Valve instability - but not as we know it.

[Adrian Tuddenham]

I have been commisssioned to design and build a one-off audio
preamplifier using E88CC / 6922 valves in most of the signal-handling
stage. (Don't ask why - it is a strange project but definitely not
intended to be a piece of cult audiphool gear)

It has to meet an exacting spec for several parameters, including noise.
I was experiencing great problems measuring the noise by means of my
workshop bench amplifier because, as I increased the system gain,
various forms of instability began to manifest themselves.

At first I thought the problem was caused by V.H.F. instability in two
of the stages which were connected as cascodes; in that mode, high gain
is maintained to several hundred megacycles and some of my wiring was
long enough to have an appreciable effect at those frequencies. I was
already using a grid stopper on the bottom triode of the pair, but,
unconventionally, I found that another grid stopper on the upper triode
was necessary to cure the problem. H.F. oscillation in the HT PSU
(which was transistorised) was found to be another cause of some
unpredicatable behaviour - but there was still another effect which
remained:

This third effect depended on the gain of the whole system, the
amplifier on test plus the bench amplifier. The bench amplifier has a
monitoring loudspeaker, with an audio gain control independent of the
main calibrated measuring attenuator; the effect was found to vary with
the setting of this volume control. When the volume was advanced
slowly, there was a pause, then the level meter slammed hard over. As
the control was turned back, after a few seconds, the meter dropped
back. The effect was just like audio feedback in a very high-Q
situation.

Eventually I came to the conclusion that this really was audio feedback,
but in the ultrasonic range. I found I could alter the oscillation
point by covering the loudspeaker with my hand. If I tapped one of the
valves with a small hard object, I could send the meter off-scale -
although very little sound was audible through the loudspeaker. An
oscilloscope showed oscillation building up at 37.5 Kc/s

Presumably the short, robust, electrode structure of these VHF triodes,
which were never intended for audio work, has a high-Q mechanical
resonance at ultrasonic frequency.

 

[Pimpom]

I have been commisssioned to design and build a one-off audio
preamplifier using E88CC / 6922 valves in most of the
signal-handling
stage. (Don't ask why - it is a strange project but definitely
not
intended to be a piece of cult audiphool gear)

It has to meet an exacting spec for several parameters,
including
noise. I was experiencing great problems measuring the noise by
means
of my workshop bench amplifier because, as I increased the
system
gain, various forms of instability began to manifest
themselves.

At first I thought the problem was caused by V.H.F. instability
in two
of the stages which were connected as cascodes; in that mode,
high
gain is maintained to several hundred megacycles and some of my
wiring was long enough to have an appreciable effect at those
frequencies. I was already using a grid stopper on the bottom
triode
of the pair, but, unconventionally, I found that another grid
stopper
on the upper triode was necessary to cure the problem. H.F.
oscillation in the HT PSU (which was transistorised) was found
to be
another cause of some unpredicatable behaviour - but there was
still
another effect which remained:

This third effect depended on the gain of the whole system, the
amplifier on test plus the bench amplifier. The bench
amplifier has
a monitoring loudspeaker, with an audio gain control
independent of
the main calibrated measuring attenuator; the effect was found
to
vary with the setting of this volume control. When the volume
was
advanced slowly, there was a pause, then the level meter
slammed hard
over. As the control was turned back, after a few seconds, the
meter
dropped back. The effect was just like audio feedback in a
very
high-Q situation.

Eventually I came to the conclusion that this really was audio
feedback, but in the ultrasonic range. I found I could alter
the
oscillation point by covering the loudspeaker with my hand. If
I
tapped one of the valves with a small hard object, I could send
the
meter off-scale - although very little sound was audible
through the
loudspeaker. An oscilloscope showed oscillation building up at
37.5
Kc/s

Presumably the short, robust, electrode structure of these VHF
triodes, which were never intended for audio work, has a high-Q
mechanical resonance at ultrasonic frequency.

Tube (valve) microphonics are/were common with audio types too.
Other components can also exhibit the effect. I've come across it
with capacitors (ceramics are notorious), even resistors and
jacks.

 

[Phil Allison]

"Adrian Tuddenham"

I have been commisssioned

** Balls - you have happily accepted a paying job of work.

to design and build a one-off audio
preamplifier using E88CC / 6922 valves in most of the signal-handling
stage.

** Another audiophool wank machine.

Don't ask why

** For the money of course.

it is a strange project but definitely not intended to be a piece of cult
audiphool gear

** I believe you - but millions would NOT !!!

It has to meet an exacting spec for several parameters, including noise.

** ROTFLMAO !!!

Fucking valves are NOISY - you imbecile !!

