
and it defies human commonsense, and it sounds well.

When a tube cuts off its current sharply, and no load is present,
the anode voltage can rise to a high value and the swing positive is
larger than the B+ voltage,
and the only thing that tends to stop it is the
shunt capacitances which limit the speed of voltage change and current
change
and the fact that as anode voltage rises, there is a tendency for the
negative grid voltage
to not be negative enough to maintain cut off with a high anode voltage
present.
Its certainly the case with triodes which are the main tube used for SE
amps.
In normal SE operation, acual cut off of Ia never occurs, and in PP
although one tube does cut right off, the other is turned on well.

Amps with pentode outputs are more prone to generating very high peak
anode voltages
when no anode load is present.

Patrick Turner.




Ian



  #6   Report Post  
Posted to rec.audio.tubes
BretLudwig BretLudwig is offline
external usenet poster
 
Posts: 696
Default Single ended tube audio output stages


Single ended tube audio output stages
by "Tio Pedro" May 21, 2008 at 06:48 PM

"HiTech RedNeck" wrote in
message

The collapsing magnetic field in the audio transformer generates a
counter
EMF that is in series with the plate voltage. So yes, the voltage at the
plate
can be considerably higher than the plate supply. If the tube is
overdriven
into cutoff, the EMF voltage can greatly exceed the breakdown voltage of
the transformer."


Look at a single ended tube amplifier and then look at a schematic of any
1925-1990 car's secondary ignition system.

"Hmmmmm".

Or,"zzzzzaappppp!!!!!".

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


  #7   Report Post  
Posted to alt.ham-radio,rec.antiques.radio+phono,rec.audio.tubes,rec.ham-radio
Tio Pedro Tio Pedro is offline
external usenet poster
 
Posts: 1
Default Single ended tube audio output stages


"HiTech RedNeck" wrote in message

The collapsing magnetic field in the audio transformer generates a counter
EMF that is in series with the plate voltage. So yes, the voltage at the
plate
can be considerably higher than the plate supply. If the tube is overdriven
into cutoff, the EMF voltage can greatly exceed the breakdown voltage of
the transformer.


  #8   Report Post  
Posted to rec.audio.tubes
Phil Allison Phil Allison is offline
external usenet poster
 
Posts: 1,444
Default Single ended tube audio output stages


"Tio Pedro"

The collapsing magnetic field in the audio transformer generates a counter
EMF that is in series with the plate voltage. So yes, the voltage at the
plate can be considerably higher than the plate supply.
If the tube is overdriven into cutoff, the EMF voltage can greatly exceed
the breakdown voltage of the transformer.



** Strange then how SE output stages were ( and still are) used in so many
low powered guitar amps.

Such amps are operated into gross overdrive as a matter of course and OT
failures are rare events.



....... Phil



  #9   Report Post  
Posted to alt.ham-radio,rec.antiques.radio+phono,rec.audio.tubes,rec.ham-radio
Phil Allison Phil Allison is offline
external usenet poster
 
Posts: 1,444
Default Single ended tube audio output stages


"HiTech RedNeck"



** This is a monstrous cross-post.

Only ****wit TROLLS do that.



...... Phil




Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FA: Jensen Triaxial Hi Fi speaker, NOS Tube amp Power Transformer, 15 Watt Single-ended Audio Output Transformer No Name Marketplace 0 August 13th 07 10:12 PM
FA: Jensen Triaxial Hi Fi speaker, NOS Tube amp Power Transformer, 15 Watt Single-ended Audio Output Transformer No Name Vacuum Tubes 0 August 13th 07 10:12 PM
The Bill May Report on Single-Ended Output Transformers for 300B etc [email protected] Vacuum Tubes 6 May 4th 05 03:16 AM
FA: Pr. Single Ended Output Transformers - 7189, EL84 Stephen F. Marsh Marketplace 0 March 15th 05 01:33 AM
FA: Pr. Single Ended Output Transformers - 7189, EL84 Stephen F. Marsh Vacuum Tubes 0 March 15th 05 01:33 AM


All times are GMT +1. The time now is 04:39 PM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"