[AMRadio] Transmitting Triode Tester Update

Merz Donald S merz.ds at mellon.com
Tue Dec 28 17:11:51 EST 2004

Several months ago I mentioned that I had started to build a transmitting tube tester. The goal was to test some old triodes, mostly 800-series and early Eimac. I found the following sources useful: AWA OTB 8/2001: Revisiting The Power Tube Testing Breadboard, and G.E. Ham News, May-June, 1951: Transmitter Tube Testing. I also received some useful help from Ed Gable at the AWA Museum, the guy who built the tube tester used at Antique Electronic Supply (whose name I have shamefully forgotten), and various members of this excellent list.

I built the entire thing out of the junkbox. I won't go into it too much here as I am planning to write it up for ER. But it's got a couple small problems. The KVDC meter is reading high. 1KV on the Simpson 260 is 1.5KV on the tester meter. So the multiplier is mis-calculated. I thought the meter was a 1ma movement. But I guess it really is a 3ma movement, just like the scale says... The HV transformer I used has many taps and I must've picked the highest ones because one touch of the Variac and you're at 350V. At 25% of the knob travel, you're at 1KV. So I think I will find some lower taps to use...

Otherwise, it works well. It's an emission tester only, of course. By default, the grid is connected to the filament so tubes are tested at zero bias. The tester has provision for connecting an external bias supply. A front panel toggle switch actuates a relay that breaks the grid-to-filament connection and patches in the external supply from a connector on the rear panel. The bias supply, and maybe even a screen supply could be squeezed into the cabinet at some point in the future. But the front panel is full. So a separate, external meter panel would have to be provided. No matter. I have no plans to get into tetrodes or pentodes anytime soon. 

First tube tested was a 203A. Filament lit, but it has very low emission--a bad tube. Second 203A tested was a dud--no filament continuity. Third tube was a known good, new-old-stock 203A which delivered a clean 180ma at 1250VDC and showed a perfect match to the zero bias curve as the voltage was varied. Obviously, it is a brand new tube, as advertised. Last one tested was a VT-4 (211) which basically has the same specs and internal design as the 203A. It also performed like a brand new tube, despite coming from a hamfest junkbox last year.

This project required more cutting, drilling and sawing than I would have liked. It seemed like I needed every power tool in the shop. Those damn meter holes again.... Plus brackets for this and brackets for that, ad infinitum. My paranoia about HV wiring didn't help things either. I've got a ton of wire ties in the thing to keep the HV wiring from shifting around.

In terms of the time it consumed, it's a project that I would say is equivalent to building a medium-power CW transmitter from scratch, with all parts on hand. In terms of cost, you need a pair of beefy 7 or 8 amp variacs plus a 200ma, 1.5KV HV transformer. If those are in your junkbox, then you've probably also got most of the other stuff lying around too. Weight will be an issue, so aluminum chassis is not advisable. The safety requirement is an irritant. Full plate voltage appears on multiple tube sockets and on the plate cap connector. Gotta have safety precautions in mind and in practice for dealing with that. The KV meter circuit acts as a bleeder so you can watch the HV bleed off the caps when the HV Variac is zeroed out. Oh, and try to remember that a 211 cranking out max plate current for a couple minutes is likely to be, as they say, "warm to the touch".

Bottom line: If you keep a lot of triodes around and want to be sure that they are worth keeping around, then this is probably a worthwhile project. Otherwise, I'd just test my tubes in the rig. That's the real test anyway.

I am taking photos but will hold them until I see if N0DMS wants this.

73, Happy New Year!
Don Merz, N3RHT
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