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Default exceeding heater to cathode voltage

I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072,
and it's maximum claimed voltage is +- 90v peak. i'm using one triode
as a cathode follower, and so the voltage at the cathode is gonna be
around 150v above ground. i'm running AC on the heaters.

so, why not cap couple before the follower you say? well, some think
even the expensive caps sound better when driven and loaded with lower
impedances. i'm saving the cap for the follower's output.

i'm actually modifying an existing mic pre, and one thing they did that
i don't understand is they return each side of the AC heater supply to
ground with a 1k resistor. what is this done for? seems like if it
were left floating, i wouldn't have to worry about the voltage limit
situation. it probably also causes ground to wiggle a tiny bit at
60hz.

thanks!

SB

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Mark
 
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well obviously if you exceed the h-c rating you could cause a h-c
breakdown.

If you float the filament at 150 V, make sure you don't have any other
tubes connected to the same filament winding and make sure the
transformer filament winding is rated for 150 V.

But I don't understand why the cathode has to be at 150 V above ground
for a cathode follower, isn't the cathode a few volts above the grid
and the grid is near ground?

Of course what I really don't understand is why you want to do this at
all , but lets not go there.

Mark

  #3   Report Post  
Scott Dorsey
 
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In article . com,
wrote:
I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072,
and it's maximum claimed voltage is +- 90v peak. i'm using one triode
as a cathode follower, and so the voltage at the cathode is gonna be
around 150v above ground. i'm running AC on the heaters.


Depending on the tube, it could be anything from hum to premature heater
failures.

so, why not cap couple before the follower you say? well, some think
even the expensive caps sound better when driven and loaded with lower
impedances. i'm saving the cap for the follower's output.


So, raise the heater reference voltage. Hang the heater at +150V.

i'm actually modifying an existing mic pre, and one thing they did that
i don't understand is they return each side of the AC heater supply to
ground with a 1k resistor. what is this done for? seems like if it
were left floating, i wouldn't have to worry about the voltage limit
situation. it probably also causes ground to wiggle a tiny bit at
60hz.


The heater will induce more hum if it's left floating. The heater
circuit needs to be referenced to ground. It could be referenced
to something above ground, if need be.

Or you could use a different tube for the output stage.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Ben Bradley
 
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On 15 Apr 2005 12:44:00 -0700, wrote:

I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072,
and it's maximum claimed voltage is +- 90v peak. i'm using one triode
as a cathode follower, and so the voltage at the cathode is gonna be
around 150v above ground. i'm running AC on the heaters.

so, why not cap couple before the follower you say? well, some think
even the expensive caps sound better when driven and loaded with lower
impedances. i'm saving the cap for the follower's output.

i'm actually modifying an existing mic pre, and one thing they did that
i don't understand is they return each side of the AC heater supply to
ground with a 1k resistor. what is this done for?


It guarantees the capacitive coupling of the AC from the heater to
the cathode is balanced: When one side of the heater supply goes
positive, the other side is going negative. If it were left to float,
it would probably be driven by the capacitive coupling of the
transformer winding, where one side likely has more capacitance to
ground than the other.

seems like if it
were left floating, i wouldn't have to worry about the voltage limit
situation.


Does this tube have a heater winding separate from the other tubes?
If not, can you add a separate transformer to run its heater? I
presume any transformer will have at least 200 volts isolation.
If you have this tube's supply isolated, remove the ground side of
the two 1k resistors, and connect them to the cathode connection.
Even without isolation and using a single winding for all heaters,
you can perhaps 'bias' the heater voltage up to perhaps 80V DC so that
all heater-cathode voltages are within spec.

it probably also causes ground to wiggle a tiny bit at
60hz.


I doubt there's that much current to ground, it's just the
capacitively coupled 60 Hz from the secondary winding.


thanks!

SB


-----
http://mindspring.com/~benbradley


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Paul Stamler
 
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"Scott Dorsey" wrote in message
...
In article . com,
wrote:
I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072,
and it's maximum claimed voltage is +- 90v peak. i'm using one triode
as a cathode follower, and so the voltage at the cathode is gonna be
around 150v above ground. i'm running AC on the heaters.


Depending on the tube, it could be anything from hum to premature heater
failures.


I've even seen some odd distortion behavior and oscillation.

so, why not cap couple before the follower you say? well, some think
even the expensive caps sound better when driven and loaded with lower
impedances. i'm saving the cap for the follower's output.


So, raise the heater reference voltage. Hang the heater at +150V.


