strange ZA5502

At a flee market for radio parts I bought a ZA5502. It seemed to me an interesting valve because it has two penthode systems in parallel. Because I saw that the paint at the balloon was vanishing, I did scratch the number also in the socket in order not to loose the number. Now, after many years, I am measuring all old valves and did do so also with the ZA5502. When looking for the tube in the museum, I saw that it was a 2 Volts tube while I did note it was 4 Volts. Also, the base is an octal and the top connection should be fat, while mine had a Europe 4 pin base and a top connection like the A442.

I started to measure the filament, and at 2 Volts, the max cathode current was only 2 mA. The current came up too slow to be correct. The ZA5502 should take 300 mA at 2 Volts, mine did take only 130, which does not seem correct for two systems of a power tube in parallel.

I did observe the filament colour in the dark, and it was not visible. Than I started to turn up the voltage and at 3 Volts a very faint red became visible at the filaments. The Cathode current increased rapidly above 2.5 Volts and became 70 mA saturated at 4 Volts. The filament current was than 240 mA  A voltage change of +/- 0.5 Volts did not change the cathode current significantly. At 4 Volts the colour of the filaments were normal dull red and did compare with the colour of other valves at the correct voltage. So it is definitely a 4 Volts tube while the ZA5502 should be 2 Volts. The filaments of the two systems are connected in parallel inside the balloon. So the tube is different with respect to the filament voltage wich makes rebasing a little unlikely. I do not see any markings that show rebasing. There are no brand indications visible at the tube, they may have been vanished.

Does anybody have an idee where this tubes come from? A prototype?

• strange ZA5502 (38 KB)

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Dear Nicolaas,

ZA5502 was just the British Army store reference for the ATP4 transmitting pentode. Later this code was replaced by the unified British service code CV1366. I do not know where the 2 volts filament value is coming from. Anyway the CV spec for this tube, available here, gives a filament value of 2.6 volts. Oxide coated filamentary cathodes can operate at quite low temperature. Your sample does not match even the current given in the same spec, 0.27 to 0.33 amps, requiring 4 volts to operate at a mere 240 milliamps. Maybe that your tube is well beyond the end of its useful life with part of the filament evaporated. This could explain the higher voltage required to operate at a current anyway under the value specified for it.

The open question now is to see if any wrong filament values were loaded into the CV specs. I am quite sure that 2.6 volts is the right value, since the CV specs were always used by Military to run the acceptance tests of any lot delivered through the years by tube factories. Any mistake would then have resulted in 100% rejects, so to cause an immediate corrective action.

Regards, Emilio

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Dear Emilio

Thanks for that information.

I did inmedeately check how it does operate at 2.6 Volts. The filament current is than 170 mA, so far too low. I do not see any glowing in the dark. The cathode current (G2 and anode connected together) is than 8 mA. At 4 Volts, I see ALL filaments glowing in the dark, a dull red, not visible in the light. Compared to other valves the same colour). The cathode current (at 100 Volts) increases at 4 Volts to almost 70 mA. That really does not seem an end of life current to me, quite an hefty current. The anode current is at 150 Volts 33 mA with the G2 at approx 95 Volts. Also a normal current.

Kind regards

Nico

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Dear Nicolaas,

no doubt about results of your measurements but test data for this tube are detailed in its CV spec sheet. This is a partial reproduction from page 2 of the said sheet:

All listed tests but the first one are to be run on 100% of the tubes. The filament current test can be alternately run on sample lots and must give a read from 0.27 to 0.33 amps at the minimum operating filament voltage of 2 volts. The table gives also test conditions for measuring other parameters, such as the anode current or the reverse grid current. These were the tables used to accept CV1366 tubes. Any tube returning a single parameter outside the specified range was to be discarded.

Of course you can stimulate the emission of a bad tube simply rising its cathode temperature. I remember of some CRT rejuvenators, usually step-up transformers, that in the '50s gave new life to exhausted B/W television CRTs. Try to repeat your test setting the conditions as specified in rows c and g of the above table.

Regards, Emilio

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Dear Emilio

I did test according the list that you did gave me. But as I did write, at 2 volts the tube takes only a few mA. At 4 volts, the value comes close and the tube takes over 20 mA.

Please remember, at 2 Volts the filament only takes 130 mA, far too low. At 4 Volts it takes a little below 240 mA filament current..

It should be nice to know if you see the filament glowing in the dark of an other ZA5502 at 2 Volts and at 2.6 Volts Mine is invisible at 3 Volts and below. I never saw a tube that does not glow visible in the dark.

I also did revive old B&W TV tubes with transformers. With exhausted thoriated tungsten transmitting tubes the trick works a lot of times as well in order to migrate the thoriumm to the surface of the filament.  But with vintage tubes I don' like to do that if the cathode glow becomes more bright than normal. Rather an exhausted tube than a toasted one!

Kind regards

Nico

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