Re: Orange AD 15 Combo ...


Arfa Daily wrote in message ...



Oscillation means feedback. There has to be a feedback path, and the gain
has to be high enough to sustain oscillation. I found a web page that
mentioned something about a 1000p cap placed across a plate load resistor
specifically to avoid oscillation. The schematic didn't appear to be from
an
ad 15 combo. Some people were removing that cap as a mod to get a
brighter
sounding amp.
Another thought - the schematic I downloaded for this amp shows a
connection
from the 2nd stage grid back to the first stage input jack. It appears
that
the purpose of this is to ground the second grid when nothing is plugged
in.
If something is a bit bent in that jack, it might be possible for this
2nd
grid to be connected to the 1st grid, which will make a feedback loop
when
the volume control is turned up.
Positive feedback will also increase the noise. Something has to have
changed because I don't think they would have been able to sell a crappy
design and stay in business.



Good thoughts. I will check them out Monday. Interestingly, I did try
hanging 1000pf across both of the load resistors, and they did produce a
significant improvement, but not enough to actually totally stop it
hooting.
You could also definitely hear these caps starting to knock the
'brightness'
on the head. As you can now see from the schematic, one or two aspects are
a
little 'unusual' compared to the norm - that additional connection for
grounding the second stage grid for instance, or the dual gang master pot.

I must admit that if it is down to poor design, the effects do seem a
little
extreme, even given that the "Orange" name is just a badge, and it *is* a
Trace Idiott design ... It's certainly not the best constructed item I've
ever seen. Thanks all for the continued interest and suggestions. I'll post
back if I get anywhere further with it as a result.

Arfa

I tried to find the website again but had no luck. The cap appeared to be
connected across r4 of the phase inverter. You could try yanking V2 out and
see what happens with the gain cranked up.


.

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