And no two are quite the same !!!

Eventually I came to the conclusion that this really was audio feedback,
but in the ultrasonic range. I found I could alter the oscillation
point by covering the loudspeaker with my hand. If I tapped one of the
valves with a small hard object, I could send the meter off-scale -
although very little sound was audible through the loudspeaker. An
oscilloscope showed oscillation building up at 37.5 Kc/s

** No fooling ?????????

Who woulda ever thunk a small, thin walled glass tube could resonate at
such a frequency...


.... Phil

 

[Adrian Tuddenham]

Adrian Tuddenham wrote: [...]

Eventually I came to the conclusion that this really was audio
feedback, but in the ultrasonic range. I found I could alter
the
oscillation point by covering the loudspeaker with my hand. If
I
tapped one of the valves with a small hard object, I could send
the
meter off-scale - although very little sound was audible
through the
loudspeaker. An oscilloscope showed oscillation building up at
37.5
Kc/s

Presumably the short, robust, electrode structure of these VHF
triodes, which were never intended for audio work, has a high-Q
mechanical resonance at ultrasonic frequency.

Tube (valve) microphonics are/were common with audio types too.
Other components can also exhibit the effect. I've come across it
with capacitors (ceramics are notorious), even resistors and
jacks.

I had expected some microphony, but not at ultrasonic frequencies. I
was quite prepared for a bit of audible 'ponging' when I tapped the
input valves, a small amount of which did occur.

The effect of turning up the monitoring amplifier volume control was a
sudden *drop* in background noise (as it became overloaded with the
ultrasonic signal), simultaneously the meter hit its end stop. It was
initially very puzzling.

 

[Pimpom]

Adrian Tuddenham wrote: [...]

Eventually I came to the conclusion that this really was
audio
feedback, but in the ultrasonic range. I found I could alter
the
oscillation point by covering the loudspeaker with my hand.
If
I
tapped one of the valves with a small hard object, I could
send
the
meter off-scale - although very little sound was audible
through the
loudspeaker. An oscilloscope showed oscillation building up
at
37.5
Kc/s

Presumably the short, robust, electrode structure of these
VHF
triodes, which were never intended for audio work, has a
high-Q
mechanical resonance at ultrasonic frequency.

Tube (valve) microphonics are/were common with audio types
too.
Other components can also exhibit the effect. I've come across
it
with capacitors (ceramics are notorious), even resistors and
jacks.

I had expected some microphony, but not at ultrasonic
frequencies. I
was quite prepared for a bit of audible 'ponging' when I tapped
the
input valves, a small amount of which did occur.

The effect of turning up the monitoring amplifier volume
control was a
sudden *drop* in background noise (as it became overloaded with
the
ultrasonic signal), simultaneously the meter hit its end stop.
It was
initially very puzzling.

I can't recall a personal experience of instability caused by
microphonic feedback at ultrasonic frequencies, but I do remember
some occuring at high audio freqs. One case that comes to mind
happened in a movie theater run by a friend back in the 70s. I
isolated the offending valve by tapping them in turn and cured it
by inserting a wad of cottonwool between the tube and its
retaining clamp. The owner was amazed by what seemed like magic
to him.

 

[Tauno Voipio]

I have been commisssioned to design and build a one-off audio
preamplifier using E88CC / 6922 valves in most of the signal-handling
stage. (Don't ask why - it is a strange project but definitely not
intended to be a piece of cult audiphool gear)

It has to meet an exacting spec for several parameters, including noise.
I was experiencing great problems measuring the noise by means of my
workshop bench amplifier because, as I increased the system gain,
various forms of instability began to manifest themselves.

At first I thought the problem was caused by V.H.F. instability in two
of the stages which were connected as cascodes; in that mode, high gain
is maintained to several hundred megacycles and some of my wiring was
long enough to have an appreciable effect at those frequencies. I was
already using a grid stopper on the bottom triode of the pair, but,
unconventionally, I found that another grid stopper on the upper triode
was necessary to cure the problem. H.F. oscillation in the HT PSU
(which was transistorised) was found to be another cause of some
unpredicatable behaviour - but there was still another effect which
remained:

Why on earth cascodes for audio?

The cascode is a way to get a good VHF/UHF amplifier
with triodes, simulating the properties of a pentode
with two triodes. The tubes do not know that you're
aiming at a frequency well below the capability of
the amplifier. If you're building an UHF-capable
amplifier, you have to have the layout, shielding
and bypassing to match. Have you paid attention to
proper bypassing of the heaters?

 

[Adrian Tuddenham]

Why on earth cascodes for audio?

