Omega - trimpot settings?

[mattpocket]

I hear that the trimpot can be set to 4.5v for a LPB kind of sound, and at 7v its more reminiscent of a rangemaster... I like, and could use both of these sounds, so is there a way I could use a pot so that I can adjust bias without having to open up the enclosure... The trimpot is a 100k, could I use a pot with the signal going into the middle lug, a resistor to bias the transistor to 4.5v on one outside lug, and one to bias the tranny to 7v on the other leg, and then the pot would sweep between the two values, would this work?

If it wouldnt work is there a way I could do this? To make a pot sweep between the two values?

Thanks in advance...

Matt

[slacker]

you could just replace the trim pot with a normal pot wired the same way, that would let you set the bias over a wide range of voltages. This would probably give some interesting sounds and some unusable sounds or no sound at all at certain settings.
If you just want the pot to sweep between 4.5 and 7 volts then breadboard the circuit and set the trimpot for 4.5 volts and measure the resistance then set it for 7 volts and measure the resistance again, this would give you 2 resistance reading that relate to the 2 voltages. You could then use a resistor to set the lowest resistance and a suitable valued pot in series with it to set the higher resistance. Depending on the resistance required for the pot you might need to put a resistor in parallel with the pot to give the correct resistance.

[mattpocket]

What do you mean in parallel? Across the two middle lugs? How do I know what value to use?

Thanks for the info! 

Matt

[petemoore]

  Her'es what I'd do...
  Figure out what voltage range you want by ear, measureing the resistances needed at the pot [min/max for chosen sound variations within bias parameters] by measureing the pot 'hi and low' at the settings you choose/would use.
  say you don't want anything below the voltage that 22k gets, and nothing above what 56k gets, you want a pot when turned CCW = 22k or so, Cw = 56k.
  Use a stop resistor [spliced in an outside lug connection] of 22k.
  Use a 100k pot to find the resistance parallel to a 100k pot which makes the pot you want modified to have a max setting of ~34k, pick a close value fixed resistor and solder that to the pot.
  That way you'll always have operational' bias wherever the pot is set, ballparking the values should be close enough to whatever you choose with the 'regular' pot settings.

[slacker]

If you wire it up like the trimpot is shown on the Omega schematic then you put it across the middle lug and the bottom lug.
To work out what value you need the formula is

Total resistance = (R1 x R2) / (R1+R2) 
Where Total resistance is the value of pot you want R1 is the value of the pot you have and R2 is the value of the resistor in parallel.
Or you could cheat and use the calculator at the bottom of this page. Put the value of the pot you have as R1 and the value of the pot you want as "Total R" then R2 will be the value of the parallel resistor.
Hopefully though one of the standard pot values will be near enough to use without having to mess about with parallel resistors.

[mattpocket]

Thanks guys, very helpful...

Matt

[petemoore]

  Yupp, generally and ballparked, lesee, I want that value on...the outside [of the measured chosen top value] since that's what I got as fixed R that's close...I can get what I want then.
  Just makes it a little easier/finer tuning to adjust, and can prevent a misbias, blocking sound.

[mattpocket]

Thanks guys, I will let you know how it goes!

Matt

[MartyMart]

Couldn't you do it also with a switch ?
Measure bias A and bias B values with a pot, remove and use 2 x fixed resistors !
Switch up= 4.5v and switch down= 7v rangemaster, probably quieter and less likely
to drift too.

SPDT, middle lug to 9v outer lug bias A resistor to Drain other outer lug bias B resistor to Drain
MM

[mattpocket]

I had thought about it, but then I thought a pot would offer more adjustment inbetween the values?

Matt

[petemoore]

  Tells...
  I didn't have a trimpot for that, but did have a ton of room, I used a regular boxtop pot for the bias on Omega, only about 1/3 of it's rotation adjusts to usable bias points...which is fine enough.