[Amps] Choices for valve linear project 1st timer!

[Mark Hill]

Simon,

Apologies, I sent my reply (below) direct to you and not via the 
reflector.  It just might be of interest to someone else.

Ian 'SEK has given the correct reference for the Russian valve site I was 
trying to remember.  Modesty however prevents Ian from telling you that he 
also makes a well engineered bias/control board suitable for most any RF 
power triode, GS-31 included.  More details, including the manual for the 
'Triode Board' that contains lots you should know when embarking on a first 
amp. project, may be found at:

http://www.ifwtech.co.uk/g3sek/boards/triode/triode-1.htm

I have looked at the other references for a design that you quote in your 
original post.  I personally would not advocate using transformers out of 
old microwave ovens unless novelty is a prime design goal.  Peter Rodmell 
at Linear Amp UK would I'm sure be pleased to sell you some 'off the shelf' 
parts to help get you off the ground quickly.  Take a look under components at:

http://www.lauk.karoo.net/

Whatever you do, do it safely and get good quality (and cross-checked) 
advice - there's plenty available from the contributors to this reflector.

Regards.

Mark.

---------------------------------------------------
Simon,

My advice would be to go for a 'KISS' design with a single Russian GS-31 
(or GS-35) valve in grounded grid.  The valves are cheap; there's no 
expensive base to buy; the heater requirements are easily satisfied; the 
list of plusses goes on and I'm struggling to think of any negatives!

For a first PA (any PA!) multiple valves are a hassle, metalwork can be a 
hassle and tetrodes are definitely a hassle, as for them, you would need to 
build screen and bias supplies, plus an amount of protection in case of 
'accidents'.  In my opinion, the era of the 4CX250 is well and truly 
over.  There are far better, cheaper alternatives now (such as the Russian 
4CX800 if you must use a tetrode) and for an HF linear, using a valve with 
loads of gain at VHF can turn into a struggle.

Type 'QRO via Russia' or similar into Google and you will find a website 
full of ideas.

I well remember the 'JST 4CX250 design.  Still have the article I think.  I 
remember thinking at the time that the design, whilst somewhat innovative, 
was not one that I personally would be happy to commend to a public audience.

Regards.

Mark.

----------------------------------------
At 20:28 31/01/2005, you wrote:
>Hi all,
>
>I'm a UK ham who is limited to 400W & would like to build my own linear, one
>as I've always wanted to play with valves and two because I want to learn
>more from the experience.
>
>Now googlising has brought up many designs + I also have a few old magazine
>articles for similar. A couple that have caught my eye are:
>
>1/One published in the UK mag Ham Radio Today & was designed by G4JST. It
>uses three 4CX250B's with a HT of 1.3kv to produce 400W which is apparently
>very linear output. I know the HT is low compared to what it should be so
>I'm prepared to uprate it accordingly if this is feasible. I can scan the
>article in if any of the more experiences users would like to see it's
>design? There are some holes in the article though so it may not be 100%
>suitable for my 1st project - I quite like it though :)
>
>2/ Anyways the 2nd I've seen is on
>http://www.mbc.co.uk/RADARS/G3RIK/g3rik.htm#linear & uses the Gi-7B valve.
>Again not  great deal of constructional info but I've mailed the author so
>hopefully he can assist should I need it. The circuit diagram for this beast
>is http://www.mbc.co.uk/RADARS/Pictures/HF_Linear/HFLinear1.jpg - again I
>need more info, it only shows one valve on the diagram but two are used in
>the project - being dumb I'm not sure if I'm totally missing something
>here??
>
>OK so I've managed to find a couple of potentially suitable circuits. Are
>there any others I could use? For background I'm using a Kenwood TS690S with
>10-100W available to drive the linear. I'd like something like 600-800W
>output which when I use at the legal 400W limit *should* produce very clean
>output power with low spurious harmonics etc. I also don't want to spend an
>absolute fortune on the project and also need projects where the components
>are available or easily made!
>
>Any suggestions greatfully appreciated!
>
>Regards
>
>Si
>G0IQC
>
>
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>Amps at contesting.com
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********************************************************
Mark Hill - G4FPH
E-mail: g4fph at mjha.co.uk
New web site: www.qsl.net/g4fph
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Dipoles resonant on 1940/3700/7080 kHz
2 ele delta loop beam resonant on 14180 kHz

Remember: A Texas Gallon is actually less than a UK one!
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