C.B. - Boudio
Well-known member
That'll be in a couple of weeks, because of exams ;-)
Small valve mixing console
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C.B. - Boudio
Well-known member
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!
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
Well-known member
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.
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.
C.B. - Boudio
Well-known member
I'm almost there! I seem to reach decent gain now (which used to be the biggest problem), and noise is surprisingly low. I don't get remotely the impressive noise and distortion figures of, say, an ITB product, but I am quite pleased. Of course, there is still a lot of tweaking possible.
I'm still looking for a way of gain control. At the moment, I use a 1K pot as V1 cathode resistor with the bypass cap connected to the wiper (recipe for scratchiness, I am aware), which doesn't really work.
The EQ section needs some revision (especially capacitor value-wise), and my PL95 cathode follower stage could be improved a bit, too.
Next week, I won't have time to do anything at all, but I'll try to upload the revised schematic at the end of the week. Won't win a beauty contest, with resistors paralleled until I get a defintive value, for example, but, again, it finally is coming to life!
I'm still looking for a way of gain control. At the moment, I use a 1K pot as V1 cathode resistor with the bypass cap connected to the wiper (recipe for scratchiness, I am aware), which doesn't really work.
The EQ section needs some revision (especially capacitor value-wise), and my PL95 cathode follower stage could be improved a bit, too.
Next week, I won't have time to do anything at all, but I'll try to upload the revised schematic at the end of the week. Won't win a beauty contest, with resistors paralleled until I get a defintive value, for example, but, again, it finally is coming to life!
