>
>
>-----Original Message-----
>From: 2@vc.net [mailto:2@vc.net]
>Sent: Wednesday, July 31, 2002 19:09
>To: Jim Thomson; W6frmarv@dslextreme.com
>Cc: w8ji@contesting.com; stevek@jmr.com; AMPS
>Subject: RE: FW: diode strings...bias
>
>>
>>### (VE7RF) Ur idea of replacing the stock chokes with 30 ohm
>grid
>>resistor "fuses", just creates unneeded, and varying bias.
>
>>
>The V-drop across the 30-ohm resistors amounts to c, 5% of the
>cathode-grid voltage. What effect did you observe?
>
>
>(VE7RF) I used 10 x 1N5408'S Bias went from 7V to 8V. (14%
>reg)
>
>-throw in the 30 ohm grid resistors and it's now 150Ma (per tube)
>x 30 ohms = 4.5V of even more bias.
>-bias will now go from 7V to 12.5V , with amp driven, which is
>79% regulation. (8V+4.5V=12.5V)
>
>-Now 79% reg is complete nonsense.... and doesn't work.
You forgot to figure in the 120v of RF. 1V + 4.5V = 5.5V. . . .
5.5V/120V = 4.6% regulation - however, it's RF-NFB, which reduces IMD.
>
>- I'd say your 30 ohm grid fuse/floating grids/nichrome concepts
>are flawed.
>
>-Now I'm overhearing on 75M, that the 30 ohm "grid fuses" will
>PREVENT cathode to grid shorts !! This is like the blind leading
>the blind !
No one who replaced the 1A grid chokes with 30-ohm frangible resistors
has so far reported a grid-fil short after a glitch that wiped out the
30-ohmers. Sure, they ain't sophisticated or nuthing like that, but
cheap resistors do the job.
>
>
>-If the tube is gonna internally short, it will, and fuses,
>resistors, transistor grid/plate over current protection devices are
>not going to stop it from happening.
>
So don't use it aeronautical mobile in a stunt plane.
>Grid fuses, if used, should consist of super fast ones placed in
>series with the grid shunt. Preferably in a 3AGC type rear
>chassis mount, for quick replacement.
3AGC fuses are undoubtedly faster than frangible resistors and they would
be easier to replace if fuse holders on the rear panel are added.
>
cheers, Jim
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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