[AMPS] strange idling current

[John T. M. Lyles]

Dick,

I will respond to these in reverse order, even though you apparently have
proven that the 3CX800A7 isn't really drawing 17 mA. Still, had it been,
you could be assured that it was trouble.  For instance, that current is
making about 50 Watts of DC plate dissipation for a 3 KV DC supply. (I just
guessed that voltage, sorry). It is not good to leave that going even in
standby. As Rich said, it should be below a mA. I would think that you'd
get some noise during receive with a tube like this, and of course, some
extra heat coming out the cabinet.


>Date: Sun, 14 Mar 1999 00:17:10 -0500
>From: "Dick Green" <dick.green@valley.net>
>Subject: [AMPS] 3CX800A7 plate current leakage
>
>What's a reasonable value for plate current leakage for a healthy 3CX800A7
>in GG with cathode bias voltage at cutoff? Is 17 mA OK, or would that
>indicate a leaky tube?

Now, on to the real problem you are dealing with:

>6. With the HV reconnected to the plates, I measured the voltage drop across
>the shunt resistor at about 30 mV, indicating a plate current of about 3 mA.
>Deducting the 1.5 mA that was already there, perhaps the tubes are each
>leaking about 750 uA. Rich says it should be 500 uA max, so it's high, but
>this is a lot closer to nominal than the 35 mA per tube the LED Ip meter and
>CPU would have me believe!

Good.

>7. FYI, cathode bias is about 33 volts in standby and 8.8-9.1 volts in
>transmit (0 to 1000W out.)

>From the tube curve from CPI/Eimac, (I had the U7 datasheet, and none from
Svetlana) you would expect to be below the 0.001 Ampere plate current for,
say 2 KV DC, with > 20 Volts of cutoff bias on grid (or cathode in your
case). That agrees with the 500 - 750 uA assumption. I didn't expect a tube
to get 'soft' and start conducting - besides some horrible internal
mechanical change such as warped filament or grid. Gas is another thing,
with high enough gas, you would also get some tube arcs or pings, not just
steady DC current, no?

>8. The meter circuit is composed of an op amp in what I believe is a
>differential amplifier configuration, with the inputs connected across the
>shunt resistor through some 1M resistors. There are a bunch of 1M and 10K
>resistors on the input side, but I don't know enough about op amps to know
>their function. The output from the op amp goes through a resistor divider
>pair and out to the CPU's monitoring circuits. There's a 5.1V zener
>connected across the output and ground, but I think that's just for
>overvoltage protection. The output from the meter runs through an analog
>multiplexer, then an A/D converter, and is fed to the CPU. It looks like the
>CPU's digital version is then converted back to analog by a D/A converter
>and fed to the LM3914 bar/dot drivers for the meter. Kinda complicated, huh?
>
>9. The output from the op amp is kinda suspicious. Here are some
>measurements for different operating states and power levels:

It does appear that your diff amp is showing an offset in the DC output level.

>The op amp supply voltages are taken from the same 36V supply used for the
>cathode bias. This supply originates on the low voltage board as +36V
>and -37.4V. Both voltages are dropped through a 1K resistor before getting
>to the op amp supply pins. In standby, the op amp gets +34V and -35.4V. In
>operate, the op amp gets +30.5V and -35V. Key down, the op amp gets +29V
>and -32V. I'm a little suspicious about the voltage discrepancy between the
>plus and minus supply rails, but remember that the op amp's output seems
>pretty high even in in standby mode, where the voltage difference isn't as
>great. Again, I don't know squat about op amps -- is that discrepancy
>between the plus and minus supply rails important to a differential
>amplifier? Each supply pin is has a 10uf electrolytic shunt to ground. I
>thought maybe one of them was leaking, but they measure OK on a capacitance
>meter (in circuit, though, and not at operating voltage.)

The answer to the question is yes, maybe. The diff amp should have a power
supply rejection ratio, if it is a commercial Op Amp. Some call it PSRR.
I've seen good numbers like 10 uV/V of power supply change. This is a 10 uV
output change for a volt of PS change. Some spec it as a ratio of 100-120
dB PSRR.  Off hand, I don't know how it relates to differerences in the 2
rails. I would suspect a diff amp is sensitive to this. Your readings are
much higher. A lot of these specs are more related to AC noise on the
output related to PS noise, for low noise applications in audio and
instrumentation.

I am very supprised that you have +/- 30 volt rails for an op amp. What
kind of device is this? Most op amps would die with more than about +/- 20
volts, or a total of 40 volts. Yours is more like 60 to 70!

If you have a DC present on the input of both pins to the diff amp, over
the manufactures stated common mode range, the op amp can do all sorts of
unpleasant things, including failing. What is the DC level at the pins,
related to power supply ground, on each input (+ and -) of the amplifier.
Common mode voltages are usually below what the supply rails are, for the
op amp to have a chance of working.

Finally, is there an offset trim pot on the amplifier? It is possible that
it is drifting (DC offset) or that it has a differential gain resistor
changing. If this happens, it will become a common mode amplifier too, and
offset will show up on the output for even small CM voltage on the inputs.
Those 10 K resistors around the op amp (should be in pairs that are closely
matched to be a diff amp) are critical.

John
K5PRO





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