[DIY] OT: Tube amp disaster

[Don Tillman]

   Date: Thu, 17 Aug 2000 08:42:47 -0700 (PDT)
   From: Don Tillman <don at till.com>

   I've noticed that most, or maybe all, tube guitar amps have a shorting
   speaker connector so that the output transformer is shorted when there
   is no speaker connected.  Can you tell us why this is?  I'm guessing
   the effects of running a tube amp into a short are less harmful than
   running into an open, but what are those effects?  One would think
   that running into a short would be bad for the transformer.

[From Eric...]

Date: Thu, 17 Aug 2000 23:24:21 -0700 (PDT)
To: Don Tillman <don at till.com>
From: Eric <synth at metasonix.com>

A short is safe, though you don't want to run full power
on the tubes (probably this would not be done if no speaker).

But opencircuiting the secondary can lead to enormous
back-EMF appearing on the primary. One little tick or
disturbance, and it becomes a huge transient, which can
be thousands of volts--resulting in arcing on the primary.
Then destruction.

Remember, the average push-pull output transformer is
wound with a 4000-6000 ohm primary--that can be as much
as 1000 Henries of inductance......

(Would you post this to DIY for me? I have to get off it--
lot of work to do. Thanks)

Eric Barbour
METASONIX