[Dave xxxx]

When the amplifier is in use its got two 6922 in pre amp stage, and solid
state power amp

If you touch the amp its like touching a microphone you hear a noise through
the right speaker

swapping the valves gets rid of it, so yes its a faulty valve but what's
wrong with the valve to cause that fault ?


ta in advance

[Eeyore]

Dave xxxx wrote:

When the amplifier is in use its got two 6922 in pre amp stage, and solid
state power amp

If you touch the amp its like touching a microphone you hear a noise through
the right speaker

swapping the valves gets rid of it, so yes its a faulty valve but what's
wrong with the valve to cause that fault ?

It's called microphony and is a common problem with valves.

Graham

[Patrick Turner]

Dave xxxx wrote:

When the amplifier is in use its got two 6922 in pre amp stage, and solid
state power amp

If you touch the amp its like touching a microphone you hear a noise through
the right speaker

swapping the valves gets rid of it, so yes its a faulty valve but what's
wrong with the valve to cause that fault ?

The fault could be one or two things, or both together.

The first problem with using tubes for preamps is
their tendency to be microphonic, so gently tapping a preamp tube
will often generate a noise in the speaker just like someone tapping a
microphone connected to the input. Some tubes are more microphonic than
others,
and depending on the amount of gain, a very microphonic tube
will be sensitive to vibration and even to sound in the room,
so the sound will be affected by acoustic feedback.
It is entirely possible that the very slight acoustic feedback that
occurs between
speaker and input tubes gives the tubes chosen their characteristic
"voice".
Tested with a resistance load, the microphonic tube will show low noise,
low THD/IMD,
but will have a random frequency ring tone or tones when struck with a
pencil, like a bell in fact,
and some ring tones could be at a harmonic in the music, or maybe
not....
Tubes in phono amps or microphone amps especially are
the most sensitive to vibration, and you have to pick through a batch of
tubes for the quietest
and least microphonic for best results.
Once well chosen, microphony is not a problem that seems to be at all
audible.
If you take a speaker and lead out into the back yard, and shut the
doors, and have your
friend scream at the tube directly, inches away,
***You noisy little critter!!!!***, and you cannot hear the criticism
from the backyard speaker,
chances are the tube is fine.
But if you hear your friend muttering quietly, " ****, why to i get
asked to do the stoopidest things?..."
Then you have a really bad tube. The idea is to build amplifiers, not
microphones.

The second problem could be that of oscillations occurring in the preamp
tube.
The frequency of the oscillations are above the audio band, and could be
at some radio frequency.
Such oscillations occur in a stop start fashion because the amp becomes
saturated, shuts down for a moment,
then starts again, and so consecutive bursts of oscillations at say 1MHz
might
stop and start at an audio frequency rate, and a squeal is heard from
the speaker.
This is called squegging, and its more common that you'd think.
Cascode topology phono front ends are notorious for oscillations at HF
and
sometimes they indicate their presence only by slowly touching an input
tube,
and the stray hand capacitance stops the oscillations,
so a click is heard, and again when you remove your hand.
Touching the earth rails near the preamp input with a screw driver blade
can have the same results.
If the RF oscillations are above 20MHz, you won't see them on many CROs,
so usually some
alteration to the earth pathing must be done along with bypassing heater
circuits, B+ supplies, grounded grids et all
with plastic and ceramic caps and done all very close to electrode
connections, ie, within 10mm.
Some high gain circuits can oscillate because at say 20MHz, innocent
looking pcb tracks of 50mm long
have enough inductance to act like a phase shifting inductor with high
impedance, and in fact at
20MHz, the circuit is in fact a very different looking circuit to the
one built for audio
when you take into account all the impedances of the small amounts of L
and C around the circuit.


Often such obscure bugatious behaviours are beyond the capability of
most DIYers to diagnose,
and never even noticed during normal use. If some supersonic tone is
being produced by a preamp,
and the power amp bandwidth is wide enough, tweeters can be destroyed
without knowing why.

DIYers don't expect things to go wrong they can't understand, but sorry,
****e happens, right!

So you should have a good tech whom you trust will diagnose the problem,
will not rip you off to cure it, and won't apply a solution which
compromises the audio bandwidth, or
increases the existing noise and distortion.

Such people are very rare, mainly because DIYers and audiophile hate
paying experts.

I know a lot of unpredictable ****e happens with audio gear.
And it is often with temperamental hi-end makes, CJ, ARC, etc, just to
name two.

