This Week on the Breadboard: the Subdecay Variac (Brownout OD)

[temol]

And some oscilloscope shots.
IN - 1kHz, 200mV p-p. Yellow trace - output, cyan trace - out from TRE potentiometer. Different settings of GAIN and PENTODES pots. TRE @ 15, LOW @ 9.
MMBF5457 and MMBT5088

G @ min, P @ min


G @ 9, P @ 12


G @ 9, P @ max


G @ 12, P @ min


G @ 12, P @ max


G @ 8, P @ max

 

[temol]

By the way - Q1 and Q2 with Vp about -1.4V. I lowered the value of the source resistors from 47k to 43k. Now Q1 and Q2 are biased around 5.4V. With 47k it was almost 5.7V.

 

[Chuck D. Bones]

Good info!
The asymmetric clipping from the preamp section is quite apparent. Interesting that higher setting of PRE causes the preamp clipping to become more symmetric. The clipping at the output becomes more and more symmetric as PENTODES is turned up.

 

[temol]

Good info!
The asymmetric clipping from the preamp section is quite apparent. Interesting that higher setting of PRE causes the preamp clipping to become more symmetric. The clipping at the output becomes more and more symmetric as PENTODES is turned up.

We have two inverting stages here. I'm sure both are capable of clipping the signal. So maybe at some point we have more or less symmetric signal. I'm on a vacation now so can't test the circuit to check what's going on.

 

[Chuck D. Bones]

That was my thinking too.