Comments to responses from my posting:
From: Jon Ogden <jono@enteract.com>:
>That's another thing I fogot to mention. A class AB amp is going to
>produce much more heat than a class C amp. Depending on how large your
>heatsink is, you might need to add additional cooling (a fan) or even put
>the amp on a larger heatsink. Many class C heatsinks I have seen would
>be totally unacceptable for class AB.
Rather than Class C, I discovered that the amp was intended for low duty
cycle pulse AB service. I've added two cooling fans to the heatsink.
From: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>:
>The big book of 'Motorola RF Application Reports' (HB215/D) is
>obtainable free from the Motorola web site (sorry, I don't have the
>address for the USA).
Thanks for the tip; I've ordered it.
>Why is this amp "obviously" Class C, Dave? If it has bias pots, it's
>already capable of being forward-biased, especally if the bottom leg of
>each bias network includes a thermistor.
You are quite correct, Ian. I learned from W8NF that the amplifier is
biased off normally to reduce heating. One must apply a ground to the
"blank" input to activate the bias circuits. It then works great!
>As D&G say, "the idle current required to place the device in the linear
>mode of operation is usually given in the data sheet."
Thanks for the tip. I had overlooked the table in the data sheet that
gives sample parameters for SSB operation. I set the MRF148s to 100mA
and the MRF150s to 250mA, per those conditions. The amplifier now
looks good on the scope, but has high 3rd harmonics. I'll wait to put
it on the air until I have a LPF. Power output is now a very satis-
factory 500W with my IC706 test radio cranked as low as it will go.
From: dhaupt@bewellnet.com (W8NF):
>1) The bias circuit to a FET is always a high impedance. There are
>valid RF reasons for making this true. Therefore, it is a simple matter
>to ground all the FETs except the one whose bias you wish to adjust. In
>your case, ground 7 of the gates, and the remaining one is what you'll
>be biasing.
That's what I did and it works great. In fact, the amp has jumper blocks
for this very purpose!
>6) Both spectroscopy and single-frequency MRI amps actually are class
>AB to begin with. However, it is often the case that they required an
>external signal from the rest of the medical equipment before they would
>turn the bias "on." Our customer called this "unblank". So look at the
As noted above, that was the secret to making the amp work! I'll wire
a relay in with my T/R to activate the bias when I key it up.
>9) Although some manufacturers run the MRF148 and MRF150 as high as 65
>volts, don't do it with a ham amp. The device breaks down at about 130V
>(per testing - the spec is actually more like 110, if I remember
My loaded voltage is 46V at full output, so I think that will be safe
without VSWR protection, per your comment. On the other hand, those
transistors sure are expensive...
I would like to thank everyone for the wealth of information provided me
on the reflector. The amp is now working great and I hope to have it
on the air as soon as the LPF arrives. These MRI boxes are a very
inexpensive route to high power on 6, and I encourage others to look for
them at the swaps.
--
...Dave Phillips | Internet: phillipsd@saic.com
The *MAGIC* band: 6 meters | Amateur Radio W7GZ, grid DM42nf
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