[VHFcontesting] AM-6155 PS Question

Michael Sapp wa3tts at verizon.net
Sun Dec 11 17:47:52 PST 2011

Hi Kent (& All):

Thanks for the thought on paralleling the screen resistors. The one AM-6155 
unit here was using only one of the 100K-ohm resistors on the left side of 
the PS module (the other
100K resistor bypassed per AM-6155 mod info). The "working" 100K-ohm 
resistor opened up, so I moved the bypass jumper to use the other screen 
resistor.  I had to pull the PS module to do that. Fortunately, there is an 
access hole in the side of the PS module to do that, so it was an easy fix.

The second AM-6155 PS module had the center one of the  three 130V/10W Zener 
diodes fail to short condition, which took only a few minutes to figure out. 
I have another parts unit here so I will
likely scavenge a zener from it this evening to get it going.

Neb Surplus Sales has those cavity anode HV feedthrough caps with the 
correct 3/8 thread. I picked up two of them last week to replace 
broken/failed ones in my other cavity amps that got scavanged in prior years 
to keep the 432 Mhz unit running years back....

After running the tube filament for an hour or so I started out with 1W or 
drive and peaked input and output power controls on 222MHz. Then went to 4W 
of drive from a handheld FM rig and had 125W or so showing from the Bird 
meter, which is about 177 "ham" (peak) watts with 150ma plate current, so it 
looks like it will make rated power with 8~10W of drive...

One more cavity amp to clean and then replace the HV cap and unit number two 
will be ready to go.  It's good to see these old friends come back to life 
an make good RF power.....

73  Mike wa3tts

----- Original Message ----- 
From: "Kent Tiburski" <k6fq at arrl.net>
To: "'Michael Sapp'" <wa3tts at verizon.net>
Sent: Sunday, December 11, 2011 6:18 PM
Subject: RE: [VHFcontesting] AM-6155 PS Question

> Hi Michael,
>  I did a suggested mod to the 6155 I used to own.  I don't remember about
> the zeners on the screen supply, maybe because they weren't there or I
> removed them (been a long time).  Took the two big resistors coming off of
> the B+ and paralleled them.  All was good!  Remember the voltage was 
> around
> 325 or so.  8930 liked it.  Mine was used on 2 meters and could put out 
> 400
> watts and not break a sweat.  GL!
> 73, Kent, K6FQ
> -----Original Message-----
> From: Michael Sapp [mailto:wa3tts at verizon.net]
> Sent: Friday, December 09, 2011 9:23 PM
> To: vhfcontesting at contesting.com
> Subject: [VHFcontesting] AM-6155 PS Question
> Hi All: I decided to blow the cobwebs out of my AM-6155 amps that had not
> been used in 20 years.  I have HV ok on two units but the screen V is low 
> on
> one (275V) and not present on the second.  I will check the screen 
> dropping
> resistors and Zeners tomorrow, I'm just curious if others have run into
> similar issues..... Actually, not having the HV caps go BANG on start-up
> after 20 years was a pleasant surprise.... Mike wa3tts

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