Out of Circuit Transistor Biasing ?

[Phend]

Hello;
Assume a circuit like the BMpi (green), 4 transistors, collectors to plus thru 12k resistors (as shown on circuit drawing).
Question: Can the transistors be biased to 4.5 v out of the circuit (on a breadboard) without all the other components around it ? Or is the best solution to use a temp pot to dial it in and measure and then solder in the close resistor ? I am sure you wouldn't want to solder in resistors and then measure and then de-solder......

[GibsonGM]

...trim pot in-circuit?  (shrug)

[duck_arse]

breadboard with trimpot. tune by ear. measure collector voltage for fun. measure trimmer resistance for fun and profit. fit that transistor to board with nearest standard value. sleep easy, and don't shout at anyone.

[Phend]

thanks,
Breadboard, trim pot C to plus, E thru (resistor in this case) to gnd.
B blank.

[mozz]

Take out all caps, isolate the section you are working on, bias just that part. 

[duck_arse]

thanks,
Breadboard, trim pot C to plus, E thru (resistor in this case) to gnd.
B blank.

B blank? how will it bias? B to C via resistor. B to ground via resistor. you can leave the diodes out for DC.

[antonis]

Except last stage BJT, BMPi transistors are biased via feedback resistor..

https://semesters.in/collector-feedback-biasing-a-bjt-electronics-engineering-notes-1st-year/

[Phend]

Thanks again:
Got it, simple enough to bread board.
In this BMPi case,
Q123, 4 resistors ea.
E R Gnd
B R Gnd
C R +
B-C R

Q4 :
No B-C
B to Gnd and B to +

9v in and bias C to Gnd to 4.5v

[m_charles]

why would you want to bias them all to 4.5v? almost all muffs aren't biased to 4.5v at the collector, and usually the last trans is biased higher (sometimes up to 6v). Thats part of the sound.
Sure, 4.5v will work, just not sure why you'd want them that specific.

[Phend]

^ Thanks, I see the voltage recommendations now, maybe I should have not used 4.5, but instead "recommended bias volatage". Good catch. Anyway the idea of the topic is out of circuit bias. But thanks for stopping me from making a mistake.

[antonis]

the idea of the topic is out of circuit bias.

Why..??

You can't bias them without bias configurations while all other components are bias-useless..

[Phend]

Thanks Antonio, I will print that out for the reference file.

[antonis]

If so, save the following for your case..