[Amps] TL-922 80m load issue

[Vic Rosenthal 4X6GP]

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 at comcast.net>
> To: Amps at 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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