The voltage gain of a single triode is about 30, which would not have
been enough for the required open-loop gain before feedback. A cascaded
pair of triodes (both in common-cathode mode) could be expected to give
a gain of 900, but the output signal would be in the same polarity the
input, so feedback would have to be returned to the cathode of the first
stage. Two of the stages in this design needed the 'virtual earth'
configuration to obtain a response which fell to zero at certain
frequencies or certain gain control settings, so I needed a
configuration which gave inverted output polarity.

A pair of triodes in cascode gave sufficient open-loop gain (about 100)
for the feedback to work properly and still allowed enough voltage swing
(about 20v p/p) to give a good overload margin. I could have obtained
even more gain from the pair by increasing the anode load resistor and
by-passing the upper valve with a resistor so that the lower valve could
work at higher anode current. This gain would have come at the expense
of bandwidth, which I could easily have afforded to lose, but
unfortunately the D.C. conditions of that configuration are not
particularly stable with valve ageing.


The input stage of the chain had to be designed for the lowest possible
noise and had to present a load of 47k to the source. If a physical
terminating resistor had been used between signal and earth, the S/N
ratio would have been worsened by 6dB; so a virtual source input
impedance was generated instead, using separate current and voltage
feedback paths. The virtual earth current feedback from anode to grid
determines the signal input current; a resistor in series with the
cathode bypass capacitor allows the correct voltage swing at the cathode
to make the grid circuit look like 47k. (This means that D.C. heaters
are an absolute necessity.)

http://www.poppyrecords.co.uk/other/images/FrontEnd.gif

Again, this could have been done with a cascaded pair of triodes, taking
the inverted polarity current feedback from the anode of the first
stage, but the cascode gives a reduced component count and a tidier
layout.

The cascode is a way to get a good VHF/UHF amplifier
with triodes, simulating the properties of a pentode
with two triodes. The tubes do not know that you're
aiming at a frequency well below the capability of
the amplifier. If you're building an UHF-capable
amplifier, you have to have the layout, shielding
and bypassing to match.

By using a grid stopper on the upper triode, the vhf response is killed
and the design becomes a lot less layout-critical. It seemed a pity to
lose all that hard-earned bandwidth - but I still had enough left to
allay any fears about whether the H.F. response would be high enough for
audio.

...Have you paid attention to
proper bypassing of the heaters?

Very helpful advice. During my attempts to track down the cause of the
instabiity, I tried by-passing the heaters to earth with a short-leaded
capacitor, but that didn't make any difference. If the cathode-heater
capacitance is low, the inductance of long cathode leads can improve
stability by reducing VHF gain - although I wouldn't recommend this as a
design technique.

 

[Phil Allison]

"Adrian Tuddenham"

The input stage of the chain had to be designed for the lowest possible
noise and had to present a load of 47k to the source.

** So it is a MM phono pre-amp - cos nothing else has that requirement.

Why be so fucking coy about it ???

If a physical terminating resistor had been used between signal and earth,
the S/N
ratio would have been worsened by 6dB;

** Absolute bollocks !!!!!!!

Nowhere in audio are the source load impedances made the same.

And no phono PU has a 47kohms source impedance.

so a virtual source input
impedance was generated instead, using separate current and voltage
feedback paths.

** Guaranteed to be noisier than the usual circuits.

My god you are a tedious idiot.

http://www.poppyrecords.co.uk/other/images/FrontEnd.gif

** A bonkers schem.

Again, this could have been done with a cascaded pair of triodes, taking
the inverted polarity current feedback from the anode of the first
stage, but the cascode gives a reduced component count and a tidier
layout.

** And far more noise.

Measured the EIN yet ??



.... Phil

 

[Adrian Tuddenham]

Yes, of course! I would have expected this immediately. Vacuum valves
are notoriously microphonic,...

The thing which surprised me was that the microphony was so marked and
of such high Q (it took several seconds to die down when the gain was
cut).

The other surprise was that a cheap and nasty loudspeaker, squirting
sound through a few holes drilled in the bottom of a measurement
amplifier casing, which was screwed to the underside of a shelf, should
give enough output at ultrasonic frequencies to cause the effect. I
suppose I had not taken account of the loop gain, which was being used
to measure the noise level of the input stage.

The voltage stage gains of the pre-amp on test were approximately x10,
x1, x1, x2, x5, x10; giving a total gain of 1000. The bench amplifier
was set to "50dB" , which is actually a voltage gain of x316. A further
10dB could be added by the volume control on the bench amplifier to give
a total of 10^6 voltage gain betweeen a 47k input load resistor and the
3-ohm moving coil loudspeaker.

When measured this way, with a bandwidth of 25 Kc/s, I am happy to say
that the noise level of the pre-amp was equivalent to the Johnson noise
of the 47k resistor within the limits of my measuring equipment.

