# Puzzling Pentode Problem, 1951 radio

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A Pye P43U 4 tube radio: it works, but all is not well with the output pentode section and I'm not fully clear why.

https://archives.doctsf.com/documents/feuilleter_document.php?num_doc13%349&ref14%933
shows the circuit.

Problem: Ia should be 48mA, Is 9mA so Ik 57mA. It starts off at 66mA and rises in a few minutes to 77mA, which is the abs max Iq value for the UL41 pentode according to UL41 datasheet. Why is it rising uncontrolled?

Grid: goes to 0v dc-wise. There are 2 caps but if either were leaky or even shorted, Vgrid would go -ve not up. This is not the traditional cap from previous anode circuit.

Screen: connects direct to the secondary smoothed B+, as recommended in the valve data sheet.

Cathode: Rk is 120R nominal, 110R real. This is what I suspect, UL41 datasheet recommendes 170R, which would explain the raised i, but... Why are Pye using 120R? Why is Ia rising? Is it a faulty valve? (I don't have a spare UL41).

PS 3 of the 5 power Rs were burnt out, presumably due to overcurrent in the UL41.

NT

Re: Puzzling Pentode Problem, 1951 radio
On 8/1/2020 12:23 AM, Tabby wrote:

Of course the grid is going to be at ground, what's the voltage on the
cathode? Don't leave us in suspense, my dude...

Re: Puzzling Pentode Problem, 1951 radio
On Saturday, 1 August 2020 07:01:30 UTC+1, bitrex  wrote:

pentode section and I'm not fully clear why.

13%349&ref14%933

d rises in a few minutes to 77mA, which is the abs max Iq value for the UL4
1 pentode according to UL41 datasheet. Why is it rising uncontrolled?

even shorted, Vgrid would go -ve not up. This is not the traditional cap fr
om previous anode circuit.

the valve data sheet.

tasheet recommendes 170R, which would explain the raised i, but... Why are
Pye using 120R? Why is Ia rising? Is it a faulty valve? (I don't have a spa
re UL41).

the UL41.

Started off at 7.5v, rose to 8.5v in a few minutes. That's how I know Ik :)

The grid has a 100k stopper R on it, working with the valve's Cg to form an
RC IF remover. IF is 470kHz. I don't know if there might be valve leakage
increasing Vg, the 100k would surely make the stage sensitive to any such i
ssue.

But since the bias is at odds with the datasheet I don't see how it's meant
to work correctly out of the box. I'm just puzzled why the bias is off, an
d thus don't know how to go about solving it. I could replace Rk, but it's
clearly meant to be 120R and work ok.

Having said that I did encounter other questionable points of design & cons
truction in the set.
1. On 240v mains (peak 330v) the hot chassis to AE terminal cap was rated 3
00v
2. The output stage was miswired: Rk, Ck were present but shorted to ground
giving zero bias, & the screen grid was connected to the -ve agc line inst
ead of the smoothed B+ line. It looked like it was that way from new, ie ne
ver worked. Shrug, I just  haven't got my head properly around this output
pentode section.

NT

Re: Puzzling Pentode Problem, 1951 radio
Tabby wrote:

=============

nd giving zero bias, & the screen grid was connected to the -ve agc line in
stead of the smoothed B+ line. It looked like it was that way from new, ie
never worked. Shrug, I just  haven't got my head properly around this outpu
t pentode section.

** That set seems like a factory dud.

Why not increase Rk to say 180 ohms and see.

At near 70 years old, all the tubes are likely a bit gassy and the output t
ube hasscreen-grid leakage via the mica soon as it get hot.

....  Phil

Re: Puzzling Pentode Problem, 1951 radio

[...]

Grid stoppers are usually around 10k.  I've never heard of valve stray
capacitance being deliberately used as part of the I.F. filter, the
stopper is usually there to prevent oscillation at V.H.F.  With 100k and
the valve strays of 10pf, this would form a low-pass filter with a
turnover of 160 Kc/s, but that may be intended to give a dominant pole
to stabilise the audio feedback loop.

It depends on which data sheet you have seen.  The Mullard sheet
suggests it is working well within its safe limits.

It might have been 300v A.C. rated, in which case it has a small safety
margin.

