[AMPS] TL-922 Filament Transformer Protection

[Tom Rauch]

> Personally, I  disagree in general with Rich about parasitics and killing
> tubes,  although I concede that a parasitic could boil gold on a tube with
> low grid dissipation. 

That's why the design should employ a grid-current overload tripoff. 

It's just funny that commercial manufacturers of
> amplifiers (commercial meaning for commercial market, as opposed to the
> amateur market) just don't seem to get a problem. 

That's probably because commercial PAs aren't driven with 
775DSP's and aren't retuned by people constantly. Adding a grid 
trip circuit will greatly reduce those problems, although a rig like a 
775 will still slowly erode the gold plating of PA grids.  

I also question whether
> overheating of parasitic suppressors isn't down to them getting very hot
> because of physically where they are, and to them being under rated for
> the amount of dissipation of the fundamental they get, especially on 10,
> 12 and 15 metres.

Absolutely. That's what ruins the suppressor resistors. Bake a 
carbon resistor in a conventional over, and watch what happens. 
Carbon resistors are very unstable with excessive heat, and go into 
a resistance value spiral.

That's why carbon resistors should never be used to equalize 
electrolytics, and why Magnavox had TV sets that caught fire in the 
60's and 70's.

73, Tom W8JI
w8ji@contesting.com

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