Portaflex Build - Pot Question

[GregS]

Although (to my understanding) the pots are supposed to be audio taper for all three (bass, treble, volume) I ordered 3 audio taper and 3 linear taper just in case. I ordered Alpha brand and one type of pot is marked "A1M" on the part and the other "B1M". The A1M has a larger diameter nut. Naturally I would assume the "A1M" would indicate audio taper because of the A but the "B1M" creates some uncertainty. Is the letter B associated with linear taper?

It's important for me to be clear as to what is what as the required hole sizes are different between the two. I don't want to drill larger holes in my chassis for the A1M and then discover that I should be using the B1M as the holes would now be too big.

Thanks,

Greg

[Enzo]

1. B generally means linear, but there are excepions, so...

2. You can always check a control and see how it is tapered. Measure end to end - and don;t expect the value to be right on, tolerances are pretty loose on most controls - for the total resistance of the pot. Now venter the wiper and measure from either end to it. If the wiper sits at about half the total resistance it is linear - halfway up is half the resistance. If the resistance is maybe 15-20% of the total or something like that, you have an audio taper.

3. So you don't know if one pot or the other should be used? Drill the small hole and mount the small shafted pot. If it works OK, leave it. If you want to try the other one anyway, then take the wires of the small shaft one and transfer them to the larger pot - but don't mount it. Leave it in mid air or fold a towell under it. Try it now. If you like this pot better, then enlarge the hole and mount it.

Point being you can wire in a control without installing it. Run the amp on the bench sitting open while experimanting.

4. Weren't the pots in little bags that identified them? If it didn't spell out the taper, it should at least show the catalog part number so you can look it up to see which it is.

[tubeswell]

Yep A is the audio taper pot. If you are in doubt, hook the wiper and one side up to your R-meter with some 'gator clips, and you will see that the resistance measurement responds to your turning in a logarithmic fashion, whereas the linear taper pot will respond in an "even" fashion [- which is not the same thing as an evening fashion ;-) ]

[txstrat]

I had a look on the schematic of the B15-N of '68 and found there are no indications of using audio taper pots. Might be that I have a different schematic.
When I built my clone of it I remember having used the audio taper pots, anyway.
The amp sounds great and the pots are pretty effective with a smooth response all the way.
If you're unsure you should go for Enzos advice and try both the pots to decide what you like more.

Hope this helps

Matt

[GregS]

Thanks a lot for the replies. That helps a lot.

4. Weren't the pots in little bags that identified them? If it didn't spell out the taper, it should at least show the catalog part number so you can look it up to see which it is.

Unfortunately the bags weren't marked and nothing on the parts themselves correspond to the part number.

It sounds to me that, although I can conclusively determine which are audio and which are linear from the advice I've got here, I should try them out in the amp first before drilling out the holes. I was hoping to do all of the drilling before assembling the inards so I wouldn't risk hitting something with the drill, getting metal dust all over everything, or having to take it apart and re-assemble. However, it's probably better to do a bit of extra work and be sure.

Thanks again,

Greg

[txstrat]

You could at least drill the smaller holes before. If, after testing, you decide to go for the pots which need the bigger holes you wouldn't have to drill as much and therefore have less dust and metal all over you amp inards.

[GregS]

Good Suggestion! Thanks. That saves the hassel of having to dis-assemble and re-assemble.

Greg