This Week on the Breadboard: the Subdecay Variac - highly modified

Yes you did. That's just not what I read for some reason. I'd wager a guess as to why, but I'm not trained in that field either.

They are stock per the schematic.

Related question: Which resistor(s) would be considered the source resistors for Q1?

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If we're interested in the DC bias, then it's R3. If we're interested in the gain, then it's everything between the source terminal and ground.

Back when I was in college, I walked out onto the quad and saw five PhD candidates standing on each others' shoulders next to a flagpole with a long tape measure. I asked them what they were doing and the guy on the bottom told me "We're measuring this flagpole."
Then I asked, "Wouldn't it be easier to lay the flagpole down down on the ground and measure it that way?"
The guy on the top shouted down "that would work if we wanted to know the length, but we want to know the height."
 
On this build, yes sir. Hang on... I think one rail lost continuity. I added another wafer so I could include the Volume pot.
I'm not certain how many people as ignorant as I come through here asking you questions they should go read about for themselves, but I am not going to take advantage of your patience nor good nature. I will return when it is working. Thanks so much!
 
I just rechecked my Maniac breadboard. Using a 2N5457 with Vp = -0.86V, Vcc = 8.99V, R3 = 22K and R5 = 100K, Vd = 7.51V, which is way too high. When I reduced R3 to 6K, Vd was 4.9V. Since R3 is in parallel with R13 from an AC perspective, lowering R3 that much increases the low freq gain to the point that we might as well delete R13 & C9. Which is exactly what Temol did and why he did it.
 
It's working....power to the people (and components). The configuration above biased fine Vcc = 9.03V 2N5457s R4 = 220KΩ / R8 = 56KΩ. Q1 Vp = 4.485V, Q2 Vp = 4.190V.
I've got a new scope I'm itching to try out (don't tell FF). This and the auto-bias circuit should be good break-ins.
 
Chuck D. Bones said:
Have you unpacked that scope yet?
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Nope.

I was putting together that new micro-protoboard. I need to send @temol one for pincushioning on the go. It's pocket-sized.

Edit: I actually bought two scopes by accident. I had them both in the cart to compare. One is 2 channel, the other 4. The 4-channel offers a digital channel as well, so I'm likely to keep that one (which was my original intention). I've been playing with these fruit computers and shield boards a bit.
 
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Not a lot different, but different enough. I think you're better off keeping the BJT output stage.

If you want the MID knob, then disconnect BASS pin 2 from Vref & disconnect C11 from GND. Hook up BASS pin 2 and C11 to a B50K MID pot. Arrange it as either a Fender or Marshall tone stack, your choice. I recommend using TSC and/or a breadboard to verify that the tone stack does what you want. You'll probably need to adjust the value of C11. Make sure that pin 1 of the MID pot goes to Vref and not GND.
 
Chuck D. Bones said:
Not a lot different, but different enough. I think you're better off keeping the BJT output stage.

If you want the MID knob, then disconnect BASS pin 2 from Vref & disconnect C11 from GND. Hook up BASS pin 2 and C11 to a B50K MID pot. Arrange it as either a Fender or Marshall tone stack, your choice. I recommend using TSC and/or a breadboard to verify that the tone stack does what you want. You'll probably need to adjust the value of C11. Make sure that pin 1 of the MID pot goes to Vref and not GND.
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Thanks Chuck. I'll build another and set up the mid control as you outlined. I'd also like a bit more from the Pentodes control, and I may delete it completely. Can I change R22 and R23 (voltage divider before V1 input of diff pair)?

Perhaps lower R22 to something like 1.2k... Should R23 stay proportional to R22?

 
You can try increasing R23, but honestly, I think the circuit already goes about as far as it can go. Try R23 = 100K. If that doesn't get you there, then maybe you should try out the Suburban Decay Maniac.

We've talked about JFETs, the MID control and increasing the gain of the last stage. What else are you contemplating?

If you're not doing all of this on a breadboard, you should be.
 
Chuck D. Bones said:
You can try increasing R23, but honestly, I think the circuit already goes about as far as it can go. Try R23 = 100K. If that doesn't get you there, then maybe you should try out the Suburban Decay Maniac.

We've talked about JFETs, the MID control and increasing the gain of the last stage. What else are you contemplating?

If you're not doing all of this on a breadboard, you should be.
Click to expand...
100K? That's quite a jump up from 3k3. Can you explain some of the theory behind the functionality of R23?

I would eliminate the toggle switch as it doesn't do much. If the potential of the Pentodes control is pretty much maxed out with the original design, then I'd remove the pot so it's wide open all the time. I'm more interested in the overdriving power amp AC/DC kind of sound than anything.
 
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