Gates Sta Meter Issues

Hey guys, I had originally posted about some issues I was having with my stepped attenuator in my drip dual sta build, and once that was resolved I started having some metering issues and thought it would be best to start a new topic with the appropriate title.

The previous thread was here: https://groupdiy.com/index.php?topic=71011.0

Originally I was trying to use some old Simpson VU's and had tried them without the rectifier or resistors, and was advised that a 1mA meter would work best, so I ordered a few old Simpson 29 1mA meters. I've already cut the front panel out for the large meters so I wanted to find something that would fit correctly. The problem I'm having seems to still be there.

I have the meter zero'd and as I turn the input up I am measuring the CV from R35 and ground until I get 20db of compression, which reads -19VDC as per the note on the schematic. That should be metering right in the middle of the scale. Instead the scale seems to be log and every meter I've tried seems to do the same thing. The original meters of the Sta have a linear scale, as does the 1mA meter I am now using. Starting from the right, 0 is the .9mA, and -20db is right in the middle at .5mA. I just can't seem to get the meter to work in a linear way.

I don't know if I am missing something very simple here, but it's driving me crazy. Am I measuring the CV incorrectly, or do I need to remove the 10K trimmer that is in series with the positive side of the meter and use a shunt across the meter instead? I have experimented with using a 25 turns 2k trimmer for the shunt and winding in different amounts of resistance but all it seems to do is move the 0 calibration, which I then have to recalibrate using R38 (series trimmer).

I should also note that this happens no matter where I zero the meter, e.g. if I make the centre marking (.5 mA) of the meter my zero, it still moves in a log manner rather than linear.

If hope this all makes sense. I've attached a photo of the 1mA meter I am using so the scale markings make more sense to anyone that may be able to help.

Thanks so much!

So the meter is measuring current linearly. You can see that from the markings on the meter scale.
The sta level meter scale  is log: it goes from 0 dB (at the right) down to 40 dB (at the left).  Markings in dB are log, since dB ~ 20*log(V) So to zero the meter, you should bring the meter over the right side of the meter.  Under compression, the current decreases.  The meter MEASURES linearly, but the scale translates this to a log representation.

The meter circuit is simple and can be figured out using ohm's law (V=IR).  The original schematic says at 0dB the voltage is 0.6V and at -15 dB the voltage is 0.33V
The meter is in series with the 10k trimmer and in parallel with a 68 ohm resistor.
So to zero the 1 mA meter, we need R_trimmer=600 ohm. Then under -15 dB compression, the 0.33V gives you  0.55 mA (assuming the meter impedance is negligible)
The original sta level meter was 200 microAmps I believe. Since the trimmer only adjusts the zero and not the tracking, you might need to adjust the 68 ohm in parallel to get more driving voltage for the meter, and then re-zero it I think. But this changes how the tube is biased so you'd want to adjust R8/R9 to compensate and keep the tube bias at the same place.

So in summary, with a 1 mA meter you are having trouble getting the meter tracking correct, since you only have a zero control. If you can get a .2 mA meter it should work. Or you could  adjust the 68 / 130/ 130 resistors to increase the driving voltage for the meter.

dmp said:

The original sta level meter was 200 microAmps I believe.

Click to expand...

No, it's a 1mA meter. 

This thread has a lot of discussion on the meter:
https://groupdiy.com/index.php?topic=59017.0

Consensus seemed to be that the Sta Level used a 200 microAmp meter and that a 1mA could not be calibrated correctly. Some comments indicated a 500mA could be calibrated correctly.
I have a Sta Level build but I haven't started it yet so I can't say personally what works and what doesn.t
Doug, do you have a Sta Level with a 1mA in it?
What is r7 (68 ohm on schematic)?

https://www.don-audio.com/Vintage-Simpson-Level-VU-Meter-STA-LEVEL-Model

This replacement is 200 microAmps

dmp said:

https://www.don-audio.com/Vintage-Simpson-Level-VU-Meter-STA-LEVEL-Model

This replacement is 200 microAmps

Click to expand...