If you're limited to a single heater circuit for all the tubes, float it at
+75V. That'll mean the non-CF tubes will be at about -70V H-C and the CD
will be at about +75V. Everyone should be happy.

Peace,
Paul


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Or you could use a different tube for the output stage.
--scott
--


do you think the 6072 makes a poor follower? it'll just be driving the
5 or 10k load of an AD converter, and i don't plan on using long cables
between the two. do i need a higher current tube, like a 12AU7 or
something? since i'm modding, i'm trying to minimize the
pain-in-the-ass factor, and also not bludgeon the PCB too much. the
preamp used 12ax7s before.

SB

  #8   Report Post  
Scott Dorsey
 
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In article .com,
wrote:

Or you could use a different tube for the output stage.


do you think the 6072 makes a poor follower? it'll just be driving the
5 or 10k load of an AD converter, and i don't plan on using long cables
between the two. do i need a higher current tube, like a 12AU7 or
something? since i'm modding, i'm trying to minimize the
pain-in-the-ass factor, and also not bludgeon the PCB too much. the
preamp used 12ax7s before.


I don't know, I never did the math on the 6072. But if you look up the
follower section in the Radiotron Book, there's a method for calculating
basic linearity from the load lines, and there's a formula for figuring
the effective output impedance with a given cathode resistor. Your whole
goal is to balance voltage swing with output impedance and get the best
combination for the device you're using. My personal feeling is you
probably want the load to be at least four or five times as high as the
output impedance of the stage.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #9   Report Post  
Mark
 
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which A/D converter?

some A/D's have a non-linear input Z and need to be driven by a low Z.

While this article is not audio related, depending on the A/D you are
using, some of the concepts may apply.

http://www.sensorsmag.com/articles/0303/20/main.shtml

Mark

  #10   Report Post  
Paul Stamler
 
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"Scott Dorsey" wrote in message
...
In article .com,
wrote:

Or you could use a different tube for the output stage.


do you think the 6072 makes a poor follower? it'll just be driving the
5 or 10k load of an AD converter, and i don't plan on using long cables
between the two. do i need a higher current tube, like a 12AU7 or
something? since i'm modding, i'm trying to minimize the
pain-in-the-ass factor, and also not bludgeon the PCB too much. the
preamp used 12ax7s before.


I don't know, I never did the math on the 6072. But if you look up the
follower section in the Radiotron Book, there's a method for calculating
basic linearity from the load lines, and there's a formula for figuring
the effective output impedance with a given cathode resistor. Your whole
goal is to balance voltage swing with output impedance and get the best
combination for the device you're using. My personal feeling is you
probably want the load to be at least four or five times as high as the
output impedance of the stage.


Not really -- you want it to be a good deal higher than that. Typically a
cathode follower circuit has an output impedance of some 300-600 ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up most of
its open-loop gain.

Sean's wanting to drive a soundcard input. If it's 5k input impedance I'd go
for something with some real muscle, like a 12FQ7 (same tube as a 6SN7 but a
different filament setup, and it's a 9-pin). If it's 10k a 12AU7 will do.
The goals are linearity and good current delivery, and those two tubes are
good in both respects. The 6072 is basically a mil-spec version of the 12AY7
which has low microphonics, but only so-so linearity and current delivery.

Peace,
Paul




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Paul Stamler wrote:

Not really -- you want it to be a good deal higher than that.

Typically a
cathode follower circuit has an output impedance of some 300-600

ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up

most of
its open-loop gain.


Hi Paul! how much higher? and what 'loop' are you refering to?
there's no global feedback happening in this pre. and what
performance change are you refering to when you say 'it sucks'? a load
that's 10x greater Z than the follower will have a small effect on its
output level. but of course, it may turn into a fuzz box. soon i
will investigate the effects of loading on distortion behavior with
the power of FFTs, i just haven't gotten the unit to my practice space
yet where my computer is.

also, can you recommend a transparent sounding zener diode series, for
the purpose of output clamping? with the swing available from tubes,
they really could wipe out any subsequent solid state stage that wasn't
input-protected. go over to a friends house, "dude check out
my...fftfftttttttssszzzz". oops, toasted your AD. sorry!


Sean's wanting to drive a soundcard input.


well, it's a Lavry Blue AD. let's call it a fancy, expensive
soundcard. it's good to have the blues!


SB

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Scott Dorsey
 
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wrote:
Paul Stamler wrote:

Not really -- you want it to be a good deal higher than that.

