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Default Oldtimer question (valves/tubes)

Gentlemen,

I've got this RF sig gen I'm testing at the moment. It was made in 1955
and is all valve (I say "all valve" but there are only two tubes in it
and only one of those generates the RF - the other's 400Hz for the
modulation).
Anyway, it has 8 ranges in total covering 100KHz to 240Mhz. The
oscillator tube is a double triode, a 12AT7. One half handles range from
100Khz to 30Mhz and the other takes care of 30 - 240Mhz. Now, it all
works great EXCEPT for one range (the 3rd lowest) which has appreciable
distortion present on the output. It looks a bit like it's being over-
driven on my scope. My question is, does a valve like a 12AT7 require
different DC biasing points for every range of frequencies? Obviously the
range switch is switching in different combinations of coils and
capacitors, but is it likely to be also switching in different cathode-
grid DC biasing at the same time?

TIA



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On 5/12/19 6:04 AM, Cursitor Doom wrote:
My question is, does a valve like a 12AT7 require
different DC biasing points for every range of frequencies? Obviously the
range switch is switching in different combinations of coils and
capacitors, but is it likely to be also switching in different cathode-
grid DC biasing at the same time?


A make and model would help considerably.
But, I'm gonna generalize here, if it's cheap enough to only
have two tubes, they certainly didn't waste any money on
complications like switchable bias.

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Keep in mind that a 2-tube SG is, was and ever will be a hobby device. With that in mind, a little, even a lot of distortion which is still "on frequency" hardly matters to a hobbyist aligning a consumer grade radio.

Peter Wieck
Melrose Park, PA
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On Sun, 12 May 2019 11:04:06 +0000, Cursitor Doom wrote:

[...]

Sorry, guys, it's a Taylor 68A/M. I can't find the exact model on line
but this one is as close as it gets:

https://www.radiomuseum.org/r/taylor...tor_68a_2.html

The grid/Cathode bias does change with range selection; I just measured
it. There doesn't seem anything obviously amiss with the readings:

Range 8 (lowest frequency) -25V
7: -7V
6: -3.4V
5: -1V
4: -0.3V
3: -1.6V
2: -1.8V
1: -1.0

Anode is +80 to +90VDC vs. Cathode over all ranges.




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On 2019/05/12 6:05 a.m., Cursitor Doom wrote:
On Sun, 12 May 2019 11:04:06 +0000, Cursitor Doom wrote:

[...]

Sorry, guys, it's a Taylor 68A/M. I can't find the exact model on line
but this one is as close as it gets:

https://www.radiomuseum.org/r/taylor...tor_68a_2.html

The grid/Cathode bias does change with range selection; I just measured
it. There doesn't seem anything obviously amiss with the readings:

Range 8 (lowest frequency) -25V
7: -7V
6: -3.4V
5: -1V
4: -0.3V
3: -1.6V
2: -1.8V
1: -1.0

Anode is +80 to +90VDC vs. Cathode over all ranges.





If you are troubleshooting this then be aware that plate resistors often
fail high and are (after capacitors) the next suspects in repairing old
tube/valve equipment. Schematics are a huge help of course to be able to
verify meeting manufacturers BOM as older parts colours have usually
faded to obscurity....antique radio societies will be an aid in tracking
down schematics...

If you can't find a match check out EICO or Heathkit of the same vintage
as chances are someone may have copied the schematics to their product.

John :-#)#

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On Sunday, 12 May 2019 14:05:35 UTC+1, Cursitor Doom wrote:
On Sun, 12 May 2019 11:04:06 +0000, Cursitor Doom wrote:

[...]

Sorry, guys, it's a Taylor 68A/M. I can't find the exact model on line
but this one is as close as it gets:

https://www.radiomuseum.org/r/taylor...tor_68a_2.html

The grid/Cathode bias does change with range selection; I just measured
it. There doesn't seem anything obviously amiss with the readings:

Range 8 (lowest frequency) -25V
7: -7V
6: -3.4V
5: -1V
4: -0.3V
3: -1.6V
2: -1.8V
1: -1.0

Anode is +80 to +90VDC vs. Cathode over all ranges.


If you've eliminated other faults as possible then time to adjust the bias on the iffy range.


NT
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On Sun, 12 May 2019 08:46:42 -0700, tabbypurr wrote:

If you've eliminated other faults as possible then time to adjust the
bias on the iffy range.


