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Saturday, 26 November 2011

Valve testing.

This is an exchange I'd like to share from the G-QRP email list that some might like to read as it contains some information that I found of use...

Hi Dom,
I cannot offer either a valve tester or some spare valves, but the following might help you to cope without a tester..
My working life started in 1957 as a MN Radio Officer, so I cut my teeth on valves.   Ships were not supplied with any means of testing valves as such, but We managed adequately without.   Generally there are two main types of faults with valves.   Either the heaters goes o/c, or the cathode emission reduces.
An o/c heater is easy to check - either there was a lack of glow in the glass or, if necessary, the test meter can be used!
Low emission can be determined by the valve clearly not conducting as much current as it should, determined by "wrong" pinout voltages.   The tests are relatively simple.  
First check that h.t. is correct.   At the time, being too low usually indicated either valve h.t. rectifier gone low emission or a vibrator with burnt contacts. (Ships of the 1950's commonly had d.c. supplies, or possibly the receivers were run from the 24v energency battery supply - hence the need for the vibrator).
Assuming the h.t. was correct the next checks are voltages at cathode, screen grid and anode.   Assuming there was some resistance in series with the pin, either cathode volts low, or screen and anode volts high can be taken as general indications of low emission.
The next test was simple to a seagoing "Sparks" - try a new spare valve.   Fortunately the "Merchant Shipping (Radio) Rules" dictated that spares were carried to a minimum quantity calculated by a defined schedule.   If found faulty, the offending valve was left to swim home!
In some equipment, I have in mind particularly the magnetron in the radar which operated on a very low duty cycle - short high intensity pulses at relatively long intervals, the manufacturer actually wired in a series resistor, duely shunted by a suitable smoothing capacitor, with the handbook stating the correct voltage drop for a valve having normal emission.
If you want to make a tester, don't get too technical as it isn't necessary.   Uswe the heater and h.t. supllies from any suitable piece of kit you have and using a spare valveholder wire up the base with suitable values of anode, screen and cathode resistors to vause the valve to conduct a modest amount of current, that is well below full dissipation rating for the valve in question.   Measure the current with a milliameter in the cathode circuit (it keeps h.t. from both your fingers and the meter movement).  Record readings of anode current, screen voltage and cathode voltage.   The voltage at the cathode is in effect the same as the negative voltage at the control grid.   Because the cathode is at a few volts above zero, remember to subtract this voltage from the readings you get at anode and screen grid.
Now compare what you get to the characteristic curves for the valve in question, remembering that is the valve was from a "Quality" manufacturer, you can expect the actual characteristic to be within about 5% of the publlished data.   If you are getting anode current readings substantially lower than you should for the values of grid (cathode), screen and anode volts, then the valve is low emission.
If you need valve characteristic curves, feel free to ask, as I have quite a lot of valve data books.
I hope this helps.
Alan   g3rmz  in Plymouth

From: DomBaines
Sent: Friday, 25 November 2011, 12:02
Subject: [GQRP] Valve tester anyone?

Hi all,

Anyone got a valve tester capable of testing the metal 6??? series of octal tubes 6K7, 6J7, 6SQ7, 6V6GT (OK this one is glass)? Either I could borrow or I could send you the valves and you test them for me? Or I could travel to you in person if local to me... I am in Cambridge and not working right now.

I can test if the filaments/heaters are intact with a meter but that is about it and want to know if any of the valves in my HRO-5 are duff or not. I think most are fine and the AF and IF sections seem to be working, the LO seems to be creating a signal as you can pick it up with a MW radio but it is still not working and the signal is lost in the mixer but not really sure.

The PSU heaters are fine, the HT is fine, the capacitors and resistors seem to be testing fine (although building an ESR meter to confirm that)

Or anyone advise a simple series of tests I could do? I'd feel a bit stupid shelling out for a valve only to fine the original is fine.

I have also thought about trying to build one :-)

Or...a nyone built 'The Sussex VT' valve tester? or got a part build they can spare.



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