Oldtimer question (valves/tubes)

[Cursitor Doom]

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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[Fox's Mercantile]

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.

--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

 

[pfjw@aol.com]

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

 

[Cursitor Doom]

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_afrf_signal_generator_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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[John Robertson]

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_afrf_signal_generator_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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MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
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www.flippers.com
"Old pinballers never die, they just flip out."

 

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_afrf_signal_generator_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

 

[Cursitor Doom]

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 fucked around with it and since I
only paid a fiver for it, it's time to consign it to recycling.





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[Mike Coon]

In article <qb9fhd$gun$2@dont-email.me>, curd@notformail.com says...

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

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...

Mike.

 

[Cursitor Doom]

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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[Look165]

ECC81 is the same.

Cursitor Doom a Êcrit le 12/05/2019 à 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

 

[Cursitor Doom]

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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[Fox's Mercantile]

On 5/12/19 10:46 AM, tabbypurr@gmail.com wrote:

On Sunday, 12 May 2019 14:05:35 UTC+1, Cursitor Doom wrote:
The grid/Cathode bias does change with range selection; I just measured
it. There doesn't seem anything obviously amiss with the readings:

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

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.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

 

[Cursitor Doom]

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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[Ralph Mowery]

In article <-LqdnbRNIbQ8wUXBnZ2dnUU7-bPNnZ2d@giganews.com>,
jdangus@att.net says...

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.

Most likely the difference in the Q of the circuits and maybe the gain
of the tube at different frequencies. That would change the power of
the osicllator and voltages. One thing I would look at would be all the
reisitors in the generator. Those old carbon ones often change value
after a number of years, most seem to go up in value.

 

[Ralph Mowery]

In article <qb9m0d$gun$7@dont-email.me>, curd@notformail.com says...

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.

In the past some coil problems have been caused by the change in the
cores. Not due to being shattered but just that the material ages.

 

[Mike Coon]

In article <qb9hfa$gun$3@dont-email.me>, curd@notformail.com says...

That valve tester of yours was probably an AVO. I've got a couple of
those in my collection.

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.

Mike.

 

[Cursitor Doom]

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

 

[Mike Coon]

In article <qba4km$gun$9@dont-email.me>, curd@notformail.com says...

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.

Oh, thanks, I might look that up. But the valve tester would certainly
be older than that. It did have a lacquered wooden case, after all!

Mike.

 

[Cursitor Doom]

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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