Hi RATs,

Two questions here.

I have an AVO MkII valve characteristics meter. Does anyone have
calibration info for this model?

Secondly, I just got a CT-160 with a service manual. I haven't had a
close look at the manual yet but I saw on the first page that a
reference tube with a known transconductance is needed (I assume to
calibrate the Gm test). Obviously these reference tubes aren't
available anymore. Is there any way to calibrate without this tube?

Any help is appreciated.

Thanks,

John

"Johnny C" wrote in message
om...
Hi RATs,

Two questions here.

I have an AVO MkII valve characteristics meter. Does anyone have
calibration info for this model?

Secondly, I just got a CT-160 with a service manual. I haven't had a
close look at the manual yet but I saw on the first page that a
reference tube with a known transconductance is needed (I assume to
calibrate the Gm test). Obviously these reference tubes aren't
available anymore. Is there any way to calibrate without this tube?

Any help is appreciated.

Thanks,

John

Hi John,


Yep, if you check these then you shouldn't need a test valve.
Use a DVM to check between grid & cathode sockets, then with any given
setting of the grid voltage the mean DC reading obtained between grid &
cathode sockets should be 0.52 x the nominal setting of the grid control,
this should be maintained over the full span of settings.
i.e control at -6V the DVM should be -3.12V. Similarly pressing mA/V button
should result in a positive voltage change of 0.52VDC thus the grid volage
set as above should read -2.6V i.e. (-3.12 + 0.52).
Similiarly the applied anode & screen voltages may be checked by taking a
reading between anode (or screen) and cathode sockets of a suitably set up
valve holder with an ac voltmeter. These can be compared with the
appropriate anode or screen voltage switch settings as follows:
Nominal voltage (DC) setting of switch = ACV apparent at valveholder/1.1
Finally the accuracy of the movement can be checked via the link on the back
panel using milliameter.

Any Internal adjustments required cover Vg, mA/V and 'S' to adjust panel
meter as marked on the internal pots.

Hope this helps

Mike

"Mike Gilmour" wrote in message
...

Hi John,


Yep, if you check these then you shouldn't need a test valve.
Use a DVM to check between grid & cathode sockets, then with any given
setting of the grid voltage the mean DC reading obtained between grid &
cathode sockets should be 0.52 x the nominal setting of the grid control,
this should be maintained over the full span of settings.
i.e control at -6V the DVM should be -3.12V. Similarly pressing mA/V
button
should result in a positive voltage change of 0.52VDC thus the grid volage
set as above should read -2.6V i.e. (-3.12 + 0.52).
Similiarly the applied anode & screen voltages may be checked by taking a
reading between anode (or screen) and cathode sockets of a suitably set up
valve holder with an ac voltmeter. These can be compared with the
appropriate anode or screen voltage switch settings as follows:
Nominal voltage (DC) setting of switch = ACV apparent at valveholder/1.1
Finally the accuracy of the movement can be checked via the link on the
back
panel using milliameter.

Any Internal adjustments required cover Vg, mA/V and 'S' to adjust panel
meter as marked on the internal pots.

Hope this helps

Mike



Forgot to say that these details are for the calibration of the AVO valve
characteristic meter Mk II

"Mike Gilmour" wrote in message ...
"Mike Gilmour" wrote in message
...

Hi John,


Yep, if you check these then you shouldn't need a test valve.
Use a DVM to check between grid & cathode sockets, then with any given
setting of the grid voltage the mean DC reading obtained between grid &
cathode sockets should be 0.52 x the nominal setting of the grid control,
this should be maintained over the full span of settings.
i.e control at -6V the DVM should be -3.12V. Similarly pressing mA/V
button
should result in a positive voltage change of 0.52VDC thus the grid volage
set as above should read -2.6V i.e. (-3.12 + 0.52).
Similiarly the applied anode & screen voltages may be checked by taking a
reading between anode (or screen) and cathode sockets of a suitably set up
valve holder with an ac voltmeter. These can be compared with the
appropriate anode or screen voltage switch settings as follows:
Nominal voltage (DC) setting of switch = ACV apparent at valveholder/1.1
Finally the accuracy of the movement can be checked via the link on the
back
panel using milliameter.

Any Internal adjustments required cover Vg, mA/V and 'S' to adjust panel
meter as marked on the internal pots.

Hope this helps

Mike

Thanks for the info Mike. I'll get into it soon.

What about the functioning of the "Set Zero" pot which nulls the plate
current before pressing the mA/V button? The reason that I think this
needs adjustment is that for some valves with highish current (eg a
KT66 passing 76mA), the set Zero knob can't get the current nulled all
the way to zero so I can't do this test.

Do you think that this is a problem with the "set Zero" circuit or
just that the plate current is too high? The plate current isn't
above one hundred so I'm not sure if high plate current is the
problem.

Also, is the reset circuit breaker adjustable? Mine tends to trip if
the tube is passing above 60mA and you switch it into test. It works
OK if you put it into test while the tube is heating up and the
current increases slowly.

It sounds like you've worked on one of these before so hopefully you
know something about them.

Thanks for your help,

John

"Johnny C" wrote in message
om...
"
Mike

Thanks for the info Mike. I'll get into it soon.

What about the functioning of the "Set Zero" pot which nulls the plate
current before pressing the mA/V button? The reason that I think this
needs adjustment is that for some valves with highish current (eg a
KT66 passing 76mA), the set Zero knob can't get the current nulled all
the way to zero so I can't do this test.

Do you think that this is a problem with the "set Zero" circuit or
just that the plate current is too high? The plate current isn't
above one hundred so I'm not sure if high plate current is the
problem.

I think you need to check out that the accuracy of the meter movement first,
as backing off is just applied from a secondary winding on the mains
transformer via the backing off pot pot and half wave rectifier to one side
of the meter, so really not much to go wrong.


Also, is the reset circuit breaker adjustable? Mine tends to trip if
the tube is passing above 60mA and you switch it into test. It works
OK if you put it into test while the tube is heating up and the
current increases slowly.

The trip winding responds to anode and/or screen overs. Before going any
further I would really check out the measurements in my original posting
first and recalibrate if necessary. If any adjustments are made now without
knowing the overall accuracy it will only put you further into the mire :-(
Once you've completed the checks and recalibrated its possible your troubles
may go. If not get back to me and we'll take it from there.


It sounds like you've worked on one of these before so hopefully you
know something about them.



Thanks for your help,

You're very welcome,

Mike


John

"Johnny C" wrote
...
Secondly, I just got a CT-160 with a service manual. I haven't
had a
close look at the manual yet but I saw on the first page that a
reference tube with a known transconductance is needed (I assume
to
calibrate the Gm test). Obviously these reference tubes aren't
available anymore. Is there any way to calibrate without this
tube?
...

I have a CT160 and after much consideration decided it's not worth
calibrating unless it is miles out.

Adjustment, if necessary, is done by rotating the dials with respect
to their shafts. This merely centres the error on whatever valve you
are using as a reference. The valve data they publish is no
different for the CT160 so clearly doesn't take into account which
valve is used for calibration. Use one somewhere in the middle of
the tester's range, bearing in mind that the scale for Vg is log
linearised in a few steps. You can use the other tester to calibrate
the valve.

There was a disclaimer published by AVO at the time, saying that the
CT160 should not be taken too seriously, and that suspect valves
should always be retested at base.

In my case, "base" was probably Portsmouth and, after a life at sea,
the wiring is brittle. If yours is in better condition it may be
worth blueprinting some of the resistors. There are a couple of
little rectifiers in there that might be worth checking, but AFAIK
they last forever.

cheers, Ian