transistors and gain

[Sleeper]

.ok.
got some voltages... but I think it's moot at this point, I replaced the gain pot cause it was a bit scruffy and put in a socket for the transistors.

I am not a fan of the 360 bias setup grounded emitter, even with the feedback from collector to base. That type of bias is too device dependant IMO.

geezus Gus, you are right about that being device dependant setup.
I tried about a dozen or so in that last slot and then, blammo.
I guess I found the right transistor.
the thing has almost too much gain. but it sounds killer (and about equal to the bypassed fuzz setting) with the fuzz gain control all the way up its
oscillating a bit, but it's an amazing sound. I'm not selling these so if it blows up, it's on me, but wow, I'm not touching a thing. F**kin A.
I had a Strat plugged in just to test things out. My Hendrix poster nearly fell off the wall.

these are measured with a 1k sine test tone at 0db measured on a vu meter.
q1-2n5210
b .4dc
c 11.5
e ov

q2 2n1306
b -10.46
c 3.99v
e 0v

q3 2n5210
b 18.3v
c 25v
e 18.9v

Kelly

 

[Gus]

I see some issues from the voltages. It looks like the transistor as a zener is not working and Q2 voltages don't make sense -10.46V?

First the 2n2926 that goes to B in Joe Piazza's schematic is being used as a zener of about 7 or 8Vs I would use a 9.1VDC zener cathode to node B and anode to ground

SO Q1 and Q2 should not have more than the zener voltage on them. 25VDC goes to the 6.8k resistor then to the B node and the zener(transistor) goes to the same node as a shunt voltage regulator for about 7 to 8VDC. So this part of the 360 runs at about an "effect standard battery" 9VDC

Next Q3 is run at 25VDC so it does not clip I would guess. The operating points of 25VDC at the collector and 18VDC at the emitter gives an OK swing. It is an OK setting for an effect circuit

I would drop in the foxey lady bias circuit and collector and emitter resistor values. Cap values to taste almost any transitor would bias up with this more stable bias string

With the stock circuit I would aim for
about 9VDC at node B I don't think the transistor you built it with is zenering drop in a zener. Get that right then check the voltages again
2.5VDC to 3VDC at Q1 and Q2 collectors(this is an effect so you might want to be offset more than you might think from a book etc) I have built a circuit like this and I like the collectors at 2.5 or so.

You can adjust the operating points by adjusting the 680k and/or 470K for Q1(this type bias is to remove the signal fom the feedback the .1uf at the node shunts the signal away from the base leaving more of the part at DC and lows going to the base) I would adjust the 680K

The 1meg is one part to adjust for Q2

I think the clones should have a foxey lady bias setup for Q1 and Q2.

For Q3 I would use a transistor like a MPSA18(high Hfe 800 or higher) and add a 2.2meg from base to ground for a better bias setup. 1 meg +25VDC to base then 2.2 meg from base to ground. You want the offset of the E from the middle more toward +25VDC.

Look in the groove tube books for the old sears silvertone solid state bass amps they have interesting preamps sections.

 

[Sleeper]

Hi Gus,
you are right about the 2N2926 not zenering.
I don't have anything at all there.

On the moosapotamus DPDT version ( that's the variation I built) the 2n2926 and 100uf are out of the circuit completely. Charlie notes that the transistor was used to stabilize the circuit when it was switched off. the dpdt version shorts the input to ground when switched off, etc. so it doesn't need the 2n2926.

I'll stick a zener in there.

I think I'm really starting to "get" this. I'd gotten really confused because when I first built the unit- way before this post,... I'd been probing around
and paralleling resistors to see if gains or distortion flavor changed and I got very confusing results...

thanks for all the hand holding.
Kelly

 

[moosapotamus]

In my experience, the SPDT version with the cap and transistor-as-zenner does not completely silence the fuzz. There is a little bit of fuzz bleed-through when you switch the fuzz off.

The DPDT version (without the cap and transistor-as-zenner) that grounds the input to the fuzz section works much better and does completely silence the fuzz when you switch it off.

~ Charlie

 

[Sleeper]

Hiya Charlie.
yeah, the dpdt version works as you say.
Gus has me pretty convinced that maybe a zener in there would be worth a try.... I was just about to do it and realized I don't have any 9v zeners.
I'll stick a 12 in there and see how it goes. like I posted earlier, my fuzz is way rad now without anything in there, but the usable range I've got now is from 0 to 3 on the gain pot. maybe the zener will chill this out just a little.
Sleeper

 

[Sleeper]

Uh Oh, the phone rang with a new project, diy is got to stop for a while.
But:
i stuck an 8.2 zener in there and added a 2meg from base to ground on q3. both of these changes stabilized the circuit.

Q3 I would use a transistor like a MPSA18(high Hfe 800 or higher) and add a 2.2meg from base to ground for a better bias setup. 1 meg +25VDC to base then 2.2 meg from base to ground. You want the offset of the E from the middle more toward +25VDC.

the distortion is pretty much unchanged (that's good) but I have nearly full range of motion in the gain control pot. now just the last few degrees are sort of beyond usable/totally unstable.
this also tamed some high frequency oscillation that was taking place at the higher gain settings.

I'll do some more tweaking when I can get back to the workshop.

meanwhile, thanks again for walking me through this.
a little practice goes a long way.
Kelly