AVO 160A, Operating Instructions And Maintenance Manual

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Page 9 of 17
This document is a collaboration between Martin forsberg, Sweden, and Euan MacKenzie, Australia. Copyright Martin Forsberg &
Euan MacKenzie 2010-11-18
use a combination of resistors, either in parallel, eg 1.1k // 6.2k; or in series, eg 910
Ω
+ 24
Ω
, to adjust the
total current.
If you wanted to measure the current, you could insert a current meter in series with the circuit, but you
would then have to adjust the circuit for the additional resistance introduced by your current meter, as it
will also produce a voltage drop. It is therefore easier to measure the voltage drop across each resistor
with a digital multimeter, as that will have a high input resistance, typically 10M
Ω
, which will not affect the
circuit as much as the current measurement would do – quite apart from that, you do not need break the
circuit for voltage measurements!
This also means that if you need to replace the 90
Ω
anode current potentiometer, with another value, say
100
Ω
(since 90
Ω
will be hard to find nowadays) you can do that, but then you will need to reduce the
series resistor, R6, by the additional resistance in the new potentiometer, in order to keep the current at
12.5mA through the backing-off circuit; in this example, by 10
Ω
. However you will need to make a new
scale for the anode current potentiometer, and you will get a larger overlap on each range, but it will still
give the correct measurements for anode current. The practical minimum value will be close to 90
Ω
, as it
was originally. If for any reason, you need to go lower than 80
Ω
, then you will have to lower each 80
Ω
resistor, and increase the current, to maintain exactly 1V. A typical reason for replacing it would be that
the old one is open circuit, or perhaps has become non-linear, due to wear, or is otherwise damaged.
Making a new scale is quite easy if you use a 360° protractor, together with a multimeter to measure out
each step of either 0.1V, or 8
Ω
, and mark them on the protractor, then transferring them to a paper scale.
Or, alternatively, you can drill a hole through the centre of a fairly large protractor, or a piece of PCB, then
fasten the potentiometer in the hole and using a large knob on the potentiometer, and as you turn it, mark
each point on the protractor/pcb, which you can then transfer to a paper scale.
NB:- There is one case that has not been checked thoroughly so far, and that is whether the gm
measurements will be affected, if the potentiometer is changed to any value other than 90
Ω
! There is only
a slight risk of that, since each of the three 240
Ω
resistors, R24 to R26, are used to compensate for the
anode current control resistances - this should be investigated further, before I can recommend changing
the potentiometer for another value; but my guess is that the change doesn't matter, as it is the voltage
delivered between the two points that form the backing-off circuit, is what the measurement is compared
with, and since that is unchanged, so then is the current and resistance in that circuit path.
However, there also the possibility to put a potentiometer with a higher value in, and then shorting out the
last part of the track above 90
Ω
, and then make a new scale to fit the new potentiometer; then this will
work just as well as the old one, except that the new scale will be more cramped!
For the anode current control measurements to be accurate, you must ensure that the remainder of the
components in the AVO CT160 are within tolerance, and also that the tester is calibrated; however for the
anode current controls in themselves to be accurate, you must ensure that a voltage of 1V Mean DC is
developed across each of the 80
Ω
resistors.
The other diodes in the CV140 valves, V1 & V2, can also be replaced with Silicon diodes, and fortunately
the value used for the SET Vg potentiometer, RV3, is large enough so that nothing needs to be changed
or added there. Each rectifier in the CV140 has a forward voltage drop of approximately 2.2 - 2.9 V, at the
currents involved (which range from close to 11mA, to up to 14mA, through each diode), whereas a
Silicon diode has close to 0.7V at these currents. The difference between these voltage drops will be
taken up by RV3 as it has enough resistance for this adjustment.
A quick calculation of this follows:- 55V RMS is equivalent to 55 ÷ 1.1107 = 49.52V Mean, which half wave
rectified, becomes 49.52 ÷ 2 = 24.76V Mean DC.
For SET Vg calibration purposes, AVO state in their calibration procedure that a voltage of 20.8V Mean
DC should be present across the Grid Volts control, RV2; this then means that the diode in the CV140