preamp parts

[saintjohnbarleycorn]

I am looking to replace some pots on a mic preamp. They are 1.5 meg, and I am having a hard time finding them. Panel mount, shaft for knob. I have tried mouser, anybody have any other ideas? thanks.

[Enzo]

ANy reason you can't make them 1 meg? or 2 meg? Adapt with the more common value.

SInce pots are almost always custom made for OEMs, it is rare to find an exact part in the general commodity market.

[saintjohnbarleycorn]

I was not sure if it was ok to do that, if I make them a different value, how would that affect the circuit. I guess to be safe it would be better to go up then down? thanks.

[mike_mccue]

It depends on the circuit but a common use of potentiometers is to make a variable resistance shunt to ground. Most volume knobs work this way. For any pots that are used to shunt to ground the 0ohm resisitance position diverts all the current to ground so you create the impression that you have "turned down" the sound. In this case the 1meg would not *turn up* as loud, while the 2 meg would go a little louder than your original 1.5 meg spec.

There are other scenarios but they are all based on how various resistances effect the circuit.

best regards,
mike

[saintjohnbarleycorn]

I don't know a whole lot about it, but it is from the 70's with the epoxy type board( if that is the right name for it, kind of greenish color) a very simple circuit by todays standards, I would think. A mike mixer with a spring reverb. I will go with a more common pot. Can you tell me what the correct name for the pot is. I want one thats panel mount so a knob can go on it. Its about 3/8" or so long through the board and about 1/4' round. It just has two taps on it. With the proper name I could easier find it. thanks again for all the help.

[Enzo]

Hmmm. In my experience, most volume controls are simple voltage dividers, not shunts, and so the value of the pot doesn't matter that much as long as you don't wander too far and get impedance loading effects.

DO we possibly know the make and model of this mic preamp?

[mike_mccue]

thanks for the correction.

mike

[saintjohnbarleycorn]

Yes it is a tapco, 6000r. 6 channel preamp mixer. Thanks for help.

[saintjohnbarleycorn]

I found a schematic for the preamp, here is the link. I have not downloaded them yet, I am limited to 36bps on the phone line.

[Enzo]

Oh, well that is very different. In a solid state circuit using op amps, the gain controls aer usually in the feedback loop, and their value IS important.

Your link didn't appear, try again? My Tapco files are buried.

[saintjohnbarleycorn]

try this one. http://www.speakers-audio.com/news/s...hematic-mixer/

[saintjohnbarleycorn]

here is the link again ...http://www.eskimo.com/~bgudgel/6000/6000.html

[52 Bill]

Looking at the schematic, the volume control can be replaced with a higher value pot (2M) with a resistor paralleled across it to create the 1.5M value.

I don't know what the taper of the original pot is, but you can measure the original one with your meter and order the closest version from someone like Mouser.

I seem to recall that these Tapcos used PC mount pots with long legs that acted as the PC board standoffs. You will probably have to hard wire the replacements.

[saintjohnbarleycorn]

thanks so much bill. That is what I will be doing, and yes they will be hardwired in. thanks again for all the help.

[Enzo]

We sometimes call those spider pots, especially when they have a support leg opposite the three main legs.

When i have to invent one, I usually make three leg extensions from solid wire and add them to a common pot. Then I can solder the extensions into the board. The stiff wire holds its shape well enough to guide the pot into its hole, where a nut secures it.