It breaks my heart to do this, but I think it's a junker. On further
inspection some screwdriver jockey's ****ed around with it and since I
only paid a fiver for it, it's time to consign it to recycling.





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On Sun, 12 May 2019 17:02:11 +0100, Mike Coon wrote:

The computer museum at Bletchley Park has (or had) a valve re-cycling
exchange. I bequeathed (or dumped) my father's collection of valves and
his (Taylor?) big wooden-cased valve tester to them and they made
appropriate gratitude noises...


Oh, I save the tubes alright; I'm not that much of a philistine.
That valve tester of yours was probably an AVO. I've got a couple of
those in my collection.





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Default Oldtimer question (valves/tubes)

ECC81 is the same.

Cursitor Doom a √©crit le 12/05/2019 √* 13:04¬*:
Gentlemen,

I've got this RF sig gen I'm testing at the moment. It was made in 1955
and is all valve (I say "all valve" but there are only two tubes in it
and only one of those generates the RF - the other's 400Hz for the
modulation).
Anyway, it has 8 ranges in total covering 100KHz to 240Mhz. The
oscillator tube is a double triode, a 12AT7. One half handles range from
100Khz to 30Mhz and the other takes care of 30 - 240Mhz. Now, it all
works great EXCEPT for one range (the 3rd lowest) which has appreciable
distortion present on the output. It looks a bit like it's being over-
driven on my scope. My question is, does a valve like a 12AT7 require
different DC biasing points for every range of frequencies? Obviously the
range switch is switching in different combinations of coils and
capacitors, but is it likely to be also switching in different cathode-
grid DC biasing at the same time?

TIA






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On Sun, 12 May 2019 19:28:17 +0200, Look165 wrote:

ECC81 is the same.


Yes, and fortunately I did have some spare new ones to try (on the
suggestion of a pal of mine who I would have ignored were it not for the
fact that he's 20 years my senior and has a doctorate in electronics). It
didn't fix the problem - unlike him I didn't expect it to - but did tend
to suggest that the issue was one of poor biasing (for whatever reason)
as I'd expected.
Sadly the thing has passed its prime by many years. It's not worth
continuing with when you discover the many and various pot cores have
nearly all been shattered by some dimwit who didn't know what he was
doing.



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On Sun, 12 May 2019 12:49:21 -0500, Fox's Mercantile wrote:

There is NO adjusting the bias. It's NOT adjustable.
The difference in readings is based on the difference in output for each
range. It's entirely likely it was like that new from the factory.


Whist it's true there are no trim pots involved, the fixed resistor
values will have changed over the last 65 years, which is quite possibly
responsible for the fault. They would need to be hooked out individually
and replaced, but it's not worth it.




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On Sun, 12 May 2019 22:00:16 +0100, Mike Coon wrote:

Highly unlikely. I still have Dad's "Universal Avometer Model 7", so if
the valve tester was from the same stable I would have linked the two.
Unfortunately I can't find any record of my offer to Bletchley, to a
chap who IIRC has since died.


I think you'll find AVO took over Taylor in the late 1950s so it's all
just about dates of manufacture.




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On Sunday, 12 May 2019 19:29:45 UTC+1, Cursitor Doom wrote:

Whist it's true there are no trim pots involved, the fixed resistor
values will have changed over the last 65 years, which is quite possibly
responsible for the fault. They would need to be hooked out individually
and replaced, but it's not worth it.


One needs replacing. Sounds like a trivial job. Send it to me if you want.


NT
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On Sun, 12 May 2019 11:04:06 -0000 (UTC), Cursitor Doom
wrote:

Gentlemen,

I've got this RF sig gen I'm testing at the moment. It was made in 1955
and is all valve (I say "all valve" but there are only two tubes in it
and only one of those generates the RF - the other's 400Hz for the
modulation).
Anyway, it has 8 ranges in total covering 100KHz to 240Mhz. The
oscillator tube is a double triode, a 12AT7. One half handles range from
100Khz to 30Mhz and the other takes care of 30 - 240Mhz. Now, it all
works great EXCEPT for one range (the 3rd lowest) which has appreciable
distortion present on the output. It looks a bit like it's being over-
driven on my scope. My question is, does a valve like a 12AT7 require
different DC biasing points for every range of frequencies? Obviously the
range switch is switching in different combinations of coils and
capacitors, but is it likely to be also switching in different cathode-
grid DC biasing at the same time?