That doesn't mean it's right.    I've fixed a lot of original Stalevels, built a few too.  If you pull an original meter face off, you'll find it's the stock 1mA face flipped over with the Gates screening on the back.

But of course, yes, a 200 microamp gives you room to work shunt resistances as opposed to series resistance. 

R7 is part of the cathode resistance, the tap point for the meter.

EmRR said:

That doesn't mean it's right.    I've fixed a lot of original Stalevels, built a few too.  If you pull an original meter face off, you'll find it's the stock 1mA face flipped over with the Gates screening on the back.

But of course, yes, a 200 microamp gives you room to work shunt resistances as opposed to series resistance. 

R7 is part of the cathode resistance, the tap point for the meter.

Click to expand...

Good to know - thanks.

Like I said I don't have first hand knowledge of it.

I looked at old pics, and the face of an original meter also has in red a stamp in the lower corner which says

FS = 1MA

Thank you guys for helping. I am a bit confused about the original Sta meter being a log scale. The replacement meters available from don-audio looks like it has linear markings to me, in 5db increments, all evenly spaced from each other. 20db compression looks to be right in the middle, so i am trying to achieve the same result with my meter if possible.

the cathodes need 200 ohm to ground, so am i right in thinking changing r7 and then making up the needed resistance with different values of r8 and r9 should affect how my meter is tracking but keep the same resistance to ground for the cathodes. I could always make a custom scale so that it's accurate with the markings, but i just thought using a 1mA meter should work just as it is if that's what the original meters are.

Looking much closer i can see small variances in the spacings between the 5db increments which suggests it's not perfectly linear, although they are still very different to where the needle is pointing to for 20db compression on my meter. It is not near the centre at all

Wouldn't you know it, after using a 'mystery meter' on my diy Sta Level for the past five years (don't know the spec but it kind of works)  I just ordered an old stock 200 microAmp meter off ebay based on that Don Audio spec.

EmRR said:

But of course, yes, a 200 microamp gives you room to work shunt resistances as opposed to series resistance. 

Click to expand...

Could you explain this? I'd like to be able to use the 200 microAmp meter if possible.

Thanks,
Paul

So I'm probably extra dense at this hour, but re-reading I'm not terribly clear on exactly how you are determining you have 20 dB of compression, or where the meter is reading exactly.   

I think I follow the logic you're taking from the later schematic, with it's voltage scale for various amounts of compression. 

3.3:1 ratio means 20dB past threshold will result in a 6dB increase in gain and 14dB compression.  It's almost +30 over threshold to get 20 dB compression. 

To confuse it further, if the 0B2 is off, or the voltage divider across it, the threshold and ratio are different and that impacts the meter scale.  Any time you hack variable ratio into a vari-mu like this, the meter becomes a 'general indication of action', and not a thing resembling meaningful precision.  Single versus double will affect the way it reads too. 

It sounds like you are getting consistent results across various meter types.  Have you changed the 6386 for another to compare?  They can be notoriously different from another. 

That's all I have for now....

even increments in dB is log scale

The audio loss is a log function of the cathode current. Use a linear meter and it will tend to read nearly linear-in-dB.

I'm currently working on my meter scale / issue...

I've measured the compression curve of my unit:
https://groupdiy.com/index.php?topic=59017.msg973559#msg973559

I use a 200uA FSD Simpson Meter.
I had tried an additional shunt resistor (to fit 1mA), but it didn't made any difference.

My meter scale curve is in no way linear:
https://groupdiy.com/index.php?topic=59017.msg973619#msg973619

I've already started to make a new scale based on my measurements,
but I'm hesitating if this is the best way to do so...

EmRR said:

Any time you hack variable ratio into a vari-mu like this, the meter becomes a 'general indication of action', and not a thing resembling meaningful precision. 

Click to expand...

Any advice would be great...


ROCK-ON!

Sorry to fly around like a vulture but I am looking for a Simpson Model 29, 1mA… if you ever have your unused