[Amps] RE: FW: diode strings...bias

2 2@vc.net
Fri, 2 Aug 2002 07:40:25 -0700

>-----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
>>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%
>-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,