Puzzling Pentode Problem, 1951 radio...

[Adrian Tuddenham]

Tabby <tabbypurr@gmail.com> wrote:

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.

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.

[...]

...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.

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!.


--
~ Adrian Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[Tabby]

On Sunday, 2 August 2020 03:29:17 UTC+1, Mike Perkins wrote:

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?

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


NT

 

[Tabby]

On Sunday, 2 August 2020 07:01:46 UTC+1, Jan Panteltje wrote:

On a sunny day (Sat, 1 Aug 2020 13:56:52 -0700 (PDT)) it happened Tabby
tabbypurr> wrote in
350820f6-f987-4267-8d17-176d98657894o@googlegroups.com>:

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...

I wondered about that. Might try it, and add a fusible R to the output anode in case Ia rises too much.

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.

could use the dial lamps as a 6v dropper. Not sold on that much modification tho.

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.

no, it fries itself and will only get worse.

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

Contemplated that too. Shouldn\'t be hard to do, I note that Cin is way higher on fets tho. I\'d use a C for the heater.


NT

 

[Tabby]

On Sunday, 2 August 2020 09:08:25 UTC+1, Adrian Tuddenham wrote:

Tabby <tabbypurr> wrote:
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.

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.

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

...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.

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!.

I measured right on, Rk, Ck rather than counting on correct pin counting. It was definitely shorted good & hard. Initially I assumed it was the C (which needed replacing) so removed it. Still shorted. So removed R thinking that\'s not common but it must be faulty. Still a dead short. The way the valve 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

 

[Jan Panteltje]

On a sunny day (Sun, 2 Aug 2020 07:33:29 -0700 (PDT)) it happened Tabby
<tabbypurr@gmail.com> wrote in
<ceb63e4b-04dc-4d22-bd65-74cf2a2183eao@googlegroups.com>:

On Sunday, 2 August 2020 07:01:46 UTC+1, Jan Panteltje wrote:
On a sunny day (Sat, 1 Aug 2020 13:56:52 -0700 (PDT)) it happened Tabby
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

Contemplated that too. Shouldn\'t be hard to do, I note that Cin is way higher on fets tho. I\'d use a C for the heater.

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:
https://www.radiomuseum.org/tubes/tube_pl802t.html
seems to have a BJT in the input, not hard to design / make something like that.

 

[Tabby]

On Sunday, 2 August 2020 09:08:25 UTC+1, Adrian Tuddenham wrote:

Tabby <tabbypurr> 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.

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.

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).

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


NT

 

[Mike Perkins]

On 02/08/2020 15:41:02, Tabby wrote:

On Sunday, 2 August 2020 09:08:25 UTC+1, Adrian Tuddenham wrote:
Tabby <tabbypurr> wrote:
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.

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.

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

...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.

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!.

I measured right on, Rk, Ck rather than counting on correct pin
counting. It was definitely shorted good & hard. Initially I assumed
it was the C (which needed replacing) so removed it. Still shorted.
So removed R thinking that\'s not common but it must be faulty. Still
a dead short. The way the valve 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.

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
www.videosolutions.ltd.uk

 

[Ingvald44]

On Sat, 1 Aug 2020 14:03:34 -0700 (PDT), Tabby <tabbypurr@gmail.com>
wrote:

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

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.

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Adrian Tuddenham]

Ingvald44 <noone@nowhere.com> wrote:

On Sat, 1 Aug 2020 14:03:34 -0700 (PDT), Tabby <tabbypurr@gmail.com
wrote:

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


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.

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.


--
~ Adrian Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[Tabby]

On Sunday, 2 August 2020 16:57:23 UTC+1, Mike Perkins wrote:

On 02/08/2020 15:41:02, Tabby wrote:

I measured right on, Rk, Ck rather than counting on correct pin
counting. It was definitely shorted good & hard. Initially I assumed
it was the C (which needed replacing) so removed it. Still shorted.
So removed R thinking that\'s not common but it must be faulty. Still
a dead short. The way the valve 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.

If you remove the valve is there still a dead short?

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

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.

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

 

[Tabby]

On Sunday, 2 August 2020 21:24:30 UTC+1, Adrian Tuddenham wrote:

Ingvald44 <noone@nowhere.com> wrote:

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.

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.

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


NT

 

[Adrian Tuddenham]

Tabby <tabbypurr@gmail.com> wrote:

On Sunday, 2 August 2020 16:57:23 UTC+1, Mike Perkins wrote:
On 02/08/2020 15:41:02, Tabby wrote:

I measured right on, Rk, Ck rather than counting on correct pin
counting. It was definitely shorted good & hard. Initially I assumed
it was the C (which needed replacing) so removed it. Still shorted.
So removed R thinking that\'s not common but it must be faulty. Still
a dead short. The way the valve 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.

If you remove the valve is there still a dead short?

