Re: Orange AD 15 Combo ...


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.



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.

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


.

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