Fairchild 670's R115 (AC threshold) infos needed

Hi there, I' looking for some informations about R115 pot, used to determine the AC threshold of the compressor.
I'm attaching a snip of he schematics for reference.

My questions are:

1) the schem is not very clear about the actual value of the pot, is it 100K or 180K?

2) I've seen around that the center tap of a pot is often rated as a percentage, and most of the center tapped pots I've came across (mainly used for volume+loudness control) are 60% 40% - Do you know what percentage is the center tap rated in R115? Is this value vital to the operation of the device?

3) These pots - 100k dual linear with center tap - are quite hard to find, maybe only on custom order thus costing a lot, what is the drawback here if using a pot with the aforementioned specs but lacking a center tap?

4) Can a center tap be "faked"? i.e. can I fake the operation of a center tapped pot using extra components as voltage dividers?

thank you very much for taking the time to read this post

niccolo

The schemo I have clearly indicates 100k. Anyway, this is not a critical value, since these pots control the level of signal to the grids of the 12ax7, and these could be as high as 1Meg without any problem. I suspect a Lin pot had been chosen because the tracking is much better than Logs, and they added a tap in order to steer the law from Lin to some kind of more progressive. Tracking is very important because the balance of negative and positive going alternances is part of the distortion optimisation.
I think you should use a standard dual 100k Lin without tap, and connect 47k resistors between wiper and bottom of the pot that will give a retarded-Lin law that should be relatively satisfactory. There is room for experimentation there; reducing the 47k would give a pore Log-like law, but at the top of travel of the pots, the load applied to the secondary of T103 would be excessive. Probably a combination of 220k pots and 47k res would be preferrable in terms of smoothness of the curve. Increasing the pot value to 470k may not be possible, because the HF frequency response may suffer due to the large Miller-effect in the first stage.

Thank you very much for your response.
I will now read it with great attention, lots of food for thoughts here..

thanks again

niccolo

Good stuff.. thanks
I have a couple of questions though:

1)

you should use a standard dual 100k Lin without tap, and connect 47k resistors between wiper and bottom of the pot

Click to expand...

Could you please elaborate what do you mean by "bottom of the pot"?
I understand the wiper must be the center pin, right?

2)

Probably a combination of 220k pots and 47k res would be preferrable in terms of smoothness of the curve.

Click to expand...

cool.. I found a 250K dual linear pot, will it be ok? should I vary 47k accordingly if using the 250k instead of the 220k?

thanks again

niccolo

niccolo gallio said:

Good stuff.. thanks
I have a couple of questions though:

1)

you should use a standard dual 100k Lin without tap, and connect 47k resistors between wiper and bottom of the pot

Click to expand...

Could you please elaborate what do you mean by "bottom of the pot"?
I understand the wiper must be the center pin, right?

Click to expand...

The bottom is the pin at the beginning of the travel of the pot, the top is at the end of the travel. If you turn the pot fully CCW (counterclockwise), the wiper and the bottom measure zero ohms; conversely, if you turn it fully CW, the wiper and the pot should measure zero ohms. In fact they never measure exactly zero ohms because  of the wiper-to-track residual resistance.

2)

Probably a combination of 220k pots and 47k res would be preferrable in terms of smoothness of the curve.

Click to expand...

cool.. I found a 250K dual linear pot, will it be ok? should I vary 47k accordingly if using the 250k instead of the 220k?

Click to expand...

No. As I said, these resistors are used to "twist" the law, so you can start with 47k and if you find the gain progression is not to your liking, you could alter their value. The lower the value, the smoother the volume progression should sound, but as I mentioned earlier, too low a value would introduce too much load on the transformer.

Thanks again,
it all makes sense to me now, I'm sorry I could have just googled for top and bottom pins in a pot..

R115 is a 100k.  You will be fine with a good quality dual log taper.

Thanks!