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Default Tail Resistor in long tail pair with CCS.

I'm working on a long tail pair phase inverter using a 12ax7 with a current
sink
on the tail. I've noticed in some other designs (ARC) they use a resistor
~5k between the 2 tails
and the CC sink. What's the benefit of this resistor? I find very little
difference with or without it.
It causes the CCS to work a little harder to make up for the series
resistance but other than that
I see no change in the bias or operating point.
There's very little written about using a CCS with long tails. I've
searched the web
searched the web, but nothing.
Any Ideas??

RonL


  #2   Report Post  
Phil Allison
 
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What's the benefit of this resistor?




** Imagine the CCS device fails short cct.



.......... Phil


  #3   Report Post  
Patrick Turner
 
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wrote:

I'm working on a long tail pair phase inverter using a 12ax7 with a current
sink
on the tail. I've noticed in some other designs (ARC) they use a resistor
~5k between the 2 tails
and the CC sink. What's the benefit of this resistor? I find very little
difference with or without it.


But what are you saying?

Are there two CCS, one for each 1/2 of the 12AX7?
If so, a 5k between each cathode acts like a current feedback R
to linearize the current change in the two tubes;
there is none in the CCS.

Have you checked the gains of the 12AX7?

What *exactly* is the circuit configuation?




It causes the CCS to work a little harder to make up for the series
resistance but other than that
I see no change in the bias or operating point.


CCS never "work hard"
There is no current change; the current is constant, so no
signal power is liberated in the CCS, only the DC power
of Vdc x Idc across the device.




There's very little written about using a CCS with long tails. I've
searched the web
searched the web, but nothing.
Any Ideas??


A CCS **is** a "long tail", because it acts exactly like a resistor of huge
value to some mythical
very negative voltage, ( except for stray capacitances ).
A typical CCS would have a **real** finite R = 20Mohms, and if there is 5mA of
current,
then the CCS acts like 20Mohms commom Rk taken to -100,000 volts dc supply,
which of course is difficult to arrange in a little box.

But you could have an LTP with each grid at 0V bias, cathodes at say +5V bias
and
a 5mA common cathode current taken to -12V, or taken to some lower negative
supply
as in the case at
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Patrick Turner.


RonL


  #4   Report Post  
Ian Iveson
 
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Patrick Turner wrote

I'm working on a long tail pair phase inverter using a 12ax7 with
a current
sink
on the tail. I've noticed in some other designs (ARC) they use a
resistor
~5k between the 2 tails
and the CC sink. What's the benefit of this resistor? I find
very little
difference with or without it.


But what are you saying?

Are there two CCS, one for each 1/2 of the 12AX7?
If so, a 5k between each cathode acts like a current feedback R
to linearize the current change in the two tubes;
there is none in the CCS.

Have you checked the gains of the 12AX7?

What *exactly* is the circuit configuation?


There are enough clues, as long as you don't get totally hung up on
the meaning of "tail".


It causes the CCS to work a little harder to make up for the
series
resistance but other than that
I see no change in the bias or operating point.


CCS never "work hard"
There is no current change; the current is constant, so no
signal power is liberated in the CCS, only the DC power
of Vdc x Idc across the device.


What a jumble of nonsense.

It's class A. Instantaneous power varies, although mean power will
not, assuming a symmetrical signal and a perfect CCS which yours, by
some considerable distance, is not.

You may find these equations useful:

Ohms Law: V=R.I
Power: W=V.I

So, to the extent that a CCS is not perfect, it will work harder if
the voltage across it varies more. To the extent that it is perfect,
it will work harder some of the time, and less hard some of the
time, and although that amounts to the same amount of work
altogether, peak work rate will be higher.

You should take some lessons from Phil, who has seen sense twice in
just one week.




There's very little written about using a CCS with long tails.
I've
searched the web
searched the web, but nothing.
Any Ideas??


