"n cook" wrote in message
...
Arfa Daily wrote in message
...

"n cook" wrote in message
...


--
.
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



Swapping each pair over, improved matters a lot.
The bias stayed about 6 volts more negative and much less noise for the
same
time intervals from switch on. So to save buying new
bottles , a matter of switching off for 2 minutes every hour or so.
I can only assume it is due to over driven valves - well something had to
burn up those resistors and knock out the HT fuses.
Is there a name for this leakage? effect


Not that I'm aware of. I suppose that there probably is if you're a valve
manufacturer, though. Might be worth looking into the cost of some new
valves. Prices have come down a lot since the chinese have got involved.
Apparently, much of the old valve manufacturing equipment from UK plants,
was sold to Russian and far east countries. There's often some good deals to
be had on e-Bay. Also worth checking with your local music shop, if you've
got one. Valve PA amps are still as popular as ever with ' real '
guitarists, and my local shop buys in valves in quite large quantities, so
prices are not too bad.

Arfa