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
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