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RE: [Amps] 2m Linear Amp Enquiry

To: jeff millar <wa1hco@adelphia.net>, Jerry Muller <k0tv@arrl.net>,dobricic@eunet.yu
Subject: RE: [Amps] 2m Linear Amp Enquiry
From: "Skram, Helge (MED)" <helge.skram@med.ge.com>
Date: Tue, 14 Sep 2004 17:03:59 +0200
List-post: <mailto:amps@contesting.com>
The "easy-build" amps by YU1AW are interesting, and at first I could not
believe the quoted efficiencies.
Ouput power is remarably high for such simple designs.
The efficiency is typically in the 60-65% range.

Bandwidth indicates typically Qloaded=9, which is much lower than usual
stripline designs.
With this low value it is possible to use a fairly lossy inductor.

Looking at the output circuit design, it is similar to the K2RIW for 432MHz
when we convert the stripline to an inductor as it really is.

My problem is, how is it possible to achieve Qloaded as low as 9 at 144MHz ?
Qloaded beeing approximately Rp/Zcout, where Zcout is the impedance of the
tube capacitance pluss tuning capacitance transformed to the tube pluss
spread capacitances.
For a GS-35B:   Cout=4.5pF  When we add another 4.5pF for tuning and spread
C -> Zc=123 Ohms @ 144MHz. 
RP = EP /1.8 X IP =2500/1.8*1.2=1157 Ohms, then assuming K=1.8 for class
AB1.
Qloaded=1157/123=9.4
This matches the bandwith in graph.

Output network efficiency should approximately be (1-Qloaded/Qunloaded),
where Qunloaded will be dominated the Q value of the output inductor.
Qunloaded=Zinductor/Rloss

For a stripline it is possible to have Qunloaded=400 @400MHz.
For a simple wire inductor it may be Qunloaded=123/1=123  ,assuming 1 Ohm
loss which is reasonable including skin losses.
Then output circuit efficiency=(1-9,4/123)*100%=92% which is on line with
what is necessary for amp eff 60-65%.

So my conclusion: Seems possible to make amplifiers with the quoted
performance.
What makes it possible:
-Use tube with low output capacitance (microwave tube)
-Don't ruin this with excessive tuning capacitance (big flapper with
striplline).
-Use low plate voltage and high current to have a low plate impedance.

Harmonics can be done with in a low pass filter.

I may try to build one of the easy-build amps made for lazy amp builders.


73 

Helge

 

-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
Behalf Of jeff millar
Sent: 14. september 2004 05:15
To: Jerry Muller
Cc: amps@contesting.com; David King
Subject: Re: [Amps] 2m Linear Amp Enquiry


Pi Net's don't work on 2M, but here's a site that shows how to use HF 
techniques for building 2M PA's

    http://nd2x.net/yu1aw-2m-easy.html

Note he uses the Russian triodes like Jerry suggests.

jeff, wa1hco


Jerry Muller wrote:

>A pi network on 2 meters? Never work.
>
>I've had a couple of tetrode amps for 2, one was the old classic plumbers
>delight push pull amp with a pair of 4CX250s. That worked OK. A much better
>design is a 1/4 wavelength stripline using either 4CX250s or 8930s. That
amp
>was sold by ARCOS a few years ago.
>
>The stripline design is better with better efficiency. A problem however is
>IMD with those tetrodes. It's a bit high and those tubes require a fairly
>complex power supply with a screen supply that must not only be capable of
>sourcing voltage, but also sinking it. Negative screen current is normal
>with those tetrode amps. I'd bet the 4CX800s would work well in that
>configuration too.
>
>Triodes are much better. The W6PO amp using the 8877 is an excellent
design.
>A similar design could be constructed with the Russian tubes or the Eimac
>triodes like the 3CX800 or 3CX1200. The IMD with these amps is much better.
>
>If you're only operating CW or FM there's not much difference. On SSB the
>triode designs with lower IMD are a far better choice. Your neighbors (hams
>nearby) will appreciate it. On VHF/UHF nearby stations are REALLY loud. The
>tetrode amps will make you VERY wide at your neighbors houses even if you
>run them properly. The high order IMD products (7th, 9th, 11th, 13th) are
>still quite strong when you're 100+ dB above their noise floor.
>
>73 es GL,
>
>Jerry, K0TV
>
>----- Original Message ----- 
>From: "David King" <G6KWA@thersgb.net>
>To: <amps@contesting.com>
>Sent: Thursday, September 09, 2004 4:50 PM
>Subject: [Amps] 2m Linear Amp Enquiry
>
>
>  
>

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