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Old 13th Oct 2021, 2:04 pm   #1
Techman
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Default Mazda TV250 valve

This valve was in a job lot of valves I picked up recently. It looks like a cross between a PX4 and a PX25, but with (apparently) a 7.5 volt, 1.2 amp heater, so probably closer to a PX25 in characteristics. Pictures below show the valve and its box, which is actually peppered with woodworm holes to one of its sides - this valve has been in storage, possibly in a damp old shed for many decades. Note that like some similar valves, the actual number as printed on the valve itself is T.V.250 and not TV250.

Even though this particular valve with a 7 volt heater rings a bell with me from somewhere, I can find no reference to it anywhere on the internet, other than a mention of it on Radiomuseum (link below) with a generic picture of a four pin B4 base - nothing about the actual valve other than filament data. The valve looks to be new and never used and in its original box and it truly is a thing of beauty (as are all these large 'globe' type valves, I've got other similar types), and it would be interesting to knock up a single ended amplifier where it would not only look quite attractive, but no doubt work quite well.

I was sure I remember this valve being mentioned somewhere, but can find no reference to it now, so this thread could turn out to be unique in being the only 'on-line' reference to this valve type number. I even turned to my old valve data books and it's not listed in any of them, the only 'TV' type number valve listed being a TV4 tuning indicator. I have a feeling that there will be forum members who know this valve and probably have data and information on it and also for what purpose it was produced by Mazda in the first place.

Link to Radiomuseum page that gives filament voltage and current, but that's all:-
https://www.radiomuseum.org/tubes/tube_tv250.html

And some pictures of the valve itself:-
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Old 13th Oct 2021, 3:07 pm   #2
HamishBoxer
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Default Re: Mazda TV250 valve

Similar to the 2 lovely looking valves I have, used by the Post Office way back.
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Old 13th Oct 2021, 3:23 pm   #3
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Default Re: Mazda TV250 valve

Many thanks for that. That's jogged my memory and I think it may have been a Post Office reference that I remember regarding this valve years ago. It's strange that there seems to be absolutely no data available on it. I'll probably knock up a little circuit with power supplies, meters and croc clip leads and plot some characteristics on it when I get a spare moment.
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Old 13th Oct 2021, 3:51 pm   #4
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Default Re: Mazda TV250 valve

According to this the equivalent is a F704 (book page 17)

https://nvhrbiblio.nl/biblio/boek/Ra...1942_Brans.pdf

Philips F704 data:

https://frank.pocnet.net/sheets/097/f/F704.pdf

Lawrence.
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Old 13th Oct 2021, 4:45 pm   #5
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Default Re: Mazda TV250 valve

Lovely! And it really should be put to use/displayed!

I'm thinking a single-ended amplifier built into a squat square piano-black-finished case, with all the transformers/chokes.capacitors concealed within and just this valve sitting centrally on the top, so it is displayed without there being any other 'things' on the top surface to act as distractions.
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Old 14th Oct 2021, 9:41 am   #6
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Default Re: Mazda TV250 valve

Maybe add a couple of 615 rectifiers for extra bling?
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Old 26th Oct 2021, 5:41 pm   #7
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Default Re: Mazda TV250 valve

Thanks chaps. Just returning to this again after getting distracted onto other things.

When I started this thread I had a feeling that Lawrence would be the one to come up with something on this valve, I even nearly mentioned that he might be the one to find the data, but didn't want to put him off. That Belgian data book scan is great and I've downloaded and saved it - it would nearly be worth printing it all out as a reference. Also, the Philips data - Philips 'Miniwatt' brand name not being very applicable to this valve, there being nothing 'mini' about it

Regarding the 'look' of these 'globe' triodes, yes, a black box without any other components other than the valve mounted externally would certainly look very attractive, the only down side is that you might be accused of faking it with just a heater supply to the valve and a little solid state amplifier inside the box!

These certainly are a very attractive looking valve. I run a pair of Mazda PP3/250 valves in the LF59 in my Dynatron radiogram (as shown below), it's just a shame you can't see them without turning it round and removing the back. Removing the back is what I did the other day, as I needed to test some AC044 valves that were in the same 'lot', and in the picture below you can see just how boring these particular valves look in comparison to their 'globe' equivalents. Again, regarding how valve type numbers are written down differently in different places, data books often list them as ACO44, rather than AC044 (with an 'O' rather than a 'zero'), which I understand that they're supposed to be listed as.

Regarding testing these triodes in the amplifier, I'm tempted to try the TV250 with one of the other valves in the amplifier - perhaps not with one of the PP3/250s, but with a random PX4 just in case. I think that with running the 7.5 volt heater at 4 volts, the current will come down to around what a PX/PP/AC valve would be and it might be by coincidence, turn out to be reasonably balanced...just for testing purposes and interests sake, of course. If anyone thinks this is a really bad idea for any reason, then please shout out!