A CCS **is** a "long tail", because it acts exactly like a
resistor of huge
value to some mythical
very negative voltage, ( except for stray capacitances ).


Good!

A typical CCS would have a **real** finite R = 20Mohms, and if
there is 5mA of
current,
then the CCS acts like 20Mohms commom Rk taken to -100,000 volts
dc supply,
which of course is difficult to arrange in a little box.

But you could have an LTP with each grid at 0V bias, cathodes at
say +5V bias
and
a 5mA common cathode current taken to -12V, or taken to some lower
negative
supply
as in the case at
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm


What do you think the dynamic resistance of your CCS is? At DC and
at 20kHz? If resistance falls to a few kohm at, say 50kHz, does that
matter?

cheers, Ian



  #5   Report Post  
Ian Iveson
 
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PS

I wrote:

...Instantaneous power varies, although mean power will not...


This only takes the signal into account, BTW, which will be the same
regardless of the resistor, assuming a perfect CCS.

But looking at the DC conditions, the resistor drops the voltage
seen by the CCS, thus the mean power it dissipates is less.

All assuming Phil and I are correct in assuming the resistor is
simply in series with the CCS.

cheers, Ian




  #6   Report Post  
Ian Iveson
 
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Patrick Turner wrote
...
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm
...


Could you show the power supply for this? Or perhaps just say what
the source resistance is of the -87V bias supply?

cheers, Ian


  #7   Report Post  
 
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Hi Group:
I should have posted this before but here's a link to the actual circuit:

http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf

It will be the input/phase inverter section of an amp I'm building.
A differential driver will follow this stage and then the output section
(Kt88's)




"Patrick Turner" wrote in message
...


wrote:

I'm working on a long tail pair phase inverter using a 12ax7 with a
current
sink
on the tail. I've noticed in some other designs (ARC) they use a resistor
~5k between the 2 tails
and the CC sink. What's the benefit of this resistor? I find very little
difference with or without it.


But what are you saying?

Are there two CCS, one for each 1/2 of the 12AX7?
If so, a 5k between each cathode acts like a current feedback R
to linearize the current change in the two tubes;
there is none in the CCS.

Have you checked the gains of the 12AX7?

What *exactly* is the circuit configuation?




It causes the CCS to work a little harder to make up for the series
resistance but other than that
I see no change in the bias or operating point.


CCS never "work hard"
There is no current change; the current is constant, so no
signal power is liberated in the CCS, only the DC power
of Vdc x Idc across the device.




There's very little written about using a CCS with long tails. I've
searched the web
searched the web, but nothing.
Any Ideas??


A CCS **is** a "long tail", because it acts exactly like a resistor of
huge
value to some mythical
very negative voltage, ( except for stray capacitances ).
A typical CCS would have a **real** finite R = 20Mohms, and if there is
5mA of
current,
then the CCS acts like 20Mohms commom Rk taken to -100,000 volts dc
supply,
which of course is difficult to arrange in a little box.

But you could have an LTP with each grid at 0V bias, cathodes at say +5V
bias
and
a 5mA common cathode current taken to -12V, or taken to some lower
negative
supply
as in the case at
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Patrick Turner.


RonL




  #8   Report Post  
Ian Iveson
 
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r.laury wrote

Hi Group:
I should have posted this before but here's a link to the actual
circuit:

http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf


Yes Ron, that's what Phil guessed.

Have you followed his advice? Current through ECC83 should be about
1mA per side, giving 2mA through the CCS.

Take failure of MJE340 to collector/emitter short as your starting
point with the present circuit.

2mA through 15k would give -30V at cathodes, so clearly the current
will rise. By how much? What will the situation be if the current
rises to 4mA? That would give 0V at the cathode but less than 0V at
the anodes (!), so it couldn't be quite 4mA. Somewhere between 3.5
and 4mA will give a Vak which allows that current with the
consequent largish positive Vgk.

What happens at the grids?