 

[Adrian Tuddenham]

"Adrian Tuddenham"


** Absolute bollocks !!!!!!!

You seem very angry that the electrons don't seem to have learned your
laws of physics - perhaps you should take the trouble to learn theirs.

 

[Phil Allison]

"Adrian Tuddenham"

You seem very angry that the electrons don't seem to have learned your
laws of physics - perhaps you should take the trouble to learn theirs.

** So you snip my post to just one line and write a totally stupid reply.

Electrons are embarrassed to know fools like you.

Piss off.


.... Phil

 

[Tim Williams]

Why on earth cascodes for audio?

Why not?

A friend of mine is building <0.01% THD amplifiers, before global
feedback, using just a couple garden variety JFETs and BJTs. That's not
bad at all. Okay, it's not tube, but the same principle applies. No need
to use hundreds of transistors like ICs do, just a responsible excess, in
the 20s or so for a gain block or what have you.

Tim

 

[Tauno Voipio]

Why not?

A friend of mine is building<0.01% THD amplifiers, before global
feedback, using just a couple garden variety JFETs and BJTs. That's not
bad at all. Okay, it's not tube, but the same principle applies. No need
to use hundreds of transistors like ICs do, just a responsible excess, in
the 20s or so for a gain block or what have you.

Tim

The devil is in the details.

Tubes are different beasts due to the impedance levels
and physical dimensions. 100 MHz and up are challenging
with tubes, but with semiconductors even far higher
frequencies are pretty easy.

The tube used, E88CC is a twin triode designed especially
for TV receiver VHF front-ends, and using it needs VHF
-compatible layout, shielding and by-passing techniques.

For parasitic taming, I'd attempt using ferrite beads.

 

[Adrian Tuddenham]

"Adrian Tuddenham"




** So it is a MM phono pre-amp - cos nothing else has that requirement.

Why be so fucking coy about it ???




** Absolute bollocks !!!!!!!

Nowhere in audio are the source load impedances made the same.

And no phono PU has a 47kohms source impedance.




** Guaranteed to be noisier than the usual circuits.

My god you are a tedious idiot.





** A bonkers schem.




** And far more noise.

Measured the EIN yet ??

Yes.

With a measurement bandwidth of 19 Kc/s and a room temperature of 12
Celcius it is 3.7 microvolts (with about 20% uncertainty) when the input
terminals are terminated with a 47k resistor. The calculated Johnson
noise of the 47k resistor under these conditions is 3.74 microvolts.

I think this refutes your previous comments and would be grateful for
an apology.

 

[Phil Allison]

"Adrian Tuddenham"

With a measurement bandwidth of 19 Kc/s and a room temperature of 12
Celcius it is 3.7 microvolts (with about 20% uncertainty) when the input
terminals are terminated with a 47k resistor. The calculated Johnson
noise of the 47k resistor under these conditions is 3.74 microvolts.

** A noise figure of close to 0dB in the audio band is not possible with
valves.

And you still will not say what the signal source really is.

I think this refutes your previous comments ....

** A crazy, unsupported assertion refutes nothing.



.... Phil

 

[Adrian Tuddenham]

"Adrian Tuddenham"

With a measurement bandwidth of 19 Kc/s and a room temperature of 12
Celcius it is 3.7 microvolts (with about 20% uncertainty) when the input
terminals are terminated with a 47k resistor. The calculated Johnson
noise of the 47k resistor under these conditions is 3.74 microvolts.

** A noise figure of close to 0dB in the audio band is not possible with
valves. [...]
** A crazy, unsupported assertion refutes nothing.

Checkmate.

 

[Phil Allison]

"Adrian Tuddenham"

Checkmate.

** What sort of asinine fuckwit are you ???

Yet again, you snip everything you do not feel like answering and post
another arrogant, shithead remark.

Guess what - that makes you an arrogant shithead.




.... Phil

 

[Phil Allison]

"Bitrex = TROLL "


** FOAD

Asshole

 

[Jamie]

Sometimes when I read your replies I think of these WAV files:


http://tf2wiki.net/w/images/2/22/Sniper_DominationEngineer05.wav

http://tf2wiki.net/w/images/1/1f/Sniper_DominationSoldier01.wav

http://tf2wiki.net/w/images/c/c9/Sniper_revenge21.wav

nice files, I saved them

Jamie

 

[Fred Abse]

The tube used, E88CC is a twin triode designed especially
for TV receiver VHF front-ends

ECC88 is/was the consumer, TV frontend thing.

E88CC is/was a special quality industrial variant, which probably cost
twice as much.