That is very strange - but some Philips circuits of that era used a
system of grid bias which connected all the cathodes to chassis and put
one big dropping resistor in the negative H.T. lead.  The bias from this
was divided by a resistor chain and fed to the various stages as fixed
bias.  It worked very well and got rid of a lot of electrolytics because
the unwanted ripple and audio coupling could be removed at high
impedance by smaller-value capacitors.   (Very similar to the old
grid-bias system on directly heated valve receivers.)

It wouldn't surprise me to find that some Pye sets used that bias
circuit, although the 'Trader' sheet shows quite clearly that this one
didn't.  I don't suppose it was modified during production to take a
valve with different pin-out and the cathode was actually on a a
different pin?  (UCL41, UBL41,  ...something like that just to keep the
production line running during a shortage?)

--
~ Adrian Tuddenham ~
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Re: Puzzling Pentode Problem, 1951 radio

The high self-capacitance of the primary winding (and reduced coupling
due to eddy currents in the core deflecting the primary flux) causes
some attenuation of the upper audio frequencies, which can be partially
corrected by the feedback, but it also causes phase shift as it
approaches the self-resonant frequency and this can cause a peak in the
response or total instability.  That is why only limited feedback is
possible.

[...]

There is one fitted now, but it might have left the factory with
something else - most unlikely, but things like that did occasionally
happen in the era of post-war shortages.

Possibly a previous repairer altered the wiring in an attempt to
'correct' the fault or, more likely, you suffered a momentary brainstorm
and mis-counted the pins on the B8A valveholder (no easy way to locate
Pin 1) through the clutter of components - the number of times I have
done that!.

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Re: Puzzling Pentode Problem, 1951 radio
On Sunday, 2 August 2020 09:08:25 UTC+1, Adrian Tuddenham  wrote:

form

e
ve to
ay
and
e
n that.

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

he

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

Re: Puzzling Pentode Problem, 1951 radio
On 02/08/2020 15:41:02, Tabby wrote:

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.

--
Mike Perkins
Video Solutions Ltd
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Re: Puzzling Pentode Problem, 1951 radio
On Sunday, 2 August 2020 16:57:23 UTC+1, Mike Perkins  wrote:

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

Re: Puzzling Pentode Problem, 1951 radio

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

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

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Re: Puzzling Pentode Problem, 1951 radio
On Monday, 3 August 2020 10:31:01 UTC+1, Adrian Tuddenham  wrote:

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

Re: Puzzling Pentode Problem, 1951 radio

[...]

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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Re: Puzzling Pentode Problem, 1951 radio
On Monday, 3 August 2020 12:35:34 UTC+1, Adrian Tuddenham  wrote:

und
the

ht
I

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

Re: Puzzling Pentode Problem, 1951 radio
On a sunny day (Mon, 3 Aug 2020 14:01:19 -0700 (PDT)) it happened Tabby

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.....

Re: Puzzling Pentode Problem, 1951 radio
On Tuesday, 4 August 2020 06:46:46 UTC+1, Jan Panteltje  wrote:

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

I got an early roundie, prefer radios generally though

NT

Re: Puzzling Pentode Problem, 1951 radio
On a sunny day (Tue, 4 Aug 2020 01:23:48 -0700 (PDT)) it happened Tabby

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.

http://panteltje.com/pub/Tecsun_PL-600_IXIMG_0508.JPG
Just dial in a frequency.

Re: Puzzling Pentode Problem, 1951 radio
On Friday, 7 August 2020 07:20:45 UTC+1, Jan Panteltje  wrote:

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

Re: Puzzling Pentode Problem, 1951 radio
On a sunny day (Sun, 9 Aug 2020 00:44:24 -0700 (PDT)) it happened Tabby

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?
Was just bull?

Re: Puzzling Pentode Problem, 1951 radio
On Sunday, 9 August 2020 11:38:33 UTC+1, Jan Panteltje  wrote:

tage

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

Re: Puzzling Pentode Problem, 1951 radio
On Fri, 07 Aug 2020 16:07:07 -0700, Tabby wrote:

I too would like to know which _transistor_ radio is "far better"! Or
perhaps you were thinking _transformer_?