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[Amps] Phase and amplitude of 2 tube amplifiers

Subject: [Amps] Phase and amplitude of 2 tube amplifiers
From: John Lyles <>
Date: Thu, 13 Oct 2022 23:08:12 -0600
List-post: <>
Over the past decade I designed a system with three high power transmitters that each use a pair of tetrodes in combined operation. These are TH628L, which run at 23 kV DC each, and produce about 1.5MW pulsed per tube, average power is approx 180 kW per tube, at ~200 MHz. Combiners and Splitters had limited possibilities and this is a fixed frequency operation for driving high Q cavities for a particle accelerator. As such, reflected power can also be horrendous at times. I chose quadrature combiner, with a 4 port hybrid custom made by a company in 12 inch diameter hard coax. The combiner is called a branchline coupler. The splitter is also a hybrid, fed with 3 1/8 inch hard coax. It uses a pair of coupled lines 90 deg long inside a box. Both of these are 90 deg devices. If the amplifiers are running at about the same power, with the same voltages, the phase shift through them is similar enough. We built a custom vector phase meter that measures the real time phase difference between the PAs. When I ramp the power up from zero, there is approx a 10 deg phase variation in each amplifier but its repeatable and the same in each one. Once the combined system is near the optimal 2.1-2.8 MW total power, the phase variation vs power level is a few degrees at any time.

Since we have VSWR all times, the length from the load (source of the reflected power) to each tube is off by 90 deg if the lines were all cut the same length. This would create a messy operation, as one tube would respond differently than the other due to the phase of the reflected power being different. To fix this, there is extra piece of line on the output of one amplifier, so that they are both in phase with respect to the load, but there is a similar 90 deg line on the opposite input splitter port. This maintains the 90 deg relationship of two feeds to the combiner, while staggering the amplifiers apart by 90 deg so that they both respond to VSWR the same. It works well like this.

We use the screen voltage (from separate power supplies) as a knob to adjust the gain of each PA to be the same. Could also use the ouput loading control, but that introduces a phase change. There is a 3-1/8 inch coaxial phase trimmer, a line stretcher, after the first splitter that follows the driver stage, in the feed to one PA input side. On the other side there is a nearly identical line without adjustable phase. We have run these transmitters for 6-8 years and not seen problems due to phase variation. In fact, the curves of phase vs power falloff, of a 90 deg combiner is quite tolerant. I monitor the 4th port of the hybrid combiner, which feeds a water cooled resistor. The power on this port occasionally gets to 5 kW peak which is nothing when the total power is 2.5 MW.

It is important to note that our system is a fixed freq setup, where you will likely be tuning around. I can imagine that things get more complex when varying frequency and also maintaining phase. Using in phase combining and Wilkinson devices will help there. If you are driving two antennas for circular polarization, small phase difference should just alter it to be more elliptical polarization.

You can find some images from a 2017 paper on this system here. Let me know if you want any further info on this.




Date: Thu, 13 Oct 2022 12:37:10 +0000
From: Conrad PA5Y <>
To: "" <>
Subject: [Amps] Phase and amplitude of 2 tube amplifiers

I am going to attempt to use 2 identical monoband tube amplifiers to generate 
CP but before I start I have a question. The 90 deg delay for CP comes from the 
array orthogonal physical boom separation. So the amplifiers must be in phase 
at the outputs.

I presume that this has been done in broadcast applications and I would like to 
know, assuming everything is tuned correctly how close the phase likely to be?

I have line stretchers for the inputs, I have extremely good phase matching on 
the Wilkinson splitter and 2 identical directional couplers that I have tuned 
for good phase and amplitude matching. I can also measure the relative 
amplitude and phase of the 2 amplifiers accurately. Small phase and amplitude 
adjustments are ok but I have no idea what to expect with tube amplifiers.

This was not really planned but a 2nd amplifier became available, so I thought 
why not? The tubes are becoming scarce so this way I can be a lot more gentle 
on them.

Regards, Conrad PA5Y
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