[Stretto]

I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

[dre7]

Not sure what it could be, but you shouldn't have the amp on without a dummy
load or speaker hooked up.

Andrew

"Stretto" wrote in message
...
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

[Sjouke Burry[_2_]]

Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?
Even a wire close to some steel will start singing at high currents.

[Jeff Johnson]

"dre7" wrote in message
...
Not sure what it could be, but you shouldn't have the amp on without a
dummy load or speaker hooked up.

Andrew


Yeah, I competely forgot about that until I saw the smoke. Lol, luckily I
seemed to have caught it before any real damage was done. Not sure where the
sound was coming from but it was probably that resistor that started to
smoke.

[[email protected]]

On Jan 9, 4:46*pm, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

Your transformers are resonating - very common... if you drop the
signal level it will disappear. Never operate without a dummy load.

[Pimpom]

Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was
using a
freq. sweep input and noticed than when I turned the gain up
the I was
hearing the sound. There is no speakers hooked up and the sound
seems
to be an exact replica. I did not try other sounds as it seems
pretty
obvious that something is making reproducing this sound and it
is not
coincidence.

I guess I could make some type of acoustical amplifier to probe
around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I
suppose
there is the possibility of a resistor or cap producing the
sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something
pretty
straight forward(bad tube)?

Any ideas where I should start looking?

I've been using cheap medical stethoscopes for a few decades to
locate the source of a sound, in electronics and while tinkering
with my car. I remove the diaphragm-bell assembly and point the
open end of the pipe at various parts until I locate the culprit.

[HardySpicer]

On Jan 10, 10:46*am, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

It's a ****in oscillation, what do you think it is?

[whit3rd]

On Jan 9, 1:46*pm, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up...
Any ideas where I should start looking?

Well, if you have a microphonic tube (not unusual, it's why
preamplifiers often were on a different chassis than power stages),
that could be getting some acoustic input and 'singing'.
Is the noise broadband, or peaked at some frequencies?

Inductors can be noisy, if there's DC in them, too.

[Big Bad Bob]

On 01/09/11 13:46, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

transformer plate vibrations are the most likely source. the rapidly
changing magnetic field in a transformer can cause the plates to vibrate due
to differences in magnetic flux, and the rapid succession of magnetic
attraction and 'no attraction' (and other factors).

others have suggested microphonic tubes causing oscillation. I would
actually expect that a single-ended relaxation or negative-resistance
oscillator circuit might be created by open-ended amplifiers that are
improperly loaded. Use a dummy load or speakers, as others also suggested.
You can soak an 1 or 2 watt 8 ohm resistor (with wires attached) in a
glass of water to keep it cool if you don't have a 50W resistor handy.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing?

yes. The 'transformer buzz' you sometimes hear from power line transformers
is the same kind of thing. Usually transformers are potted to limit this,
since it's a loss factor also. If it's happening in a toroidal transformer,
it could actually be the wires themselves (and that would be BAD because the
wire insulation would eventually wear off). In any case, potting the
transformer helps minimize wire vibrations also.

I suppose there is the possibility of a resistor or cap producing
the sound due to some strange effect

capacitors... maybe. Ceramic capacitors COULD become 'piezeoelectric
transducers' if high enough current is running through them. I doubt this
is the case in an audio amplifier. Most of the power is going through the
output transformer. that's where the sound is coming from.

[Lord Valve[_2_]]

Stretto wrote:

I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

1) Don't run your amp without a load on it.

2) Google "magnetostriction." That's what's
happening in your output transformer.

LV

[Rockinghorse Winner[_6_]]

* It may have been the liquor talking, but
Lord Valve wrote:

Stretto wrote:

I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

1) Don't run your amp without a load on it.

2) Google "magnetostriction." That's what's
happening in your output transformer.

LV

Yes, I've heard that is bad news, also. Never run it out to an open circuit.

*R* *H*
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"Preach the gospel always; when necessary use words." St. Francis

[Big Bad Bob]

On 01/12/11 18:20, Lord Valve wrote:
2) Google "magnetostriction." That's what's
happening in your output transformer.

ah, so that's what it's called!

[Phil Allison[_3_]]

"Stretto"

I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.


** Gee whiz - most amps just hum along.

But your one is way smarter and knows the words.....



..... Phil

[NX211]

On Jan 9, 4:46*pm, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

Now - this is important to know.

