September 2003

TRF Sets: Receive Frequency

Last time I asked if anyone knew a method of measuring the frequency to which a t.r.f. receiver was tuned. From the (two) replies I received - plus my own ideas - it seems one method would be to shock excite the tuned circuit and then measure the frequency of the resulting oscillation. However, there's doubt as to whether the decaying oscillation could be measured reliably.

Another method involves the use of an automatic grid-dip oscillator. Position a g.d.o. coil near the tuned circuit and automatically sweep the g.d.o. over the receiver's tuning range. Once a dip is found, measure the g.d.o. frequency. But could the dip be found with sufficient accuracy?

If anyone else has any ideas, then I'd be glad to hear them. Remember, the frequency would need to be measured 'on the fly' and not rely on any previous calibration.

Using Ex-Military And Ex-Television Valves

My stock of 6BQ5s is running low so I've been looking for another n.o.s. alternative. While looking, I came across the 6BZ7, a curious valve made by General Electric (GE). It's electrically equivalent to two 6BQ5s in one envelope. It has an octal base, which unfortunately severely limits the number of electrode connections that can be brought out. The cathodes and suppressor grids are tied together, as are the screen grids. The valve is not useful for replacement purposes, but it can be used instead of two 6BQ5s if you're building an amplifier yourself. No ultra-linear operation though. Pity that.

Military (CV-Numbered) Valves

Why am I telling you this? Well, it pays to look around. Although some valves are still being made new, many n.o.s. valves are becoming expensive. Fortunately, there are other n.o.s. valves available and some are very similar - if not identical - to well-known types. This particularly applies to ex-military valves; those valves with CV numbers. Almost all CV-numbered valves have commercial equivalents, so it's worth checking through any sale lists as a CV-version may be on sale at a considerably reduced price compared to the commercial version. They won't be supplied in nice colourful manufacturers' boxes; they'll either be supplied in plain white boxes, or simply wrapped in soft padding. Don't let this put you off, the valves themselves should be fine. If you haven't got a CV-equivalents list, then get hold of one (there's plenty on the Web). By the way, n.o.s. = 'New Old Stock'. That's valves which were made years ago but have never been used. They're the best valves available but there is, of course, a finite supply. One day they will all be gone.

I do have one word of caution though. It seems the military are releasing stocks of valves which were manufactured in the early-to-mid 1980s. In some cases, I've noticed that these valves don't look up to the standard that I'd expect of devices made for military use. I don't know about their electrical characteristics, but physically, some look less well made than those valves manufactured in the 1970s and before. In fact, a few have electrode structures that look skewed. I'm tempted to think that these valves were made in Russia or the Far East, but the valve codes on at least some of them, indicate they were made here in England.

Television Valves

Valves designed for (UK) television receivers are also worth investigating. There are still a lot of them around and few people want them, so they're often quite inexpensive. While some are too specialised to be of much use, the only real disadvantage with them is their 300mA-series heaters. However, that's no longer the problem it once was. Nowadays, h.t. and heater transformers are usually separate, so a suitable l.t. transformer and a resistor or two, is all you need to get them glowing nicely.

There are another couple of advantages in using TV valves: in the main, they're good performers and they can operate with a low h.t. In fact, most were specified to work from a supply of 170V or less. This kind of voltage can be obtained from a low-voltage transformer (say 20-0-20V or 0-40V) driving a voltage quadrupler. Keeping the h.t. rail below 200V avoids any problem of resistor breakdown, and suitable capacitors are readily available and inexpensive.

Valves intended for series-heater chains are specified by heater current and not voltage. Always adjust the heater supply to provide each valve with its rated heater current, and let the heater voltage look after itself.

Occasionally, a 300mA-heater TV valve will have a 'twin'. The only valve I know that has an identical twin is the PCL86. Apart from the heater rating, the valve is identical to the ECL86. The pentode section of the PCL83 is identical to the pentode section of both the ECL83 and the UCL83. The latter valve having a 100mA-series heater. In fact, the ECL83 is the same as the UCL83 save for the heater rating. But most interesting, the triode section of the PCL83 is the same as a 6C4, which in turn is half an ECC82. If you do decide to try a TV valve or two, remember to get hold of a full data sheet (see links below).

General Electric Identification/Date Codes

I have a few 6BZ7 valves, all made by GE (although two have RCA and Zenith printing on them). Under the '6DZ7' identification, there are some dots. You can see them in the image on the right (I've enhanced the dots to make them stand out). They form a pattern and seem to be an identification or date code. Does anyone know exactly what the dots mean?

Data on valves is available on the Internet. Try Steve Bench's Tube Search page. There you'll find an index of much of the valve data that's available on the Internet.

One site that frequently pops up is Frank Philipse's Tube Data pages. Frank now has over 6,500 data sheets on-line, and has brief data on a further 2,000 types. This site is an invaluable resource and Frank really ought to get a medal!

There's a very interesting description of how valves work at http://www.john-a-harper.com/tubes201/.

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