Marshall 4140 problems
Steve Cowell
I am the owner of a strange bird, the Marshall 4140 combo. It has 2
12" speakers with a JCM800-like amp head. I remember hearing funny
things about the 'country western Marshall' and sure enough, the
sticker on the chassis has "C+W" stamped on it.
I got the thing used, with funny recones and mongrel output tubes (6550).
Once I got the speakers in phase, it didn't sound too bad. Then I made
the mistake of cranking it up for a while. After about 10 minutes of
abuse I heard a 'plink' and the output went way down. I looked in the
back and saw one of the finals' plate hot. I took this as a reminder
to get new finals, and promptly ordered Teslas to convert to EL34s.
Well, after the modification to the bias (I did the long one, not the
short one GT pushes) I discovered that the screens (or some other
element you can see through the gaps in the plates) glowed on loud
notes on one side, the inverted side. Upon the advice of
the most generous Tom Balon at TAS I used an A+B scope
and verified that the phase inverter was becoming unbalanced and
correlated the unbalance with the glowing (nasty subharmonic too).
I then shotgunned all components in the PI circuit, bypassing the
dual-pot master as I had no substitute. Guess what? No change.
I took this opportunity to replace the output transformer as I
had measured a 1.5 ohm difference between halves of the primary,
and found bad contact in the speaker switch (1 ohm there).
I used the 1900 ohm, 120 watt transformer from Antique Electronic
Supply, as it was less than half of the new cost. Guess what?
Right, no change. Nothing I could do to the phase inverter balance
or final bias would change the glowing screens on the inverted side.
Well, this is when I finally decided that Marshall had designed this
amp just to screw me. I rebuilt the phase inverter to resemble another
JCM800, the 2150, and *finally* got a handle on the problem. It seems
that Marshall in their infinite wisdom (!) had increased the negative
feedback to outrageous proportions by 1.) increasing the cathode resistor
of the phase inverter (the one that the feedback appears across)
from 4.7 to 22Kohms and by 2.) deleting the presence control.
I think this was done to get a 'country' sound. Well, it sounded like
the stuff we smell out in the country (or at the rendering plant), I'll
give it that! I was able to adjust the feedback by moving the speaker
tap to eliminate the glowing screen problem. I had replaced the filter
caps, the speaker transformer, the final tubes, the phase inverter tube
and circuitry, bypassed the master volume (the pot was 20% out of sync
and 30% out of tolerance, still is), the speakers, bypassed the speaker
switch, been out 6mos of use and wasted ~20 hours on the bench. I am
certain it would havve lunched my new E34Ls in short order had I played
at volume in its stock condition.
My point: Anyone else seen one of these *bastards* come through their
shop, or had any of these problems before? The speakers are another
heartache, they were 'Custom Designed for Jim Marshall Inc.' Celestions
that were discontinued, another whole story (I replaced 'em with G1285Ks),
unreconable with Celestion cones (the red ones, avoid 'em like plague).
The schematic is in two versions in the TABv4, pg 609 and pp 692-3,
same except for the errors on the pg 609 one.
I now have a pretty good sounding Marshall that was $500 plus another
$400 in parts. Any heartache I can save someone from will be worth
it (*NOT*) :) :).
--
Steve.....scowell@aoc.nrao.edu