03-08-2011, 05:37 PM
(03-08-2011, 09:58 AM)tiv48 Wrote: a lovely simple circuit transformer may prove a pig to get hold of
rob t
As shown, it would need to be custom wound and would be prohibitively expensive.
But it wouldn't need to be just one transformer.
Many mains transformers from scrap AC radios could supply the HT, 6.3V and 5V. A separate transformer for 2.8V and 1.4V could easily be wound. EG, remove the secondary from a scrap transformer with a good primary, then wind on ten turns and check the voltage. If the voltage was say 2.1V, (just a random figure for the sake of example). Divide 2.1V into ten turns and we get 4.76 turns per volt. If we want 1.4V then 1.4 x 4.76 = 6.66 turns, near enough to 6.5 turns. Hence, 6.5 turns will give us 1.4V and 13 turns will give us 2.8V. Of course, that's under no load conditions, so it would be necessary to experiment a little. However, 1.4V valve heaters generally draw no more than 50mA so I doubt the voltage under load in this example would fall much. To check that out, using Ohms Law, 1.4V/0.05A = 28 Ohms, so, rather than risk a valve, by placing the nearest preferred value resistor - 27 Ohms, across our 1.4V winding to simulate a valve, we can check that the voltage remains at 1.4V. As to the Wattage of that test resistor, again, using Ohms law: I x V = W, so 0.05 1.4 = .07 Watt. Hence, any small 27 Ohm resistor would suffice in this example to simulate a 1.4 V valve heater. Having determined the number of turns for 1.4V, the same number of turns added to it would give 2.8V.
For applications such as a valves tester, the tester is used only briefly, so ratings aren't as important as they would be for continuous duty.
The circuit shown looks quite simple, and for anyone wanting a valve tester but unwilling to fork out the astronomical sums they seem to fetch nowadays, it would be a good circuit to build, but would of course need to be adapted for UK 240V mains input. It could be built cheaply from bits and bobs, but of course you always end up with a rats nest of wires to the valve holders and switchery, so there's quite a lot of teduious work involved. Although I have a Taylor 45 Valve Tester, I've used it little over the years. It's mainly output valves and rectifiers that seem to decline in performance or fail, and that should be apprent from their performance is use. Substitution for a NOS valve (unless it's very rare and very expensive, which most valves aren't), will confirm that. Obviously you only need a multimeter to check for open circuit valve filaments, or for heater/cathode shorts.
With all valve testers, it's necessary to be cautious with the settings, or an otherwise perfect valve can be wrriten off simply by botching the test.
Just my thoughts on the subject.
David
