Date: Wed, 14 Sep 2016 23:20:48 -0400
From: K2CB <k2cb@comcast.net>
To: Amps@contesting.com
Subject: [Amps] TL-922 80m load issue
Helping a friend with a TL-922.
On 80m, it is suffering from not enough loading capacitance. The peak at low
drive levels is at 1. If driven to 1kw the peak it not attainable, as it
appears to be past the lower limit.
I suspected that maybe the padding capacitor for 80m, or its associated band
switch wiring failed Also the second portion of the load variable cap which is
switched in for 80m. All tested good, including the band switch.
The issue seemed to begin when both tubes were replaced with new RFP 3-500ZG
tubes. At that time the 4.7k resistors and 470uh inductors on the grids were
replaced. The original external wire wound style inductors were NLA, so
standard sealed resistor-like style 470uh inductors were used.
On the upside, the amp now does 1000w with only 50w drive now. So with that
in mind, I wonder if the lower drive requirement is related to the "lack of
sufficient loading" ?
I recall reading in the past that directly grounding the grids requires
increased drive. Thus my suspicion that the new inductors may be the culprit.
Could the lower drive condition be related to the lack of sufficient loading
capacitance?
Eric
K2CB
## The RFP 3-500ZG tubes are MU= 200. The eimac 3-500Z is MU=130
The RFP version will draw LESS idle current for a given amount of bias V.
## Directly grounding the grids results in LESS drive required. With all 6 x
grid pins
bonded directly to the chassis with wide copper strap, you will see a drive
reduction
of 20-25 watts. Get rid of the caps from each grid to chassis..and ditto
with the choke
from grid to chassis...you dont need em. The amp will be far more stable with
the 6 x
grid pins bonded to chassis. Directly grounding the grids has been done at
least 2 dozen times
that I know of..all on TL-922s..and all report the same results, 20-25 watts
less drive required.
( ditto with drake L4B, SB-220, henry anps etc, etc)
## The original idea with the grid caps + chokes was a NFB idea. Its fubar at
best..plus you then
get a dc Vdrop across each choke = unwanted, varying bias..on top of the
normal regulated bias.
## since the xcvr now has to put out 20-25 watts less power, the IMD on the
xcvr will improve
by quite a bit, so the drive signal to the amp is now cleaner, resulting in a
cleaner IMD from the
output of the amp. ( esp on run of the mill 100 w xcvrs).
## IF you have an LCR meter, easy matter to measure the capacitance of the
80m padder.
Ok, what about 160m ?? Even if he doesnt use that band, you might want to
check 160m..
into a dummy load. The problem is either the padder, bandswitch..or both.
Jim VE7RF
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