The items ppl cannot fix after trawling around a few repair people end
up on Ebay,
and one guy here buys a lot off Ebay, and yessir, I get the job of
fixing the crap other people can't.

Often hi-end gear is a complex mix of tubes and solid state devices such
as mosfets and j-fets,
to the extent that 3 times the device number is used, just to get an
extra zero
in the THD measurement at output voltages you will never need.
The combination of "imaginative" but over complex arrays of tubes and
high transconductance
devices with loops of NFB are a problem waiting to happen when the
slightest
thing alters; such things are over-engineered.
On many designs, you can see what the original designer has done to stop
oscillations.

To sum up, the problem with microphony you seem to have may not be as
simple as it seems.
When in doubt, get the item inspected by an expert.

And BTW, some very elaborate all solid state gear can give a litany of
faults...

Patrick Turner.

[Dave xxxx]

Dave xxxx wrote:


Thanks for answers

[west]

"Patrick Turner" wrote in message
...


Dave xxxx wrote:

When the amplifier is in use its got two 6922 in pre amp stage, and
solid
state power amp

If you touch the amp its like touching a microphone you hear a noise
through
the right speaker

swapping the valves gets rid of it, so yes its a faulty valve but what's
wrong with the valve to cause that fault ?

The fault could be one or two things, or both together.

The first problem with using tubes for preamps is
their tendency to be microphonic, so gently tapping a preamp tube
will often generate a noise in the speaker just like someone tapping a
microphone connected to the input. Some tubes are more microphonic than
others,
and depending on the amount of gain, a very microphonic tube
will be sensitive to vibration and even to sound in the room,
so the sound will be affected by acoustic feedback.
It is entirely possible that the very slight acoustic feedback that
occurs between
speaker and input tubes gives the tubes chosen their characteristic
"voice".
Tested with a resistance load, the microphonic tube will show low noise,
low THD/IMD,
but will have a random frequency ring tone or tones when struck with a
pencil, like a bell in fact,
and some ring tones could be at a harmonic in the music, or maybe
not....
Tubes in phono amps or microphone amps especially are
the most sensitive to vibration, and you have to pick through a batch of
tubes for the quietest
and least microphonic for best results.
Once well chosen, microphony is not a problem that seems to be at all
audible.
If you take a speaker and lead out into the back yard, and shut the
doors, and have your
friend scream at the tube directly, inches away,
***You noisy little critter!!!!***, and you cannot hear the criticism
from the backyard speaker,
chances are the tube is fine.
But if you hear your friend muttering quietly, " ****, why to i get
asked to do the stoopidest things?..."
Then you have a really bad tube. The idea is to build amplifiers, not
microphones.

The second problem could be that of oscillations occurring in the preamp
tube.
The frequency of the oscillations are above the audio band, and could be
at some radio frequency.
Such oscillations occur in a stop start fashion because the amp becomes
saturated, shuts down for a moment,
then starts again, and so consecutive bursts of oscillations at say 1MHz
might
stop and start at an audio frequency rate, and a squeal is heard from
the speaker.
This is called squegging, and its more common that you'd think.
Cascode topology phono front ends are notorious for oscillations at HF
and
sometimes they indicate their presence only by slowly touching an input
tube,
and the stray hand capacitance stops the oscillations,
so a click is heard, and again when you remove your hand.
Touching the earth rails near the preamp input with a screw driver blade
can have the same results.
If the RF oscillations are above 20MHz, you won't see them on many CROs,
so usually some
alteration to the earth pathing must be done along with bypassing heater
circuits, B+ supplies, grounded grids et all
with plastic and ceramic caps and done all very close to electrode
connections, ie, within 10mm.
Some high gain circuits can oscillate because at say 20MHz, innocent
looking pcb tracks of 50mm long
have enough inductance to act like a phase shifting inductor with high
impedance, and in fact at
20MHz, the circuit is in fact a very different looking circuit to the
one built for audio
when you take into account all the impedances of the small amounts of L
and C around the circuit.


Often such obscure bugatious behaviours are beyond the capability of
most DIYers to diagnose,
and never even noticed during normal use. If some supersonic tone is
being produced by a preamp,
and the power amp bandwidth is wide enough, tweeters can be destroyed
without knowing why.

DIYers don't expect things to go wrong they can't understand, but sorry,
****e happens, right!

So you should have a good tech whom you trust will diagnose the problem,
will not rip you off to cure it, and won't apply a solution which
compromises the audio bandwidth, or
increases the existing noise and distortion.