Typically a
cathode follower circuit has an output impedance of some 300-600

ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up

most of
its open-loop gain.


Hi Paul! how much higher? and what 'loop' are you refering to?
there's no global feedback happening in this pre.


There is... that's what a cathode follower is.... it's a stage with
100% feedback. The feedback through the grid circuit is what causes
the cathode to precisely follow the grid. And when you load it too
much, the cathode suddenly isn't following any more.

I think Mr. Stamler is exaggerating a little bit about a load that
is a factor of ten higher than the effective output Z being a problem.
Note that when the effective output Z is calculated, the gain of the
tube is taken into account... the more gain you have, the more the
feedback is helping you.

and what
performance change are you refering to when you say 'it sucks'? a load
that's 10x greater Z than the follower will have a small effect on its
output level. but of course, it may turn into a fuzz box. soon i
will investigate the effects of loading on distortion behavior with
the power of FFTs, i just haven't gotten the unit to my practice space
yet where my computer is.


You don't need an FFT, although it can be nice. Just stick a 1kc square wave
through the thing and watch what happens to the edges of the waveform.

also, can you recommend a transparent sounding zener diode series, for
the purpose of output clamping? with the swing available from tubes,
they really could wipe out any subsequent solid state stage that wasn't
input-protected. go over to a friends house, "dude check out
my...fftfftttttttssszzzz". oops, toasted your AD. sorry!


I don't like zener clamps if they can be avoided at all. And quite frankly,
you don't get very much output swing out of a cathode follower. At least
not if you want low output Z.

You really need to read the Radiotron Handbook section on followers.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #13   Report Post  
Paul Stamler
 
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wrote in message
oups.com...
Paul Stamler wrote:

Not really -- you want it to be a good deal higher than that.

Typically a
cathode follower circuit has an output impedance of some 300-600

ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up

most of
its open-loop gain.


Hi Paul! how much higher? and what 'loop' are you refering to?
there's no global feedback happening in this pre.


No, but there's local feedback. I was guilty of using shorthand; for
open-loop read "gain without feedback". A cathode follower basically has
100% feedback, bringing the gain down to unity.

How much higher? Basic rule is that you figure out what load the thing can
drive based on its current capacity, not its output impedance. Like I said,
10k for a 12AU7, 5k for a 6SN7 if you bias it good and hot, and if you want
to drive a 5k load with tubes you're best off setting the jumpers for -10dBV
nominal sensitivity rather than +4dBu.

and what
performance change are you refering to when you say 'it sucks'? a load
that's 10x greater Z than the follower will have a small effect on its
output level. but of course, it may turn into a fuzz box.


Exactly. Look, back to opamps for a second. A 5534 has a specified output
impedance of 0.3 ohms, but that doesn't mean you can hang 3 ohms of load on
it and expect anything but fuzz. Same goes for tubes.

soon i
will investigate the effects of loading on distortion behavior with
the power of FFTs, i just haven't gotten the unit to my practice space
yet where my computer is.


You may be horrified at what you see. Tubes really don't like low
impedances.

also, can you recommend a transparent sounding zener diode series, for
the purpose of output clamping? with the swing available from tubes,
they really could wipe out any subsequent solid state stage that wasn't
input-protected. go over to a friends house, "dude check out
my...fftfftttttttssszzzz". oops, toasted your AD. sorry!


Check with the manufacturer -- you may find the AD already has clamping
diodes installed for exactly such reasons.

Peace,
Paul


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Paul Stamler
 
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"Scott Dorsey" wrote in message
...

I think Mr. Stamler is exaggerating a little bit about a load that
is a factor of ten higher than the effective output Z being a problem.
Note that when the effective output Z is calculated, the gain of the
tube is taken into account... the more gain you have, the more the
feedback is helping you.


Except that into, say, a 6k load, the tube has a lot less gain to begin
with, so a lot less feedback when used as a cathode follower.

I did some tests a couple of decades ago to illustrate the point. The tubes
weren't completely open-loop, since they had unbypassed cathode resistors,
but still. For a 12AU7 with a 100k plate resistor and a 1.5k cathode
resistor, idling at about 2.2mA:

Load / Gain / THD at +15dBu

240k / 10.0 / 0.22
92k / 8.7 / 0.26
48k / 7.2 / 0.32
20k / 5.0 / 0.52
9.6k / 2.9 / 0.90

Note that whereas at a 240k load there's 10x gain, so the chance to reduce
distortion by that amount, at a 9.6k load, there's only 2.9x gain, so you
can only reduce distortion by that much when you turn the tube into a
cathode follower. Hence my suggestion that the 12AU7 be used with an A/D
converter of -10dBV nominal sensitivity.

