I've not looked at the Alpha 70 circuit, but from the discussion it
appears that the HV is measured by putting a 1 mA meter in series with a
high resistance voltage divider, with a shunt across the meter to give
the correct calibration. To me, that suggests Bill's surmise may be
correct: Alpha designed it badly in the first place, and for the reason
Paul mentioned. One can too easily dump 4kV onto the meter switch and
surrounding hookup wire when rotating the meter switch.
The method I prefer is to have a fixed voltage divider, with a high
resistance on the upper arm of the divider and a low resistance on the
bottom arm, the values of the resistors chosen to give a 1:1000 voltage
division ratio. For example, in my last amplifier, I wanted a 3kV full
scale meter reading. The voltage divider resistors serve double duty as
the bleeder resistor for the power supply, so I use a 200K/100W resistor
for the upper arm of the divider, and a 200 Ohm for the lower arm. With
these values, 3000V on the top of the divider gives 3V at the bottom
resistor. The bleeder current is then 3000V/200200 Ohms or about 15 mA.
To use a 1 mA meter, one then bleeds off 1 mA of this current by tapping
into the 3V point on the divider, with an appropriately chosen resistor
in series with the meter. That way, there's never more than 3V on the
meter circuit and its switch. Calibration of the meter is done by
adjusting the series resistor, which is large enough in value not to be
finicky.
In my amp, I use a 100 uA meter, which has an internal resistance of
about 2K ohms. The total series resistance (which includes the meter
resistance) is about 30K, so I use a 50 K trimpot to get the calibration
exact. In my case, the loading of the 30K on the 3V is negligible. With
a 1 mA meter the 3V is loaded down a few percent, but that is easily
compensated for by adjusting the series trimpot. The one disadvantage of
this design is that one shouldn't ground the negative terminal of the
meter directly to the chassis, but rather connect it to the B- line.
Otherwise, there will be some interaction of the HV meter with the grid
current.
73,
Jim W8ZR
On 1/20/2017 2:57 AM, Bill Turner wrote:
That still doesn't explain the original error.
I see two possibilities:
1. Alpha designed it wrong in the first place.
or
2/ Some component changed value.
So, which was it? Or was it something else?
73, Bill W6WRT
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Thu, 19 Jan 2017 20:55:32 -0500, W9AC wrote:
I must be the only person who didn’t know the Fluke DMM can source exactly 1 mA. If I
read the manual in the past, it probably didn’t register. Not sure if the internal
source is regulated as the battery ages. Time to review the manual. Anyway, that method
works great.
My favorite "grab 'n go" DMM is a Fluke 8060A from the mid-1980s. I have three of
them; two are still new on the shelf in the original boxes. It's not auto-ranging, so it
doesn’t slow me down when taking multiple measurements. I prefer to range it myself.
The 8060A's lowest DC resistance range is 200 ohms. In that position, the Modutec meter reads
34 ohms versus my manual method that shows 32 ohms. Because of various mechanical limitations,
the Fluke is probably more believable. In the 200-ohm range and with the leads applied to the
meter terminals, it reads...almost full scale as Jim pointed out! BTW, it's only the in the
lowest resistance range that the 8060A sources 1 mA.
To Bill's question, the 1% metal film resistors all measure on the high end of tolerance. 1%
of 1-meg is 10K ea. for a total of 50K, but that still doesn’t come close to the 500K
required change. With the Fluke DMM applied, it does not reach full scale and is off by
about half the discrepancy amount. If the Fluke is sourcing very close to 1 mA, then the
meter may be contributing to the error. I would like to believe it except that an identical
Modutec meter reads the same error. But I have to believe it because the math doesn’t
make sense otherwise. The meter's internal resistance of 32 ohms in parallel with the 15K
pull-down resistor is a miniscule resistance in the string.
Next, I need to source exactly 1 mA and observe the result. I may have two
identical meters that read low. Since the meter is used for HV and Ig, then
grid current is probably reading a bit low. Recall that my Alpha 70V uses the
same components as the 70A, yet metering in the 70V's HV position agrees with
the Fluke's HV probe.
