Hello Gurus!

I've completed my amplifier testing (EL34 based) and noted that the voltage
supply drops from 480VDC to 440VDC over a period of 1.5hours. Of course the
power transformer was taking quite long to get hot (still touchable) and
after 1.5hours, the supply voltage remains at 440VDC. When the supply
voltage drops, the bias voltage across the cathod resistor for the EL34
drops from 0.75VDC to 0.65VDC and stays there.

Is this normal for a tube amplifier to be behaving this way when the output
tranformer gets hot?

Is it possible to keep everything stable using a LM317 floating voltage
regulator in place of a choke?

Regards,
Mark

F.H.Chan wrote:

after 1.5hours, the supply voltage remains at 440VDC. When the supply
voltage drops, the bias voltage across the cathod resistor for the EL34
drops from 0.75VDC to 0.65VDC and stays there.

Hi, quick questions.
o What is the topology of your output circuit,
single, push-pull, triode-connection or pentode-connection?
o What is the exact biasing method for the EL34?
o Is your cathode register very hot?

I was kinda confused for the cathode resistor voltage of 0.75-0.65[V].
If this is the actual bias voltage, it is too low for EL34 with plate
supply voltage of over 400[V]. So I assume that you are also applying
grid bias voltage somehow, but I don't know for sure.

Atsunori

Hi Atsunori,

Hi, quick questions.
o What is the topology of your output circuit,
single, push-pull, triode-connection or pentode-connection?
Its a push-pull amplifier

o What is the exact biasing method for the EL34?
o Is your cathode register very hot?

I was kinda confused for the cathode resistor voltage of 0.75-0.65[V].
If this is the actual bias voltage, it is too low for EL34 with plate
supply voltage of over 400[V]. So I assume that you are also applying
grid bias voltage somehow, but I don't know for sure.
Yes there's a grid bias (negative) voltage applied. The cathod resistor is
connected to ground and it was biased at 0.75V (to determine current through
the tube). Nope the resistor doesn't get hot at all.

Regards,
Mark

Mark wrote:

Its a push-pull amplifier

I see. So the cathodes of two EL34's are connected together, and
is grounded through a resistor, over which you notice the change
of voltage from 0.75[V] to 0.65[V] after a while.

Yes there's a grid bias (negative) voltage applied. The cathod
resistor is connected to ground and it was biased at 0.75V
(to determine current through the tube).
Nope the resistor doesn't get hot at all.

Sounds like the operating current for the output stage actually
decreased after 1.5 hours, mean while the supply voltage also
dropped from 480 to 440v.

I would then check the followings over the 1.5 hours
of operation as the plate supply voltage changes.

o If the primary line voltage(AC 110v or whatever) measured at
the power transformer changes.
o If the secondary high voltage from the power transformer
changes.
o If you are using rectifier tube(s), see if the voltage
drop(s) across your rectifier tube(s) change.

Atsunori

"F.H.Chan" wrote in message
...
Hello Gurus!

I've completed my amplifier testing (EL34 based) and noted that the
voltage
supply drops from 480VDC to 440VDC over a period of 1.5hours. Of course
the
power transformer was taking quite long to get hot (still touchable) and
after 1.5hours, the supply voltage remains at 440VDC. When the supply
voltage drops, the bias voltage across the cathod resistor for the EL34
drops from 0.75VDC to 0.65VDC and stays there.

Is this normal for a tube amplifier to be behaving this way when the
output
tranformer gets hot?

Is it possible to keep everything stable using a LM317 floating voltage
regulator in place of a choke?

Regards,
Mark

Long term drift problems can be difficult to troubleshoot. Perhaps this
thought is radical (maybe Atsunori can express his opinion on this), but can
the output stage be disconnected and a large resistor (dummy load) be hung
in its place and then monitor the PS to see if there is still drift? Much
can then be eliminated as the culprit.
Back to earth. For long-term temperature drift type problems, I try to
hasten the time by alternating a heat gun and cold (Freon type) spray. If
you then narrowed the problem to a particular area, the long nozzle tube can
be directed to individual components. Food for thought (may give you
indigestion though).
Cordially,
west

west wrote:

"F.H.Chan" wrote in message
...
Hello Gurus!

I've completed my amplifier testing (EL34 based) and noted that the
voltage
supply drops from 480VDC to 440VDC over a period of 1.5hours. Of course
the
power transformer was taking quite long to get hot (still touchable) and
after 1.5hours, the supply voltage remains at 440VDC. When the supply
voltage drops, the bias voltage across the cathod resistor for the EL34
drops from 0.75VDC to 0.65VDC and stays there.

Is this normal for a tube amplifier to be behaving this way when the
output
tranformer gets hot?

Is it possible to keep everything stable using a LM317 floating voltage
regulator in place of a choke?

