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Old 28th Sep 2022, 3:26 am   #1
retailer
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Default Avo CT160 meter amplifier experiences.

While the case halves of the Avo CT160 I'm repairing are being painted I went through the calibration procedure, I'm using a 200uA movement in the original Avo meter case driven by a home built Crowthorne Tubes meter amplifier - I initially set up the meter using a battery powered dc supply for 30uA FSD measured against my DMM, the meter behaved perfectly on DC when I initially set it up and I had no issues with the CT160 calibration procedure.
With the calibration completed I was keen to see how it all worked, the valve panel has been set aside ready for an industrial clean so I soldered in a temporary octal socket wired for a common old 6V6. The initial test procedure gave expected results the meter nulled at zero with around -12V bias and 45mA anode current, but a check of the Gm came up at 2Ma/V - approx half of the expected 4.2mA/V. I've spent a good part of the last few days trying to find out where the fault lies and strongly suspect the fault lies with the meter and/or it's amplifier.

I clipped my DMM to the meter terminals at the 10K resistor - when setting the line voltage the meter needle just sits in the black 'set AC' section and the DMM reads around 88mV, this part seemed close enough - moving on to the 6V6 Gm test - with the meter nulled at zero, as I advance the mA/V scale the DMM reads 72mV when the Gm scale is at 4.5 - but the meter needle lagged behind and had not even reached the green section - if I continue to advance the Gm scale until the meter needle is resting on the 1Ma/V mark the DMM reads around 130mV which should place the meter needle way off it's scale, at this point I can see the tip of the meter needle vibrating.

After going through a lot of posts on the CT160 I read that some had wired a 'protection' capacitor and diodes across the 10K meter resistor - capacitor values varied from 10uF through to 100uF, reports were that the meter needle vibrating was greatly reduced, the biggest I had in my junk box was 4.7uF so I tried that. This time with the meter needle at the 1Ma/V mark the DMM read 72mV and the Gm scale was at 4.5Ma/V - this is what I would have expected from what I believe is a nos 6V6 valve.

I read quite a few posts about the Crowthorne and various other meter amps - most of the circuits are similar but I didn't see a post where others have had an issue like I've had, maybe in my case it is the dynamics of the meter movement itself, the original CT160 movement is very fragile, and lightly built, the coil is not wound on a former but is held together by varnish, in my original meter the coil had a few loose turns on the inside that had come away from the varnish one turn was rubbing on the centre pole piece and had eventually worn through.
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Old 28th Sep 2022, 10:05 am   #2
Craig Sawyers
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Default Re: Avo CT160 meter amplifier experiences.

Have you followed the calibration procedure? There are some oddities. Some steps call for an electronic test meter - in other words an equivalent of a DVM. And some call for an AVO7 or 8 - which are average reading, RMS calibrated instruments - and operate on signals like half-cycle rectified AC quite differently to a DVM.

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Old 28th Sep 2022, 11:12 am   #3
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Default Re: Avo CT160 meter amplifier experiences.

I almost commented previously regarding the 741 circuit which drives your 200uA meter movement.
The problem is related to the waveforms which the meter is being called upon to monitor: the backing-off waveform is half-wave rectified ac (50Hz mains derived), and the anode current is similarly half-wave but certainly not sinusoidal. So in order to reach a null balance, the resultant waveform will clearly be some complex ac waveform with a mean DC of zero.

Turing to the 741: it's configured with symetrical +/- supplies and the input reference at the centre-point. Hence to faithfully pass the signal to the meter, there needs to be sufficient op-amp output-voltage headroom feeding the resistor (R1?) in series with the meter. I don't know if this is the case here as there were no values on the schematic posted previously, but R1? should perhaps be reduced in value in order to ensure the waveform is not clipped prior to reaching the meter. Adding a capacitor in parallel with the meter is a very good idea: in this case you could split R1 and place the capacitor in parallel with the ( meter + half R1)
John
PS I discovered this issue when adding diode protection to a standard CT160. The problem only manifested itself when measuring high-gm valves, in my case EL84 pentodes.
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Old 28th Sep 2022, 1:22 pm   #4
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Default Re: Avo CT160 meter amplifier experiences.

I'm using a TL071 in place of the original 741 - my power supply and component values are as in the original Crowthorne circuit with approx +/- 9V DC derived from a 6-0-6 transformer, when I first ran into the problem I started looking at the 741 thinking it may not be able to handle the mixed wave forms, and noticed a lot of sources of online data recommend the minimum supply for a 741 is +/-10 volts, not knowing how old my 741's are I swapped out the transformer for one that gave +/- 12V DC, this made a marginal improvement, I swapped out the 741 for a TL071 this gave another marginal improvement but still not nearly good enough, the 4.7 uF capacitor has so far bought the instrument into useable condition, I'll have to wait until the valve panel is cleaned before I check it with higher Gm valves.

In my forum search I came across a circuit put forward by a forum member Karsten, I already have 99% of the components so I designed a pcb in Fusion360 and will route it tomorrow and see how it performs, the idea of protecting the meter has some appeal.

On the calibration:- I use a Fluke 87, I have an old AVO model 7 and used that along side the Fluke when I calibrated my VCM II - they were always with 0.5 - 0.75 volts of each other and no matter which one I used I noticed no difference in the end result, so I now just stick to the DMM, it's easier to read.
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Old 28th Sep 2022, 9:37 pm   #5
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Default Re: Avo CT160 meter amplifier experiences.

Did you use ferrite beads on the wires that you hooked up the socket with?

It could be a case of valve oscillation that results in incorrect measurements.

Karsten's design is sound and verified and works just as it should.
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