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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