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Re: [Amps] New Guy, Old 813 amp

To: k7fm <k7fm@teleport.com>
Subject: Re: [Amps] New Guy, Old 813 amp
From: Doug <dcoffman@iquest.net>
Date: Thu, 26 Apr 2007 13:14:19 -0400
List-post: <mailto:amps@contesting.com>
I cannot remember the company that makes it, but there is a small autotuner 
designed to be retrofitted into a 6M-HF amplifier that sells for $100.00 that 
would fit the bill.

Regards,

Doug - N9XTF

Quoting k7fm <k7fm@teleport.com>:

> Hello Scott:
> 
> Congratulations on the 813 amp.  I had one many years ago and worked about 
> 150 countries on 75 meters.  The tubes are instant on - and that is worth a 
> couple of db when you hear a rare station.
> 
> The electronics of a tuned input is relatively easy - you simply need to 
> match the impedance of the tubes to 50 ohms.  Unless the amplifier you took 
> the input from used the same tubes, it would not match, so you might as well
> 
> build your own.
> 
> The most difficult problem in retrofitting a tuned input to an existing 
> amplifier is trying to figure out a way to switch the input with the output 
> switch.  I have done that successfully on a couple of occasions and it 
> involves skill and good luck.  So, plan on having a separate switch for the 
> tuned input and remember to switch it with the output.
> 
> If you wish to reduce designing to a minimum, get a pi-network antenna tuner
> 
> and connect it right at the input - with zero length cable.  Then adjust the
> 
> match for the bands you want to operate on.  You can then simply dissect the
> 
> antenna tuner and determine what value parts you had when you had the best 
> input swr.  Then replace those measured or calculated values with mica caps 
> and small coils.  You can use a two wafer rotary switch to select the band.
> 
> Or, you can design your own.  I am guessing that the input impedance of an 
> 813 is about 375 ohms.  Two in parallel would be 188 ohms.  Use one of the 
> filter design programs to design a pi type filter that has 50 ohms input and
> 
> 188 ohm output.  Wind a toroid for the inductor and use mica compression 
> trimmers for the capacitors and you should be able to hit it right on the 
> money.
> 
> On my 813 amp, I wound a broadband toroid step-up transformer and loaded the
> 
> output with a bit of resistance to make the load constant and called it 
> good.  The driver was happy and I did not have to switch the tuned input.  I
> 
> did not have much equipment to measure the performance, but never got death 
> threats from anyone so assumed it was ok.
> 
> 73,  Colin  K7FM 
> 
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