Phil Allison said:
"Arfa Daily"
** You don't service that many valve guitar amps - do you ??
Er, that would be a yes then ... Probably three or four a week, most every
week, and have been doing them for around 40 years ... Whilst some faults
are a little obscure, most are very straightforward, and I'm sure that you
don't have any more trouble fixing them, than I do Phil. I would pick a
fault on a valve amp over a transistor one, every time, as would probably
any engineer who has ever worked with valves. Every stage is pretty much
independent, and troubleshoot-able as a separate entity. You can't say that
about most transistor amps.
The ones that were originally well made are now so old they have strange
faults most techs have never seen before - while the ones made more
recently have designed in faults never seen before by anyone.
A few examples.
1. Recent Marshall combo amps with fibreglass PCBs that become LEAKY
when hot - output valve bias goes crazy and even the phase splitter
stage goes way out of DC balance in normal operation because of leakage
from anode tracks to grid tracks. There is simply no fix for this
problem.
Care to give me a model for that behaviour ? I see a lot of Marshall combos,
as they are built just a few miles from where I live. I have not seen any
similar problems here. I used to visit Jim Marshall in his factory in the
early days. He was a very good friend of a friend of mine. As to the problem
with a - g leakage, I would have thought that you might have been able to
get away with isolating the affected tracks by cutting or even removing them
from the board completely, and then hard wiring.
2. Recent Gibson /Trace Elliot amps that develop mysterious crackling
noises all over the circuit due to the use of water soluble flux that was
never properly washed of the PCB. Removing the main PCB entirely and
washing it thoroughly in solvent is the only fix.
3. Same amps as above use 9 pin PCB mount sockets that SHRINK - yep,
the plastic material used to make the socket shrinks under the heat from
EL84s, reducing the pitch circle diameter so much it is impossible to fit
a new tube in the socket. Replacing all the output valve sockets is the
only fix.
Then there are all the issues with the valves themselves - all new
production now comes from China or the former USSR and most of the valves
have designed in faults of kinds that were never seen previously. Eg.
nearly all 12AX7s now crackle and pop if tapped with the back of a
screwdriver and/or have bad heater cathode hum or are very microphonic -
while octal power types often suffer from bad soldering (PbF ?) on the
pins causing all manner of weird intermittents when the solder joint lets
go.
Whilst I would agree that valve quality is not as good as it was when the
likes of Mullard and Brimar etc made them, I also don't think that they are
quite as bad as you seem to find them, if you buy decent quality ones, or
manufacturer's originals, which have been vetted by the amp manufacturer as
being of the required quality. The Ruby HG (Hi Grade) 12AX7s that I use,
don't crackle and pop, and are no more microphonic than any high gain
triodes have ever been. I also don't find the h - k leakage bad either -
certainly not enough to identify as being a predominant source of unwanted
hum. And as to bad soldering on octal base pins, I honestly can't say that
I've ever seen this problem on any amp that's been across my bench, or on
any replacement valves that I have fitted, which may have had their bases
connected to the leadouts by PbF. Perhaps I've just been lucky ...
Yep - fixing valve amps is pure joy.
.... Phil
Well, I find it so ...
Arfa