[Amps] SB200 Parasitic Chokes

[Carl]

Of course it requires less drive for the same output. Read up on negative 
feedback and notice that without it the stage gain is higher which may lead 
to instability.

You can add some non inductive R in series with the drive at the filament to 
restore the NFB and the original drive versus output. You also made a big 
circle for a net gain of zero.

Carl
KM1H



Subject: Re: [Amps] SB200  Parasitic Chokes


>I too have noticed that directly ground the grids lowers the drive 
>requirement. The amplifier is also more stable after conversion to QSK.
> My amplifier was the original 10-80 meter version. I left the stock 
> suppressors in.
>
>
>
> 73
> Jim W7RY
>
>
> -----Original Message----- 
> From: Paul Baldock
> Sent: Saturday, June 15, 2013 12:47 PM
> To: amps at contesting.com
> Subject: [Amps] SB200 Anti Parasitic Chokes
>
> A few weeks ago I posted asking if anyone new the optimum
> Anti-parasitic chokes (APCs) to use when modifying an SB200 for 6
> Meters. I received some interesting comments but no definitive
> answer. So I thought I would share with you what I came up.
>
> I experiment with many different combination or R and L and came up
> with something very similar to the original used in the SB200. The
> original SB200 APC is a 47Ohm/2W Carbon Comp in parallel with about
> .08uH. This actually works well a 6M except the resistor get quite
> hot. In fact I disassembled the original APCs and found the resistors
> actually measured around 65Ohms probably due to age and heat stress.
> Based on the higher value working well I finished up with 2 x
> 150Ohm/3W low inductance metal film resistors in parallel with .08uH.
> I tried 3 in parallel and it did not make any difference so I went
> back to 2. These run only slightly warm
>
> If anybody wants any info on doing the SB200 6M conversion feel free
> to contact me directly. No secrets here!
>
> BTW I did remove the chokes and caps between the Grids and Ground on
> the 572Bs and replaced them with a direct short to ground resulting
> in more output power and less drive required. No flames about this please.
>
> - Paul  KW7Y
>
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