----- Original Message -----
From: "Brett Gazdzinski" <[EMAIL PROTECTED]>
To: "Discussion of AM Radio in the Amateur Service"
<[email protected]>
Sent: Monday, November 12, 2007 8:42 PM
Subject: Re: [AMRadio] Re:Reactor Wanted
Modulator or linear amp tubes (I treat them the same here) are designed
to
operate within certain parameters to maximize plate efficiency while
maintaining acceptable distortion levels. Operating above those specs
will
increase power at the expense of tube life and increased distortion.
I am running them at 2000 volts which is where the books say to run them.
No argument there, I wasnt implying that.
I will be using 810's at 725W "advertised" audio to modulate a pair of
813's
at 750W input. There will be a common PS providing ~ 2250V. Its a bit
over
spec for the 813's but nothing that a little air cant cure; the tubes
hi-pot
at well over modulation peak voltages.
I have experimented with mine at 2500+ volts (the deck has filtered forced
air cooling)
and heavy modulation without problems.
Glad to hear it. Ive got NOS 810's (same date codes that Ive had stashed for
decades) and NOS carbon plate 813's.
The problem with that theory is that tetrodes have a higher level of IMD
increase than triodes when run above their spec. Feedback and a low
impedence regulated screen and grid supplies can gain back some of that
IMD.
Another tetrode problem at over voltage is secondary screen emission
which
contributes to even more IMD. Used tubes and especially FM pulls almost
always have screen emission.
I worked extensively with the 4X150/250 family for decades on VHF where
their cheap price resulted in high use in homebrew amps with resultant
very
poor IMD, often in the teens. It took several of us in the US and Europe
to
establish cause and effect and establish correct operating parameters.
Good 4CX250B & R tubes operated at 2500V in SSB service can deliver 28-32
dB
IMD with the above criteria. Thats about equal to an average run of the
mill
triode. It still requires careful tuning and heavy loading which equates
to
using a correct mod xfmr for both audio and RF loads.
I dont see any published specs for 4X150A/7034 operation in either AB1 or
AB2 at any voltage. Have you obtained that data in your actual setup?
Youre
not using the old glass 4X150A I hope.
I have been using new 4x150a/7034's, which I thought were equal to a
4cx250
type tube except for the vhf/uhf zone where the ceramic did better then
the
glass
4x150's.
The ceramic versions can withstand higher seal temperatures which could
relax air flow a bit for less noise. Im building for silence; Ive enough
blower noise on my regular amps from HF thru 2304 MHZ. All I want is glow,
xfmr hum and relay clatter!
The RCA book lists the 3rd order dist products level as 30db and 5th order
as 35 db down,
nor sure how that ties into THD or IM distortion.
I dont see that with my 7034 spec sheet in RCA or Eimac. It is listed that
way for the 4CX250B but I know nobody that has measured that using operating
parameters of the 50's. Also realize that the SSB ratings are for AB1; if
you are running 650W then that is AB2 which will increase IMD. How much I
dont know; Ive only run 8122's in AB2 and they are in the mid 20's.
How much that translates to actual signals on a crowded 75M with all the
other crud on the band I dont know. Its probably no worse than most
riceboxes. On a quiet VHF band poor IMD is very obvious and usually
commented on.
The distortion gets lower with higher plate voltages (in the book) up to
2000
volts, which is as high as I run them.
The screen voltage and the bias voltage are regulated and adjustable.
Except for the frequency ratings, both types seem the same.
Switching between a pair of 4x150a's and 4cx250b's does not change
anything as far as I can hear.
This deck sounds cleaner then any class B deck I have built.
I remember an article in CQ maybe bemoaning the lack of audio use of the
4X150 class and including a building article. Probably still have that issue
in the attic.
Has anyone run a more modern Class B tube such as the 3-500Z or one of the
big ceramic tubes as modulators? Ive also thought of using 572B's running at
2250V since they were originally designed by Taylor (T-160L) as an audio
tube. It would be interesting to compare with the 810's.
Good performance comes by the pound. Tailoring the audio at low level is
a
seperate part of the problem and almost always inadequate in commercial
ham
rigs. My current rig is a Viking I but others will be homebrew using the
best of the old designs and updated if necessary but not going overboard
with high tech enhancements.
The 813 rig takes up two 6 foot rack cabinets, its very heavy.
The 4 UTC cg108's and 109's must weigh 100 pounds each, plus 2
large power transformers, 2 large variac's, all the oil filled caps,
etc....
I have used and abused this rig for over 20 years without a crapout.
(now having said that....)
Sounds like a neat setup, I'll probably run into you on some band soon. Im
limiting myself to one cabinet per station even if it takes some creative
metal work. At least the 813's are bandswitching. The HK354's for the 30's
station will be PP and maybe just a single band, I dont relish changing
coils anymore.
Carl
KM1H
Brett
N2DTS
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