I have a Radiosonde weather balloon transmitter which has a
single valve. The valve is a 3A5 twin triode with one being the
RF oscillator at 72 MHz and the other an audio oscillator which
was controlled by the sensors.

I did have a one valve test oscillator which was powered by
a nine volt grid bias battery.



TIA



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On Sun, 12 May 2019 15:53:17 -0000 (UTC), Cursitor Doom
wrote:

On Sun, 12 May 2019 08:46:42 -0700, tabbypurr wrote:

If you've eliminated other faults as possible then time to adjust the
bias on the iffy range.


It breaks my heart to do this, but I think it's a junker. On further
inspection some screwdriver jockey's ****ed around with it and since I
only paid a fiver for it, it's time to consign it to recycling.


I picked up a valve paging transmitter from a rubbish tip.
It was in working order, 5 watts output on 27.212 MHz,
with the crystal in an oven.
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On Mon, 13 May 2019 20:53:55 +1000, Lucifer wrote:

I picked up a valve paging transmitter from a rubbish tip.
It was in working order, 5 watts output on 27.212 MHz, with the crystal
in an oven.


Nice find. It's amazing what gets thrown out! I dream of owning an
apartment overlooking a vast garbage tip so I can spot anything
potentially valuable being dumped. ;-)





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On Sun, 12 May 2019 18:12:10 -0700, tabbypurr wrote:

One needs replacing. Sounds like a trivial job. Send it to me if you
want.


Thanks for the offer, Tabs, but it's too far gone in other respects I'm
sorry to say. Trust me, I hate throwing anything away so it really has to
be terminal case for me to even consider junking it.



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On Mon, 13 May 2019 20:46:03 +1000, Lucifer wrote:

I did have a one valve test oscillator which was powered by a nine volt
grid bias battery.


Tust the grid alone was battery powered? Seems a bit of a strange idea. I
mean, it's entirely feasible, but if you have to generate higher voltages
for the anode from the mains....





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On Mon, 13 May 2019 11:31:05 -0000 (UTC), Cursitor Doom
wrote:

On Mon, 13 May 2019 20:46:03 +1000, Lucifer wrote:

I did have a one valve test oscillator which was powered by a nine volt
grid bias battery.


Tust the grid alone was battery powered? Seems a bit of a strange idea. I
mean, it's entirely feasible, but if you have to generate higher voltages
for the anode from the mains....


The nine volts was the anode voltage!
I had a hybrid car radio with 4 valves and 2 transistors.
The valves used the 12 volts for the anode voltage.

Valves don't need high voltage on the anode for low power
applications.


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On Mon, 13 May 2019 11:23:51 -0000 (UTC), Cursitor Doom
wrote:

On Mon, 13 May 2019 20:53:55 +1000, Lucifer wrote:

I picked up a valve paging transmitter from a rubbish tip.
It was in working order, 5 watts output on 27.212 MHz, with the crystal
in an oven.


Nice find. It's amazing what gets thrown out! I dream of owning an
apartment overlooking a vast garbage tip so I can spot anything
potentially valuable being dumped. ;-)


I have found so many amazing things at tips.
A bag of stained glass hobby bits and pieces.
Photo copiers I spent hours dismantling.

Auctions are a good cheap source of what many people call junk.
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On Mon, 13 May 2019 21:52:09 +1000, Lucifer wrote:

I have found so many amazing things at tips.
A bag of stained glass hobby bits and pieces.
Photo copiers I spent hours dismantling.

Auctions are a good cheap source of what many people call junk.


What's worth salvaging from old photocopiers? I vaguely recall there's
some exotic goodies in them somewhere.





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On Mon, 13 May 2019 21:46:56 +1000, Lucifer wrote:

The nine volts was the anode voltage!
I had a hybrid car radio with 4 valves and 2 transistors.
The valves used the 12 volts for the anode voltage.

Valves don't need high voltage on the anode for low power applications.


I'll have to look into that. I had no idea of such extraordinarily low
plate voltages as that. 9V eh? Wow! Do you happen to remember the type
number of those? I'd be fascinated to check out the data sheet for 'em.





