Mirko, YU1AD writes:
I am writing to give information about a very special Russian-made transmitting valve, named Gi7B (sounds like some special event station from Northern Ireland!). This is a coaxial-type valve, apparently developed to be primarily used in radar systems, but it is very useful for any frequency from zero to nearly 3GHz. It is a valve with ceramic insulation (pink colour!) and has a substantial heatsink at the anode connection. You can see the valves outline and the set of characteristic curves, in figure 1. Other electrical data are given in table 1.
Table 1: Gi7B valve data
|Eff. cathode current mA||-||400||600|
|Max usable freq GHz||2.7||-||-|
|Output power W||-||-||350|
|Cap Gg-k pF||10||11.1||12.2|
|Cap Ga-k pF||0.055||0.075||0.08|
|Cap Gg-a pF||4||4.6||5.2|
It is a directly heated triode, with the heater connections on two bottom round-type connectors, so it can be easily connected with the aid of appropriate shells. Above that is the grid connection, which fits a brass or bronze machined flange fixed to the chassis with, say, four M3 screws, giving a very good ground connection for a grounded grid amplifier. Above that broad round area, below the heatsink, is a silvered metal connection for the plate, which could be realised with an appropriate shell. Naturally, this valve requires forced air cooling, with a good squirrel fan, to blow the part of the valve which is above the chassis.
Figure 1 - Gi7B valve outline and characteristic curves.
Up to this moment it might sound like a fairy tale, until we discover how cheap and easily obtainable it is. A Yugoslavian ham, who runs his own trade business, has about 100 of them available to be sent the UK (note that Mirkos letter was written in May 97 - Ed).
Figure 3: G3MY's cooling chimney - a flower pot!
The price for each valve is 40 DM, plus p/p. For two valves you should send a bank cheque (not a personal one please, since they take quite a long time to clear) for the sterling equivalent of 100DM (currently about £36). If you order four valves, send the equivalent of 190DM. This is the limit for one order, since four well-packed valves weigh 1.85kg which is near the limit for shipment. If you need more, they will be shipped in two packages. I am guaranteeing that the business is absolutely legal and no-one will lose any money. If they are sold out, cheques will be returned.
Figure 2: The interior of Mike, G3MY's Gi7B amplifier.
With each shipment you will get a photocopy of an article from the Hungarian magazine Radiotechnika describing a linear for 144MHz with full schematics, plus the drawing of an appropriate flange which has to be turned on a lathe. One tip: after making the flange to fit the grid area of the valve, make a vertical cut in it and after dropping the valve in put an appropriate shell around the flange with the facility to tighten it with an M4 or 4BA screw. That will keep the valve very firmly in place and ensure solid grounding of the grid.
Needless to say, this valve is ideal for 50MHz linears as well!
The address for ordering is:
Imre Salgo, YT7RIT,
Hajdukovo 12, 24413 PALIC,
Kod Subotice, Yugoslavia.
Mike, G3MY has a daily sked on 14MHz with YU1AD. He has bought four of the valves, and writes:
Mike, GØIMG and I are both using the Gi7B tube on 50MHz. Mikes runs at 400W PEP output with 2.5kV on the anode and mine is a mini version with 1.5kV and 225W PEP.
I thought you might like some photos of this prototype, including the cooling chimney make form a small fibre plant pot cut to size - it cools the valve very effectively even at 400W+ output.
My amplifier uses a fairly conventional grounded grid circuit with zener bias. 15-16 watts of drive from the IC706 gives 225W output. In Mike, GØIMGs amplifier the same drive at 2.5kV pushes output to 400W+ PEP.
My power supply is a full wave voltage doubler with 560V AC giving about 1.35 to 1.4 kV on load. This and size limits me to the 225W output, but at 2.5kV maximum output before significant compression appears to be about 560W PEP - well within the ratings of the valve.
At the price they must be the gift of the century! And they are rated to 2.5GHz+!
While we have no reason to doubt the bona fides of this offer, please note that the UKSMG can accept no liability for any problems you may have obtaining or using these valves.
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