Matching transistors, tube sound OD

[davepedals]

I'm building the tube sound OD which requires two HFE matched transistors, 2n3906, 2n3904.  This is the first project that I have ever needed to match transistors on so I'm not really sure what I'm doing. Out of 30 transistors each the closest match I can fine find is 274 HFC for the 2n3906, and 160 for the 2n3904.  This is already a stupid question but that's just way too far apart,  right ?
Thanks!

[antonis]

Something like long tail pair, hmmmm... 

Maybe you'll have better results (closer matching) with 2N5088 & 2N5087..

[sajy_ho]

I tried a few pairs in this circuit and the closest results to tube overdrive was with a pair of NPN-PNP with hfe around 160, so I think your 3904 is perfect for the purpose but you should find another PNP with lower hfe. Remember lower hfe means smoother clipping.
Also there is a schematic out there by the name of "Night Train overdrive" if I remember correctly; it's an improved version of the circuit with tone stack and input buffer, you may wanna check it out...

[davepedals]

Thanks, I'll check that out.

[PRR]

> the tube sound OD

Link to schematic, please.

Without a clue we are just guessing. "Matching" can be many things, and may not really be needed.

[davepedals]

I read an article on this project that said if the transistors were not matched it would sound like crap .

[antonis]

IMHO, replace R3 with 100k to 150k, lower significally (or delete) R2, replace C1 with 22nF to 15nF and use the closest matched BJTs handy availiable..

(just a thought for "independence" of individual transistor β..)

[Cozybuilder]

IMHO I'd change the power section, add some adjustability to the drive, try out different input caps until you find a sound you like:

i.e. - more like this:

[PRR]

Well, I give up. Q2 works as a simple amplifier, except Q1 is clamping it. They never work under "similar" conditions, so no clue what to "match".

Get one Q1 and ten Q2, same source same brand (trying to get same foundry), breadboard, try them all.

[Cozybuilder]

I see a couple of issues with the TSO.
1) The signals are taken from the emitter on the PNP, which is non-inverted approximately unity gain. The signal from the NPN is taken from the collector, which is inverted. Thus, you have signal cancellation rather than enhancement- can't think of why, but maybe thats desirable?

2) It seems the two transistors are fighting over the base bias voltage. If you assume the 10K to +9V on the PNP drops 1V, giving 8V, and the base will bias to about 0.6V from there (B-E) you wind up with about 7.4V at the base. Now, assume the 10K on the NPN emitter allows a 1 V pullup, you have approx 1V, and the BE junction will be another 0.6, so it wants to bias about 1.6V. What if you put a resistor between the two bases which would allow a voltage difference, better yet a pot to allow adjusting the signal to each base? I drew up the following- is it feasable? It looks a bit like a modified mu amp.

[davepedals]

I was thinking about a better way to bias the two transistors, but I don't know. Here's a link to an article about this one along with some interesting mods . Apparently several have built this pedal with success.
http://revolutiondeux.blogspot.com/2012/06/tube-sound-od-briggs-paf-overdrive.html

[antonis]

I was thinking about a better way to bias the two transistors, but I don't know.

That "better" way is to bias them in respect only to Collector/Emitter currents (independently of their Base current) so no need for matching them..
(as long as the R_C/R_E gain is lower than the lowest BJT beta..)

But, as Paul said, it's just a clamping arrangement..

[allesz]

Briggs page contains the original arsenio novo's post. It states that the bjts must be perfectly matched.

That's probaly why it did't work for me

So I think you'll need to try different transistors first.

The gain control idea posted by Cozy looks very neat btw.

The base bias network is quite strange.... it looks more like an opamp's input bias 

[duck_arse]

it's beginning to look like part mu-amp, part power output stage, part percolator.

[Cozybuilder]

What to call it  The camel? The grey camel? 
Maybe I should breadboard it first 

[mac]

This is Arsenio Novo's design posted long, long ago, Harmony Central 95-96?

The 20k pot sets the sweet spot. When you get it, the PNP base to emitter voltage is zero, so it is acting as diode. Similar to a fine tuned Bazz Fuss.
Try without matching, unless you want some octave up out of it

See my gallery, Arsenico v2.

mac

[dschwartz]

I think the idea of the circuit is to get both sides of the wave clipped by transistor saturation and avoid cutoff clipping, which is harder..
If the point is that this is tube-like, I don't think so..
Triodes clip very similar to bjt (soft at saturation and hard at cutoff).there is other fenomena related to grid conducting,miller effect and bias shifting, though..
But a simple bjt buffer into a bjt common emitter amplifier wil give a much better tube like distortion ( check black fire by joe davisson).

Any circuit that needs matching sucks

[bool]

In essence, this "double bjt" circuit is not that far away from bazz fuss (spelling?).

The PNP clamps the NPN collector down in a different manner than the diode-npn-base/emitter in the bazz fuss though.

The caps at the input and R5/C2 cct fragment will probably introduce some bias shift as well (needs simulating to confirm that).

[duck_arse]

there is a similar sorta circuit at the other plaice called the "what the fuzz", but has an LM386 goose-ing the input.