"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"

Hi,

That 15 ohms is about right for a high power output transformer.

The first thing is to check the input grid coupling caps for leakage.
Then the screen grid resistors for opens. Then remove the finals and
apply power briefly to measure that the grid negative bias is healthy.
Check the tubes. Mount them back in. Then on to actual running tests.
Beware of the B+, high DC with ++joules is quite painful and lethal.
Do discharge all supply electros first thing before working inside!

A*s*i*m*o*v


nc From: "n cook"
nc Xref: core-easynews rec.antiques.radio+phono:359961
nc sci.electronics.repair:355619

nc I rarely deal with valve/tube amps and am more familiar with radio
nc output-matching transformers primary-windings having DC resistance of
nc windings of hundreds of ohms.
nc Been in a shed for years and no known history so treading cautiously.
nc Measuring the DC resistance of each side to centre tap of this
nc Marshall amp, shows only about 15 ohm each way.
nc Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each
nc side of this impedance matcher.
nc Does 15 ohm look right for each of these primary coils ?
nc Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems
nc fine.
nc --
nc Diverse Devices, Southampton, England
nc electronic hints and repair briefs , schematics/manuals list on
nc http://home.graffiti.net/diverse:graffiti.net/

... "'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