[Amps] Parasitic suppressors on GI-7B.

[Ian White GM3SEK]

Peter Chadwick wrote:
>The resistor size can be a problem, and I don't know where to get the 
>globar or carborundum type resistors - they look like a piece of carbon 
>rod about 3/8inch diameter and 1-1/2 inches long and are rated at about 
>15 or 20 watts. I got mine years ago when they were available!

2W or 3W metal film resistors are the modern substitute for solid 
carbon. The inductance is slightly higher, but not seriously so, and it 
can be reduced by paralleling two or more higher-valued resistors.

There has been a lot of discussion about this before. Click the link at 
the bottom of every AMPS message,  click on through to the Searchable 
Archives, and search for "metal film".

>Usual value is about 47ohms.

If anything good has come out of all the previous discussions, it has 
been the realization that "one size fits all" does NOT apply to 
parasitic suppressors. Different amps have different VHF parasitic 
resonant frequencies, and also different reserves of stability against 
oscillation, so it only makes sense that they're also going to need 
differently designed suppressors... which can sometimes mean no 
suppressor at all.

Let's just say that 47 ohms and "a few turns" is somewhere to start 
from.


As others have said, if the grid ring of a UHF tube like the GI-7B is 
solidly grounded (and if there are no other unexpected feedback paths) 
then it may not actually need a parasitic suppressor.


-- 
73 from Ian GM3SEK