I agree with what both Jim's replied to Alex P.s comment about trying to
get reasonable input Z measurements using a VNA and a cathode-driven
triode. Same is true of cathode-driven tetrodes, which I use extensively
at work at very high power VHF. There must be some Q on the input
matching (even if the Z looking into the cathode is close to 50 ohms) as
the cathode current pulse is discontinuous if the tube is running in
class B as many are. The instantaneous (across 360 deg RF cycle) Z is
not steady. The part of it that drives the grid to cathode voltage more
positive than the DC bias set for zero signal idling current causes the
cathode current (also anode current) to increase significantly. Input Z
is proportional to E/I, so as I rises, the Z drops. This is, of course,
only during that part of the cycle that the tube is conducting, while
the rest of the time it is higher Z. Thats why larger tubes with
significant current have lower input Z than smaller.
Grid driven tubes present a fairly high Z to the driver stage, in
parallel with the capacitances. The value doesn't change as much with
drive power. Typically we use a matching network to transform 50 ohms up
to that Z and to tune out the capacitive reactance. The untuned input
amplifiers usually add a resistive load across the grid to cathode to
try to make a lower Z to the driver.
One can try and measure it with a VNA, but you need to use a commercial
VNA that has ability to use a separate directional coupler for a test
set. For instance, I have 3-1/8 inch directional couplers for input
power measurement for one of the tetrode amplifiers at work. With the
VNA I can feed the forward into one VNA receiver channel and the
reflected into the other, and with calibration done, I can read the S11
on Smith Chart even. The power in that directional coupler is the VNA's
generator amplified by a stage or two (in this case a 100 kW smaller
tetrode). So it actually does measure the operating impedance into the
big amplifier.
If you can find one, the Delta Electronics OIB2 operating impedance
bridge is made to operate with 1kW of HF power. They sold these to
shortwave broadcasters and military who wanted to measure an antenna
under power. It is inserted in the input to your amplifier, and you just
dial in the knobs to null the meter and read Z from the markings. Delta
also makes the OIB1 and OIB3, and CPB, which can measure impedance up to
50 kW of CW power on medium wave. I had two of them, sold one.
73
John
K5PRO
From: Jim Brown<jim@audiosystemsgroup.com>
To:amps@contesting.com
Subject: Re: [Amps] Starting up a NEW Henry amp
Impedances include the tube(s) as active elements, and their
contributions depend on their dynamic characteristics.
73, Jim K9YC
On 12/12/2023 1:37 PM, Alek Petkovic via Amps wrote:
For what it's worth, I have had terrible problems getting the input
matching right on several amplifiers using the VNA, yet when using
normal drive levels, as recommended in the user/service manuals, tuning
is a breeze. Therefore, I think that there is some truth in the
assertion that tuning at low levels does not work properly.
Message: 1
Date: Wed, 13 Dec 2023 03:18:18 -0800
From: "jim.thomjim.thom@telus.net"<jim.thom@telus.net>
To:amps@contesting.com
Subject: Re: [Amps] Starting up a NEW Henry amp
### In any GG, ( aka cathode driven) high MU triode tube amp, the tube
only conducts for 210 degs when the cathode is driven negative....(think
SB-220 with it's 5 vdc zener bias). When drive is applied to the cathode,
the grid is driven positive..and the tube conducts.... but only for 210
degs, when the cathode is driven negative. The tube is cut off for the
remaining 150 degs.
( the input + output of the GG amp are in series, but 180 degs out of
phase...which is why the same tube used in cathode driven service will typ
have 6db better IMD vs the same tube in grid driven service).
The tube cathode "see's" say 50 ohms or less, over only 210 degs..... and
see's high Z over the remaining 150 degs. With NO tuned input used at
all, and when driven, you can see the distortion easily, with a scope
wired between cathode and chassis. The idea of the PI tuned input is to
supply some flywheel action, so the tube Z at the cathode averages out to
50 ohms ( or whatever the actual cathode Z is) over the entire 360 degs.
With a high enough tuned input Q, you won't see the distortion at all.
When the tube is driven harder, typ the cathode Z will drop a few
ohms...normal. You see that on the bigger metal GG tubes, when used in FM
broadcast service, and also linear service.
I see very little difference in input SWR, when driving my drake amps
initially with low power..like 10-20 watts... vs 100 watts, and in both
cases, the tune + load tweaked for max PO. But the drake amps (L4B's) all
have a much higher tuned input Q vs a SB-220. The C1 + C2 values on all
the drake tuned inputs are much higher vs the SB-220 inputs.
But when a vna is used, or any analyzer, there is simply not enough drive
power for the tube to conduct..and input Z is sky high.
....
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|