"Reverse boost" build report

[earthtonesaudio]

Jack Orman did a nice little write up here:
http://www.muzique.com/lab/reverse.htm
more:
http://www.muzique.com/images/PBX.gif

So I tried it out, or something similar.  The circuit ended up more like the Beginner Project circuit (Gus Smalley's NPN Boost), but whatever.

I tried it with several transistors, most worked but the 2n3904 did not (barely passed sound but didn't amplify).
2n2222a (metal can): nice
BC547: pretty good
BC549: pretty good
BC337: so-so

In all cases the reverse configuration was slightly quieter (I'd estimate maybe 3 to 6dB lower) and exhibited noticeable distortion.  To my ears it sounded a little bit like the "poorly biased transistor" type of background-fizz, but it seemed smoother than usual.
Also the volume knob cleanup effects were different.  It seemed like as I rolled back the volume, instead of getting the usual jangly tone, it was a more midrange heavy sound. 

Wish I had tried this sooner.  It's pretty neat.

Here's what Wikipedia says:

Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximize current gain in forward-active mode, the βf in inverted mode is several (2 - 3 for the ordinary germanium transistor) times smaller. This transistor mode is seldom used, usually being considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.

[Mark Hammer]

Hmmmmm....

Analogguru sent me a couple of notes recently in response to my thread about the Tychobrahe Octavia.  One of the things he pointed out was that most circulating schematics for it had the emitter of the first transistor going to ground (e.g., see: http://www.generalguitargadgets.com/pdf/ggg_toct_sc.pdf ), whereas other more original drawings of similar devices from the mind of Mr. Mayer had the collector going to ground: http://www.diystompboxes.com/smfforum/index.php?topic=52731.0  Perhaps the harmonic content generated by this "reverse-gain" stage produces some of the tone of the Octavia?

[caress]

i've been using reversed germaniums in fuzz face setups for a while.  a little quieter but with slightly different characteristics for sure.... easy enough to try out, although you may need to fiddle with your bias.

[earthtonesaudio]

Interesting, Mark. I never looked at the "original" Octavia schem on GGG, but they have indeed drawn it with all the transistors inverted.  ...Which is strange considering the "improved" version: http://www.generalguitargadgets.com/pdf/ggg_toct_sc_improved.pdf has no such inversions.  This makes me think the "original" schem on GGG is simply drawn wrong.  Especially when you take a look at some of the schematics from the link you posted, like this one:
http://www.danalexanderaudio.com/OutboardPic/HeliosInputSchematic.jpg
If this is really the real original, then the GGG "original" is drawn wrong. 

I don't doubt that the inverted version would sound different, I just think it's a mistake on the GGG schematic.

i've been using reversed germaniums in fuzz face setups for a while.  a little quieter but with slightly different characteristics for sure.... easy enough to try out, although you may need to fiddle with your bias.

Yeah, I didn't try changing the bias.  I just flipped transistors around.  I'll have to try that tonight.