guitar speaker/tube challenge redux

[Paul R. Higgins]

> Seems like a slow acting fuse in the plate circuit might be good protection
> for expensive output tubes.  Anyone ever seen a tube circuit fused that way?

Yes--the Mesa/Boogie company puts fuses in the cathode circuit of the power 
tubes in their big rackmount power amps (Strategy 500, etc.).  It's much safer 
to fuse the cathodes than the plates, especially when the fuses are on the front
panel.  It would probably be a good idea to fuse the cathode circuit of any amp,
however.  And with a big tube power amp, fuses are pretty much essential to 
prevent a really catastrophic meltdown.  Things can get out of hand real fast.

FWIW, I use low-watt (1/2W) 10 ohm resistors, matched to 1%, in the cathode 
circuit of my tube power amps as current-sense resistors.  This makes it really 
easy to measure idle current when I'm setting the bias; 1 mA = 10 mV.  If 
something really goes haywire, these resistors burn out and open the circuit. 
Never had a problem so far.  But I think I'm going to start using fuses as well.
(Years ago, Ampeg used this resistor-fuse trick in their mammoth SVT bass amp).

-PRH  


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Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
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