[Amps] Starting up a NEW Henry amp

[John Lyles]

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 at audiosystemsgroup.com>
> To:amps at 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 at telus.net"<jim.thom at telus.net>
> To:amps at 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.
>
> ....