I have always had a similar issue with my TL922 on the CW portion of 80
and the low end of 160. I too checked everything, including the
capacitance of the padder, and found nothing wrong. I finally replaced
the padders with higher value cspscitors. Now it tunes properly, but it
makes slightly less power than I think it should on these bands. My
suspicion is that there is insufficient inductance. Since I don't have
an antenna for these bands at present, I haven't bothered with it.
FWIW, I have directly grounded the grids, but I don't see why that
should affect the plate circuit.
If it matters, my amp is the 200/220/240v model originally sold in
Japan, and the tubes are the original Eimac tubes.
73,
Vic, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/
On 17 Sep 2016 20:59, Jim Thomson wrote:
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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