Puzzling Pentode Problem, 1951 radio

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.

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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

Reply to
Tabby
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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...

Reply to
bitrex

pentode section and I'm not fully clear why.

=13349&ref=14933

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
  1. 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

Reply to
Tabby

Also useless without screen voltage (and plate voltage to some extent).

Rising current can be a sign of gas or grid contamination.

Tim

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Reply to
Tim Williams

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

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

Reply to
Phil Allison

I presume you have calculated these current values from the voltage dropped across the cathode resistor and haven't tried breaking into the anode circuit? Sometime long meter connections to the anode can cause instability and erroneous readings.

Did you measure it and check that it really is at 0v ? If the valve has gone soft, it might well be drifting positive. I seem to remember that UL41s did tend to go soft, very often with an even blue glow inside the envelope (not small patches of blue on the glass, which are harmless). Have a look in a darkened room and see if there is anything like that.

If turning the volume control up and down alters the anode current, positive grid current is flowing. Another quick test is to turn up the volume control with no station tuned in and short-circuit the wiper to earth with a screwdriver; if D.C. is present, you will hear a 'plop' from the loudspeaker.

As suggested in other replies, check all the resistors in the grid circuit (including the pot wiper and grid stopper) to make sure they are not high resistance, which would make the valve over-sensitive to slight softness.

[...]

According to the 'Trader" sheet, the measured anode voltage to chassis is 160v. If you then look at the Mullard Datasheet for the EL41 (which is identical to the UL41 except for the heaters) you will see that an anode current of 55mA is the maximum allowable at 160v.and requires -5v grid bias. In practice that means +5v on the cathode and only 155v between anode and cathode, which brings the anode dissipation down a little bit.

The screen gid current is roughly 10mA, so the total cathode current is

65mA. To get 5v bias, the cathode resistor needs to be 77 ohms, so your resistor at 110 ohms is running the valve well below its maximum rating.

I presume R14 (which supplies HT to the rest of the set and balances the HT ripple in the output transformer) doesn't show any signs of overheating?

By the way, a lot of Pye kit was very similar indeed to Philips designs

- suspiciously so. It came as no surprise when Philips bought-out Pye.

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Adrian Tuddenham

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?)

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Adrian Tuddenham

On a sunny day (Fri, 31 Jul 2020 21:23:31 -0700 (PDT)) it happened Tabby wrote in :

In the old days composite carbon resistors were common, some would go high, some even short. Check all resistor values with an ohm meter.

Reply to
Jan Panteltje

Not sure how the bias is at odds with the datasheet, on fig 6 here:

when Va and screen are both about 170 volts -8.5 volts bias gives about 65 mA plate current, plus maybe 10 mA from the screen.

That's not how the screen is connected in the schematic though, it's connected through one half of the transformer primary and then thru a dropper R in line with the current to the rest of the set, and a bypass cap. and looks like the screen voltage should be at around 140.

So IDK man you connect the output stage differently then it's rigged in the schematic and then say it's not working like it should. Well...

Reply to
bitrex

That by connecting the screen directly to the B+ instead of thru the dropper R as it is on the schematic and it's running 30 volts too high could be part of the "problem", too...

Reply to
bitrex

R14 isn't just a dropper, it also balances the H.T. ripple in the output transformer and smooths the supply to the screen grid. The D.C. in the output transformer is also partly balanced by the current drawn by the rest of the set, allowing it to have a smaller core without risk of saturation.

That is a very clever bit of circuitry which was described in the Philips Technical Review some years before this set was made. Philips used it a lot in the post-war years. Radio repairers used to hate Philips sets because they never really understood them: typically they would replace the output transformer with a standard Radiospares component and wonder why it hummed badly and distorted too easily.

You mess about with Philips and Pye radios at your peril, nothing is ever quite what it seems.

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Reply to
Adrian Tuddenham

All Rs were checked & most Cs replaced before it saw power.

NT

Reply to
Tabby

t pentode section and I'm not fully clear why.

=13349&ref=14933

and rises in a few minutes to 77mA, which is the abs max Iq value for the U L41 pentode according to UL41 datasheet. Why is it rising uncontrolled?

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

in the valve data sheet.

datasheet recommendes 170R, which would explain the raised i, but... Why ar e Pye using 120R? Why is Ia rising? Is it a faulty valve? (I don't have a s pare UL41).

in the UL41.

k :)

m an RC IF remover. IF is 470kHz. I don't know if there might be valve leak age increasing Vg, the 100k would surely make the stage sensitive to any su ch issue.

eant to work correctly out of the box. I'm just puzzled why the bias is off , and thus don't know how to go about solving it. I could replace Rk, but i t's clearly meant to be 120R and work ok.

The datasheet calls for 170R Rk, this thing is running with 110R.

No I've corrected it so it matches the schematic. Originally Rk was shorted & the screen went to the -ve agc line.

Look closer at that transformer primary, the top end pickoff for the second ary HT comes from just 6% of the total primary winding. The HT line is very high impedance with a stack of droppers plus only 32uF of reservoir on hal f wave rectified mains. As the output pentode draws anode current that top

6% of the primary wind improves the stability of the secondary HT supply.

NT

Reply to
Tabby

The radio is run the same way as the UL41 datasheet. Component values are a bit different but fairly near.

NT

Reply to
Tabby

I appreciate the caution. I was surprised by the small size of the transformer. C24 cuts bass nfb, compensating somewhat for poor lf response. It's not needed to block dc.

NT

Reply to
Tabby

Recheck R15. It was common for these to creep up in value. If the valve is soft you might get some control grid current. Crucially, have you measured the voltage on this grid?

Is there any hint of a blue glow in the valve?

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Mike Perkins 
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Reply to
Mike Perkins

On a sunny day (Sat, 1 Aug 2020 13:56:52 -0700 (PDT)) it happened Tabby wrote in :

LOL

1) Evil that AGC circuit is decoupled via C19 (I think it is, hard to read), that AGC creates a negative voltage for strong signals to the grid of the first tube. And EVIL person could try shorting C19 as that would create negative for strong signals and high volume settings... 2) better You could also make a low power negative supply with some diodes and caps and fed it via a high value resistor into the grid. 3) leave it But in general I would not worry much about it, tube age was at least 20% tolerance for all parameters, much more over lifetime. You do not want to try an other output tube so... A monte carlo in spice would probably show worse things,,,, 4) high tech! For some tube TV sets (much later) there ware transistor replacement things for the audio video? output tube IIRC., Tried one once... You could do that with a JFET and some HV transistor, a resistor for the heater Ah! Finally back to design:-)
Reply to
Jan Panteltje
[...]

1) Isn't all that expensive and should sort it out properly. 2) & 3) Are temporary botches and further gassing may cause another runaway or flashover and another batch of burnt-out power resistors (or even set fire to your house).
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Adrian Tuddenham

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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Adrian Tuddenham

R15 is fine. Didn't see any blue glow. Grid voltage is the problem - the valve isn't healthy.

NT

Reply to
Tabby

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