Puzzling Pentode Problem, 1951 radio

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Yup. Anything that attenuates causes phase shift, assuming it's a single si mple part.

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I measured right on, Rk, Ck rather than counting on correct pin counting. I t was definitely shorted good & hard. Initially I assumed it was the C (whi ch needed replacing) so removed it. Still shorted. So removed R thinking th at's not common but it must be faulty. Still a dead short. The way the valv e holder was wired shows it was that way from new. As you point out I've no way to know if it shipped with something else - if it did though that also ran with no V on its cathode, there is no other R,C to ground anywhere on the holder. So I suspect that's not what happened.

NT

On a sunny day (Sun, 2 Aug 2020 07:33:29 -0700 (PDT)) it happened Tabby wrote in :

It is only audio, say 2 nF in 500 k is 1 ms = 1000 Hz, rules out a MOSFET but a JFET with a BJT is at most a 100 pF. I meant something these things:

seems to have a BJT in the input, not hard to design / make something like that.

It would. It also needs an unobtainable connector. Total cost more than the radio's worth. I'm not in love with this one, it's a plain postwar budget set.

This radio was already designed with features that can do that. Runaway is easy to protect against.

NT

If you remove the valve is there still a dead short? Is the pin for the suppressor grid tied to ground? [1]

[1] There may be an internal connection in the valve between cathode and suppressor grid.

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What was the ac voltage back in the 50's there? Here in the US it went from 110 to 120 v. This upset some old radios.

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It varied from town to town, but usually somewhere between 210v and 250v at 50 c/s. There were a few pockets of D.C. at + or - 200v, but anything else was extremely rare.

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~ Adrian Tuddenham ~ 
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no I fixed it. Rk works correctly now, the valve has bias.

there is. It's not a problem afaik.

Update time... I added a diode from grid to chassis. Meter on dc then read 0.35v on the grid, a massive improvement. One small problem: the sound volume has almost all gone. Sigh. I guess I could put a cap between the 2k2 at the bottom of the pot & chassis...

This mucking about with history pieces is exactly the sort of thing I generally hate. I still don't want to buy a UL41 though.

NT

I've got a 1934 set made to run on minus 100v dc.

NT

The grid current through the diode is making it appear as a low value resistor shorting the high-impedance output of the detector. The choke/mains transformer will work better, but it is still not the right answer.

What is making you so resistant to doing the job the right way?

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~ Adrian Tuddenham ~ 
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Philips offered many of their pre-war domestic models with the option of a vibrator pack for D.C. mains from about 100v upwards (including 127v and 145v, but I never discovered where those supplies were in common use). There was an immensely complex rectangular-slotted socket on the back of the mains transformer with contact sets that were separated when the plug was inserted. It was wired with rubber-covered cable that crumbled when bent and smelt like farmyard manure when soldered.

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Grid current flowed before, yet it worked audio-wise. The idea was the diode would just conduct on peaks, charging C24 so the audio fed to the grid peaked at 0.6v. But it seems that doesn't work.

This one just isn't worth the spends. And I've yet to look at the other one - I might get one good one not from the pair.

NT

The grid current is about 5 to 8 microamps if your measurements were made with a 10 megohm meter (10 to 16 if you used a 1 megohm measuring device). A current of 5 microamps through a silicon diode with (say)

0.5 volts drop appears to the audio as though it is a 100k resistor shunting the detector. If you were using a Schottky or Germanium diode, which would explain the 0.35 volts, the situation is even worse.

Botched or scrapped it is worth next to nothing; properly repaired and working nicely it is worth more than a replacement valve will cost you. The economics are in favour of doing a proper job.

If you have another one, what happens if you swap the two UL41s? Valve swapping used to be the first resort of every hard-pressed serviceman in the field - even before bothering to unpack the meter on some occasions.

You also might want to sniff the output transformer for the smell of burnt windings if the set has been used under runaway conditions for a long period.

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~ Adrian Tuddenham ~ 
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More info... The other UL41 works fine. The resistors in the other set show (less) damag e but no visible repairs. Fitted to the set I've been working on it sits at : Vg=1v Va= 163v at max volume, 169v at zero vol. Ig = 1.5uA

For the first not so healthy UL41: Ig = 9.1uA initially, 17uA after a few minutes. It pulls Va down to 139 s ilent 115v at max volume.

With either pentode the sound is equally poor. Especially poor lf response and an amount of distortion that gets unpleasant when not at low volume, bu t it seems to be working as well as it's going to. I also note there's no r f hiss, but no trivial way to measure conditions on the earlier valves.

NT

On a sunny day (Mon, 3 Aug 2020 14:01:19 -0700 (PDT)) it happened Tabby wrote in :

From a different perspective I thought: 'Why not buy a modern transistor radio?'

I have, among several others, a Tecsun PL600 with AM FM SSB long medium shortwave and a simpler smaller Tecsun PL360 without SSB. The PL600 was like 60$ on ebay, the PL360 less.

All the time spend, unless you are expecting money for it as an antique, is IMNSHU not really worth it. Nice exercise, but.. I have, in the attic, still an old CRT color monitor, but only because I see it from a physics POV as my own personal particle accelerator. Has not been on for many years.....

I can't think of any reason to get another transistor radio.

I got an early roundie, prefer radios generally though

NT

On a sunny day (Tue, 4 Aug 2020 01:23:48 -0700 (PDT)) it happened Tabby wrote in :

Yea.

*IF* the output tube in that radio is considered 'total loss' then MAYBE: Perhaps there is some cathode material coming lose and shorting the grid if the tube gets hot. In such a case burning it out might help, say a hundred volt between cathode and grid with a 25 W Edison type bulb in series for a second.

I have revived many old CRTs like this: Heater on, few hundred volt on the grid + versus cathode - via a light bulb for a few seconds to revive the cathode, but that was for low light cases, cathode contamination.

Sparking out a short could work for some cases perhaps. New tubes are 20$ on ebay..

For the price of 2 tubes you have a very good PLL transistor radio, inclusive rechargable batteries, power adaptor, and sometimes even a turnable ferrite antenna, even a wire antenna.

All that old junk, you cannot take it with you when you go.

Just dial in a frequency.

Isn't that what CRT rejuvers do? The result doesn't last, but worse it damages the picture quality when it deteriorates again.

Tubes keep getting harder to find though. Refiring the getter can also fix some.

Lol. I've got a far better tranny radio than that. Unfortunately they're obsolescent these days.

NT

On a sunny day (Sun, 9 Aug 2020 00:44:24 -0700 (PDT)) it happened Tabby wrote in :

Well you are using the tube out of spec. I have seen cases like that, CRT lighting as a lightbulb.

You never answered the question I had to your statement that you had far superior tranny radios? What radio? Was just bull?

tage

superior tranny radios?

I don't bull. If you've not realised that you've not been paying attention. Yes there's more than one contender, yes one of mine is one of them. I'm n ot much into tranny radios. The internet has obsoleted them, and really tha t's a good thing. Listening to round the world shortwave was never too ente rtaining or informative. I don't care whose is better or why.

Valve radios I still like some. Unfortunately I'm not impressed by the Pye. Yes it has some clever ways of being cheap, but that's all. I dislike the sound. The cabinet is a fine example of missed opportunity. The thing is pa cked with an excessive number of power resistors all running hot. What's to like?

NT