Arfa Daily wrote in message
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"n cook" wrote in message
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"n cook" wrote in message
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"Asimov" wrote in
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"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete
Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not
a
fan
of running tubed / valved group amps without the output bottles in
place,
no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing
current
from the HT line, this can rise easily by 20% or more, from its
nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor
HT
rectifiers, as the HT rail will be high of its nominal running value
until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will
come
up
slowly, as will the load imposed by the outputs as they warm up.
This
results in an HT rail that comes up to a value, and stays there. If
you
now
run the amp up with the outputs removed, the output of the reccy
will
come
up with no load on it, which is an unexpected condition for the
designer,
and the resulting unloaded voltage may well come very close to, or
even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in
place.
Marshall generally tend to use a negative supply of around 50v for
the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than
likely
result in excess output stage current demand, with consequent
lowering
of
the HT rail. Glowing anodes are the order of the day here, but I
would
rather have that, as the outputs will stand this for a considerable
time
without damage, rather than have exploding or shorting
electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for
that
,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
forgotten how problematic , with high current valves, to get the
initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac +
current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve
amps
with full speaker load on output with all valves in place and to
power
up
transistor amps intitially without load ?


That's my usual recommendation.

Arfa



On the slow variac power up, I usually power up to about 80 per cent
mains
with no valves in and then add the valves and then go low to 100
percent.


That's a good way of doing it, and avoiding the potential pitfalls that I
have pointed out regarding over-volting the smoothers. About the only
slight
difference that I would normally do with an amp of this type that had been
standing unused for a long time, would be to first come up to about 30% on
the variac, and leave it running like this for 10 minutes, then come up to
50% for 20 minutes to half an hour, before going up to 80% to do the
checks.
This will ensure that there are no issues with the caps reforming.

Arfa



I've replaced the burnt stuff and its back working.
Unfortunately after about 10 minutes hum makes an appearance and after half
an hour becomes excessive.
I stuck a piece of 20 to 35 degree C thermochromic paper to each of the 4
can electrolytics and one is heating up.
I assume its normal for each of the 4 EL34 to have a blue glow observable
through some of the holes in the internal metal-work as well as normal
orange heater glow ?