Is it singing the words or just humming? If it's humming it probably
doesn't know the words. In that case you should start singing a few
bars and give it a chance to learn the words. I think then it will be
OK.

Regards

[Phil Allison[_3_]]

"NX211"


Is it singing the words or just humming? If it's humming it probably
doesn't know the words.


** Wot a pathetic copycat.



..... Phil

[Patrick Turner]

On Jan 10, 8:46*am, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

The output transformer has a number of primary and secondary windings
and they always make some audible sound even with no speaker
connected. The reasons are that the magnetic currents cause
microscopic movements of the coils and core structure an the voltages
between anode wndings with high audio frequency voltages. These Vac
act into the capacitances and you get an effect like that in an
electrostatic speaker. So the OPT acts like a transducer in more ways
than one.

When you have no speaker connected and no dummy load the OPT signal
voltages can me made to rise up to about 3 times the maximum output
signals when a load is connected. This has been said to occur because
of back emfs involving the leakage inductance, or merely the back emf
with each half primary when the idle current cuts off.
When one of the 2 tubes cuts off its current when the grid goes to a
low negative value, the anode voltage rises because that's how
inductors behave. ( if you don't understand this very simple basic
property of inductors+current change then don't be lazy, do some study
via Google or some books ) The other tube is being turned on with a
positive going grid voltage but its anode voltage moves towards 0V and
continues below 0V to maybe a negative voltage = twice the supply
voltage, and during this time this tube is cut off, even with a
positive going grid. The absense of a load on the OPT allows this huge
Vswing of perhaps up to 3 times the normal Vaa with a load connected.
Normal Vaa might be 500Vaa rms, and you might measure 1,500Vrms
without a load before anything clips. Such a huge Vaa can cause arcing
across poorly insulated windings or at output tube sockets, typically
between pin 3 anode connection to pin 2 filament connection which is
earthy.

Perhaps you may be unaware of the high voltages you might be producing
when you have no speaker connected.

Beware, you might damage your test gear, and damage your OPT!

There is a solution to the problem of excessive anode voltage
generation when no load is used.
Connect two simple networks of 3 x 1n4007 diodes in series all facing
in the same current direction.
Connect the end diode anode of each diode network to the output tube
anode.
Connect the end cathode of each diode network to 0V.
Normally the diodes never conduct any current.
But when one side of anode winding tries to move below 0V because of
the back emf in the other half primary, the diodes conduct and
harmlessly shunt the low value of current causing the back emf. So the
Vaa swing at each anode is therefore limited to +/- the peak voltage
equal to the B+ at the CT. This action is due to what is called a
diode clamp, and "clamping diodes" should be fitted to every PP tube
amp to prevent excessive Vaa occuring. IN larger amps in ther past
when B+ may have been 2,500V with larger transmitting tubes, silicon
diodes would be useless. One common way to prevent excessive Vaa was
to have a pair of adjustable sparking points connected anode to anode
with a load resistance of say 10k, so that if Vaa went over say
3,000V, then there could be an arc between the points and the current
flows through the 10k load and damps the Vaa.
The distance across the gap is adjusted just right.


I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing?

Its natural, and you can't avoid what is called "OPT howl". If
anything, SE trannies howl more than PP trannies.
Potting the OPT reduces audible howl which can be quite loud while
playing music.

Excessive howl is produced in poorly varnished transformer windings
and poorly clamped up laminations.

I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

Tubes, caps and resistors make a tiny amount of howl. 95% of the howl
is due to the OPT.

Patrick Turner.

[Big Bad Bob]

On 01/15/11 15:54, Patrick Turner so witilly quipped:

Connect two simple networks of 3 x 1n4007 diodes in series all facing
in the same current direction.
Connect the end diode anode of each diode network to the output tube
anode.
Connect the end cathode of each diode network to 0V.
Normally the diodes never conduct any current.

oops - I think I understand what you're trying to do here and the diodes
should be the other way 'round. So if plate voltage goes below ground
potential, you want the diodes to conduct, right? That way you're
preventing overly negative voltages on the transformer primary.

But if you have a negative F/B loop driven from the secondary, I think
that would solve the problem too, as in no 'infinite gain' on the power
tube stage. You may still have a problem with spurious oscillation due
to negative resistances and Z mismatch.

Its natural, and you can't avoid what is called "OPT howl". If
anything, SE trannies howl more than PP trannies.
Potting the OPT reduces audible howl which can be quite loud while
playing music.

