[Moon-net] [Mw] Pre-Tuning a 8877

[Edward Cole]

Thanks Tom, and all who replied.

I will be testing the input tuning parameters with a MFJ-269 antenna 
analyzer which outputs around 10mw, so will not be running any 
HV.  Since I am interested in the full power input impedance (R+jX) I 
will use 54-ohms across the cathode-grid.  That ought to help make 
any gross adjustments to the input coils.  After sufficient burn-in 
on the filaments, then will try applying some HV looking for arcs and 
setting bias without drive.  Hopefully, in a few days I can report 
success in achieving some power.  My HV supply provides 2500-vdc at 
400-mA so hoping to get about 600+ watts with 25-30w drive.  I have a 
QRO HV supply (2900-vdc at 900-mA) that needs some TLC before 
applying power, but hopefully it will be ready by this fall before 
the ARRL EME Contest (I have to wire 240-vac to the radio room for that one).

As far as the filament in-rush circuit, I am using the design given 
in G3SEK's Triode Board Manual.  It operates on the primary side of a 
transformer and uses a simple RC time delay circuit with diode 
rectifier to drive a 12-volt relay.  The relay shorts out the current 
limiting resistor after the delay time.  I am going with a 4-second 
time constant (1750-ohm x 2200mfd).  My 5-volt transformer is rated 
for 12-amps and runs 5.15 vac open circuit.  It has multi-tapped 
primary coil.  I am selecting the full primary of 126-vac to step 
down to run at 4.9 volts (my line voltage is 120-vac).  I understand 
that running about 4.75-4.95 volts will extend tube life.

73 Ed - KL7UW

At 07:02 AM 7/24/2008, W2DRZ Thomas Mott wrote:
>Hi,
>
>Use a 54 ohm for tube impedeance.input, output is 2,000 ohm.
>
>tom W2DRZ
>
>http://www.w2drz.ramcoinc.com/
>----- Original Message ----- From: "Edward Cole" <kl7uw at acsalaska.net>
>To: "Moon-Net" <moon-net at list-serv.davidv.net>
>Cc: <microwave at echo.valinet.com>
>Sent: Thursday, July 24, 2008 10:48 AM
>Subject: [Mw] Pre-Tuning a 8877
>
>
>>I recall hearing that one can use something like a 6K reisistor 
>>across the cathode-grid on a gg 8877 to resemble input impedance 
>>for pre-tuning the input ckts using a small signal from a MFJ-269 
>>antenna analyzer.  Can anyone confrim the input impedance for a 
>>8877 (144-MHz)?  Should I have the tube inserted to include 
>>inter-electrode capacitance?  This is with HV off, of course.
>>*****************************************************
>>73, Ed - KL7UW              BP40iq, 6m - 3cm
>>144-EME: FT-847, mgf-1801, 4x-xp20, 185w
>>http://www.kl7uw.com     AK VHF-Up Group
>>NA Rep. for DUBUS: dubususa at hotmail.com
>>*****************************************************
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