For a 6SN7 with a 27k plate resistor and 475 ohm cathode resistor, running
at about 5.4mA idle current, the numbers a

240k / 13.0 / 0.072
92k / 12.0 / 0.082
48k / 10.0 / 0.10
20k / 8.0 / 0.18
9.6k / 5.6 / 0.27

Note that at 9.6k you have about twice the gain and less than 1/3 the THD,
so a 6SN7 should be about 6x cleaner as a cathode follower under those
conditions. It's a nice tube, although a power hog. But it's still happier
at higher load impedances; into 240k you have the possibility of around
..006% THD, and in fact that's what I've measured at that level into that
impedance.

By the way -- yes, I know I've written that I don't think THD is a good
measure of audio quality, and I stand by that. But I did these tests back in
the 1980s, when I was young and foolish -- and besides, the same pattern of
behavior will show up in IM tests.

Peace,
Paul


  #15   Report Post  
Ben Bradley
 
Posts: n/a
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On 15 Apr 2005 12:44:00 -0700, wrote:

I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072,
and it's maximum claimed voltage is +- 90v peak. i'm using one triode
as a cathode follower, and so the voltage at the cathode is gonna be
around 150v above ground. i'm running AC on the heaters.

so, why not cap couple before the follower you say? well, some think
even the expensive caps sound better when driven and loaded with lower
impedances. i'm saving the cap for the follower's output.

i'm actually modifying an existing mic pre, and one thing they did that
i don't understand is they return each side of the AC heater supply to
ground with a 1k resistor. what is this done for?


It guarantees the capacitive coupling of the AC from the heater to
the cathode is balanced: When one side of the heater supply goes
positive, the other side is going negative. If it were left to float,
it would probably be driven by the capacitive coupling of the
transformer winding, where one side likely has more capacitance to
ground than the other.

seems like if it
were left floating, i wouldn't have to worry about the voltage limit
situation.


Does this tube have a heater winding separate from the other tubes?
If not, can you add a separate transformer to run its heater? I
presume any transformer will have at least 200 volts isolation.
If you have this tube's supply isolated, remove the ground side of
the two 1k resistors, and connect them to the cathode connection.
Even without isolation and using a single winding for all heaters,
you can perhaps 'bias' the heater voltage up to perhaps 80V DC so that
all heater-cathode voltages are within spec.

it probably also causes ground to wiggle a tiny bit at
60hz.


I doubt there's that much current to ground, it's just the
capacitively coupled 60 Hz from the secondary winding.


thanks!

SB


-----
http://mindspring.com/~benbradley


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Ben Bradley
 
Posts: n/a
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On 15 Apr 2005 12:44:00 -0700, wrote:

I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072,
and it's maximum claimed voltage is +- 90v peak. i'm using one triode
as a cathode follower, and so the voltage at the cathode is gonna be
around 150v above ground. i'm running AC on the heaters.

so, why not cap couple before the follower you say? well, some think
even the expensive caps sound better when driven and loaded with lower
impedances. i'm saving the cap for the follower's output.

i'm actually modifying an existing mic pre, and one thing they did that
i don't understand is they return each side of the AC heater supply to
ground with a 1k resistor. what is this done for?


It guarantees the capacitive coupling of the AC from the heater to
the cathode is balanced: When one side of the heater supply goes
positive, the other side is going negative. If it were left to float,
it would probably be driven by the capacitive coupling of the
transformer winding, where one side likely has more capacitance to
ground than the other.

seems like if it
were left floating, i wouldn't have to worry about the voltage limit
situation.


Does this tube have a heater winding separate from the other tubes?
If not, can you add a separate transformer to run its heater? I
presume any transformer will have at least 200 volts isolation.
If you have this tube's supply isolated, remove the ground side of
the two 1k resistors, and connect them to the cathode connection.
Even without isolation and using a single winding for all heaters,
you can perhaps 'bias' the heater voltage up to perhaps 80V DC so that
all heater-cathode voltages are within spec.

it probably also causes ground to wiggle a tiny bit at
60hz.


I doubt there's that much current to ground, it's just the
capacitively coupled 60 Hz from the secondary winding.


thanks!

SB


-----
http://mindspring.com/~benbradley
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