If there were no extra 5k, there would be just 10k at the cathode.
What happens to the grids now?

I am surprised you measured no difference, BTW. The added 5k makes
for a poorer CCS. What did you measure, just the static current? Try
using an AC signal to the input, then measure the AC voltage at the
cathodes, and measure the AC current through the CCS. Calculate the
AC resistance. Do this at 100Hz and 20kHz if you can, with and
without the 5k.

Mosfets are better if you can avoid blowing them up.

It will be the input/phase inverter section of an amp I'm
building.
A differential driver will follow this stage and then the output
section (Kt88's)


It's all the rage...

cheers, Ian


  #9   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

Patrick Turner wrote
...
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm
...


Could you show the power supply for this? Or perhaps just say what
the source resistance is of the -87V bias supply?


There is no need for me to show a power supply.
Voltage supplies nominated are voltage supplies as nominated.
Voltage supplies are regarded as low impedance supplies.

DIY ppl like to make their own PS, with flexible values for the
voltages and emitter resistors to suit the power tranny they choose.

DIY ppl should be capable of building themsleves a PS before proceeding
with building an amp.
They should fully understand the schematic, and why the values have been
chosen.
Then they would know why it is important that where a low impedance
voltage supply is advantageous
to a circuit's performance, then they should make sure their voltage
supplies are low impedance.

I never set out to spoon feed everyone with every morsel of information
or possible situation.

People must have learnt to think for themselves and to have read and
educated themselves to a level of
competence which allows them to easily follow all of what I have said at
my website.

Where I get enquiries from time to time from ppl trying to build my
designs
of amps or transformers, I always offer them sufficient further
information so they may proceed.

Patrick Turner.









cheers, Ian


  #10   Report Post  
Patrick Turner
 
Posts: n/a
Default



wrote:

Hi Group:
I should have posted this before but here's a link to the actual circuit:

http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf

It will be the input/phase inverter section of an amp I'm building.
A differential driver will follow this stage and then the output section
(Kt88's)


The schematic above appears to be OK although an MJE340
used for only 2 mA of constant current seems a bit over the top
when a TO92 package smaller bjt or a j-fet would do.

The 5k R6 does not have to be in the circuit but it isolates the cathodes from
stray C in the bjt, and allows you to check the actual CCS current easily as
well as provide some
guard in case a tube shorts between anode and cathode, which wouldn't
hurt the bjt because there is the high value anode load RL there to limit
current.

With 50Vrms of signal out of both anodes, and only
say 1.4Vrms input to one side of the LTP, there should be about
0.7Vrms at the commom cathodes.
if you try to measure the signal voltage across the R6 5k,
you should find it to be an extremely small voltage,
so signal current in the 5k is low because below the 5k the
collector input resistance is very very high, many megohms.

Patrick Turner.



"Patrick Turner" wrote in message
...


wrote:

I'm working on a long tail pair phase inverter using a 12ax7 with a
current
sink
on the tail. I've noticed in some other designs (ARC) they use a resistor
~5k between the 2 tails
and the CC sink. What's the benefit of this resistor? I find very little
difference with or without it.


But what are you saying?

Are there two CCS, one for each 1/2 of the 12AX7?
If so, a 5k between each cathode acts like a current feedback R
to linearize the current change in the two tubes;
there is none in the CCS.

Have you checked the gains of the 12AX7?

What *exactly* is the circuit configuation?




It causes the CCS to work a little harder to make up for the series
resistance but other than that
I see no change in the bias or operating point.


CCS never "work hard"
There is no current change; the current is constant, so no
signal power is liberated in the CCS, only the DC power
of Vdc x Idc across the device.




There's very little written about using a CCS with long tails. I've
searched the web
searched the web, but nothing.
Any Ideas??


