[Premium-Rx] Tubes versus FETs

[A. Gundes]

Hello Richard,

Interesting subject, for me at least.

There are several way to protect transistors especially FETs being more
sensitive ( if you are using MOSFET they are even a lot more sensitive
to high voltage spikes ).  If the configuration you are using is the
Common Source ( grounded Source of the FET ), then you can try to
put a Choke Coil say at 1mH value between the Gate of the FET and
the Ground. Basically you are Shorting the Gate of the FET for DC and
not AC.  However lightning is a DC pulse and it contains AC components
that will still not be completely shorted out by using the choke.
You can use a Low Pass filter ahead of the Gate to help this.
An addional safety measure is to use Gas Tube Arrestors and similar
low loss devices.  They are very very effective. In addition companies
such as CP Clare sell transient protection devices which can be
used on the FETs.
Can you tell me what FET you are using ( part no and mfg ).
Also can you tell me how you are matching the antenna to the input
of the FET as well as how you are getting output matching to 50 Ohms.
FET 's are not too easy to match unless a negative feedback is used.
I am curious about the circuit.  Are you getting amplification ? Sometimes
this is very difficult to measure for obvious reasons. An input and
output matched amplifier is of course easy to measure however.
Another thing, some Bipolar transistors have much better Noise Figures
as well as Gain as opposed to FETs.  They are also by their nature
much more resistant to High Voltage Spikes.  You may want to experiment
with these as well.  They are much much lower cost.  I can also recommend
a company who makes very nice amplifier modules ( 50ohm matched )
with a 20db Gain and they claim a 2.5db NF.  All for less than $ 40.00
including the case and connectors. Very nice device.

However I really do not think that you need to go to tubes as there are
many preamplifiers mounted outside operational in perfect shape for years
without any "poof"s.

Regards
Ahmet

Richard Reich wrote:

> Greetings.
>
> For a few weeks now I have been messing around with receiver antenna 'impedance
> matching' using a super-linear JFET. Effectively the antenna wire goes onto the
> Gate of the FET, which is biased in its linear region. The output of the FET is
> then matched down to a lower impedance, which is great for most comms receivers
> at 50 or 75 Ohms. The problem is this: poof!  A little bit of static, or a
> lightning strike nearby, and the FET dies. The Gate is already 'leaky' to ground
> - but I suspect not enough to give protection, and reducing it any more defeats
> the object. The FETs I have been destroying are $15 each! So I was thinking,
> perhaps a tube would be better for this application. Does anyone have any prior
> experience of this? I am not really looking for voltage gain in itself - but
> obviously low noise is important. I have a box full of tubes which I could
> experiment with, but don't want to embark on a fruitless excerise. Anyone have
> any info on this?  Many thanks - Richard
> ***************************************************************
> Richard Reich
> Principal Engineer Hardware
> SAAT Technology Ltd
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