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On 5/13/19 6:46 AM, Lucifer wrote:
On Mon, 13 May 2019 11:31:05 -0000 (UTC), Cursitor Doom
wrote:

On Mon, 13 May 2019 20:46:03 +1000, Lucifer wrote:

I did have a one valve test oscillator which was powered by a nine volt
grid bias battery.


Tust the grid alone was battery powered? Seems a bit of a strange idea. I
mean, it's entirely feasible, but if you have to generate higher voltages
for the anode from the mains....


The nine volts was the anode voltage!
I had a hybrid car radio with 4 valves and 2 transistors.
The valves used the 12 volts for the anode voltage.

Valves don't need high voltage on the anode for low power
applications.


http://www.junkbox.com/electronics/lowvoltagetubes.shtml

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On Mon, 13 May 2019 14:23:35 +0000, Cursitor Doom wrote:

On Mon, 13 May 2019 21:52:09 +1000, Lucifer wrote:

I have found so many amazing things at tips. A bag of stained glass
hobby bits and pieces. Photo copiers I spent hours dismantling.

Auctions are a good cheap source of what many people call junk.


What's worth salvaging from old photocopiers? I vaguely recall there's
some exotic goodies in them somewhere.


In a good copier, there's an excellent lens, for a start.
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On Mon, 13 May 2019 10:57:54 -0400, Ralph Mowery wrote:

Back in the eairly 1960's I built a one tube receiver for the airplanes
that ran on 12 volts only. Plans were in a Popular Electronics of the
same years. Had to use earphones for it.


Those awful high impedance ones, by any chance?





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On Mon, 13 May 2019 09:39:11 -0500, Fox's Mercantile wrote:

http://www.junkbox.com/electronics/lowvoltagetubes.shtml


Thanks!



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On Mon, 13 May 2019 18:14:29 +0100, Mike Coon wrote:

See https://www.radiomuseum.org/r/taylor...ter_47a47.html


Very nice, that. You should hang on to it; valves seem to be making a
comeback in some areas.





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On 14/5/19 12:23 am, Cursitor Doom wrote:
On Mon, 13 May 2019 21:52:09 +1000, Lucifer wrote:
What's worth salvaging from old photocopiers? I vaguely recall there's
some exotic goodies in them somewhere.


Perhaps not exotic, but they tend to have a few nice 24V motors.
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On Mon, 13 May 2019 14:27:27 -0000 (UTC), Cursitor Doom
wrote:

On Mon, 13 May 2019 21:46:56 +1000, Lucifer wrote:

The nine volts was the anode voltage!
I had a hybrid car radio with 4 valves and 2 transistors.
The valves used the 12 volts for the anode voltage.

Valves don't need high voltage on the anode for low power applications.


I'll have to look into that. I had no idea of such extraordinarily low
plate voltages as that. 9V eh? Wow! Do you happen to remember the type
number of those? I'd be fascinated to check out the data sheet for 'em.


I don't remember other that the car radio used transistors for the
second audio and the audio output and valves for the rest.
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On Mon, 13 May 2019 09:39:11 -0500, Fox's Mercantile
wrote:

On 13/05/19 6:46 AM, Lucifer wrote:
On Mon, 13 May 2019 11:31:05 -0000 (UTC), Cursitor Doom
wrote:

On Mon, 13 May 2019 20:46:03 +1000, Lucifer wrote:

I did have a one valve test oscillator which was powered by a nine volt
grid bias battery.

Tust the grid alone was battery powered? Seems a bit of a strange idea. I
mean, it's entirely feasible, but if you have to generate higher voltages
for the anode from the mains....


The nine volts was the anode voltage!
I had a hybrid car radio with 4 valves and 2 transistors.
The valves used the 12 volts for the anode voltage.

Valves don't need high voltage on the anode for low power
applications.


http://www.junkbox.com/electronics/lowvoltagetubes.shtml


Very interesting. Thank you.
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In article , says...

On Mon, 13 May 2019 18:14:29 +0100, Mike Coon wrote:

See
https://www.radiomuseum.org/r/taylor...ter_47a47.html

Very nice, that. You should hang on to it; valves seem to be making a
comeback in some areas.


Too late; assuming they have not thrown it away, it has been at
Bletchley Park for several years, hopefully supporting their old
computer resusitation projects.

Mike.
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