A CCS **is** a "long tail", because it acts exactly like a resistor of
huge
value to some mythical
very negative voltage, ( except for stray capacitances ).
A typical CCS would have a **real** finite R = 20Mohms, and if there is
5mA of
current,
then the CCS acts like 20Mohms commom Rk taken to -100,000 volts dc
supply,
which of course is difficult to arrange in a little box.

But you could have an LTP with each grid at 0V bias, cathodes at say +5V
bias
and
a 5mA common cathode current taken to -12V, or taken to some lower
negative
supply
as in the case at
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Patrick Turner.


RonL





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

I was wondering about the one you sold, obviously. If the
unfortunate fool who paid you for it puts some NOS Mullard and GE
valves in there, he could be in for a very nasty surprise one day.
Don't assume everyone sticks with the cheapies you sell.

Please, no need to run and try and hide my questions. I'm not here
to embarrass you. I can help:-))

Perhaps you could send a 5k resistor, or perhaps a few resistors and
a mosfet, and some solder. Don't be shy: sell it as an "upgrade".

Don't you want me babe?

cheers, Ian

"Patrick Turner" wrote in message
...


Ian Iveson wrote:

Patrick Turner wrote
...
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm
...


Could you show the power supply for this? Or perhaps just say
what
the source resistance is of the -87V bias supply?


There is no need for me to show a power supply.
Voltage supplies nominated are voltage supplies as nominated.
Voltage supplies are regarded as low impedance supplies.

DIY ppl like to make their own PS, with flexible values for the
voltages and emitter resistors to suit the power tranny they
choose.

DIY ppl should be capable of building themsleves a PS before
proceeding
with building an amp.
They should fully understand the schematic, and why the values
have been
chosen.
Then they would know why it is important that where a low
impedance
voltage supply is advantageous
to a circuit's performance, then they should make sure their
voltage
supplies are low impedance.

I never set out to spoon feed everyone with every morsel of
information
or possible situation.

People must have learnt to think for themselves and to have read
and
educated themselves to a level of
competence which allows them to easily follow all of what I have
said at
my website.

Where I get enquiries from time to time from ppl trying to build
my
designs
of amps or transformers, I always offer them sufficient further
information so they may proceed.

Patrick Turner.









cheers, Ian




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

"Patrick Turner" wrote

http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf

It will be the input/phase inverter section of an amp I'm
building.
A differential driver will follow this stage and then the output
section
(Kt88's)


The schematic above appears to be OK although an MJE340
used for only 2 mA of constant current seems a bit over the top
when a TO92 package smaller bjt or a j-fet would do.

The 5k R6 does not have to be in the circuit but it isolates the
cathodes from
stray C in the bjt,


Er...to what good purpose? There are a heap of factors involved,
including the fact that the resistor reduces the DC voltage across
the CCS substantially, whilst reducing the AC voltage by a
relatively very small amount. The operating conditions of the MJE340
are so changed that it would take more than a casual half-thought to
work out whether the effective capacitance seen by the cathode is
smaller or greater. My guess is greater, and the resistor slows the
response of the CCS.

...and allows you to check the actual CCS current easily as
well as provide some
guard in case a tube shorts between anode and cathode,


Er...

which wouldn't
hurt the bjt because there is the high value anode load RL there
to limit
current.


Quite, so what "guard" does it provide?

With 50Vrms of signal out of both anodes, and only
say 1.4Vrms input to one side of the LTP, there should be about
0.7Vrms at the commom cathodes.
if you try to measure the signal voltage across the R6 5k,
you should find it to be an extremely small voltage,
so signal current in the 5k is low because below the 5k the
collector input resistance is very very high, many megohms.


How many? You claimed 22Mohm for your design, although perhaps it
was not the one you have posted now? It is very similar to the one
in question, but for the deathwish.

And at what frequency? How does it vary? Perhaps you don't think
these things are important?

cheers, Ian


  #13   Report Post  
Patrick Turner
 
Posts: n/a
Default Tail Resistor in long tail pair with CCS.



