Since the screen is the weak link in the NCL-2000 I don't run mine or
customers beyond 15-20ma in the SSB position for the pair and if the PO
is a bit lower than the usual 1200W key down it not enough to worry about.
My 6M conversion received a transplant in 67 with 67 date codes and
replacing the original 1964's that did not have the screen support structure
and finally failed. The 67's were run with care and were finally replaced in
2010 when PO dropped below 600W. With the new Burle tubes one of their
engineers gave me 1300W was easy with 15W drive.
Since it was a set and forget amp it didn't require constant tuning, once
set at 50.2 it covered 50.0 to beyond 50.4 just fine. The only tuning then
was when antennas were changed.
Ive experienced no anode discoloration at the nominal 225ma idle current
even in the modified 160-10M 9 band amp I used very heavy for decades for HF
contesting, DX chasing and some AM. It is now used with a CE -100V (with the
160 option) on vintage SSB and a lot of DSB AM. The SSB idle current has
been reduced to ~ 100-120 ma which improves efficiency and doesn't affect
the spectrum analyzer IMD display unless I push the audio too hard.
Ive modified the bias supply in several of my other brand GG amps plus
customers to improve AM efficiency for a decade + since AM activity really
took off.
Carl
KM1H
--------------------------------------------------
From: "Manfred Mornhinweg" <manfred@ludens.cl>
Sent: Wednesday, January 28, 2015 8:35 AM
To: <amps@contesting.com>
Subject: Re: [Amps] {Collins} Understanding Screen Current in a Tetrode
Under the assumption that the guys who designed and built amplifiers for
sale knew pretty well what they were doing, it might be a good idea to
simply look into the user manuals of some tetrode amplifiers.
The manual for the NCL-2000 states that:
- 40mA is the absolute limit for screen current;
- while tuning in CW, one should keep screen current between 10-25mA;
- screen current is always peaked with the tune control, then controlled
to the desired level by the load control;
- Both screen and plate current are of course affected by drive level;
- while operating CW, it's best to adjust screen current to 10mA and plate
current to the correct level to get 1kW input (that was the limit back
then), and this should be done by adjusting drive level and load control
as needed, while keeping the tune control at the screen current peak;
- During SSB operation, optimal peak screen current should be 25-35mA, at
2kW input.
In my practical use of the NCL-2000, I have to deviate somewhat from these
figures. This is because the designers also ask to set the idling current
to 225-250mA, which is about 600W idling plate dissipation! The blower was
dimensioned for 60Hz operation, but I have 50Hz, making the blower a lot
less effective. At 600W I would soon melt down the tubes! So I set the
idling current much lower, like 60mA. This should result in higher IMD,
but I have asked many hams to check my splatter, and they all report they
can't hear any, so it should be fine.
Anyway, at this lower idling current, full power SSB operation at what
seems to me to be the best settings happens with the screen current meter
only ocassionally deflecting from zero, never more than 5mA or so.
Probably this is a peak screen current of about 15 to 20mA. When I tune up
(briefly) with a carrier with the amp set to SSB mode (full voltages), I
set it for 15mA screen current.
The power output and efficiency are both good, IMD seems to be good enough
to keep the other hams happy, and I have been using the tubes for 15 years
now and they seem as good as new, and they weren't new when I got the amp.
So my guess is that my tuning practice is good enough.
I would imagine that the actual optimal screen current for other amps
depends to a good degree on the ratio between screen and plate voltage,
and of course on the specific tube.
The old handbooks, and the tube data sheets, also list recommended values
for all voltages and currents, under several different operating
conditions.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
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