Date: Sun, 24 Nov 2024 21:24:07 +0200
From: Victor Rosenthal 4X6GP <k2vco.vic@gmail.com>
To: flynth@gmail.com
Cc: Amps reflector <amps@contesting.com>
Subject: Re: [Amps] Question about average anode current in tetrode
amplifiers on higher frequencies.
<Regarding the length of the leads, I had the same problem in my 4CX1000A
<amplifier
<https://www.qsl.net/k2vco/4CX1000A/K2VCO%204CX1000A%20Amplifier.html>
<and I had to completely rebuild the tank circuit in order to get a low
<enough inductance to resonate on 10m with the correct amount of
<capacitance. My problem was the length of the leads between the
<bandswitch and the coil. The blocking capacitor can be mounted directly
<on the tuning cap. Then you only have to worry about the lead from the
<plate to the blocking cap, by way of the plate choke.
<The additional inductance is needed to get a reasonable Q with an
<achievable amount of capacitance. This will require a lower inductance
<in the tank coil.
<You do not want to use thinner wire. You should use flat strap (6 mm
<wide for the bandswitch, 10mm for the plate to blocking cap) to reduce
<the stray inductance as well as RF resistance.
<73,
<Victor, 4X6GP
Using 3mm wire for tank coil taps is a big no-no. It will add stray
L....making 10m operation near impossible. Hence he can't get it to
resonate > 25 mhz. Use 6mm / .25" wide copper straps. You also have
more stray L, via passing through the bandswitch rotor, to the center rotor
hub, then onto the COM...then back to the C2 load cap. From COM of
bandswitch to the C2 load cap should be wide, copper strap. Wide copper
strap has miniscule uh.
The trick with the tiny .1uh to .3uh coil (HD coil, has a ton of current
through it) inserted between plate block cap assy and C1 tune cap
works..... but ONLY if the tube does NOT require a parasitic suppressor.
The way it works with that config is..... the shunt tube C plus the tiny
series coil forms a step down L network. That steps the plate load Z down
to a much lower value......low enough that now a practical PI network can
be designed and built. ( the lower the Z, the higher the C is needed for
the tune and load caps...and the main tank coil between the caps will be
reduced in value.
You can see all of this on the GM3SEK PI / PI-L spreadsheet. It
factors in all stray C, like anode to chassis C, any additional stray C,
like from lower anode fins to chassis...and also stray C from tank coils /
bandswitch, to side / rear walls..and also stray C from tube to rear
/sidewalls. The software will spit out the new transformed plate load Z,
that the PI network will be designed around. You can also change the Q on
the spreadsheet, so you can see the increase in Q on 10m, when you are
forced to use a smaller than normal main 10M coil. With too big a 10m
coil, your required tune + load C will be depicted as negative values on
the software.
You can easily measure all that stray C with a digital LCR meter. Ditto
with all the stray L
Also, use a separate connection from top of plate choke to edge of anode
fins for the B+ connection. Use a separate connection between edge of
anode...to the plate block cap assy. And keep the B+ connection well
away from the RF connection, like 90 degs or more. This will enhance
stability.
But since that tube you are using requires a parasitic suppressor, you
won't be able to use the small, extra coil trick, too bad. Your only
recourse is to minimize all the stay L and stay C as much as possible, then
decrease the value of the main 10m tank coil tap, so the tank will
resonate on 10m, with the C1 tune cap barely meshed.
This might require the use of a vac tune cap with a low min C....or a 2 x
section air variable, like Alpha and other's used, with a tiny 1st section,
with low min C, used on upper HF only.... and both big and small sections
used on lower HF.
I have gone through all this mess several times before on hb amps. That's
why I stick with the metal tubes that don't require a suppressor. That 10
kw, 6M amp ( 3x6 triode) I designed for Scott, used the extra coil trick.
It's the only way to get that tube to work on 6m. In that one off case,
both 'coils' were made of 1" wide copper strap (25.4mm wide) in a 1.2
circle shape...and both at right angles to each other. The 1/2 circle
shape was used cuz the current will then flow on both sides. If a
conventional tubing coil was used, it would have to be made of 1,27" od
tubing.
On the latest 160-15m, 3x6 amp, the L coil trick was used...just to get the
loaded tank Q down on 15m. Instead of a separate coil, the extra .6 uh was
incorporated into the 20-15m coil...made from 1/2" silver plated tubing.
The C1 cap tapped the junction of the extra .6 uh portion.
Be careful when running a high Q on 10m. RF current through the 10m coil
is aprx the plate current times the input tank Q..+ 10%. On the GM3SEK
software, it's the overall network Q that is adjustable. It spits out the
input Q..and also the output Q. Those 2 added = total network Q. High
current on 10m = heat. It's just (I squared) x R...or in this case, RF
current squared X ESR of the tank coil. (ESR = effective series
resistance or RF resistance). Double the current in the tank coil..and the
amount of watts dumped into the tank coil will quadruple.
Watch out with modes used. FT-8 / data modes is an amp killer. Average
DC plate current on data modes is double processed SSB. Same deal, watts
dumped into the tank coil has now quadrupled. The heat can get so great
that the bandswitch contacts will heat up, lose tension, RF resistance in
the contacts will increase, and contacts will cook, due to thermal
runaway....and the ceramic can crack.
In the old arrl books (plus Eimac care and feeding) they just
depicted input Q..and typ a value of '10' was used. In actuality, the
overall network Q is '12'...which consists of an input Q of 10...and an
output Q of 2. The newer arrl books use overall network Q..and typ use
'12'.
Jim VE7RF
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