Ian Iveson wrote:

I was wondering about the one you sold, obviously. If the
unfortunate fool who paid you for it puts some NOS Mullard and GE
valves in there, he could be in for a very nasty surprise one day.
Don't assume everyone sticks with the cheapies you sell.

Please, no need to run and try and hide my questions. I'm not here
to embarrass you. I can help:-))

Perhaps you could send a 5k resistor, or perhaps a few resistors and
a mosfet, and some solder. Don't be shy: sell it as an "upgrade".

Don't you want me babe?

cheers, Ian


Ian, you make a boring ****wit out of yourself some days.

Patrick Turner.



"Patrick Turner" wrote in message
...


Ian Iveson wrote:

Patrick Turner wrote
...
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm
...

Could you show the power supply for this? Or perhaps just say
what
the source resistance is of the -87V bias supply?


There is no need for me to show a power supply.
Voltage supplies nominated are voltage supplies as nominated.
Voltage supplies are regarded as low impedance supplies.

DIY ppl like to make their own PS, with flexible values for the
voltages and emitter resistors to suit the power tranny they
choose.

DIY ppl should be capable of building themsleves a PS before
proceeding
with building an amp.
They should fully understand the schematic, and why the values
have been
chosen.
Then they would know why it is important that where a low
impedance
voltage supply is advantageous
to a circuit's performance, then they should make sure their
voltage
supplies are low impedance.

I never set out to spoon feed everyone with every morsel of
information
or possible situation.

People must have learnt to think for themselves and to have read
and
educated themselves to a level of
competence which allows them to easily follow all of what I have
said at
my website.

Where I get enquiries from time to time from ppl trying to build
my
designs
of amps or transformers, I always offer them sufficient further
information so they may proceed.

Patrick Turner.









cheers, Ian



  #14   Report Post  
Patrick Turner
 
Posts: n/a
Default Tail Resistor in long tail pair with CCS.



Ian Iveson wrote:

"Patrick Turner" wrote

http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf

It will be the input/phase inverter section of an amp I'm
building.
A differential driver will follow this stage and then the output
section
(Kt88's)


The schematic above appears to be OK although an MJE340
used for only 2 mA of constant current seems a bit over the top
when a TO92 package smaller bjt or a j-fet would do.

The 5k R6 does not have to be in the circuit but it isolates the
cathodes from
stray C in the bjt,


Er...to what good purpose? There are a heap of factors involved,
including the fact that the resistor reduces the DC voltage across
the CCS substantially, whilst reducing the AC voltage by a
relatively very small amount. The operating conditions of the MJE340
are so changed that it would take more than a casual half-thought to
work out whether the effective capacitance seen by the cathode is
smaller or greater. My guess is greater, and the resistor slows the
response of the CCS.


In actual fact the operation with the series R in the tail
makes no real difference in practical terms even though
one could possibly measure what the stray C might do.


...and allows you to check the actual CCS current easily as
well as provide some
guard in case a tube shorts between anode and cathode,


Er...

which wouldn't
hurt the bjt because there is the high value anode load RL there
to limit
current.


Quite, so what "guard" does it provide?


One like those guys at the Palace, big furry hat and red coat,
and probably gay.


With 50Vrms of signal out of both anodes, and only
say 1.4Vrms input to one side of the LTP, there should be about
0.7Vrms at the commom cathodes.
if you try to measure the signal voltage across the R6 5k,
you should find it to be an extremely small voltage,
so signal current in the 5k is low because below the 5k the
collector input resistance is very very high, many megohms.


How many? You claimed 22Mohm for your design, although perhaps it
was not the one you have posted now? It is very similar to the one
in question, but for the deathwish.

And at what frequency? How does it vary? Perhaps you don't think
these things are important?


Its obvious you have never measured the aspects of using the circuit
under discussion.
After 4 years of discussions about such things its time you did
something.

Patrick Turner.



cheers, Ian


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