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

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.

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...

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.

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.

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


--
~ Adrian Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[Adrian Tuddenham]

Tabby <tabbypurr@gmail.com> wrote:

On Sunday, 2 August 2020 21:24:30 UTC+1, Adrian Tuddenham wrote:
Ingvald44 <noone@nowhere.com> wrote:

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.

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.

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

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.


--
~ Adrian Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[Tabby]

On Monday, 3 August 2020 10:31:01 UTC+1, Adrian Tuddenham wrote:

Tabby <tabbypurr wrote:
On Sunday, 2 August 2020 16:57:23 UTC+1, Mike Perkins wrote:
On 02/08/2020 15:41:02, Tabby wrote:

I measured right on, Rk, Ck rather than counting on correct pin
counting. It was definitely shorted good & hard. Initially I assumed
it was the C (which needed replacing) so removed it. Still shorted.
So removed R thinking that\'s not common but it must be faulty. Still
a dead short. The way the valve 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.

If you remove the valve is there still a dead short?

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

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.

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...

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.

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

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

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

 

[Adrian Tuddenham]

Tabby <tabbypurr@gmail.com> wrote:

On Monday, 3 August 2020 10:31:01 UTC+1, Adrian Tuddenham wrote:
Tabby <tabbypurr wrote:
[...]
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...

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.

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.

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.

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.

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

This one just isn\'t worth the spends.

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.

... And I\'ve yet to look at the other
one - I might get one good one not from the pair.

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.


--
~ Adrian Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[Tabby]

On Monday, 3 August 2020 12:35:34 UTC+1, Adrian Tuddenham wrote:

Tabby <tabbypurr> wrote:
On Monday, 3 August 2020 10:31:01 UTC+1, Adrian Tuddenham wrote:
Tabby <tabbypurr wrote:
[...]
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...

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.

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.

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.

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.

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

This one just isn\'t worth the spends.

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.

... And I\'ve yet to look at the other
one - I might get one good one not from the pair.

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.

More info...
The other UL41 works fine. The resistors in the other set show (less) damage 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 silent 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, but it seems to be working as well as it\'s going to. I also note there\'s no rf hiss, but no trivial way to measure conditions on the earlier valves.


NT

 

[Jan Panteltje]

On a sunny day (Mon, 3 Aug 2020 14:01:19 -0700 (PDT)) it happened Tabby
<tabbypurr@gmail.com> wrote in
<7fe87d85-545e-48b1-be4d-d56453c4a79fo@googlegroups.com>:

More info...
The other UL41 works fine. The resistors in the other set show (less) damage
but no visible repairs. Fitted to the set I\'ve been working on it sits at:
Vg=3D1v
Va=3D
163v at max volume, 169v at zero vol.
Ig =3D 1.5uA

For the first not so healthy UL41:
Ig =3D 9.1uA initially, 17uA after a few minutes. It pulls Va down to 139 silent
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, but
it seems to be working as well as it\'s going to. I also note there\'s no rf
hiss, but no trivial way to measure conditions on the earlier valves.

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.....

 

[Tabby]

On Tuesday, 4 August 2020 06:46:46 UTC+1, Jan Panteltje wrote:

On a sunny day (Mon, 3 Aug 2020 14:01:19 -0700 (PDT)) it happened Tabby
tabbypurr> wrote in
7fe87d85-545e-48b1-be4d-d56453c4a79fo@googlegroups.com>:

More info...
The other UL41 works fine. The resistors in the other set show (less) damage
but no visible repairs. Fitted to the set I\'ve been working on it sits at:
Vg=3D1v
Va=3D
163v at max volume, 169v at zero vol.
Ig =3D 1.5uA

For the first not so healthy UL41:
Ig =3D 9.1uA initially, 17uA after a few minutes. It pulls Va down to 139 silent
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, but
it seems to be working as well as it\'s going to. I also note there\'s no rf
hiss, but no trivial way to measure conditions on the earlier valves.

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

I can\'t think of any reason to get another 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 got an early roundie, prefer radios generally though


NT

 

[Jan Panteltje]

On a sunny day (Tue, 4 Aug 2020 01:23:48 -0700 (PDT)) it happened Tabby
<tabbypurr@gmail.com> wrote in
<d3e571d2-2bfb-4ed5-9fdf-2129f25e726co@googlegroups.com>:

>I got an early roundie, prefer radios generally though

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.

 

[Tabby]

On Friday, 7 August 2020 07:20:45 UTC+1, Jan Panteltje wrote:

On a sunny day (Tue, 4 Aug 2020 01:23:48 -0700 (PDT)) it happened Tabby
tabbypurr> wrote in
d3e571d2-2bfb-4ed5-9fdf-2129f25e726co@googlegroups.com>:

I got an early roundie, prefer radios generally though

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.

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.

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.

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


NT