DIY 901b oscillator question...

[noddyspuncture]

Hi folks,

I have a question about the 901b which I am currently attempting to build.

The schematic tells us that Q7, Q8 & Q9 are "selected". I have a component layout diagram which tells me the hfe's needed for Q7, Q9 & Q6.

Now, on this diagram Q6 & Q8 are next to each other so it occurred to me that the hfe stated for Q6 could actually be meant for Q8...!?

Can any one tell me please, if Q6's hfe is critical and, even thought Q8 says "selected" on the schematic, it's hfe is actually not important..?

Cheers,
Tom

[analogmonster]

Well, I cloned the 901B with actual BC5xx transistor types. I didn't care about the "select" directive as the newer types surely have higher betas as the originals. If you use original transistor types it could be more interesting. But Q6 is a suspect concerning another issue - the small signal plateau I got on the secondary waveforms of two of my three oscillators. I don't know whether original 901Bs produced a similar effect. The effect is barely audible and adds a light warm timbre to the sound, so I decided to keep it.

More details on http://www.analog-monster.de/mmt901_en.html