yet another dumb newb question...mps6515

[pinkjimiphoton]

hi guys, i was messing around with the one transistor electra fuzz circuit, and went thru all the transistors i had on hand one by one trying to find the best sounding one..

it was, hands down, an mps6515....at least in my circuit, to my ears. magical. had "something" in the midrange that to my ear was magical, more tube like than transistor, with a warmth i usually would associate with ge.

well, it was a pull out...from something...no idea what.. and the legs of the tranny were kinda bunched up so i tried to straighten them. and broke the damn collector clean off.

so...i figured its time to hit mouser and order me some.

what i found was it was a 1 watt amplifier transistor...do ya think that's why it sounded so good, and was so much louder in the circuit?

can this powerful of a transistor damage anything? i'd like to buy a bunch of 'em to have on hand, but figured i'd better check...it seems to have a pretty hefty output compared to the more usual transistors i've played with.

thanks for the advice!!

[John Lyons]

It's just a small signal amplifier like other transistors we use.
I have found the mps6513 to sound good, should try the 15 now. 

[Gus]

When you change transistors in a circuit like the electra the C to B resistor value matters.  Can you post a link to the electra you built?   I found different schematics.

It is helpful to measure the power supply voltage and the collector voltage to ground base and emitter voltage can help as well.  That circuit uses C to B biasing so it is sensitive to value of resistor C to B and the transistor used.

I don't think it is magical you found a transistor that had the right hfe etc for a tone you liked.  Now if you measured the transistor voltages of the build you liked you could adjust the C to B resistor value for the same C to ground voltage an then compare the tone.

[R.G.]

The MPS6512-6515 are general purpose, if lower noise devices. They were selected for only a 2:1 range in current gain, so there is some chance that what you get is going to be similar to others of the same type number. It's the highest Hfe member of the family, gains of 250-500. They're 150MHz devices, so the chances that it's doing something to the tone of the circuit as opposed to what the circuit is doing to it are small.

As compared to the 2N3904, which is otherwise similar, it has higher minimum gain, less gain variation, lower withstanding voltage (25V as I remember) and lower noise.

I think that Motorola's "MPS" numbers were an attempt to provide more consistent groupings of devices in the late 60s and early 70s.

Check the device for a stylized "M" on the front. If it's a real Motorola device, as opposed to some of the later makers, you have a better chance of getting a similar device by getting the same brand.

However, it's likely that the circuit conditions themselves (voltages, bias, etc.) contributed a lot to the magic sound.

[Gus]

Sim screenshot that might help show why the C to B resistor is important with the transistor used.
I changed the C to B  resistor value and recorded the collector voltage for three transistors in LT spice

[John Lyons]

Wow, that IS a pretty big biasing shift.
Time to do some testing.
Thanks Gus.

[pinkjimiphoton]

this is the thing i built, sorta



i built it with random parts i had on hand.

r2 2 x 4.7m parrallel

r3 4.7k

c2 22u

c4   68u

d3 green and yellow mini leds series

d4 red normal led

bias 10k with two 10k resistors from outer to inner lugs

nte 46 atm

e  .53vdc
b  1.61vdc
c  1.69vc

sounds really nice.