Mods to Arsenio Novo's design

[mac]

I found this circuit long time ago at harmony central and I liked it very much. The original version in somewhere in the gallery. Do a "arsenio" search to find links.
I had some free time so I played with it for a while. The first thing I did was to measure the voltages when the circuit was fine tuned. I noticed that the sweet spot is when the base and the collector of the npn are equal. Then I connected a big resistor from base to Vcc, of the order of some megs. With the last condition ib*R3 = ic*R1 so it is like having a common hFE = ic/ib =R3/R1.
I used a matched pair of 2N3904/06, hFE=232 and R3 was about 2.4M. Higher gain transistors like BC549C/559C need  R3 of about 4M - 5M. One interesting thing is that the tuning condition seems to be independent of Vcc and the value of R2, so R2 can be anything you like, an original 10K resistor or pot, series diodes or shorted. A 10K pot will reduce the power consumption at the expense of a little final gain which is maximun when the npn emiter is shorted. But not much. The 1K pot cleans the distortion and the volume pot is big because it has not much volume.



Reading the original post Arsenio Novo may be italian. Not sure. I named this mod Arsenico. Hope you like it.


mac

[mac]

Today I tried using a Ge 2SB176 PNP hFE=200, leak=200uA (Matsushita). The tuning conditions is now Vc - Vb = ±0.05V, because of the leakage I guess.
It sounds a lot better but the temp is a problem as it moves the sweet spot up and down.
Two Ge may sound terrific but how to keep Vc - Vb = constant?

mac