guitar speaker / was Re: Tube challenge!

Harry Bissell harrybissell at
Fri Nov 26 21:25:43 CET 1999

Of course a properly designed amplifier will include "some" resistance
(even 100 ohms) in the chassis to prevent the amp from ever being
If the speaker is soldered in, then that could be the "minimum" load.

Is everyone aware of the flyback diode modification for the transformer
primary??? A couple of 1000V diodes in anti-parallel from the B+ (center
tap) to each plate in the push-pull stage... This will "defend" against
this problem.

:^) Harry

Bill Layer wrote:

>  Hello,
>>   The problem with Transformers with open secondarys mainly
>> comes from the effective resistance that is presented to the
>> primary, you effectivly get a massive standing wave problem and
>> a tonne of relfections and if its too bad, say good by to the
>> thing driving the primary. (wow look ma, I used my transmission
>> line/transformer theory I leant at college!!!)
> All you need is SOME type of load at the output to prevent this.. I
> imagine even  a 100Ohm resistor would do it.. But it is REALLY
> important to have something if there is the chance that the amp might
> get signal with no load.
> I accidentally touched the input once of an EL84 based amplifier, that
> had an open output. The ball of plasma that engulfed the transformer
> was truly impressive.. looked just like one of those plasma balls that
> RS sold. The room smelled of ozone later. Reminds me of the time I put
> the glass bowl with gold leaf rim in the microwave...
>  +----------------------------------------------------------+|
> "The" Bill Layer - Frogtown, Minnesota. U.S.A.      || Vacuum tubes,
> Analog, Motorcycles and Other Alternatives
> |+----------------------------------------------------------++---------------------+
> +---------------------------------+| <blayer at> |  |
> <b.layer at> |+---------------------+
> +---------------------------------+

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