Date: Sat, 15 Sep 2018 20:02:34 +0000
From: "Dr. Andres Goens" <YS1AG@hotmail.com>
To: "amps@contesting.com" <amps@contesting.com>
Subject: [Amps] PASSIVE NEWORK CATHODE INPUT
<Greetings guys. I am building an amplifier around a GS35B. Recently I came
across the schematics of Heathkit SB230 using a passive network to the cathode
, doing away with the ubiquitous PI and the need of a cumbersome switch. The
solution is very <tempting but I wonder if is there a catch? Any advice will be
welcome
<Andy YS1AG
## Passive networks sorta work on GG triodes that require very little drive
power, or in the case of a SB-230, a single tube..with high gain. The purpose
of the PI tuned input is to not only match Zs, but also provide for some
flywheel action. The GG triode is only driven 210 degs out of 360.
Basicly its only driven on half cycles, with the remaining 150 degs being a
sky high Z..and no tube conduction. The tube cant source anything, it can
only sink current.
Henry Radio tried a passive network on its 3K and 8 k ultra..and both are a
dogs breakfast. Pssive inputs wont work on a GS35B..which requires a lot of
drive to begin with.
## On a HB amp, you typ have more room to play with. Instead of 9 x tuned
inputs, and either band switched or relay switched, I opted for another
approach, and that was to use a pair of broadcast variables....and a tapped
coil. I used 4 x section broadcast variables, each section is 17-540
pf....then strapped all 4 x sections in parallel. So each variable ends up
being 68- 2160 pf. Both variables are padded on 160m via the dual wafer
small bandswitch used to change taps. The 160m position is not used on wafer
#1, since the precise, exact amount of coil is installed. Instead, the unused
switch position is used to switch in 1500-2000 pf worth of small doorknob
caps onto air variable #1. Wafer #2 is not used for anything.... except
to switch in 1500-2000 pf worth of padders onto air variable #2. Small hb
4 uh coil used, wound with either 6 gauge or 8 gauge solid copper wire.
17 turns onto a 1.5 inch OD former. Space between adjacent turns.
.. then former removed. Most of use GM3SEKs PI / PI-L software to
determine the uh taps for each band. 4 uh was used for 160m, then 1/2 of
that for higher bands.
## We also used a pair of jackson brothers 6:1 ball drives....complete with
0-100 calibrations marks across 180 degs of the skirt. Then the bandswitch
knob in between the ball drives. This all resides below the chassis. Then
you end up with
dead flat 1:1 swr across each ham band. You can get the 6:1 jackson ball
drives with just the small 6:1 drives by themselves.... or complete with
either 2 inch OD skirts, 3 inch skirts or 4 inch skirts...and all marked
0-100, in increments of 1, IE: 1-2-3-4-5.... 100, across the top 180 degs
of the skirt. Then its all silky smooth tuning, just make up a cheat sheet...
and dial it up by the numbers. The above was for a 50 ohm input
Z...3CX-3000A7. To test, we used bird wattmeters on either side of the tuned
input..then into a 50 ohm dummy load. Dead flat swr on all bands. On 15M
only, with 200 watts applied to the input, we only saw 160 watts on the
output side. The 6 gauge solid cu wire ran warm. 15M tap was increased by
just .25 of a turn...and both caps had to be reduced a tiny bit in value to
obtain 1:1 swr. Ok, problem solved. Power shot up to 195 watts on the
output side. On all the other bands, power was 200 watts on the output side.
In the final test, 800 watts of cxr was applied for 10 mins..on each band.
We also tested with 2 kw. It only takes a few seconds to throw the bandswitch
to correct band, then tweak both caps to their pre-described numbers. High
powered PI output consisted of 2 x vac caps and a tapped coil, with a 2nd,
smaller diameter coil used for 20+ meters. To switch the entire mess from
15m to 160m takes exactly 45 secs....which is good enough for my
application. The above works superb, but eats up chassis space, below the
chassis, hence the hb 5 inch high chassis..and RF deck fits into a 19 inch
wide rack.
## Most setups using 9 x separate PI tuned inputs will use a myriad of
relays to switch the 9 x networks..typ all on one compact board. Fixed caps
can be used, but I have never had a lot of success with fixed caps..and an
adjustable slug tuned coil. I ended up having to solder in several caps till
I got it correct. ARCO compression trimmers work a lot easier.... 2 of em
used on each band, plus a small torroid. On the lower bands, like 160 +
perhaps 80m, the arco compression trimmers are padded. A T network,
consisting of 2 x coils in series, with a shunt capacitor at junction point of
caps..to chassis will also work, BUT you end up having to carefully tweak the
value of each coil to get a perfect match... pita...and typ only used on
monoband amps, like 10 + 6M. Even then, you end up with the loss of 2 x
coils.
## Typ bandswitched tuned inputs, with 5-9 tuned inputs, and relay
switched, or bandswitched, imo, use too low a Q, and typ tuned for the
center of each band, so they cover the entire band. However, one board is
fairly flat,and can be mounted below a shallow chassis. On the setup I use
with the pair of air variables and tapped coil, I can use a slightly higher
Q, which results in better IMD, and higher overall amp eff, but it wont
cover an entire band..on the lower bands. No big deal, since the high powered
PI net has to be retuned between ssb and cw anyway. So the cheat sheet
is modified, with several tuned input settings across each band...and ditto
with the PI settings for the high power PI net. The turns counters I use for
each vac cap, not only count each turn, but also the entire skirt
circumference is marked 0-100. Each revolution of both vac caps is marked
0-100....which makes for precise tuning. IF instead, air variables are used
for
both the high powered tune and load caps, then 6:1 jackson bros ball
drives are used..with their 0-100 calibration marks across the upper 180
degs..... then u still get to..dial it up by the numbers.
## If you decide to use GS35B tubes, make sure you have several of them,
like half a dozen at least. Too much effort goes into any hb amp.... only to
find that replacement tubes down the road become unobtainium, or prices have
skyrocketed. GS35B is a good tube, but some will arc between anode and grid
at varying B+ levels. Some are good to 4500 V..and some are only good to
3700 V. They really need to be hi-pot tested before use. You also require
a variable bias supply to set the idle current. Simplest is just a series
string of either 1N5408 or 6A10 diodes. Idle current can be reduced a bunch
on CW, like 10-25 ma....but more drive required to overcome the higher bias
voltage used. You cant cut off the idle current on a GG triode when on CW
mode, you will end up with key clicks. You can cut it off on FM. I wire a
big electrolytic between 1st and last diodes in the series string, like 10-40K
UF. V drop across each diode increases with more plate curre
nt drawn. With the big cap in there, the bias V does not budge. A simple
12 position rotary switch can be used to change taps on the bias diodes. Typ
the 1st bunch of didoes is always left in the circuit, and then the switch taps
every 2nd or 3rd diode. A small spst toggle can be used across the last
diode, for a ultra fine bias adjust. For an even simpler bias switching
setup, a spdt center OFF toggle can be used, which will result in 3 x bias
settings... IE: all the diodes used..when in OFF position, or 2 other bias
settings. 5408 is rated for 3A..with a 200 A surge...and the 6A10 is rated
at 6A with a 400A surge. You can also parallel duiodes for more current
handling. On my test bench, the 1N5408 runs fairly warm with 1A CCS.
The 6A10 runs fairly warm with 2 A CCS.
Later.... Jim VE7RF.
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