Marshall 4140 Amp question

"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" <diverse8@xxxxxxxxx>
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/

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