Re: Orange AD 15 Combo ...


"bg" <bg@xxxxxxxxxx> wrote in message news:gml4st$f3k$1@xxxxxxxxxxxxxxxxxxxx

Arfa Daily wrote in message ...

Anode is AC coupled to the top of the gain control. Wiper of said control
is
AC coupled to the grid of the second half of the first ECC83. No stopper.
No
grid leak.

The grid has to have a DC path to ground, otherwise the coupling cap won't
have a path to charge and discharge. Eventually , the tube will be cutoff
from the stream of electrons charging the grid negative
There has to be a resistor from grid to ground or perhaps the wiper is not
coupled through a capacitor. In any case , I don't think think that this
would be the cause of your feedback problem.



Yes, absolutely true. I didn't put it very well. The grid has a DC return
path, as you say, via the gain pot. The stage does not have additional AC
coupling between the pot and the grid, with a separate fixed grid leak, as
you might reasonably expect. So if the gain pot is at minimum, the grid will
actually be DC grounded.



All four HT rails have their own R-C
decoupling network, each following on from the previous one, and none of
the
rails has excessive ripple.

The decoupling RC networks prevent audio from passing through the B+. The
high voltage is common to several stages, if audio couples from one stage
to
the other through the B+, the amp will break into oscillation.The ripple
might look low but when the amp breaks into oscillation, there is a good
possibility that an audio signal is present on the high voltage. Many cap
meters only test at power line frequencies. It is possible that the
decoupling caps are not working as caps at higher audio frequencies, and
it
is also possible that the caps might be borderline. I would substitute
those
caps with any junk laying around just to rule them out.

That is a possibility. I checked the caps with an ESR meter, which operates
at an equivalent frequency of around 100kHz, so I was not just declaring
them good by value, or the fact that there was little in the way of 100Hz
ripple. Just as a matter of interest, none of the individually filtered HT
rails are 'shared' as such by more than one stage. First filter nearest the
reccy, feeds just the output anodes. Second, just the output screen grids.
Third, just the phase splitter and fourth just the preamp valve, although I
suppose that you could argue that represents two stages. I'm pretty sure
that when it was howling, I did 'scope the four HT rails, and there was
nothing significant on any of them, but perhaps not. It's just the sort of
check that you make automatically in these cases without even thinking about
it. I'll take a better look at this angle on Monday.



Another possibility is that the ground for the preamp and the ground for
the
power amp might share a common path. That path might haved aged to a point
where it is no longer well grounded. Tighten any hardware, like jacks ,
grounding screws etc...
If possible, seperate the two grounds by running seperate wires from those
stages back to the negative terminal of the power supply.


bg


Not especially easy to do, as the power supply is all part of the same PCB
with the whole of the rest of the amp. There are no signs that any hardware
is loose, or that any mechanically made grounds, such as via pot bodies etc,
are not good. I repair a lot of valve amps for a living, and have done for
many many years, so these are all suggestions that are basic second nature
to me, hence why I'm finding this an 'odd' problem. Still, thanks for all
the suggestions anyway.

This is one of those situations where you would really like to have another
one to compare by, which is why I asked if anyone had any direct experience
of this amp.

Arfa



.

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