# Puzzling Pentode Problem, 1951 radio...

T

#### Tabby

##### Guest
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_doc=13349&ref=14933
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

B

#### bitrex

##### Guest
On 8/1/2020 12:23 AM, Tabby wrote:
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_doc=13349&ref=14933
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
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...

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 07:01:30 UTC+1, bitrex wrote:
On 8/1/2020 12:23 AM, Tabby wrote:

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_doc=13349&ref=14933
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

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...
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 issue.

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, and 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 & construction in the set.
1. On 240v mains (peak 330v) the hot chassis to AE terminal cap was rated 300v
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 instead 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 output pentode section.

NT

T

#### Tim Williams

##### Guest
\"bitrex\" <user@example.net> wrote in message
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...
Also useless without screen voltage (and plate voltage to some extent).

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

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

P

#### Phil Allison

##### Guest
Tabby wrote:

============
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 instead 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 output 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 tube hasscreen-grid leakage via the mica soon as it get hot.

..... Phil

A

##### Guest
Tabby <tabbypurr@gmail.com> wrote:

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_doc=1334
9&ref=14933 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?
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

> Grid: goes to 0v dc-wise.

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.

[...]
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).
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.

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

--
www.poppyrecords.co.uk

A

##### Guest
Tabby <tabbypurr@gmail.com> wrote:

On Saturday, 1 August 2020 07:01:30 UTC+1, bitrex wrote:
On 8/1/2020 12:23 AM, Tabby wrote:
[...]

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 issue.
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.

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

Having said that I did encounter other questionable points of design &
construction in the set. 1. On 240v mains (peak 330v) the hot chassis to
AE terminal cap was rated 300v
It might have been 300v A.C. rated, in which case it has a small safety
margin.

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 instead 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 output pentode section.
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?)

--
www.poppyrecords.co.uk

J

#### Jan Panteltje

##### Guest
On a sunny day (Fri, 31 Jul 2020 21:23:31 -0700 (PDT)) it happened Tabby
<tabbypurr@gmail.com> wrote in

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_doc=13349&ref=14933
shows the circuit.
In the old days composite carbon resistors were common,
some would go high, some even short.
Check all resistor values with an ohm meter.

B

#### bitrex

##### Guest
On 8/1/2020 6:04 AM, Tabby wrote:
On Saturday, 1 August 2020 07:01:30 UTC+1, bitrex wrote:
On 8/1/2020 12:23 AM, Tabby wrote:

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_doc=13349&ref=14933
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

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...

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 issue.

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

<https://frank.pocnet.net/sheets/046/u/UL41.pdf>

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...

Having said that I did encounter other questionable points of design & construction in the set.
1. On 240v mains (peak 330v) the hot chassis to AE terminal cap was rated 300v
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 instead 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 output pentode section.

NT

B

#### bitrex

##### Guest
On 8/1/2020 6:04 AM, Tim Williams wrote:
\"bitrex\" <user@example.net> wrote in message
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...

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

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

Tim
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...

A

##### Guest
bitrex <user@example.net> wrote:

On 8/1/2020 6:04 AM, Tim Williams wrote:
\"bitrex\" <user@example.net> wrote in message
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...

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

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

Tim

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...
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.

ever quite what it seems.

--
www.poppyrecords.co.uk

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 15:56:43 UTC+1, Adrian Tuddenham wrote:
Tabby <tabbypurr> wrote:

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_doc=1334
9&ref=14933 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?

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
Yes, measured Rk.

Grid: goes to 0v dc-wise.

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.
Well, that was the problem. Vk rises as it heats up, but Vg rises worse. Started at 4.7v, rising in a few minutes to 8v before I switched off.
And yes, Ia changed as volume was adjusted.

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.
100k stopper and 0.9M volume pot - that\'s about as sensitive to grid current as it gets.

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).

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.

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

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?
no it\'s perfect - but only because I think it\'s a replacement!

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.
Right. And the Mullard valves are also Philips. It has 3 Mullards and one RTC, whoever they are. So I get the impression it\'s low hours but fried itself. Why it wasn\'t repaired under warranty who knows.

That leaves the question of what to do. Possibles include:
1. Get a new UL41. I don\'t really want to.
2. Connect primary of very small mains transformer from grid to chassis. With such a high impedance feed I doubt that\'ll work tolerably.
3. Connect resistor, diode from grid to chassis to help discharge the grid. Might stop it rising too much. 1Mohm?

NT

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 17:16:50 UTC+1, Adrian Tuddenham wrote:
Tabby <tabbypurr> wrote:
On Saturday, 1 August 2020 07:01:30 UTC+1, bitrex wrote:
On 8/1/2020 12:23 AM, Tabby wrote:

[...]

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 issue.

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.
I assumed the output transformer would attenuate at lower frequency than that.

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, and 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.

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

Having said that I did encounter other questionable points of design &
construction in the set. 1. On 240v mains (peak 330v) the hot chassis to
AE terminal cap was rated 300v

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

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 instead 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 output pentode section.

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?)
It\'s a UL41 fitted, so no.

NT

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 17:42:29 UTC+1, Jan Panteltje wrote:
On a sunny day (Fri, 31 Jul 2020 21:23:31 -0700 (PDT)) it happened Tabby
tabbypurr> wrote in

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_doc=13349&ref=14933
shows the circuit.

In the old days composite carbon resistors were common,
some would go high, some even short.
Check all resistor values with an ohm meter.
All Rs were checked & most Cs replaced before it saw power.

NT

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 18:18:31 UTC+1, bitrex wrote:
On 8/1/2020 6:04 AM, Tabby wrote:
On Saturday, 1 August 2020 07:01:30 UTC+1, bitrex wrote:
On 8/1/2020 12:23 AM, Tabby wrote:

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_doc=13349&ref=14933
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

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...

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 issue.

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, and 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.

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

https://frank.pocnet.net/sheets/046/u/UL41.pdf

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

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...
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 secondary 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 half 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

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 18:23:20 UTC+1, bitrex wrote:
On 8/1/2020 6:04 AM, Tim Williams wrote:
\"bitrex\" <user@example.net> wrote in message
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...

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

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

Tim

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

NT

T

#### Tabby

##### Guest
On Saturday, 1 August 2020 20:12:20 UTC+1, Adrian Tuddenham wrote:
bitrex <user@example.net> wrote:

On 8/1/2020 6:04 AM, Tim Williams wrote:
\"bitrex\" <user@example.net> wrote in message
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...

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

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

Tim

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...

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.

ever quite what it seems.
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

M

#### Mike Perkins

##### Guest
On 01/08/2020 05:23:31, Tabby wrote:
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_doc=13349&ref=14933
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.
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?

--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

J

#### Jan Panteltje

##### Guest
On a sunny day (Sat, 1 Aug 2020 13:56:52 -0700 (PDT)) it happened Tabby
<tabbypurr@gmail.com> wrote in

That leaves the question of what to do. Possibles include:
1. Get a new UL41. I don\'t really want to.
2. Connect primary of very small mains transformer from grid to chassis. With such a high impedance feed I doubt that\'ll work
tolerably.
3. Connect resistor, diode from grid to chassis to help discharge the grid. Might stop it rising too much. 1Mohm?
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

A

##### Guest
Tabby <tabbypurr@gmail.com> wrote:

[...]

That leaves the question of what to do. Possibles include:
1. Get a new
UL41. I don\'t really want to.
2. Connect primary of very small mains
transformer from grid to chassis. With such a high impedance feed I doubt
that\'ll work tolerably.
3. Connect resistor, diode from grid to chassis to
help discharge the grid. Might stop it rising too much. 1Mohm?
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).

--