Wah pedal transistor gain choices

[WoundUp]

So I was reading the thread by @paulyy's about using different transistors from quite a few years ago and I had a question. I wasn't sure if I should bump that thread so I'm starting a new one. See thread linked below

https://www.diystompboxes.com/smfforum/index.php?topic=94107.msg811289;topicseen#msg811289


He said that gain should be around the 350-400 range for best results. I've got some 2n2222a's @ 340 and was wondering if those would possibly work? @paulyy? How much different are 2222s? I read where you tried them in yours. Do you remember what they sounded like vs more commonly used wah transistors? Only other thing I have is an extra bc109c from the fuzz faces I built and it's around 850-900 IIRC so it's out of the question. Thanks.

[mozz]

2n2924

[Mark Hammer]

It's not the gain/hfe of the transistor itself, but rather whether it can be a drop-in value to achieve the required gain using all the other associated components. 

I built my wah from scratch, using a basic CRy Baby circuit, and had to play with the value of the Q1 emitter resistance to ground until I achieved wah.  Try any old NPN in there with a 10-turn 500R-1K trimmer and tweak away until you get what you want.

[Rob Strand]

When you dig into wahs there's a whole lot of details that make small differences.   Not even the "real" units are consistent in what they do.   For example some units used the same transistor on each position, others had a higher gain transistor in the buffer position and others had the higher gain transistor in the gain position.    Other factors are the inductor, the resistor in parallel with the inductor and the emitter resistor.     The best you can hope for is to start on the right foot.

The recommendation of 350-400 is quite reasonable.   However a good deal of real units used "B" gain transistors like BC109B (and other transistors with equivalent gains).    The gains for those is around 290 (range 180 to 460) at 2mA.    The 2mA test current is also significant.    The circuit operates the transistors at 200uA to 400uA, less than the 2mA spec.   The gain on "olden days"  transistors tended to drop off at lower currents  the hFE 290 at 2mA might drop to 200 to 220.   Modern B transistors might not drop so far.

The thing is the 350-400 is likely to be at a 1mA to 2mA test current and modern transistor might maintain the gain somewhat in circuit at 200uA to 400uA.      When you test  the transistors you will also be testing at 1mA to 2mA, so it doesn't given an idea what the gain is at 200uA to 400uA, you could only guess it will be a bit less.   If you use the 1mA to 2mA gain as a measure then you can narrow the box down to  220 to 430.

So you have,
- not all units are built the same
- other parts make a difference, albeit small
- even what we mean by transistor gain is up for question.
- personal preference to what is "good" sound

[WoundUp]

When you dig into wahs there's a whole lot of details that make small differences.   Not even the "real" units are consistent in what they do.   For example some units used the same transistor on each position, others had a higher gain transistor in the buffer position and others had the higher gain transistor in the gain position.    Other factors are the inductor, the resistor in parallel with the inductor and the emitter resistor.     The best you can hope for is to start on the right foot.

The recommendation of 350-400 is quite reasonable.   However a good deal of real units used "B" gain transistors like BC109B (and other transistors with equivalent gains).    The gains for those is around 290 (range 180 to 460) at 2mA.    The 2mA test current is also significant.    The circuit operates the transistors at 200uA to 400uA less than the 2mA spec.   The gain on "olden days"  transistors tended to drop off at lower currents  the hFE 290 at 2mA might drop to 200 to 220.   Modern B transistors might not drop so far.

The thing is the 350-400 is likely to be at a 1mA to 2mA test current and modern transistor might maintain the gain somewhat in circuit at 200uA to 400uA.      When you test  the transistors you will also be testing at 1mA to 2mA, so it doesn't given an idea what the gain is at 200uA to 400uA, you could only guess it will be a bit less.   If you use the 1mA to 2mA gain as a measure then you can narrow the box down to  220 to 430.

So you have,
- not all units are built the same
- other parts make a difference, albeit small
- even what we mean by transistor gain is up for question.
- personal preference to what is "good" sound

Yea, I noticed a lot of the differences during my previous reading among the different forums. Thank you for taking the time to go into detail. Being new to this all, it helps to really understand what's going on. Before I ever put a soldering iron to it, I plan on doing a lot more reading so I have a better grasp on which mods do what and whether or not that's what I'm looking for. Thanks again. I really appreciate it!

[Big Monk]

My PedalPCB Tearjerker uses Q1 at 160 hFE and Q2 at 312

[Rob Strand]

Yea, I noticed a lot of the differences during my previous reading among the different forums. Thank you for taking the time to go into detail. Being new to this all, it helps to really understand what's going on. Before I ever put a soldering iron to it, I plan on doing a lot more reading so I have a better grasp on which mods do what and whether or not that's what I'm looking for. Thanks again. I really appreciate it!

There's quite a few posts about what parts affect what,

This one shows how the inductor DC resistance affects the height of the low frequency peaks,
https://www.diystompboxes.com/smfforum/index.php?topic=120336.msg1141118#msg1141118

This one show that the emitter resistor changes the gain but when you take the (signal) gain variation
away it's actually changing low end range of the pedal sweep and not changing the shape of the response,
https://www.diystompboxes.com/smfforum/index.php?topic=126565.msg1210650#msg1210650

IIRC, the resistor in parallel with the inductor affects the HF peaks. (you might find another post saying that.)

You also might find posts about the affect of the transistor gains.   I've posted stuff over the years and other's, like Gus, have done the same.

You can compensate the effect of one aspect by tweaking another but only for small amounts as eventually each aspect has its own individual effect.

[Rob Strand]

I haven't got all my stuff on this computer but  I found this on my laptop,

Suppose you have a wah with a Q1 of gain 300 and RE set to 470 ohm.  That's the reference design.

Suppose now Q1's hFE varies.  The aim is to adjust RE to compensate for the Q1 hFE changes so the resulting response is as close as can be.  We get this table showing the required RE value   

hFE    RE
150    455
300    470
600    480

You can see you only need to make small adjustments to RE to compensate, less than an E24 resistor step.  The change in the response due to hFE are pretty small anyway.  The adjustment makes the response peak for the lowest frequency of the sweep the same the reference design.   As it turns out amount of boost at the lowest frequency also matches.    The amount of boost at the highest frequency if the sweep varies by about 1dB from hFE = 150 to hFE = 600.