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From: N_Cook on 27 Oct 2009 09:03
Once again roadies throwing things around. Previously they managed to shear
an ECC83 in half, where they pass through the chassis "protected" by rubber
grommet. Thius time broken springline and choke bent over, by G-force, so
nearly touching the rectifier valve. Anyway the main problem was
intermittent squeel on the output with all controls at minimum.
Squeel would occur with first 2 preamp valves removed and also the
splitter/driver valve removed. I expected to find serious CH/R problem on an
EL84. Anyway all output and preamp (and rectifier ) valves test absolutely
fine on gain and CH/R over my FSD of 30 Megohm.
Replacing everything, the squeel has gone, trying the amp upside down also.
Earthing, Cs and Rs and Vs seem fine and no squeel emerges with serious
twizzle-sticking.
Assuming not a valve problem and something wrong in valvebase/ wiring /
soldering what/ where to look for something amiss that could cause a squeel
? Of course, not wanting to replace 4 x EL84 if there is a lurking problem.
I cant even swap pairs as the problem has "gone" Looks as though I'll have
to desolder the pa pcb to have a look underneath. While doing that does
anyone replace with slightly longer wires? or is that likely to introduce
oscillation problems as well.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


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