I have in the past thought about trying a PX25 with a PP3/250 etc. in this amplifier, just to see the results, but the heater current at 4 volts is 2 amps as opposed to 1 amp for the original valves. I only had one spare example of one of these, but I've found another one in this lot of valves, so it's tempting to try a pair. However, although it's a meaty transformer I'd be pushing my luck a bit with double the heater current for each valve, even with disconnecting the tuner unit to take some of the strain off it, which would also involve messing about with the input circuit, as the audio goes in at the tuner, so definitely going to give that one a miss, even though it would be interesting just to see if it would work as a test.
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Old 1st Nov 2021, 5:09 pm   #8
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Default Re: Mazda TV250 valve

I've now done the test with the TV250 paired up with a PX4 in the LF59 amplifier and it actually worked (perhaps surprisingly) well.

I had a PX4 that from the valve 'lot' that needed testing, so after testing for heater continuity and internal shorts, I paired it first with one of the PP3/250 valves and it was perfect. I did some initial tests on this valve to prevent any nasty surprises like I had a while ago with one with a broken heater string that was shorting to the grid - that made a horrible racket in the amplifier and on a later bench test with power supplies, a bright flash as the short was blown clear. That particular valve did later work quite well on its remaining good heater string, although with some level of hum in push-pull due to the obvious imbalance between it and a good 'heater complete' valve, but ok for single ended use in an ordinary radio.

After confirming that the PX4 was ok, I paired it up with the TV250 and to be honest there was negligible hum on zero volume setting, although some was just audible as you would expect there to be at least some imbalance between these two valves, but it performed well with the music! The TV250 with its heater fed from a bench power supply at around 4 volts, had a current draw of around 0.8 amps, so slightly down on the 1.0 amp draw of a PX4 etc.

I think that a TV250 would probably match up well with a PX25 if the heater was supplied with its correct 7.5 volts, but this little test with 4 volts and a PX4 shows that a TV250 (if you happened to have one) could be pressed into service as a replacement for a PX4 if need be and I guess that as long as the valve had good emission (you can't really see the heater glowing at all at 4 volts) it would work acceptably well. The heater string consists of two inverted 'V' runs with two hooks supporting the top parts, so this is unlike the PX4 and PX25 in this respect.

I've been looking at the data that Lawrence found, and roughly translating the first part says that it's a modulator for anode voltage. I don't know whether this is suggesting that the valve was originally designed for use as a modulator for transmitters or is just another way of saying that modulated grid voltage modulates the anode voltage as would be the case in any such valve - thoughts on that one? It later states that saturation current is 900 mA - could this suggest possible use in pulse operation? PX4 & 25 don't give a figure for this in the data that I can see. It will operate at a higher anode voltage than a PX25 and it gives an internal resistance of 1800 ohms.

It certainly is a pretty valve and you can understand why people like these valves on an open chassis on full display and arguably the Dynatron LF59 amplifier is one of the best looking amp chassis out there, hence why they get ripped out of radiograms for this purpose. They may be regarded as better looking than the RGD, HMV or Decca variants, with nothing on the chassis top that shouldn't be there. Some LF59s have odd holes drilled for other components that may or may not be there, but mine is untouched in this respect, although the extra holes (when there) are original from the factory for such items as an electrolytic capacitor and whistle filter, if fitted. The chassis is certainly built like a tank with thick gauge metal and fully welded corners - you could probably park a tank on one and it would support it!

In the tests there was virtually no speaker hum whatsoever at zero volume setting from the PP3/250s or AC044s, but very slight hum noticed with the PX4 and TV250 due to the heater imbalance between these valves, but it was no more than you'd expect from most other sets when working normally. The LF59 amplifier has extremely good HT smoothing from its original block paper in oil capacitors - unbelievably, there are some complete heathens out there that actually replace or try to re-stuff these...there's no need to touch them unless they're genuinely proved to be faulty!

The pictures below show the TV250 working in the amplifier with the 'just tried out' PX4 - shame you can't actually hear it! I didn't try it with a PP3/250, as it's so large that it might have actually 'banged heads' with another 'globe' style valve plugged in beside it on that chassis, although I didn't actually confirm that this would be the case. They also show the three 'globe' style valves together as a size comparison with the TV250 in the middle with a PP3/250 on the left and a PX25 on the right - it's around the same size as a 'globe' PX25. The last picture shows the Dynatron radiogram back in place after the tests - it's a good looking radiogram, but some out there would say that the amplifier would look even better sat next to another one for stereo as part of a bespoke Hi-Fi system. I did a quick search on PP3/250 valves and LF59 amplifiers and although I was aware of the so called 'desirability', I am a bit out of touch and was quite shocked at a couple of recent sales of each. However, this amp and its valves are staying in the cabinet, although having seen the current value of the most desirable of the power triodes being the PP3/250 globe types, I did wonder whether I ought to put them away somewhere safe for the time being and run the gram with those less desirable, but equally good (possibly better - they're Mullard) electrically, AC044s.

Any other thoughts on the original use of the TV250? As David said, Post Office use, and that did ring a distant bell with me. You'll have to tell us more about your two valves, David.
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Last edited by Techman; 1st Nov 2021 at 5:24 pm. Reason: Typos etc.
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