Small valve mixing console

[C.B. - Boudio]

That'll be in a couple of weeks, because of exams ;-)

 

[C.B. - Boudio]

So, the last revision of the first channel is almost done!


The rudimentary EQ under construction.


The Grand Scheme of Things. (nevermind the crappy paint job on the right, it's just some primer to protect the bracket from rusting).



Back part, with connections and the PF86 stage.
In the back, there's a small 220/12-0-12 transformer I use instead of a real Edcor one (which are being ordered), secondary terminated with a 27k resistor. There's also a switching jack: when a jack is inserted, the transformer secondary gets decoupled and a 10M grid resistor comes into circuit.
The switch right behind the pot, with the small print attached, is a +-20db pad.
Every signal wire is shielded.

I have to test it all yet, and I'm very curious as to how it'll work out!

 

[C.B. - Boudio]

So, some tests have followed, and there is amplification! At least, up to the aux send - it doesn't seem to get past the EQ...
A quite distorted signal because of it being a 0,7V line source, but that's to be expected. So, the instrument input seems fine, the mic input still needs some work (probably either the transformer, the phase, the pad, or the switching of the jack - or a combination of those...). At least, up to the aux send - it doesn't seem to get past the EQ...
The g3 NFB at the PF86 is a tad disappointing, so I just connected g3 to the cathode and completely did away with the NFB there (for now).
The gain control (2M rheostat between ground and the g2 decoupling cap of the PF86) doesn't do much; should I install a lower value pot, or stop trying there and go for another system altogether?
There was a lot of oscillation going on, mainly about 100Hz I think (even after rebuilding the B+ supply, which should be really quite clean now), and using NFB from the cathode of PC(F)802 to g1 of P(C)F802 via a 220k resistor got almost entirily rid of that.