Tonebender MK2 NPN build (MP35 transistors)

[bmsiddall]

Hi all.

Since I'd had success using them for Q3 in a MK1, I decided to try building an NPN TBMK2 from the plentiful supply of cheap, low leakage MP35 transistors I've collected.  Used the Sola Sound OC81D schematic with 100K Q1 base/Q2 collector resistors as a starting point:-



For Q1 and Q3, I tried different combos from 30Hfe to 60 (Q1) and 60-80 (Q3)- all sounded good.  Q2 took a lot more transistor measuring/swapping before I found one with high enough leakage (still low by old tonebender standards) to get Q3 collector >8v.  I know I could have fast tracked this by biasing Q2 or by using one of my Tesla NPN germanium's (popped one in around 60 Hfe 135uA and it was perfect), but I was kind of enjoying myself swapping transistors and testing.

Here are the vitals from the final version:
Hfe/leakage/collector V (Vcc 8.8V)
Q1 57/56/8.5
Q2 64/128/0.170
Q3 80/70/8.4

Q3 collector voltage measures around 7.3 first thing in the morning, rising to 8.4v @ 25 deg C in the afternoon.  Cleanup is a little better at the lower voltage, but that's not why I'm here. 

Compared with my previous NPN version using leaky SD352 (10K/10K resistors), it's a little brighter in the higher mids and sounds killer (better with MP35 as Q2 than the Tesla actually).  Actually prefer this low leakage version.

For anyone wanting to take a swing at it without the bother of expensive mojo transistors, I also had my 1st shot at a tagboard layout (attack pot bypass cap is mounted on the pot).