Useful source of switched log pots (EG for DAC90A etc)

[Yorkie]

Re the shaft size, it's neither 6mm nor 6.4mm - it's 6.25mm, which is 0.246" so it's just 4 though undersized. Needless to say, the push on 'D clip' thumbwheel knob is a nice snug fit on the shaft, as the pic below will show. As to the shaft itself, though I'll be cutting it down to 20 mm, though the shaft is rigid plastic - not the least bit 'bendy' so I'd be more than happy if need be to use the full 50mm length.

I've had the type that Joe mentioned from the likes of Maplin.

As to using the correct law (log or linear) which Tony mentioned, when I restored a Portadyne Princess and Noble and 'lookalike' I suspected that the track for the reaction/vol control was anti-log, so I checked it by drawing a graph at intervals of rotation, and indeed it was. I wonder how many 'restorers' have assumed that Vol control = Log? We'll never know. No idea what the effect on performance might have been had a linear pot been substituted. As it was, I was able to repair the switch so didn't need to change the pots, which had duff switches on both sets.

Drifting off topic a bit, but for what it's worth, I've attached the graph of the original Portadyne antilog pot, a 50k antilog pot, a linear pot, and a log pot. Utterly irrelevant of course, but might be of passing interest.

[Aerodyne]

That's interesting, David. I'm surprised the Noble used an anti-log pot. As for shaft sizes, it would seem that scant regard for precise accuracy has been involved in the making of the plastic shafts you mention. Its neither one thing or't'other. I worked to half-thou limits in printing - accuracy was essential in those letterpress days with the top-notch stuff we had to produce. Maybe it isn't important now - but I wonder what anyone who tried to fit a 6mm knob on the shaft would have thought?
You must know this, but I'll tell you anyway: a lin pot fitted as a volume control where a log pot should have been puts the control's greatest effect right up at one end, most disconcerting.
I have absolutely no idea what would happen if a log pot was fitted where an antilog should be!
Tony

[pwdrive]

If you fitted a log pot instead of an anti log pot as a screen grid regeneration control then it would be very critical to adjust to just below oscillation (max signal)

You can get a reasonable aproximattion to anti log from a linear pot by including a taper resistor, in the case of a screen grid regen the resistor would go between the HT side of the pot and the pots slider, a typical value would be around 8.2kk for a linear pot value of say 50k.

Also note that you can't do an angular advance versus resistance plot with an ohm meter and expect an anti log plot... It has to remembered that it is a voltage divider therefore to get the correct plot it has to be connected to a voltage source across it's two ends and then angular versus voltage can then be read off the slider.

Using an ohm meter from any end to the slider to plot angular versus resistance will always give a linear plot because the resistors are linear....

I've used this method in a regen I built and although it's not a true anti log it's much better than linear and most certainly better than log.

Lawrence.

[Yorkie]

Also note that you can't do an angular advance versus resistance plot with an ohm meter and expect an anti log plot... It has to remembered that it is a voltage divider therefore to get the correct plot it has to be connected to a voltage source across it's two ends and then angular versus voltage can then be read off the slider.

Using an ohm meter from any end to the slider to plot angular versus resistance will always give a linear plot because the resistors are linear....

Ah, of course Lawrence, using and Ohmmeter with no voltage applied is where I ('scuse the pun!), lost the plot when I was trying to use a taper resistor and seemingly failed - see post 12 at this thread:

http://golbornevintageradio.co.uk/forum/...p?tid=2177&highlight=Noble+Radio

What surprised me about the Noble and Princess, was how well they performed with such a low component count.

[Radio Fixer]

Sure Blore will do pots with 1/4" shafts and of course any value and log or lin as well with or without a switch. I thought they were fine; certainly better than you would get anywhere else.

And on radios like the Ekco AC97 a real get you out of trouble company. This uses a twin gang type, with switch, having a 500K Ohm log section and a 50K Ohm section, used for volume related bass boost or ‘loudness’. And with switch !

Where else but Blore are you going to get one of these?

Gary