[Amps] snubbing diodes for vac relays / notes.

[Pete Smith]

Why not just use a high voltage MOSFET to switch the bias, controlled 
with the same voltage that runs the input relay?

73, Pete N4ZR

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On 3/11/2010 5:12 PM, Bill VanAlstyne W5WVO wrote:
> Hi Jim,
>
> I was particularly interested in this part of your response:
>
>    
>> In any event, the AUX  contacts on it are only rated for 28 vdc !!    You
>> will destroy
>> them, if  using + 120vdc  for  switching cut off bias.  +120 vdcv  for cut
>> off bias is
>> dinosaur technology.  Better  to simply use a 100 K  3 watt  MOF  in the
>> CT of the
>> fil xfmr.. and short  the same 100 k R  out  with the  contacts   of the
>> bias relay.
>> Only a spst relay is required for this.
>>      
> I discovered the 28VDC rating of the aux contacts AFTER I bought the damned
> thing... D'oh!! The rating may have more to do with total power dissipation
> than with voltage above ground, so it might work anyway... I'm open to
> trying your idea, though, once I understand it.
>
> In the SB-220 (and also I think other similar GG amps from the same era),
> the center-tap (C.T.) of the filament transformer is connected to the common
> pole of the SPDT contacts. In RX mode, the normally-closed contact provides
> the C.T. with cut-off bias (120VDC). In TX mode, the 8+V operating bias is
> connected to the C.T.
>
> Can you be more specific in your recommendation? What are you connecting
> through the 100K ohm resistor to the filament transformer C.T.? The
> operating bias, I would have to think -- with the resistor shorted out in TX
> mode. In RX mode, then, the tube cathodes would get the same operating bias,
> but through the 100K ohm resistor. I'm not quite visualizing how that would
> work in cutting off the tubes. Please explain?
>
> Thanks,
> Bill W5WVO
>
>
>
>
>    
>> On another note.  He is using  2 x vac relays  +  3rd relay... with all
>> their coils in series, plus
>> a  shunting resistor across the bias relay coil.  This will work  with
>> +120 vdc.. and
>> speed all 3 x of em up.
>>
>> On the 2 x GS35B  6m amp,  I  helped a friend with... we used  3 x
>> Gigavac GH-1's
>> [like a rj1a]   one  for input, one for output, 3rd one for the bias [
>> shorts out the 100k R]
>> All of  em have 12 vdc coils. All are very easy to speed up  with an
>> overvoltage.. and just one
>> series  dropping resistor.  If u want to speed  em up even faster, install
>> a 50 uf lytic  across
>> the  dropping resistor. Then u end up with a 'RC  hotshot speed up
>> circuit' .
>>
>> On the 6m amp.. both GH-1's  mounted side by side, with 2 x SO-239's above
>> them.
>> Bypass  swr on 6m is dead flat.
>>
>> For his proposal,  I would deep 6 the idea of using +120 vdc  for cut off
>> bias.. and
>> use the 100k  R  in the CT  of fil xfmr.   We always use 2 x 100 K  in
>> parallel,,, for
>> redundancy.
>>
>> later... Jim   VE7RF
>>
>>
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>>
>>      
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