The AM Forum

So I put my Magnehelic on the grid chamber of my Dentron Clipperton V, to verify that I'm getting back-pressure to spec. Eimac wants 0.2" of water for their 4CX250B, and Svetlana also wants 0.2" for their 4CX400A. I'm reading 0.3" with either tube, so at face value that seems fine. But I also find that the Eimac air-system socket accounts for 0.1" all by itself, with no tube installed. Assuming that air pressure drops behave like voltage drops in that they are additive in series, that gives me 0.2" across the tube. That's got me thinking, though: the actual airflow through the tube at any value of pressure depends on knowing the airflow "resistance" of the tube, and the "resistance" of the socket is in series with the tube, so both need to be taken together. Do manufacturers typically account for the pressure drop across the socket when calling for a given pressure drop for the tube? I would think that they must, since in actual application you can't use the tube without the socket, and measuring pressure drop between the socket and the tube isn't practical.

This is just a guess, but I'm betting that the specification includes the tube socket.  My reasoning is that in the so called West Coast Handbook series, edited and written by Bill Orr, there are diagrams showing the use of the manometer in a chassis and tube setup, presumably with the matching socket in place.  Orr was tight with Eimac and would have been familiar with their in-house procedures and that's most likely what he depicted.  Some of the Eimac notes that they used to provide have the same drawings.
One old timer whose name I don't recall once stated that "If the tube stays in the socket you may not have enough air on it."  Either way, it would appear that you are in good shape with the numbers provided.
73 de Norm W1ITT

Orr was tight enough with Eimac that he received paychecks from them.....

Yes, the tube is to be installed in the final position when testi g with the magnahelic gauge.  This would necessitate it being in a socket unless using one of the grid flange style tubes that are socket less.  This is info received from Reid Brandon years ago when I posed a similar question.

--Shane
KD6VXI

So I guess I'll just settle for the blower it came with. It's right at spec - like to have a bit more - but cramming a bigger blower in there would involve more drilling and blasting than I want to get into. I bought that amp pretty cheap, intending it to be a quick way to get a hundred watt carrier on six meter AM with my lower powered rigs, but it's already gotten into some "mission creep": metering problems, screen voltage too low, redesign lousy bias circuit - I'm just going to leave it as is and run it.