Re: Noisy A100

[ddillon@renaissance.cray.com (David Dillon)]

On Thu, 01 Aug 1996 21:12:19, John Harris <john@skynet.co.uk> wrote:

> I have an early 1960's A100 hooked up to a Leslie 760. After the organ
> has been running for approximately 2 hours, I get a noise that can
> only be likened to sub bass feedback when the vibrato is switched on
> even with no keys pressed which is present on both the Leslie and the
> internal speakers.  Removing the rear panel (just the frame/fabric
> cover and not the generator cover) seems to stop it (I know it sounds
> stupid). I have read the FAQ and I don't think that this is
> 'motorboating' more like 'supertankering'. I can live without the rear
> cover but am concerned that it might be causing damage. Any ideas?

Lots of clues here, but it isn't clear which are the most significant.

Heat buildup is evidently involved, since the failure doesn't occur for
two hours.

Rear panel removal may stop the problem by improving air circulation, or
by NOT reflecting an acoustic wave back into a vibration-sensitive or
"microphonic" component.  If the organ back usually faces a wall, the
wall could also contribute to heat buildup or acoustic reflection.

Ignoring the vibrato-on clue, I'd suspect an electrolytic decoupling
capacitor (a common cause of motorboating), a microphonic tube or other
component, or possibly an early stage of the infamous metal-whisker
migration problem in the vibrato scanner or vibrato switch.

Do let us know the real culprit when you find it!

> Secondly, is it possible to remove the internal speakers, do I have to
> put a dummy load on the amp? This is for reasons of weight as I would
> love to gig with it ....

This depends on which Leslie model you have and how it's connected to
your A-100.

If you have a balanced-input Leslie, such as a 21/22/31/122, driven by
the G-G terminals of your A-100 preamp, you can remove not only the
A-100 speakers but also the A-100 main and reverb power amplifiers,
thereby saving even more weight.  (Note that A-100 reverb is lost.)

But if you have an unbalanced-input Leslie, such as a 47/51/147/251,
driven by your main (and possibly reverb) power amplifiers, then you'll
need to keep at least the main and perhaps the reverb amp, removing only
the speakers.  It is _ESSENTIAL_ that those amplifiers have proper dummy
load resistors.  Operating an amplifier unloaded (no speaker or dummy
load) can destroy its output transformer.

Some Leslie models have built-in dummy load resistors.  But I prefer
dummy loads in the A-100 console, regardless of the Leslie model.  This
is so that an accidental unplug of the Hammond-to-Leslie cable won't
leave the A-100 running unloaded, with attendant risk of damage.

Another option for an unbalanced-input Leslie is to convert its power
amplifier to balanced-input.  After this conversion, you can get rid of
both A-100 power amps and speakers, as above.  Write for details.

--
Dave Dillon
San Jose, CA, USA