Arfa Daily wrote in message
...

"Asimov" wrote in message
...
"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 ?