[Amps] Re: 8171

[R.Measures]

On Jan 28, 2005, at 11:39 AM, David G4FTC wrote:

> Hi Rich,
>
> Most of your questions are answered in the following pages:

David -- I did not find a schematic of the grid circuitry.
>
> http://www.qsl.net/g4ftc/amplifiers/8171/8171_circuit_description/ 
> 8171_circuit_description.htm
>
> http://www.qsl.net/g4ftc/amplifiers/8171/8171_construction/ 
> 8171_construction.htm
>
> http://www.qsl.net/g4ftc/amplifiers/8171/8171_testing/8171_testing.htm
>
> The operating parameters are the same as my 8170 amp as per:
>
> http://www.qsl.net/g4ftc/amplifiers/8170/8170_testing/8170_testing.htm
>
>
>> I have used both and I can not see why the SK300 should have less  
>> inductance than a SK-300A.
>
> Firstly I must say that I’ve never actually had a SK300 in my hands –  
> it’s only my assumptions based on visually comparing the Svetlana  
> SK300As (which I use) with the datasheet photos of the Eimac SK300.

The SK-300 is similar to the SK-300A except that it has much unneeded  
metal on the grid collets that restricts air flow.  Relieving air  
restriction through a SF-300 requires disassembly, sawing the collets  
with a 32-tooth/inch coping saw, and reassembly..   Total elapsed time  
is c. 30-minutes.
>
> (This is really where photos are required to accompany this text –  
> what I am trying to describe would then become self evident).
>
> Looking at the filament connection side, the SK300 has 8 radial  
> supports whereas the SK300A only has four. Also the SK300 skirt goes  
> the full vertical length of the base, whereas on the SK300A this is  
> reduced to zero except where the four radials are located, all of  
> which are no doubt to improve airflow. Now my background is really  
> VHF/UHF where all attempts are made to try and provide the shortest  
> connections to ground to minimise inductance. To my way of thinking,  
> the radial supports, which on the SK300A are about 2-1/2 inches long  
> from the chassis to the outer filament ring, are less than ideal in  
> assuring a good RF ground.

Agreed.  Measuring screen self-resonance with a dip-meter will give a  
clue as to whether screen-grounding modification is needed for the band  
being used.
>
> What I would like is a good quality photo of a SK360 base – I have a  
> scan of the Eimac datasheet but the detail in the photo isn’t  
> sufficient to work out how it’s better at VHF than the SK300/300A.

The Eimac SK-330B is a VHF socket for the 8169.  The grid contact is  
made with a short piece of fingerstock instead of with a 360º collet.   
My guess is that this was done to reduce total input C.  The VHF  
version of the SK-300A might be similar?

cheerz
>
>
> Regards
>
> David G4FTC
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org