Valve Curve Tracer
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Transcript of Valve Curve Tracer
Minimalist Valve Curve TracerWhen I first started getting into
electronics, one of the first really useful things I designed and
built for myself was a curve tracer. A curve tracer is like the
ultimate valve tester, displaying the anode characteristic curves
in real time on an oscilloscope with X/Y capability (most
two-channel scopes have this).
My first design was understandably naive in its implementation and
would only test small triodes, but it worked OK (and still does).
All the curves in my book were done using it. Since then, more and
more DIY curve tracers have begun to appear on the internet. These
have tended to increase in complexity, often interfacing with a PC.
A particularly fine example is theu-Tracer. I decided it was about
time I built a new tracer that would test small pentodes as well as
triodes, but instead of increasing the complexity, I chose to go in
the opposite direction: What is the simplest curve tracer I could
come up with? The result is a minimalist design which actually uses
fewer parts than my very first beginner efforts!
The circuit can be broken into four functional blocks:
Anode supply
Control-grid (bias) supply
Screen-grid supply
Heater supply.Anode Supply:
The anode voltage needs to sweep from zero up to a high value. A
classsic way to do this is simply to rectify the AC voltage from a
transformer and apply it directly to the valve, as shown in the
firgure. To trace a curve we need to measure the anode current and
anode (to cathode) voltage, and plot them using the X/Y
oscilloscope function.
The current is measured by adding a 'current sampling resistor' in
series with the valve, and measuring the voltage across it. Ohm's
law tells us the current: I = V/R.
The anode voltage is scaled down with a potential divider rather
than expose the voltmeter (oscilloscope) to the full high-voltage
pulses. I used several resistors in series so they share the
high-voltage stress, so 1/4W resistors can be used throughout the
design, except where noted.
This divider is partially bypassed by capacitor C1. This
compensates for cable capacitance which otherwise leads to
looping/double traces. You may need to adjust the value of C1 to
suit your cables (my cables are rather long which is why I needed a
rather large 100pF of compensation).
Notice that the negative voltmeter probes are both connected to the
same point (the anode). This is an awkward necessity, because most
oscilloscopes share the same 'ground' connection between both
channels. Therefore, during use, the anode is safe to touch while
the grid and cathode are at high (negative) voltages- the oppsite
of what we are used to!
I used back-to-back 15V transformers to generate the required
voltages. Not only is this a cheap solution, it also minimises the
leakage currents between the mains and high-voltage supply which
otherwise flow to earth via the oscilloscope, also causing some
'looping' or double traces.
I also added a resistor to limit the peak current (power) that can
flow in the valve. This is useful when setting up, in case the
valve has been configured wrong. It is also useful for looking at
the curves of triode-connected pentodes when the anode is not used,
i.e. when the screen grid is used as a low-power anode.Control-Grid
Supply:
The control grid supply is the bias supply, and is usually the
trickiest part of a curve tracer to do well, especially if you're
aiming for simplicity. To trace each grid curve the circuit must
step the grid-cathode voltage from 0V down to negative voltages in
a staircase fashion. Ideally it should jump from one value to the
next when the anode voltage is at its highest or lowest peak,
otherwise you see the transitions on the screen as fine 'hairs' or
interference. To test various different valves, we need the size of
the bias voltage steps to be variable or switchable between say,
0.5V steps, 1V steps, or 2V steps.
In my design the staircase waveform is generated using a 4040
binary counter IC. The binary output from this chip is fed into an
R2R-ladder. This is a standard way of converting a binary number
into an analog voltage, and is really a type of digital-to-analog-
converter! This circuit uses only three bits of the counter, which
means there are 2^3 = 8 bias steps. Each step happens one every
mains cycle (1/50Hz), and I found that eight steps produces a
reasonably steady display on screen. Using more steps (i.e. tracing
more grid curves) leads to a flickering display because it takes
too long to go through the complete bias cycle. To synchronise the
bias steps with the anode voltage pulese, the pulses themselves are
fed to the clock input of the counter (via a high resistance to
protect the chip!). Some tweaking was required to get the counter
to transistion when the anode voltage drops close to zero. It's
still not absolutely perfect -you can see some retraces on pentode
curves- but for a simple circuit it's good enough.
You may be used to thinking of logic chips running off positive
supply voltages, where a binary count of 000 would correspond to
zero vots while a count of 111 would result in the maximum positive
voltage. Be we neednegativevoltages here, so in this case the chip
operates from a negative supply. A binary count of 000 now
corresponds to the maximum negative voltage, while a count of 111
result in the maximum voltage, which is zero (relative to
cathode).
Changing the size of the voltage steps is easy: just change the
supply voltage to the counter IC. This is done with a 337 negative
voltage regulator and a three-way switch (actually a double-throw
centre-off switch). This means the circuit requires no calibration
since the accuracy of the negative supply voltage is determined
quite satisfactorily by the accuracy of the regulator and resistors
(use 1% devices). The awkward-value resistors shown in the
schematic are in practice made up from the following standard
values:
640 = 620 + 20 ohms
1280 = 1.1k + 180 ohms
440 = 220 + 220 ohms
Diode D1 between grid and cathode protects the bias supply from a
direct short between anode and grid (if this happens the HT fuse
with blow).Screen-Grid Supply:
To test pentodes the screen voltage must be held constant relative
to the cathode. This sometimes requires a completely separate power
supply just for the screen, which I wanted to avoid. Therefore, in
my design the screen-grid supply comes from the main anode supply.
