Date: Sat, 2 Dec 2017 10:12:45 -0500
From: Peter Bertini <radioconnection@gmail.com>
To: amps@contesting.com
Subject: [Amps] Using 813 tubes on 17 meters and above...
<I'm getting serious about building an 813 amp as winter project and need
<some input from other OT's who've played with these tubes on 17 meters and
<above.
Plans are to go with three or four tubes in parallel. Power transformer is
rated 1500va and I'll be near 2.8 kV plate voltage, in grounded grid.
Plate load for two is around 3500 ohms, so I'll be around 1750 or so with
four of the beasts in parallel... which is doable using a Pi L on 80 and
160 to keep the load capacitor size reasonable, and then going over to a Pi
Net for the upper bands. I was more concerned with the combined plate
capacitance, which is going to be close to 60 pF ??? That is value is
over the design value for a Pi Net with a reasonable Q at that load
impedance. I am still very curious how others have made out using two or
three tubes in parallel on 17 meters and above. I've heard, anecdotally,
that tapping the plate tune cap down on the tank coil helps, but I've not
seen that in any published literature. Any problems with using Amidon
T255A-2 core for a combined shorted-turn inductor for 80 and 160 meters on
the low Z side of a Pi-L, and another core for the 160 meter inductor on
the plate side of the Pi L?
Any advice? By the way, I am going this way because I have plenty of NOS
RCA tubes and sockets, and I can always change over to a more modern tube
in the future.
73
Pete
## Are u sure there is 15 – 20 pf of stray C between the anode and the
grounded screen and
also grounded grid ?? You are using a tetrode in a high- mu triodr config,
with both screen +
grid bonded to chassis.
## Download GM3SEKs excellent fee PI and PI-L software. Then you can
introduce a tiny bit
of uh anywhere between the anodes..and the main C1 tune cap. Typ this tiny
bit of uh, like .5 uh,
is inserted between the plate block cap and the main C1 tune cap. As others
have noted, the tube
anode to grounded grid + screen C and this extra coil, form a step down L
network. This transforms the
plate load Z...down to a much lower value.... low enough that a practical PI or
PI-L can be designed.
## To get 1.5 kw out, you require 2.5 kw dc input...assuming 60% eff. With
2.8 kv under load, you require
893 ma of plate current, That equates to a plate load Z of aprx 1844 ohms.
## I would not mess with a PI-L on 160 or 80m. Use a plane PI on all bands,
and just pad the load cap on 160M.
If you use a Torroid, use the type 2, which are red. T-225-2A...and you
require THREE of em. Difference between
the A and the 2A suffix is... the 2A is an inch thick, the A is only .5 inch
thick. Im talking about when the torroid is
laying flat on the desk, the thickness is from table to top of torroid.
With 3 of em, you end up with a mess that is
2.25 inchs OD x 3 inch long.... and thats before you start winding. Its no
light weight. IMO, you may as well use
a piece of airdux for the job, like 12 gauge, close spaced, for the 160 + 80m
portion of a 160-10m PI net. Then use
heavier gauge for 40m, and tubing for 20-10m.
## PI-L is a pita. You end up with that extra L coil that follows the C2
load cap. It has to be shunted out when using the
higher bands. If the L coil is a 160 and 80m affair, then half of it has to
be shunted out when using 80m. Its one more
thing to have to switch, which complicates the bandswitching to no end. It
also puts the load cap at a higher peak V
point, since the load cap on a typ PI-L is 200-400 ohms....and not the 50 ohms
you have on a plane PI net. Using the
software you will see that the PI-L is not all its cracked out to be. The load
cap value will reduce quite a bit, but the
main coil between the tune and load cap will end up larger. You will see at a
glance. The software is very easy to use.
It can also be used for designing the PI tuned input. The only thing u cant
do with the software is to try and match two
identical impedances, like 50 ohms to 50 ohms, say on a tuned input. The best
it will do is 50-49 ohms...or 49-50 ohms.
## when playing with the GM3SEK soft ware, you will fast find what works, and
what doesnt. But you really need a
digital lcr meter to build any hb amp. Without it, you are dead in the
water. You need to be able to dial in the C1, L,
C2 cap on any tuned input..and ditto with the exact values for any kw rated
PI or PI-L network components.
## U also need the lcr meter to actually measure the stray C from anode to
the chassis. This will tell you the exact value
of anode to grounded grid + screen. Do that measurement with all four tubes
installed in their sockets..and all grids
and screen well bonded directly to the chassis. Ok, now that you know the
actual C with 4 tubes plugged in, then
you can input that value into the software. Along with the other parameters,
like loaded Q, freq, plate load Z, etc.
## You will see that on the 12 and 10m bands, it all goes to hell, since your
typ min C1 tune cap value is still too big for
10M. But by increasing the loaded Q, it will work, but a sky high loaded Q
on 10M is not what we want. High loaded
Q just means narrow banded, has to be re-tuned every time u qsy, and high
circulating RF current through the coil, and
also the bandswitch, or roller contact, if a roller coil used. By inserting
somewhere between .2 and .7 uh, between the
plate block caps and main C1 tune cap, the Q can be reduced a bunch. That
drop in plate load Z will be displayed
in software, but u have to play with several vales of loaded Q..and also
several values of that small coil b4 main PI net.
This trick has been around for decades in the commercial world. One caveat,
the tiny coil used has to be the same size tubing
as what you are using for the main 10M tank coil, it has a lot of current
flowing through it. You will also see that the small
coils effect diminishes rapidly when below 10m band. You will still see a
very small effect on 12m, but it will vanish on 21
mhz. The software will depict the transfomed Z on each band. On 10m, where
the issue is, your 1844 ohms will be transformed
down to something like 500-1000 ohms, that a practical PI can be designed
around.
## I have done the extra coil by winding a coil, and also just tapping into
the main tank coil..with the hot side of the tube cap,
both methods work. But u need to know the exact value of coil u are inserting
b4 the tune cap...and the exact value of coil,
between the tune and load cap. Once u are happy in software, build it, it
will work every time. To dry test it, wire a temp
resistor, equal to the original calculated plate load Z of the tube, say 1844
ohms, between anode and chassis. Then with mfj-259
on output of amp, tweak the tune and load caps for flat swr on the MFJ. If
that test works, it will work, with temp resistor
removed, B+ and fil V applied..and drive applied.
http://www.ifwtech.co.uk/g3sek/
Later... Jim VE7RF
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