[AMPS] Building AMP with four valves in parallel (4-250) - any guidance ?

[paul]

In message <000801c061d5$c99fdf40$d84cef9b@telkomsa.net>, Wynand van Wyk
<zs6arf@telkomsa.net> writes
>    Greetings,
>     
>    I have started work on a new amplifier (80 - 10 meters) utilising 
>    the popular grounded grid design but with a slight difference than 
>    the norm as I use four valves in parallel, of the AMPEREX 4-250 
>    type. The main reason is I have ample supply of 4-250's (as 
>    compared to the usual 4-400 or 4-500 one reads about in designs).  
>    I still would like to break the 1 KW barrier on output.
>     
>    I have a similar working one I build in 1999 but although it is 
>    working it is not very efficient and prone to destroying chokes and 
>    by-pass capacitors.  Power output is also way below what I expected 
>    .. and not breaking the 1KW barrier.
>     
>    I run the valves with +/- 3,400 Volts on the plate and the 
>    filaments have two transformers connected in parallel giving the 5 
>    Volts at +/- 15 Amps.
>     
>    I need advice on how to improve performance and limit component 
>    failures.
>     
>    I know I have to look at the capacitance of the four valves in 
>    parallel as this will impact on efficiency especially at 10 meters.
>      I AM JUST NOT SURE WHAT I AM SUPPOSED TO BE LOOKING AT.
>     
>    My plate choke will also have to be properly designed, as 
>    previously I took it out of a YAESU 2100Z and just transferred it 
>    to the homebuild version.  It works but surely is partly the cause 
>    of the lower than expected power output.
>     
>    I also now want to add a tuned input circuit as previously I just 
>    put a ATU beween the rig and amp input. 
>     
>    Any advice, especially on the use of the four valves in parallel be 
>    it practical or an indication that I should use two valves will be 
>    appreciated.
>     
>     
>    Wynand van Wyk
>     
>    ZS6ARF
>     

Have a look at http://www.vcnet.com/measures

This will tell you all you need to know about plate chokes and more. I'm
currently re-building my 4-250 amp and have hit the very same problem,
so at this moment I've redesigned the plate choke and added a secondary
choke in series at right angles.

What Richard says about chokes is spot on - try it yourself with a dip
meter!

For 10 metres 4 x 4-250's maybe too much capacitance as even 2 is just
manageable.

You will probably need a blower as opposed to muffin fans with 4 anyway.

Is it grounded grid or grid driven?

Paul 

GW4WWE

-- 

To try and not succeed is better than not to try at all (Anon)

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