Such people are very rare, mainly because DIYers and audiophile hate
paying experts.

I know a lot of unpredictable ****e happens with audio gear.
And it is often with temperamental hi-end makes, CJ, ARC, etc, just to
name two.

The items ppl cannot fix after trawling around a few repair people end
up on Ebay,
and one guy here buys a lot off Ebay, and yessir, I get the job of
fixing the crap other people can't.

Often hi-end gear is a complex mix of tubes and solid state devices such
as mosfets and j-fets,
to the extent that 3 times the device number is used, just to get an
extra zero
in the THD measurement at output voltages you will never need.
The combination of "imaginative" but over complex arrays of tubes and
high transconductance
devices with loops of NFB are a problem waiting to happen when the
slightest
thing alters; such things are over-engineered.
On many designs, you can see what the original designer has done to stop
oscillations.

To sum up, the problem with microphony you seem to have may not be as
simple as it seems.
When in doubt, get the item inspected by an expert.

And BTW, some very elaborate all solid state gear can give a litany of
faults...

Patrick Turner.

Can a tube shield eliminate or reduce some types of microphony, such as
acoustic feedback rendering the tube usable again? Thanks.

west

[Eeyore]

west wrote:

Can a tube shield eliminate or reduce some types of microphony, such as
acoustic feedback rendering the tube usable again? Thanks.

They usually incorporate a spring so the valve is held better.

It's a palliative measure though, not a cure.

Graham

[Ruud Broens]

"Eeyore" wrote in message
...
:
:
: west wrote:
:
: Can a tube shield eliminate or reduce some types of microphony, such as
: acoustic feedback rendering the tube usable again? Thanks.
:
: They usually incorporate a spring so the valve is held better.
:
: It's a palliative measure though, not a cure.
:
: Graham
:
Here's an example of a socket mounting, designed to minimize
chassis to tube vibrations passthrough:
http://www.askjanfirst.de/eindex.htm
click on the image of socket FAS154
R.

[Patrick Turner]

west wrote:

"Patrick Turner" wrote in message
...


Dave xxxx wrote:

When the amplifier is in use its got two 6922 in pre amp stage, and
solid
state power amp

If you touch the amp its like touching a microphone you hear a noise
through
the right speaker

swapping the valves gets rid of it, so yes its a faulty valve but what's
wrong with the valve to cause that fault ?

The fault could be one or two things, or both together.

The first problem with using tubes for preamps is
their tendency to be microphonic, so gently tapping a preamp tube
will often generate a noise in the speaker just like someone tapping a
microphone connected to the input. Some tubes are more microphonic than
others,
and depending on the amount of gain, a very microphonic tube
will be sensitive to vibration and even to sound in the room,
so the sound will be affected by acoustic feedback.
It is entirely possible that the very slight acoustic feedback that
occurs between
speaker and input tubes gives the tubes chosen their characteristic
"voice".
Tested with a resistance load, the microphonic tube will show low noise,
low THD/IMD,
but will have a random frequency ring tone or tones when struck with a
pencil, like a bell in fact,
and some ring tones could be at a harmonic in the music, or maybe
not....
Tubes in phono amps or microphone amps especially are
the most sensitive to vibration, and you have to pick through a batch of
tubes for the quietest
and least microphonic for best results.
Once well chosen, microphony is not a problem that seems to be at all
audible.
If you take a speaker and lead out into the back yard, and shut the
doors, and have your
friend scream at the tube directly, inches away,
***You noisy little critter!!!!***, and you cannot hear the criticism
from the backyard speaker,
chances are the tube is fine.
But if you hear your friend muttering quietly, " ****, why to i get
asked to do the stoopidest things?..."
Then you have a really bad tube. The idea is to build amplifiers, not
microphones.

