[AMPS] Portable linear using a DAF?

[jeff millar]

comments included below

----- Original Message -----
From: "sm5ki" <sm5ki@algonet.se>
To: <amps@contesting.com>
Sent: Sunday, October 29, 2000 4:53 AM
Subject: [AMPS] Portable linear using a DAF?


>
>  Hej all:
>
> I want to collect together a light weight and compact ( low profile to get
> into a normal suit case) shortwave station for portable use including a
> light weight linear that I have to build myself. Here my thoughts and your
> comments are most welcome:
>
> 1. Where do I find a light weight HV power supply and why are there no
> switched power supplies for 2 -3 kV?

Key design problems include
    - transformer resonates below switching frequency due to high
capacitance in
    large number of turns required
    - slow switching speed of high voltage diodes
>
> 2. As alternative use a number of paralell cheap russian TV sweep tubes
> with filaments in series to get rid of the filament transformer weight.
Use
> voltage qudrupling straight from 230 VAC mains with a well isolated
chassie
> to avoid electrical chocks. At the expected 500 W level use an outdoor
500W
> SMART TUNER and a long wire.

Creating a low voltage, low impedance output has a number of advantages.
Consider getting the parallel plate impedance down to 200 ohms and then
you can use a 4:1 toroidal transformer to match to 50 Ohms.  This
creates a broadband, no tune amp suitable for external antenna tuners.
Don't forget the harmonic filter.

Some calculations on place impedance, voltage and current:

    Rp = Ep / (Ip x 1.5)  or Ip = Ep / (Rp x 1.5)

    Rp        Ep    Ip    W
    300    300    .67    200
    300    450    1.0    450
    300    600    1.5    900
    200    300    1.0    300
    200    450    1.5    675
    200    500    1.7    800
    200    600    2.0    1200
    100    300    2       600
    100    450    3       1350

Looking at the 6KD6 data sheet, it looks like 150 ma max plate current
which implies 10 in parallel at 450V yeilds 200 ohm plate impedance
and can produce 675W.

Just using a voltage multiplier off the mains will create very narrow
high current spikes on the mains, leading to worse regulation that expected
and lots of HF noise in the environment.

But eliminating the plate transformer has appealed to me for quite some
time. Here's some things to think about
    - RF deck will have to float with respect to safety ground
    - Fullwave rectify the AC line and generate 300 VDC
    - Use a little bit of filter caps to fill in the nulls in the ripple
    - Use a boost type switching regulator and chop the AC line at 20-100
KHz
      This broadens the current surges, improves power factor
    - Boost the voltage to 450V or so.
    - Put filter caps on the output, sized to eliminate the switching
      ripple

Use a buck type switching regulator to produce the filiment voltage,
probably can purchase off the shelf to do the job.

> 3. Alternative would be a number paralellt 8072 tubes cooled with flat
> volume blowers. Then I get the additional weight of the filament
> transformer, of course.

Way too expensive, too high an impedance, too much plate voltage
>
> 3. Use a DAF that dissipates less heat and requires less cooling and
weight.
> Which may mean a number of flat light weight computer type blowers.

With a low voltage plate supply and sweep tubes, just generate a regulated
screen supply.

> 4. I know there have been similar linears on the market but they are too
> heavy.
>
> 5. Transistorised linears are out as the cooler adds weight although a
> switched power supply is OK.
>
> 6. A heavier alternative would be a heavier GU 74 DAF with my QRO HV
toroid
> transformer. The tubes are very cheap in Europe and reliable.
>
> 73s de Hans SM5KI
>
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>
>


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