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Help Needed: 12AX7 Single Gain Stage Preamp with Tone Controls & Cathode Follower

Leestereo

Leestereo

Super Member
I'm working on a 12AX7 preamp with tone controls and a cathode follower output. I've attached the schematic, its a modification of a small SE integrated amplifier that had trashed output transformers. I'm wondering if I implemented the cathode follower properly: I measured about 4V at pin 8 (against chassis ground) when I expected about to see about 100V. The preamp does function; I hooked it up to a solid state power amp (47kohm input impedance) and the quick listening test was fine.
 

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  • AG9016 Schematic mod.jpg
    AG9016 Schematic mod.jpg
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I'm no expert but I would expect to see 100V too and not 4V. What kind of voltage are you seeing at pin 7?

R14 is a trim pot; what do you have that at? Maybe it's pulling the pin 7 voltage too low.
 
You need a coupling cap for the second stage - as-is, the grid of the cathode follower is at DC ground - it needs to float for the stage to self-bias. And do you need two gain controls?
 
Its not quite at DC ground, its at whatever amount above DC ground you end up with after adjusting R14. I'd probably go for ditching that pot, but if you do need it, a capacitor between the center wiper of the pot and the grid resistor (R15) should sort you out. As it sits now, you should see the voltage at the cathode change with adjustments to R14.
 
Thanks for the comments and suggestions.

R14 is the balance control; the first gain control (R4,R5) is the volume pot with a loudness tap.

The consensus seems to be a capacitor on the wiper of R14 before the grid resistor R15 and the 1M resistor? What size capacitor? The funny thing is that I had initially drawn the schematic with a 10nF cap there but then thought it wasn't necessary.
 
OK, .01µF it is.

I think I understand that the grid in the cathode follower has to be isolated from DC ground; but does the cap have to be before the 1k grid resistor and the 1M resistor, or between the 1M resistor and the 1k grid resistor? Does it even matter?
 
Leestereo said:
OK, .01µF it is.

I think I understand that the grid in the cathode follower has to be isolated from DC ground; but does the cap have to be before the 1k grid resistor and the 1M resistor, or between the 1M resistor and the 1k grid resistor? Does it even matter?
Click to expand...

You want the 1 meg and the 1k to remain connected. Thats what sets the bias on the tube. The cap needs to isolate the grid from the wiper of the pot. Basically run the cap from the center wiper of the pot to the junction of the 1 meg and 1k and you're good.
 
gadget73 said:
You want the 1 meg and the 1k to remain connected. Thats what sets the bias on the tube. The cap needs to isolate the grid from the wiper of the pot. Basically run the cap from the center wiper of the pot to the junction of the 1 meg and 1k and you're good.
Click to expand...

Thanks for the clarification, makes perfect sense. I was getting confused with the 1k grid stopper in place; I guess I don't even really need the 1k grid stopper.
 
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