"n cook" wrote in message
...


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Arfa Daily wrote in message
...


Yes, most likely problem, if it's not the EL34s themselves ( and if both
on
one half are doing it, it's probably not ) is the coupling cap between
the
phase splitter valve and output valves' grids. It is very common, and
will
often result in hotspots on the anodes and even grid glow. I would just
replace both caps anyway. Check also that the bias is not drifting, and
if
there is a balance pot for the two halves of the output, check that this
is
set correctly. Check also, any resistors involved in splitting the bias
between the two sets of valves. As far as I recall, Marshall usually use
a
single -50v bias supply, adjustable with a single preset, then
distributed
to both grid circuits via a couple of fixed resistors. Check that someone
hasn't turned the bias down so that the valves are thrashing a bit
anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased
dissipation
from low bias.

Arfa



Not a problem with the DC blocking caps at final push-pull separator
stage.
Replaced and still hum after 3/4 hour.
No DC on the ganged volume pots.
Monitoring the negative bias for the output EL34s.
10 minutes in, the -ps voltage at smoothing cap is -52V.
The voltage to the (schematic marked) B pair is -39.2V before going to
1.5K
then g1 of each EL34
For C pair -39.2V also
25 minutes in B = -37.8, C=-34
45 minutes in B= -36.9 , C=-32.5
hum is getting quite noticable and -ps rail is still -52
Increasing bias pot from its original of 8K to max of 22K
brings the -ps down to -53.5
now B=-39 and C=-33 less hum but still going more positive over time and
hum
increasing.
I stopped before the B pair started glowing like before.
I was expecting from these voltage readings that it would be the C pair
that
would start complaining.
Switching off the amp, not just to standby, for a couple of minutes ,
brought things back to original cold situation and another 3/4 hour
presumably before hum gets too much.

One thing that concerns me is each of these 1.5K g1 resistors has one end
to
the valve base pin and the other floating in space , not soldered to an
insulated pin just the wire connecting through - would that be as
original?
The schematic shows 1.5Ks but could someone have replaced 4 of a
different
value? or is it EL34s leaking too much and need changing.




I think that it's getting to the point now where you've got to start trying
valve substitution. However, before rushing out and buying new ones, you can
start by removing both the C valves ( these are the ones that glow
ultimately - yes ? ), then removing one of the ( likely ) OK B valves and
putting it in the C side. The amp will run quite happily with just one valve
in each side ( it's a trick that I teach owners to allow them to finish a
gig if they have a serious valve failure ).

You will then be running it with two valves that have performed OK when they
were both in the B side, so if it now works ok, next put them into the two
unoccupied sockets instead. If it still continues to work ok, then the
chances are that it is a valve problem. If it doesn't, then it must be a
bias issue. This assumes of course, that it's nothing to do with the output
tranny, which could suffer a partial insulation breakdown, resulting in
shorted turns, when it has been running a while. Again, this could be
checked by swapping the winding ends betwen anode pairs, and seeing if the
bad behaviour swaps sides.

Arfa