Varian VTH-6079H1 (10W, 4-10GHz) Anyone use this TWT on 10.368? - Page 1
Forum section: electronics / rf-microwave

author: Uky
author_rank: Regular Contributor
timestamp: May 03, 2021, 03:29:29 pm
content: Greeting Forum members.I have been given a few VARIAN VTH-6079H1 Travelling Wave Tubes specified at ~10W between 4-10 GHz.The tubes are supposed to be only slightly degraded. The question is if this tube stillperforms at 10.368 or if it drops sharply above 10GHz. Following the spec-sheet, the gainis 61.8dB at 6.5GHz, 41.2 at 9.5 and 40.7 at 10.0 with a saturated Pout of 40.7dBm at 10.0Has anybody been able to use this tube on X-band?

author: coppercone2
author_rank: Super Contributor
timestamp: May 04, 2021, 06:15:20 am
content: you will get a drop but if its operating band is 6GHz and you go over by 400MHz you will have a drop. Not sure how much, 8dB maybe (like 2 watts out)? It is sharp in most tubes IIRC. When you keep going over its gonna fall hard. What you might also have though is distortion, but I never tested the limits. Pretty paranoid about those things, their not easy to find parts for. I am only interested because of power dependence of device characterization.Operating a 'linear' amplifier on the 'cliff' region of the curve is not really advisable. Their not really saving themselves on the curve, or trying to sell more amplifiers, they want to give you the best bang for that kind of money, so that thing gets pretty weak pretty fast. It's kind of a bastard of a device if you work on it, considering what it is, and what its connected to (that part runs on 3.3V and the other part runs on like 2kV+ lol, they know no one wants to buy it in the first place, the last thing they want is a 'derating conspiracy')

author: radiogeek381
author_rank: Regular Contributor
timestamp: May 04, 2021, 09:21:38 pm
content: Whether this tube can be wiggled or not is a matter of experimentation. It may work just fine out-of-the-box. I've had a couple of other tubes (watkins-johnson and collins) that did fine out-of-band or at the nominal edges of the spec.Back when Marconi was still fighting with his homeowner's association, this article: "Jim Vogler WA7CJO: Optimizing TWT Power Output for Narrow Band CW-SSB Operation" appeared inMicrowave update '94,the Central States conference proceedings in '93and some version of it in the Proceedings of the 9th int. EME conference, Ro de Janeiro 2000.If it is the paper I'm remembering, he describes a number of techniques for tuning the tube with magnets to adjust the band edges. It apparently takes some patience, but only rudimentary test equipment.Oddly, I can't find my copy of Microwave Update '94 anywhere. The league may have it in inventory, as they published it (on paper!) back then.---FWIW, for terrestrial contacts, I found the weight-bother-mechanics product too high to justify the 7dB gain over the 2W solid state amplifiers I had. In general, I work almost all the stations that I hear. On top of that, the control for a TWT is more involved than they make it seem in the movies. At idle, with no input power, the tube makes a whole lot of noise. That's just the nature of the beast. And it conspires against hearing those weak stations.Portability is compromised somewhat by the tendency of TWTs to draw a whole lot of power. That means managing power-on/power-off cycles in ways that you don't need to do with solid state amplifiers.For a fixed station, the weight isn't such an issue. And the mechanics just adds a little more to the story. For EME it might be the only economical path unless you can find a 10W solid state amp that fell off the back of a truck.---I'm assuming you have the matching power supply. Building one of those from scratch is a whole lot more complicated and far more work that it would be worth to anyone I know. Significant enough that you could spend the time instead working nights in a liquor store to earn the cash to buy a 10W solid state brick.

author: coppercone2
author_rank: Super Contributor
timestamp: May 05, 2021, 01:52:53 am
content: HP tube amplifiers also have a gain circuit, unlike the higher power ones, so you can adjust the gain from the front to turn it down if you want. But it also smoked the transformer and its literately the last thing on my repair list (2-4GHZ 1W amp)I never checked if it lowered noise, but IIRC its not an attenuator, they varied the power to something. If you want to learn about the control electronics, that is a good one, because most of the tubes are available (but I never looked into the weird stuff like vaccum relay, but I am sure you can get something modern to replace it). The other more modern ones have a modestly complicated control block with a bunch of normal transistors and something deep and hidden in coating to do the HV stuff, wheras the tube ones, have the HV in the open, so its much nicer for learning, so long youwork safely. Like screw terminals you can probe running at 1500V. they are actually great if you have a big old HV probe, you can actually use it comfortably without trying to fight wire harnesses. I am not sure I found the right picture because its just a google archive, but the one you have looks to be a fixed gain block, without the gain control.It's also noticable for having PCB that have tubes and card edge connectors together. So you get an actual vacuum tube PCB you can take out and swap. Not sure how common it is to have actual tubes on a PCB, its not built like an audio amplifier.I imagine where TWT get interesting is if you get to work on a circuit that goes into a satellite, those would cover all the things mentioned by the above post in a nice way I think.