The second problem could be that of oscillations occurring in the preamp
tube.
The frequency of the oscillations are above the audio band, and could be
at some radio frequency.
Such oscillations occur in a stop start fashion because the amp becomes
saturated, shuts down for a moment,
then starts again, and so consecutive bursts of oscillations at say 1MHz
might
stop and start at an audio frequency rate, and a squeal is heard from
the speaker.
This is called squegging, and its more common that you'd think.
Cascode topology phono front ends are notorious for oscillations at HF
and
sometimes they indicate their presence only by slowly touching an input
tube,
and the stray hand capacitance stops the oscillations,
so a click is heard, and again when you remove your hand.
Touching the earth rails near the preamp input with a screw driver blade
can have the same results.
If the RF oscillations are above 20MHz, you won't see them on many CROs,
so usually some
alteration to the earth pathing must be done along with bypassing heater
circuits, B+ supplies, grounded grids et all
with plastic and ceramic caps and done all very close to electrode
connections, ie, within 10mm.
Some high gain circuits can oscillate because at say 20MHz, innocent
looking pcb tracks of 50mm long
have enough inductance to act like a phase shifting inductor with high
impedance, and in fact at
20MHz, the circuit is in fact a very different looking circuit to the
one built for audio
when you take into account all the impedances of the small amounts of L
and C around the circuit.


Often such obscure bugatious behaviours are beyond the capability of
most DIYers to diagnose,
and never even noticed during normal use. If some supersonic tone is
being produced by a preamp,
and the power amp bandwidth is wide enough, tweeters can be destroyed
without knowing why.

DIYers don't expect things to go wrong they can't understand, but sorry,
****e happens, right!

So you should have a good tech whom you trust will diagnose the problem,
will not rip you off to cure it, and won't apply a solution which
compromises the audio bandwidth, or
increases the existing noise and distortion.

Such people are very rare, mainly because DIYers and audiophile hate
paying experts.

I know a lot of unpredictable ****e happens with audio gear.
And it is often with temperamental hi-end makes, CJ, ARC, etc, just to
name two.

The items ppl cannot fix after trawling around a few repair people end
up on Ebay,
and one guy here buys a lot off Ebay, and yessir, I get the job of
fixing the crap other people can't.

Often hi-end gear is a complex mix of tubes and solid state devices such
as mosfets and j-fets,
to the extent that 3 times the device number is used, just to get an
extra zero
in the THD measurement at output voltages you will never need.
The combination of "imaginative" but over complex arrays of tubes and
high transconductance
devices with loops of NFB are a problem waiting to happen when the
slightest
thing alters; such things are over-engineered.
On many designs, you can see what the original designer has done to stop
oscillations.

To sum up, the problem with microphony you seem to have may not be as
simple as it seems.
When in doubt, get the item inspected by an expert.

And BTW, some very elaborate all solid state gear can give a litany of
faults...

Patrick Turner.

Can a tube shield eliminate or reduce some types of microphony, such as
acoustic feedback rendering the tube usable again? Thanks.

The last case of very bad microphony I know occured in a phono
amp's second stage tube, a 12AT7. Usually its the first stage that is
the problem.
In this amp, the first stage is a j-fet & triode in cascode, and such MC
stages are
rarely microphonic, but its most unusual that a second gain stage is
microphonic.
We noticed a squeal at about 2kHz.
I went over to the amp and held a hand around the tube. The problem
slowly subsided,
so a couple of tube damping rings were put on, the type that look like a
simple heavy O-ring.
They did almost nothing, so we tried a rubber mat under amp,
and nothing stopped the oscillations completley except a change of
tubes.
Holding the amp in each hand so it wasn't resting on the bench and with
low level sound
still allowed the problem to continue.
The microphonic tube had been supplied by a friend who said he'd bought
it on Ebay as a NOS tube,
but he has no idea how to test tubes for noise and microphony, and
probably
it was anyhting but NOS, and he was victim for someone simply lying
about his tubes and selling
them at a high price. This happens all the time.
An enclosing shield with a spring or boll of foam in the top may have
worked but the
tube socket used has no provision for shielding.
It was just a very crook tube.

It will never be any good for anything.

Anything bad cannot be rendered good.

There are no reform schools for vacuum tubes.

Patrick Turner.


west

[Patrick Turner]

Eeyore wrote:

west wrote:

Can a tube shield eliminate or reduce some types of microphony, such as
acoustic feedback rendering the tube usable again? Thanks.

They usually incorporate a spring so the valve is held better.

It's a palliative measure though, not a cure.

Graham

The sprung metal shield cans so oftne fitted to many amps in the past
are a rarity now because tubes want to be seen.

But in phono amps and microphone amps the shield mainly acts to
screen out electrostatically sourced hum into high impedance anode
circuits.
Holding a hand around a 12AX7 1st stage phono amp tube will often bring
a hum.
But an EF86 and some other pentodes have a shield inside, so need no
screen.
Other tubes used to use moulded can to close fit around the tube;
all kept noise out.

Many old radios have RF, mixer, and IF amps all well shielded.

Patrick Turner.