By the way, with the meter at 4/5 scale for 4KV of HV, that results in 0.64
watt of 1-meg resistor heat dissipation. It looks like Alpha used 1-watt/1%
metal film resistors. My change brought the cold-end resistor in the string
down to 500K from 1-meg. The computed dissipation from that resistor is 0.32
watt. I used a 1/2 watt/1% metal film resistor, slightly elevated from the PCB.
Paul, W9AC
-----Original Message-----
From: MU 4CX250B [mailto:4cx250b@miamioh.edu]
Sent: Thursday, January 19, 2017 4:46 PM
To: Paul Christensen <w9ac@arrl.net>
Cc: amps@contesting.com
Subject: Re: [Amps] Alpha Seventy HV Meter Readings
Paul, your method of measuring internal resistance of the meter is just fine,
though it's easier just to use a DMM and measure the resistance directly. I
imagine you have a fluke DMM and these normally provide exactly 1.000mA in the
resistance mode. Actually, you're killing two birds with one stone because you
can see if your panel meter reads full scale.
73,
Jim w8zr
Sent from my iPhone
On Jan 19, 2017, at 12:12 PM, Paul Christensen <w9ac@arrl.net> wrote:
Here's a quick update. The math worked and the replacement resistor
value of 550K results in a HV reading of 4KV. This matches a Fluke DMM with HV
probe. The new HV meter divider string is now 4.55 Meg, slightly reduced
from the 5 meg design.
As previously indicated, the multimeter is a Modutec 1.0 mA DC
movement. I decided to make the effort and measure its internal DC
resistance. For the measurement, I first selected a series R and
applied a few DC volts from a bench power supply. The supply voltage
was increased until the meter read full scale (i.e., 1.0 mA DC).
Next, I shunted the meter terminals with a 2K pot as a rheostat and adjusted it
until the meter read exactly 1/2 scale.
At that point, current is evenly divided between the meter coil and
rheostat. Finally, I removed the rheostat from the circuit and
measured its resulting resistance. The answer is 32 ohms. That 32
ohms consists of the meter coil and any other internal resistance inside the
meter enclosure.
So, terminal-to-terminal, DC resistance is 32 ohms.
Back to the Alpha 70A: In addition to the original five, 1-meg HV
metering divider resistors, a 15K resistor shunts the meter terminals
when the multimeter is in the HV position. At least with this
amplifier, the 15K resistor is definitely NOT being used to sample current for
the HV reading.
The resistor is well more than 10x the meter's internal resistance. I
again verified HV multimeter accuracy by comparing results with the
15K resistor in and out of the circuit. As expected, there's little
change in deflection since the meter's internal resistance is swamping the 15K
shunt resistor.
In looking at other amplifier schematics from Ameritron, Heath and
others, most use the same HV metering configuration: the multimeter
coil is shunted with a resistor when the multimeter switch is in the
HV position. Now, it's possible that in those amplifiers that the
resistor may be used as a sample which has an additional benefit of
stabilizing readings -- but only IF the meter coil has a high internal
DC resistance that approaches the value of the shunt.
In the Alpha 70 series, the 15K meter shunt in the HV position is
performing only one function: The resistor is floating-down 4KV of
high voltage that would otherwise be present at the moment the
multimeter switch is engaged in the HV position. Otherwise, with no
meter current, the full 4KV supply potential appears on the multimeter
switch, which may lead to arcing in addition to it being a safety
concern. Well, it's already a concern in an openly exposed amp. But
few folks would intuitively think that the full HV potential could
appear on a small multi-meter switch. With phenolic used as the
insulating material on most multimeter switches, then all the more
reason to have the resistor in place. With the shunt resistor in
place on the HV supply side, that level is brought safely down on the cold end
of the HV divider string.
This is probably way more than anyone wants to read, but I wanted to
close the loop with my findings and report a solution that now results
in accurate HV readings.
Paul, W9AC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|