[AMRadio] UPDATE: Testing Transmitting Tubes


Merz Donald S merz.ds at mellon.com
Tue Oct 19 12:48:33 EDT 2004


Just thought I'd throw out an update on this. I am building an emission tester for older, mostly 800-series transmitting triodes. It will be able to measure emission current at zero bias or adjustable bias (using an external bias supply) at any plate voltage up to about 2500V. The HV is full-wave, lightly-filtered DC. Filament voltage is adjustable 0-10VAC. I have an old metered, rackmount regulated supply that uses dual 6L6's for regulation. It will do 100V--so he becomes the bias supply.

I gutted an old Beckman, tube-type frequency counter to use the cabinet and chassis. I just happened to have this unit laying around here. It has a unique cabinet design with sides that come off and modular chassis construction--even though, underneath the covers, it's a conventional 8.75 inch high, 19" rackmount unit. This design allowed me to remove sections of the chassis to accommodate the 2 big Variacs and the big HV iron. The top of the cabinet pops off with just 2 Dzus fasteners. I am going to put holes in the top with the tube sockets underneath, mounted on an inverted chassis bolted to the top cover. This way the whole unit won't have any protrusions or non-internal wiring and none of the tube socket connections will be exposed or accidentally accessible. 

I drilled and painted the panel Sunday. I then immediately discovered that the 0-300ma meter I was using did not have enough range at upwards of 2500V. I needed at least 6-800ma. So I replaced that meter with a 0-1000ma Simpson. But of course, the hole size is different. So now I have to cut a larger meter hole on my already-painted aluminum front panel. I took comfort from the fact that the black wrinkle didn't turn out that great anyway...

Most of the parts for this thing came from a supply I bought at a hamfest in 2003. For $40, it had a 7.5 amp 120V Variac, 2700V transformer (amps rating unknown but probably in the 500ma range), 4 K2AN silicon HV rectifiers and a bodacious capacitor bank of paralleled 800MFD, 450VDC caps. The meters I am using are from the junkbox (note to self: Don't buy any more meters at hamfests...), as are the sockets and miscellany. But anyone seeking to build one of these with new parts would have a big bill on their hands. Check the prices on new 7.5 amp Variacs these days...

I expect to be able to test to following tubes in this unit: 203, 211, 805, 808, 810, 838, 852, 25T(G), 35T(G), 100TH, 250TH and RK-65. The test is far from definitive. But it will tell me whether the spares I am saving are worth saving.

The top cover is big enough that I will still have some space for tetrode and pentode sockets. Screen and suppressor supplies would have to be external. But that is phase II--or something I may never do at all.

73, Don Merz, N3RHT


-----Original Message-----
From: owner-glowbugs at piobaire.mines.uidaho.edu
[mailto:owner-glowbugs at piobaire.mines.uidaho.edu]On Behalf Of Merz
Donald S
Sent: Tuesday, September 21, 2004 3:27 PM
To: 'Amradio (E-mail); 'Glowbugs (E-mail)
Subject: GB> UPDATE: Testing Transmitting Tubes


Well, here's where I am with this...

-- The guy who built the tester that AES is using says that he has some information on that unit that he will pass along.

-- Another guy says that he has built an adapter for the Tektronix 577 curve tracer for transmitting tubes. He says that he will write this up for Electric Radio.

-- Two guys said that they built custom testers for the 4CX250 tube. One of them was expanded to test other tubes and was demo'ed at Dayton. The other was used in the guy's job. No details seem to be available for either of these.

-- I checked the AWA index and found only one reference to testing transmitting tubes. It's a 50 word mention on page 21 of Volume 22, number 1. There is one diagram. The test the guy is proposing is a minimal emissions test using a known good tube as a reference. He does this for triodes only using AC both on the filament and on the plate & grid (which are connected together). It has the virtue of being simple. But it hasn't many other virtues. 

-- Finally, there is the e-mail below from the AWA's Ed Gable. This seems to be closest to the lines along which I am thinking--a scaled down emission tester.

The GE Ham News Hartley oscillator tester is more complicated than I want to get into. And the Ham News static tester is very similar to the AWA tester described below. 

So this is where I am headed, though still open to better ideas. Keeping it simple, so triodes only at this point.

73, Don Merz, N3RHT


-----Original Message-----
From: Edward Gable [mailto:EGABLE at rochester.rr.com]
Sent: Monday, September 20, 2004 2:11 PM
To: Merz Donald S
Subject: Re: Testing Transmitting Tubes


Hi Don:  We built a tube tester for Big TX tubes at the AWA Museum.

1.  Use big filament Xfmr and variac to accomodate any FIL voltage.  Use FIL
voltmeter.
2.  Use variace on HV supply for 0 to about 1500 volts.  Need about 300 Ma,
but not too much filtering.
3.  Put voltmeter and ammeter in HV supply.
4.  Parallel wire a bunch of convenient tube sockets and allow for clip
leads for socket-less tubes like 833.
5.  To test, get tube data for tube.  Look at chart and see what the tube
current should be for a
tube with zero bias.  For example, a type 810 might draw 200 Ma @ 1500 volts
with zero bias.
6.  Connect the grid to the filament (zero bias condition)
7.  Turn the FIL on and set proper voltage for the tube under test.
8.  Turn up the HV variac and watch the current & voltmeters.  See if the
current vs voltage is close
to the spec.  It varies a lot from tube to tube, but low emission,
non-useable tubes really show up
this way.

SAFETY FIRST !!!

73,

Ed Gable  k2mp
Curator, AWA Museum

----- Original Message ----- 
From: "Merz Donald S" <merz.ds at mellon.com>
To: "'Amradio (E-mail)" <amradio at mailman.qth.net>; "'Baswaplist' (E-mail)"
<baswaplist at foothill.net>; "'Glowbugs (E-mail)"
<glowbugs at piobaire.mines.uidaho.edu>
Sent: Monday, September 20, 2004 10:19 AM
Subject: Testing Transmitting Tubes


> Has anyone built a transmitting tube tester? Or seen any plans on the web?
The only thing I have seen on this subject is the issue of GE Ham News that
covers the subject (Vol 6, #3, May/June, 1951). I have some power supplies I
can use to run the static test. But I need to build something to hold the
tube sockets, filament transformers and plate current meter. I hate to
reinvent the wheel and would be happy to follow a pre-existing design if I
could find one...
>
> Any ideas?
>
> 73, Don Merz, N3RHT
>
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