Harmonic P type circuit resistor selection circuits

[Gus]

http://www.aronnelson.com/gallery/main.php/v/gus/HP_resistors_selection.PNG.html

Two selection circuits to help select the bias resistors for the HP before the final build.
The collector voltages selected are from a post in one of the threads.
You can adjust them for what you want
Use the transistors and selected bias resistors(c to B resistors) in your build

Say you built a HP like circuit and you like the sound.  Take the voltage measurements and calculate the Ic of the transistors.  Then use ohms law to calculate the collector resistor value needed to obtain both the voltage drop needed for the C to E voltage and desired Ic with a 9VDC supply.

Measure the voltage at the C, B and E of both transistors to find the two C to E drops.  Measure the Ic. You want the same C to E voltage drop at the same Ic in the selection circuit.  Use ohms law to calculate the collector resistor needed.  This will need to be redone if you change the C to E drop and/or Ic wanted

Use Ohms law to adjust for Ic and collector voltage a regulated 9VDC supply would be a good thing.  A temp reading at the time you select the bias resistor would be a good note to take.

Or if you know how you can make a CC set to .05ma or what ever Ic you want and adjust for the C to E voltage drop wanted for the transistors

Have not built the test circuit but it should work, it is basic stuff.   Look at the curve of hfe vs Ic in some transistor datasheets.  I drew the selection circuits up because people might not have adjustable regulated power supplies but they might have 9VDC regulated supplies.

[edvard]

Hmm... So it would be prudent to use trimpots in those positions?
And would doing this allow the use of transistors other than those specified?

Trimpots might also help in situations where temperature fluctuations affect the Ge transistor, i guess.

[alex frias]

Very interesting, I believe the guy behind the HP-1 used to vary those in order to accomodate different transistors hfe's.

[Gus]

Depending on the hfe vs Ic curve and the transistor(s) used you might need a wide range of C to B resistor values.