Screen grid dissipation V peak screen current? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 21, 2013, 12:08:58 pm
content: I am trying to get my head around this. I have a Polish valve (tube) known as a Q-1P/42. It's a ceramic version of the Russian GU43B. I also have a G3SEK (Ian White) "Tetrode Board" kit, that provides interlocking safety features, voltage and current control and clamping, for use in linear RF amps. The kits call for some component value calculations. I can work them all out bar one. The instructions ask for a peak screen current in mA. I cannot find this is any of the spec sheets, but the GU43B sheet does cite a screen grid dissipation of 28 Watts. At the screen grid DC voltage I am using of 350 volts this calculates to a current of 80 mA. Is that the "peak" screen grid current? It sounds and looks high. Looks high as in comparison to some more common valves Ian cites as examples. I think I'm misunderstanding any relationship between dissipation and peak current.... Can anyone help please? Here's the pdf of the GU43B valve specs. Thanks!http://www.gatesgarth.com/gu43b.pdf

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 21, 2013, 01:03:59 pm
content: That discrepancy is actually easy to explain.For better or for worse the eastern European power tubes are generally built with all the qualities of farm equipment: Mechanically rugged, works even in the face of large mechanical tolerances, huge spread in real world performance.The Russians couldn't be bothered to make the internal mechanical structure of their power tubes to terribly high standards. Their philosophy was to build mechanically rugged tubes, inexpensively and relatively quickly, and then design their equipment to compensate for the resulting spread in the individual electric parameters of each tube.In a beam power tetrode like yours, the screen grid current is hugely dependent on how mechanically well aligned the wires of the two grids are. In principle the screen grid is supposed to be located in the 'shadow' of the control grid, as seen from the cathode (this is the 'beam' part of beam tetrode). Doing this hugely reduces the intercepted electron flow of the screen grid, and thus screen grid dissipation. So for a given power level the American tube grids require less metal, resulting in lower capacitances.In an American (Eimac) tube this mechanical alignment is pretty much spot on, to a tiny fraction of a mm. However, this comes at a cost of increased overall manufacturing cost, as the whole structure has to be built to exacting standards, to make the tube sufficiently mechanically rugged.The Russians designed their tubes to not care about this too much. For that reason their screen grids are designed to dissipate higher power, compared to a similar American model, which is what you are noticing. The downside is the huge spread in all parameters depending on the exact grid and cathode geometry, for instance capacitances and overall transconductance.It is the same reason why the Americans specify 0 (zero) dissipation on the control grid for similar tubes (4CXwhatever). Their control grids are gold plated to reduce secondary emission, yet that means the grids needs to be run at zero power at all times, as the gold plating can tolerate almost no heating. The Russians didn't bother with the gold plated control grids, so they can take a bit of heat. And you frequently need it.However, whether the full screen grid dissipation is a good limit to set is a bit hard to say, without access to the constant current curves. You are running the tube at nearly half the maximally allowed screen grid voltage, so it may be that the screen current is quite a bit below the maximally allowed at your chosen operating point. Screen current close to the thermal limit may signal you have another problem, which requires a shutdown. It might be a good place to start though, if you have nothing else to guide you.I would try to find other people's experiences with your tube, and check what the limits are on their actual operating parameters, including during tube-up. Alternatively, try to dig out the constant current curves.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 25, 2013, 08:28:30 pm
content: Thanks for the detailed reply, for completeness the creator of the "Triode Boards" Ian White saw my question on another forum and replied in a similar vein, hopefully any future forum search from anyone else wondering might find this, and your reply helpful. Thanks again ElectroIrradiator!Ian replied: "The "peak" screen current needed for these calculations is the value at peaks of modulation. This is more easily measured as the key-down value at maximum RF drive.The GU43B has a maximum allowable screen dissipation of 28W. At the recommended screen voltage of 350V, 28W dissipation occurs at a screen current of 80mA - exactly as you already calculated. Note that the data sheet does mention a screen current of "<80mA" in the section on AB1 Output.If you set the screen voltage to 350V and the screen current trip to 80mA, it will be impossible to exceed the maximum allowable dissipation of 28W without causing an overcurrent trip.Many Russian tetrodes are specified to operate at higher screen currents that we are accustomed to in Western tubes. It's nothing to worry about, just a feature of their internal design, and in practice they may draw less screen current than the data sheet suggests.73 from Ian GM3SEK "