Regards,
Mark

Long term drift problems can be difficult to troubleshoot. Perhaps this
thought is radical (maybe Atsunori can express his opinion on this), but can
the output stage be disconnected and a large resistor (dummy load) be hung
in its place and then monitor the PS to see if there is still drift? Much
can then be eliminated as the culprit.
Back to earth. For long-term temperature drift type problems, I try to
hasten the time by alternating a heat gun and cold (Freon type) spray. If
you then narrowed the problem to a particular area, the long nozzle tube can
be directed to individual components. Food for thought (may give you
indigestion though).
Cordially,
west



There is more analysis than you suggest required for finding out why the
B+ sags by 40V after 1.5hrs.

Just how hot is that power tranny getting? A hot B+ winding on the brink of
destroying its insulation has more DC resistance than when cold after being on
for only 2 minutes.

Presumably the resistor at the cathodes is 10ohms, so hence the
0.75V volts and 0.65 volts measured;
it means the Ik is reducing from 75mA to 65 mA due to the reduction of the
B+ by 40V.

But something is grossly WRONG!

Just what is a mystery.

There is something sucking power and causing the B+ to fall so much.

Is it leaky electros? one would expect they are also unduly hot
to be able to dissipate the power needed to make the B+ sag by the 40V,
even though when that happens the tubes draw less current!

Patrick Turner.

"Patrick Turner" wrote in message
...

There is more analysis than you suggest required for finding out why the
B+ sags by 40V after 1.5hrs.

Just how hot is that power tranny getting? A hot B+ winding on the brink
of
destroying its insulation has more DC resistance than when cold after
being on
for only 2 minutes.

Presumably the resistor at the cathodes is 10ohms, so hence the
0.75V volts and 0.65 volts measured;
it means the Ik is reducing from 75mA to 65 mA due to the reduction of the
B+ by 40V.

But something is grossly WRONG!

Just what is a mystery.

There is something sucking power and causing the B+ to fall so much.

Is it leaky electros? one would expect they are also unduly hot
to be able to dissipate the power needed to make the B+ sag by the 40V,
even though when that happens the tubes draw less current!

Patrick Turner.


The Power Tranny is only suppling a 5V and 440-0-440 @ 750mA (as per written
on the tranny).
Its hot but I still can place my hands on it for a whole 1 minute yet I
don't have to worry about getting
burned.

The Cathode resistor is a 15ohms, hence at 0.75V = 50mA

While using the amp, I had a 500mA meter attached across the B+ and the amp.
It initially went to
190mA and later dropped to 170~160mA. At this point, the voltage stays at
440V ~ 450V. I left it
for more that 2 hours and it stayed there.

All of the caps at the power supply is cold. None appears to be warm when
touched.

Warm regards,
Mark

in article , Mark at wrote on
9/2/05 9:26 AM:


"Patrick Turner" wrote in message
...

There is more analysis than you suggest required for finding out why the
B+ sags by 40V after 1.5hrs.

Just how hot is that power tranny getting? A hot B+ winding on the brink
of
destroying its insulation has more DC resistance than when cold after
being on
for only 2 minutes.

Presumably the resistor at the cathodes is 10ohms, so hence the
0.75V volts and 0.65 volts measured;
it means the Ik is reducing from 75mA to 65 mA due to the reduction of the
B+ by 40V.

But something is grossly WRONG!

Just what is a mystery.

There is something sucking power and causing the B+ to fall so much.

Is it leaky electros? one would expect they are also unduly hot
to be able to dissipate the power needed to make the B+ sag by the 40V,
even though when that happens the tubes draw less current!

Patrick Turner.


The Power Tranny is only suppling a 5V and 440-0-440 @ 750mA (as per written
on the tranny).
Its hot but I still can place my hands on it for a whole 1 minute yet I
don't have to worry about getting
burned.

The Cathode resistor is a 15ohms, hence at 0.75V = 50mA

While using the amp, I had a 500mA meter attached across the B+ and the amp.
It initially went to
190mA and later dropped to 170~160mA. At this point, the voltage stays at
440V ~ 450V. I left it
for more that 2 hours and it stayed there.

All of the caps at the power supply is cold. None appears to be warm when
touched.

Warm regards,
Mark




Mark,

Question: Does this amp have a C-L-C filter?

Jon

"Jon Yaeger" wrote in message
...
Mark,

Question: Does this amp have a C-L-C filter?

Jon


Oh yes - sorry forgot to mention that earlier

Mark wrote:

"Patrick Turner" wrote in message
...

There is more analysis than you suggest required for finding out why the
B+ sags by 40V after 1.5hrs.

Just how hot is that power tranny getting? A hot B+ winding on the brink
of
destroying its insulation has more DC resistance than when cold after
being on
for only 2 minutes.