Since the main anode supply is already full-wave rectified, all
that is needed is a reservoir capacitor to get smooth(ish)
high-voltage DC. An extra diode is needed to block this DC voltage
from 'getting back' onto the anode supply, though.
The charging pulses into the first 47u capacitor lead to ringing in
the power transformer (aka diode switching noise). This leads to a
small but noticeable 'bump' in the grid curves. This is cured by
using UF4007 fast rectifiers, and by adding an RC snubbing network
across the transformer. The value of the resistor should be tweaked
to suit your transformers (i.e. use a trimpot at first, then
replace it with a fixed resistor). I found 10k to work best for
me.
The raw DC is regulated by an LR8 high-voltage adjustable regulator
IC. This is the only part that might be hard to find, especially
outside America (I got mine on eBay). Nevertheless, I decided it
was worth using because it results in an extremely simple circuit
and, more importantly, has built-in protection against thermal and
current overloads. Screen grids tend to be fragile, and I like the
fact that the LR8 won't deliver more than about 20mA and will shut
down if it overheats. OK, this also limits the testing only to
small pentodes that don't need lots of screen current, not big
bottles (except at very low screen voltages), but I would rather
have a bullet-proof circuit that is immune to user error. The diode
in series with the scren grid protects the regulator (and
potentiometer) from reverse bias such as would occur with an
anode-screen short circuit. This makes the circuit practically
indestructible. The screen voltage can be varied up to about 218V,
and I added a 10k resistor in series with the incoming DC to share
some of the dissipation, so the regulator does not start shutting
down so soon when the screen voltage is set to typical values. A
cheap moving-coil analog meter indicates the screen voltage (not
shown on the schematic).Heater Supply:
The first power transformer provides 15Vac which is rectified to
produce about 20Vdc for the heater and grid supplies. In my old
curve tracer I used an LM317 to make a variable heater supply. This
of course wastes rather a lot of power, and since I used a fairly
small power tranformer, it could only manage about 500mA for short
periods. In this new design I used a switching regulator. This
makes far more efficient use of the available power, so I can
supply at least 1A heaters without trouble, from the 15VA
transformer (note that the switching regulator must use a Schottky
diode; an ordinary diode won't work). In my prototype I actually
used a ready-made regulator PCB which are readily available from
Chinese sellers on eBay; cheaper than I could build it from
scratch. A cheap digital voltmeter indicates the heater voltage
(not shown on the schematic).
It is important to note that the heater regulator shares a common
connection with the valve cathode, so the heater-cathode insulation
is never stressed. Remember, in this design the anode is actually
earthed through the opscilloscope, so everything "below" the anode
is actually bobbing up and down below zero volts, including the
heater and grid supplies!External Connections:
I used three valve sockets: 7-pin (B7G), 8-pin (octal), and 9-pin
(B9A), but you could add more. All the pins are connected in
parallel, that is, pin-1 to pin-1, pin-2 to pin-2 etc. The pins are
then brought out via nine colour-coded wires (using the standard
resistor colour code). In other words, the brown wire is connected
to pin-1 on every socket, the red wire is connected to pin-2 on
every socket, etc. These have banana plugs that can be inserted
into panel-mounted sockets.
I used two banana sockets for the anode connection, with a switch
to swap between them. This way I can quickly flip between two
triodes in one envelope (grids and cathodes connected
together).
I further added a switch to connect the screen-grid socket either
to the screen-grid supply for ordinary pentode operation, or to the
anode supply through a 100R resistor for triode-connected
operation.
One final socket has no connection and simply provides a place of
rest for any unused cables.Using the Instrument:
The curves will appear in mirror image to how we normally view
them, but this is easy to put up with. If your scope has an
'invert' option on the X-input then you can use it to reverse the
curves back to normal (my scope doesn't).
I have chosen values so that 1mA through the valve corresponds to
1V vertical deflection on the scope, and 25V anode-to-cathode
voltage corresponds to 1V horizontal deflection. Since most scopes
have ten squares on the graticule, this means the whole width of
the graticule can correspond to 250V, which is about as high as the
anode voltage will go in this circuit.
There is no PCB for this project as it was purely experimental.
You'll have to lay out your own!Below is a picture of DanGu's
version of the ValveWizard minimalist tube curve tracer. He chose
to add a rotary switch to select different values of compensation
capacitor C1, to optimise the tracing. He also found it necessary
to use a 1nF capacitor (instead of 100pF) connected to the CLK pin
of the 4040, to ensure proper triggering on the 2V/grid curve
setting. His curves look even better than mine!
http://www.valvewizard.co.uk/curvetracer.htmlPagina muy interesante .te vas a calentar el coco!!!!!!