FYI your explanation as piezioelectric rather than magnetic vibration is
interesting. I would have expected plate vibration to be the primary
source, and counter EMF on the windings (like you get in a motor or
generator) to be the secondary factor (for wire motion). Still, no
matter what the root cause (which appears to be a number of things, no
surprise here) potting is the solution to prevent it and reduce losses.

[Iain Churches[_2_]]

"NX211" wrote in message
...
On Jan 9, 4:46 pm, Stretto wrote:
I am trying to diagnose a problem with an amplifier. I was using a
freq. sweep input and noticed than when I turned the gain up the I was
hearing the sound. There is no speakers hooked up and the sound seems
to be an exact replica. I did not try other sounds as it seems pretty
obvious that something is making reproducing this sound and it is not
coincidence.

I guess I could make some type of acoustical amplifier to probe around
the circuit to find where it is coming from but besides that is
anything I should look for? Maybe this is a natural thing? I suppose
there is the possibility of a resistor or cap producing the sound due
to some strange effect or it could be the sum of all the minute
effects from each component but most likely it is something pretty
straight forward(bad tube)?

Any ideas where I should start looking?

Now - this is important to know.

Is it singing the words or just humming? If it's humming it probably
doesn't know the words. In that case you should start singing a few
bars and give it a chance to learn the words. I think then it will be
OK.

An old friend of mine in the UK, who used to build very fine,
and rather expensive tube/valve amps for an exclusive clientele, always
used to play the same Jethro Tull song through and amplifier that
had completed final test and was ready to be packed for delivery
to its new owner. He even included a copy of the lyric with the amp
manual. The song was "Slow Marching Band" The lyric went
as follows:

"Take a hand and take a bow.
You played for me; that's all for now, oh, and never
mind the words just hum along and keep on going"

They did. In ten years that I knew him he never had an amp
returned for anything other than scheduled maintenance/bias checks/
valve replacement.

Iain

[Patrick Turner]

On Jan 17, 4:51*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 01/15/11 15:54, Patrick Turner so witilly quipped:

Connect two simple networks of 3 x 1n4007 diodes in series all facing
in the same current direction.
Connect the end diode anode of each diode network to the output tube
anode.
Connect the end cathode of each diode network to 0V.
Normally the diodes never conduct any current.

oops - I think I understand what you're trying to do here and the diodes
should be the other way 'round. *So if plate voltage goes below ground
potential, you want the diodes to conduct, right? *That way you're
preventing overly negative voltages on the transformer primary.

Thanks for pointing out my terrible error!!!!!!

Its easy to become cornfused in the aftermath of the silly season.

If anyone did what I said the diodes would short the B+ supply to 0V
at each end of the OPT.

One could be a purist and use vacuum diodes as the clampers, but you
need biased heater supplies, and single diodes, and its all too much
trouble, so nobody ever did it. Makers just instructed owners to
always have a speaker connected.
The other thing that often happens without a load in an amp with NFB
is the problem of LF and HF oscilations because the open loop gain
rises without a load but the amount of effective FB increases.
Many amps made by ppl who don't understandunconditional stability will
oscillate with no load.


But if you have a negative F/B loop driven from the secondary, I think
that would solve the problem too, as in no 'infinite gain' on the power
tube stage. *You may still have a problem with spurious oscillation due
to negative resistances and Z mismatch.

I think one will find the NFB does not cure the problem and the
secondary voltage will rise enormously above normal levels with no
load especially with a pure tetrode/pentode configured amp.
UL and triode connection reduces the problem though.

Its natural, and you can't avoid what is called "OPT howl". If
anything, SE trannies howl more than PP trannies.
Potting the OPT reduces audible howl which can be quite loud while
playing music.

FYI your explanation as piezioelectric rather than magnetic vibration is
interesting. *I would have expected plate vibration to be the primary
source, and counter EMF on the windings (like you get in a motor or
generator) to be the secondary factor (for wire motion). *Still, no
matter what the root cause (which appears to be a number of things, no
surprise here) potting is the solution to prevent it and reduce losses.

Try testing a tube with a resistance load only to a high value B+. You
should find the anode hardly makes any noise at all.
A choke makes very little noise because there is little capacitive
interface between high voltage swings and something earthy.
But in a OPT there are highly changing voltages against earthy
secondaries, and things try to move a little, like in an ESL. But the
majority of howl is magnetically caused.

Patrick Turner.