Hey guys,
Recently i've been busy building a alembic f2b like tube preamp according to this scheme:
My first build produced a horrible amount of hum, buzz and other strange noises. So i read a whole lot of articles and rebuilded it.
I just finished and tested the preamp. Its better than the first build, but has still too much hum.
When i turn it off the hum is gone.
Here is a picture of the preamp:
Do you guys see things that should be wired different?
Here is a quick drawing of the PSU:
I hope you can give me some tips and help me out! Thanks in advance!
Tobias
Recently i've been busy building a alembic f2b like tube preamp according to this scheme:
An externally hosted image should be here but it was not working when we last tested it.
My first build produced a horrible amount of hum, buzz and other strange noises. So i read a whole lot of articles and rebuilded it.
I just finished and tested the preamp. Its better than the first build, but has still too much hum.
When i turn it off the hum is gone.
Here is a picture of the preamp:
An externally hosted image should be here but it was not working when we last tested it.
Do you guys see things that should be wired different?
Here is a quick drawing of the PSU:
An externally hosted image should be here but it was not working when we last tested it.
I hope you can give me some tips and help me out! Thanks in advance!
Tobias
Last edited:
How is the tube heater conected ? did you use 2 resistors of 100 ohms from the heater terminals to ground ?
I connected the heater directly to the 6.3v from the transformer. I didn't use those resistors. Should i connect them, even though i haven't rectified the heater supply? Do i have to connect them as close as much to the tube?
You have to connect this resistors to put the heaters at ground level , and they dont have to be close to the tube. without this resistors, you have a lot of hum in the output .
I can't see any tightly twisted twisted heater wiring, so you will get hum.
I can see what appears to be intended as an input grid stopper but it is sitting in mid-air and nowhere near the grid pin. This risks both RF instability and hum pickup. Put the resistor right on the grid connection, then screen/shield the wire from it to the input socket.
The other common source of hum/buzz is poor grounding in the PSU but I can't see exactly how you have done that.
I can see what appears to be intended as an input grid stopper but it is sitting in mid-air and nowhere near the grid pin. This risks both RF instability and hum pickup. Put the resistor right on the grid connection, then screen/shield the wire from it to the input socket.
The other common source of hum/buzz is poor grounding in the PSU but I can't see exactly how you have done that.
Moved to Instruments & Amps
You've also not got a grid resistor to ground. For some reason it's the wrong side of the (likely unnecessary) input capacitor.
First order of business is to clean up the heater (and keep it waaay clear of everything else).
First order of business is to clean up the heater (and keep it waaay clear of everything else).
The circuit doesn't have an input capacitor. It does have an output caapcitor with a ground leak resistor.
Whoo yeah, i just added the heater resistors and its really quiet now! I mounted everything on a aluminum plate. The heater wires are running twisted between the chassis and the aluminum plate. To be sure i will move the grid resistor closer to the tube, and i will look for some shielded wire.
Thanks, guys!
Thanks, guys!
To save you some space and money on a possible next project, the amount of capacitance is the PSU is overkill to say the least. A typical 12AX7 stage will draw a max of 2mA. There's two in the alembic, so 4mA max.
There's another trick for the heater reference. You could use a 250 ohm potentiometer instead of the two 100 ohm resistors. The center tap goes to ground. This way, you can 'dial in' an even better balance between the two heater leads, improving the EMI cancellation effect. This pot is known as a humdinger.
There's another trick for the heater reference. You could use a 250 ohm potentiometer instead of the two 100 ohm resistors. The center tap goes to ground. This way, you can 'dial in' an even better balance between the two heater leads, improving the EMI cancellation effect. This pot is known as a humdinger.
I'd be surprised if it didn't 😀
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