Tone Vendor Transistors

[Hexjibber]

Hi all,

I’m hatching a plan to build a MKI Tonebender using the Tone Vendor board for a band mate’s birthday. I’ve read a lot about the circuit being picky with transistors, even with ideal hFE and leakage specs. The predominant advice seems to be buy a bunch and keep swapping them around until it sounds good. I see from the BOM that the Tone Vendor calls for 2SC381 in Q2 & Q3, is there a particular reason for that? They seem to be less common in TB builds/schematics, just wondered if they are perhaps a bit more stable and easier to come by or some other reason?

Would appreciate any advice on some good transistor choices for this build and gain and leakage recommendations.

 

[Ratimus]

I breadboarded a mark 1 the other day, which I plan to box up this weekend. The hfe's I've seen recommended are something along the lines of 70;120;100 for Q1;Q2;Q3.

The one I put together was selected from a stash of unmarked mystery germaniums from who knows where. The approach I went with was to select each transistor as I followed the signal path. Monitor output of Q1, find a few that work, move on to Q2, etc. I went through a few permutations and eventually settled on
Q1: [hfe 20, leakage 200 uA];
Q2: [hfe 250, leakage 100 uA];
Q3: [hfe 100, leakage 100uA].

Q1 is an emitter follower so hfe is far less critical here. However it's the most sensitive to leakage as far as biasing is concerned. (I wonder if leakage does more than just allow it to bias though; there's probably some nonlinearity introduced as well).

Q2 has the greatest impact on the sound and probably the least sensitivity to leakage. I've heard that hfe>150 isn't supposed to sound good here, so grain of salt with my hfe 250. Actually, I play a trashy cigar box guitar with a $15 pickup into a Monoprice tube amp so take EVERYTHING I say with a grain of salt!

I've also heard it said Q3 mostly affects treble response, though I don't think it's as simple as that. I tried 4 or 5 different transistors here and while they all sounded VERY slightly different, I ended up unscientifically choosing one based on my gut reaction without being able to objectively tell you why I chose it.

Does it sound like a dead-on, historically accurate mk1? Who knows. Probably not at all. I don't have $4,000 to find out what I'm missing, though. Do I like how it sounds? Yes! And what's more, I chose the way it sounds and can always change it if I want.

 

[Hexjibber]

Awesome thanks for the reply, that’s great info and will help guide my transistor buying!

 

[Travis]

HI!

Im going to build one MKI with one NOS OC75 and two 2SC1815-Y NOS Toshiba.

Some recommendation for HFE?

 

[peatpatrick]

On the Mk1 board is there any way to determine which orientation the transistors are supposed to be mounted? I've measured and determined which legs of the transistors are the E,C, and B but there's no indications where those go on the board. It has the "Q" shaped transistor emblem silk screened on the board but are those always the same layout?

 

[caiofilipini]

The tab indicates the emitter.

 

[spi]

I ordered a matched set from smallbear for this circuit last summer.
What they sent me was:

Q1: OC76 (G=60, L=40),
Q2: 7941? (G=82, L=250),
Q3: OC42 (G=110, L=35)

Couldn't really figure out what Q2 is, the only markings I could make out looked like 7941.

 

[peatpatrick]

The tab indicates the emitter.

Thanks!!

 

[peatpatrick]

So I noticed the build document calls for Q2 and Q3 to be 2SC381, which are NPN's... But I think those need to be PNP's, correct? I was wondering why the symbols in the schematic were PNP's.....