[MRCA] grc109 into coax fed 1/4 wave 40m vertical

[Tim]

Hi Jeff - Yep your 109 is putting out some power!  But it was designed to
drive a higher impedance antenna like the 100' "hank" wire provided.  With
a lower impedance antenna like a dipole / 50 Ω or lower, the lamp can be
over voltaged and eventually fail.  Carry spares...
If you want to routinely use a low impedance antenna you could change R-13
to a lower value or shunt it as Chris suggested.  But then later with a
higher impedance antenna you might not get enough glow to properly tune the
TX.

I put a 12 ohm 2W resistor in mine vs the 20 Ω, works fine.  The RS-6
transmitters have the same issue but with a different lamp.  I also reduced
the shunt resistor in mine as well.  It's a tradeoff when not used "by the
book".
When running the system off a nominal 120 VAC source setting the PS
selector to 130 Volts is correct.
Everything I have learned about the GRC-109 and field ops here if
interested:
 https://www.n6cc.com/angrc-109-special-forces-radio-set/

Have fun with it - It's my Daily (OK weekly) Driver...  I just had mine out
for Winter Field Day - works great.
https://www.n6cc.com/winter-field-day-2025/
Tim
N6CC

On Tue, Feb 25, 2025 at 5:32 AM comcast via MRCA <mrca at mailman.qth.net>
wrote:

> folks. finally fired up the grc109 from Joe Munson’s estate. It appeared
> to load up good on 40 m  however the #47 output bulb (LO Z indicator) was
> real bright and eventually burned out.  the neon bulb worked as it should.
> I had the power supply on the 130v tap. does this seem normal?   I was not
> using the end fed wire setup as shown in the manual.   do I need a
> different method of matching the 109 output when using it on coax fed
> antennas so the 47 bulb doesn’t burn out again ?
>
> Jeff N2IA
> Sent from my iPhone
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