Presumably the resistor at the cathodes is 10ohms, so hence the
0.75V volts and 0.65 volts measured;
it means the Ik is reducing from 75mA to 65 mA due to the reduction of the
B+ by 40V.

But something is grossly WRONG!

Just what is a mystery.

There is something sucking power and causing the B+ to fall so much.

Is it leaky electros? one would expect they are also unduly hot
to be able to dissipate the power needed to make the B+ sag by the 40V,
even though when that happens the tubes draw less current!

Patrick Turner.


The Power Tranny is only suppling a 5V and 440-0-440 @ 750mA (as per written
on the tranny).
Its hot but I still can place my hands on it for a whole 1 minute yet I
don't have to worry about getting
burned.

Well that suggests the T rise is letting the tranny rise to about 40C.




The Cathode resistor is a 15ohms, hence at 0.75V = 50mA

So the current is even les than thought.



While using the amp, I had a 500mA meter attached across the B+ and the amp.
It initially went to
190mA and later dropped to 170~160mA. At this point, the voltage stays at
440V ~ 450V. I left it
for more that 2 hours and it stayed there.

All of the caps at the power supply is cold. None appears to be warm when
touched.

When you turn on a tube amp, the B+ often goes rapidly up to a maximum voltage,
and then it falls to a lower value when the output tubes begin to conduct and
the bias
is properly established.


A 440v-0-440v winding on a transformer which measures this Vrms voltage with no
load should give
you a maximum of +616V DC with no load.

Then when the output tubes conduct, this voltage should fall as you say it does
to a stable value
which you say is about +450V DC.
I would say you are using a tube rectifier because with silicon diodes the DC
voltage
does not fall as much from the maximum DC voltage with no load;
ie, if maximum is 616Vdc, then it would fall to maybe 560Vdc with silicon
diodes.
But the tube rectifier would have considerable effective series resistance so
this is why you get only
+450V from a tranny winding of 440Vrms, which is 616Vpeak voltage swing.



The initial current required to charge up the PS capacitors may reach a peak
value for a short time
and then reduce until the output tubes conduct when the current increases to a
steady value.
All this should happen happen in the first 20 seconds of operation.

There should be no DC current flow into the CT on the OPT for
about 8 seconds after turn on when the amp is cold

So I don't know why you are measuring 190mA flowing into the CT at switch on and
then seeing it drop back to 170mA.

You may not be measuring the current at the right point.

There may not be anything wrong with your amp.

You say there is B+ drop from 480V to 440V over 1.5 hrs.

But does this drop occur after 20 seconds? if so, the amp is OK,
but if the B+ starts at +480V at after say 30 seconds, then very slowly falls
40V after 1.5 hrs,
something is causing it.
It's never happened in amps I have built or serviced.

Patrick Turner.





Warm regards,
Mark

"Patrick Turner" wrote in message
...
There should be no DC current flow into the CT on the OPT for
about 8 seconds after turn on when the amp is cold

So I don't know why you are measuring 190mA flowing into the CT at switch
on and
then seeing it drop back to 170mA.

You may not be measuring the current at the right point.

There may not be anything wrong with your amp.

You say there is B+ drop from 480V to 440V over 1.5 hrs.

But does this drop occur after 20 seconds? if so, the amp is OK,
but if the B+ starts at +480V at after say 30 seconds, then very slowly
falls
40V after 1.5 hrs,
something is causing it.
It's never happened in amps I have built or serviced.

During initial turn-on, the current meter shows 0 as you mentioned. It will
slowly reach 190mA after 30 seconds or so. At this point, the voltage
measured is 480VDC. The drop occurs gradually over a period of 1 hour till
1.5 hours.

The Amp meter is fitted between the output of the power supply and the input
supply to the amp.

Regards,
Mark

Mark wrote:

While using the amp, I had a 500mA meter attached across
the B+ and the amp. It initially went to 190mA and later
dropped to 170~160mA. At this point, the voltage stays at
440V ~ 450V. I left it for more that 2 hours and it stayed there.

In any event, Mark, I think it's better to check the power
transformer's secondary AC voltage at two time points.

- When the amplifier just started and the power transformer
is still cool.
- After 1.5 hours, where the B+ dropped to 440v and stable.


If the secondary AC voltage does not change, then other part
of the amplifier is causing this phenomenon, possibly your
rectifier circuit for one of many candidates. But since I don't
know what rectification circuit is used, I can't go further.

If the secondary AC voltage changes, there are also many
possible scenarios. But I think it's worth checking out
the presence of transformer's secondary AC voltage change.

Atsunori

I vote for Jon Yaeger's diagnosis. Cheers, John Stewart

Another possibility is to check for the "aw ****" test.

Swap out the rectifier tube and see what happens.


"John Stewart" wrote in message
...
I vote for Jon Yaeger's diagnosis. Cheers, John Stewart