[Amps] Options

[Ian White GM3SEK]

Steve Thompson wrote:
>
>
>Martin Sole wrote:
>> Start with an assumption.
>> You have a quantity, i.e. more than 10, Eimac 8930's (4CX350R some say)

The 8930 is a 'special' version of the 4CX250R with an over-sized 350W 
anode cooler.

>> with bases and teflon chimneys. Tubes are unknown condition but were
>> removed from service a couple of years ago as working. You have a
>> quantity of transformers marked 1752 volts rms 275 madc, a similar
>> quantity of oil filled capacitors rated 5uF 3000 volt and assorted other
>> componentry, non tank circuit.
>>
>> Question.
>> What do YOU think might be the most efficient way of utilising this
>> hardware assuming the goal is some minimally specified HF band amp (80
>> through 10 metres).
>What's minimal - modest power or no frills, or?
>What are you wanting out of it - a simple amp for minimum effort or some
>fun experimenting?
>
>If it's the former, I think there's incompatibility in the hardware you
>have. The transformers will give you 2.2-2.4kVish, which will work the
>valves in the region where you need comprehensive protection etc. Where
>you have the quantity, I'd put extra in parallel and run them at much
>lower voltage, say 1500V, and all the headaches go away.
>
>As a gross generalisation valves are cheap, transformers expensive, so
>parallel up the transformers (use a separate bridge on each and common
>the bridge outputs) to get the power you want, then choose valve(s) to
>suit. A pair of GI7 triodes will cost next to nothing and give you 800W out.

Steve is right - your HV transformers are not well suited to the tubes, 
so think about them separately.

Other major factors in your part of the world are shipping costs, and 
poor availability of the extra components that you'd need to operate 
tetrodes (like screen and grid bias transformers).

Build yourself a HV power supply using the big, heavy parts that you 
have. With multiple transformers and caps it's going to be huge, so plan 
to put it under the bench and control it remotely. Then find some tubes 
to suit that supply.

Sell the tubes, bases and chimneys to people who really want and need 
them, and go for a pair of GI7Bs as Steve suggests. Shipping costs on 
those parts will be relatively much smaller. Also the triodes need much 
less support electronics.




-- 
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek