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Volume pot question

brad martin
 

I see that the ubitx uses a 10k pot for the volume but I can’t seem to find a good knob for it. I do however have a 4.7k pot with a switch that has a more standard shaft size. Would I experience any ill effects by swapping the 10k for a 4.7k?

Brad

Nikos SV1IYF
 

No, you won't face any surprises.
For the same reason I used a 4K7 /A (linear) I had with dual pole power switch.
Really a robust pot from another era.
Naturally, the AF IC was toasted, but this has nothing to do with the pot.
Replaced it with an equivalent type and works ok since.
I also hooked up an RCA terminated audio cable at the extremes of this pot in order to connect my ADC sound converter at a fixed audio level for digital work. The ADC is high impedance input, but none the less it is in parallel with the 4,7KOhms of the pot.
No loading problem to the preceding stages, Cleaner audio signal is a bonus.
73 Nikos

Clark Martin
 

It will work but for best results it should be a log (AKA audio) taper, not linear. If you can’t tell, hook up an ohmmeter to the wiper and either end, set the pot to the midway point. The meter should NOT read around 2.35K ohms. It’s hard to say what the proper value for a log pot is, it depends on which side of the pot is connected and I don’t know the formula for resistance vs shaft position off hand.


Clark Martin
KK6ISP

On May 26, 2018, at 8:48 AM, brad martin <emclinux@...> wrote:

I see that the ubitx uses a 10k pot for the volume but I can’t seem to find a good knob for it. I do however have a 4.7k pot with a switch that has a more standard shaft size. Would I experience any ill effects by swapping the 10k for a 4.7k?

Nikos SV1IYF
 

You should go way-way back, Clark, to find true log rotary potensiometers.
Vacuum tube era, when power amplification was voltage related.
They were also higher value units, typically 47 to 100 KOhms, as far as I remember.
Then came the transistor, a current amplification device.
Using a tube log pot in a transistor circuit was something equivalent to a rollercoaster ride in sound (I know first hand).
Succeded by low impedance linear pots, or dual slope linear ones for closer correlation of angular position to output sound level.
Linear (long) throw faders* was another story until VCAs where established.
(*) Penny & Giles faders were the most expencive part in a pro console.
73 Nikos

brad martin
 

Thanks guys. I’ll probably install it them after I get done painting and shielding the case. I was sure it was a log and went out and tested it and yep it sure was I think it came out to 500 ohm mid range or something. Definitely wasn’t 2.35k.

Brad

On Sat, May 26, 2018 at 6:49 PM Nikos SV1IYF <info.mauve@...> wrote:
You should go way-way back, Clark, to find true log rotary potensiometers.
Vacuum tube era, when power amplification was voltage related.
They were also higher value units, typically 47 to 100 KOhms, as far as I remember.
Then came the transistor, a current amplification device.
Using a tube log pot in a transistor circuit was something equivalent to a rollercoaster ride in sound (I know first hand).
Succeded by low impedance linear pots, or dual slope linear ones for closer correlation of angular position to output sound level.
Linear (long) throw faders* was another story until VCAs where established.
(*) Penny & Giles faders were the most expencive part in a pro console.
73 Nikos