Hello,
I could use help with a preamp circuit I have built. This is a 12AU7 cathode follower buffer into a 3-band tone control from the Max Robinson website: https://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html
The output of the tone circuit goes to a John Broskie CCDA line-stage: https://www.tubecad.com/2009/03/blog0161.htm
Functionally everything works, but with the tone control section in the circuit I am getting hum on the output. With the scope probe, the excessive hum appears starting at the plate of the first CCDA triode. There is hum on the grid, but it is very, very, small signal. If I disconnect the output of the 12AX7 on the tone circuit and connect the cathode follower out directly to the CCDA section, no more audible hum. (the green line on the schematic). The signal gain of the circuit remains relatively the same whether the tone section is connected or not.
The power supply is very well filtered. The 12.6VAC heater supply is elevated 48V.
The hum signal on the scope, when spread out to 20usec shows about a 50kHz signal.
Could it just be heater noise? Thank you for any ideas
I could use help with a preamp circuit I have built. This is a 12AU7 cathode follower buffer into a 3-band tone control from the Max Robinson website: https://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html
The output of the tone circuit goes to a John Broskie CCDA line-stage: https://www.tubecad.com/2009/03/blog0161.htm
Functionally everything works, but with the tone control section in the circuit I am getting hum on the output. With the scope probe, the excessive hum appears starting at the plate of the first CCDA triode. There is hum on the grid, but it is very, very, small signal. If I disconnect the output of the 12AX7 on the tone circuit and connect the cathode follower out directly to the CCDA section, no more audible hum. (the green line on the schematic). The signal gain of the circuit remains relatively the same whether the tone section is connected or not.
The power supply is very well filtered. The 12.6VAC heater supply is elevated 48V.
The hum signal on the scope, when spread out to 20usec shows about a 50kHz signal.
Could it just be heater noise? Thank you for any ideas
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That circuit is very high impedance, and will naturally pick up hum and noise from the environment,
if it is not well shielded in a metal enclosure, and preferably constructed on a pcb to minimize wiring.
The connections directly to the tone tube grid are especially critical, and must be kept direct and compact.
if it is not well shielded in a metal enclosure, and preferably constructed on a pcb to minimize wiring.
The connections directly to the tone tube grid are especially critical, and must be kept direct and compact.
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Thank you rayma, chrisng
The chassis is aluminum. The power transformer is an Antek toroid mounted on the outside, in a steel case, directly under the power supply board that you see in the photo. The filament wire is tightly twisted right against the chassis. There is only a couple of inches of transformer wiring coming through the chassis from the top. It is under the power supply board. The power supply board is several inches away from the tone control circuitry. I suppose since I have an aluminum chassis that doesn't matter. I have some copper foil, maybe I should try shielding beneath the power supply board.
It's interesting that I don't measure the excessive hum on the grid or plate of the 12AX7.
Would scaling everything to use 100K tone pots work better?
The chassis is aluminum. The power transformer is an Antek toroid mounted on the outside, in a steel case, directly under the power supply board that you see in the photo. The filament wire is tightly twisted right against the chassis. There is only a couple of inches of transformer wiring coming through the chassis from the top. It is under the power supply board. The power supply board is several inches away from the tone control circuitry. I suppose since I have an aluminum chassis that doesn't matter. I have some copper foil, maybe I should try shielding beneath the power supply board.
It's interesting that I don't measure the excessive hum on the grid or plate of the 12AX7.
Would scaling everything to use 100K tone pots work better?
Possibly, if the wiring were made as compact as possible at the grid of the the tone tube.
In other words, for each resistor connected to the tone tube grid, the resistor body
should be very close to the grid pin, with short leads.
First, you could more the present 470k resistors closer to the grid to see if that helps.
Copper foil probably won't help.
In other words, for each resistor connected to the tone tube grid, the resistor body
should be very close to the grid pin, with short leads.
First, you could more the present 470k resistors closer to the grid to see if that helps.
Copper foil probably won't help.
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chrisng, I like the idea of the steel tape. Definitely going to order some and try it. Thank you for the link.
Hello Gerrit, the heaters are elevated to 48VDC but not DC heaters yet. I should try it and see what happens, although with the tone section of the circuit bypassed the noise level is fine. Unless it's just a heater issue with 12AX7. I have swapped tubes, and I even tried a 12AU7 in the 12AX7 position with the same results.
rayma, in your opinion do you think the main cause of the issues I am having are caused by the high impedance tone circuit? What are your thoughts on scaling everything to use 100K pots instead?
Hello Gerrit, the heaters are elevated to 48VDC but not DC heaters yet. I should try it and see what happens, although with the tone section of the circuit bypassed the noise level is fine. Unless it's just a heater issue with 12AX7. I have swapped tubes, and I even tried a 12AU7 in the 12AX7 position with the same results.
rayma, in your opinion do you think the main cause of the issues I am having are caused by the high impedance tone circuit? What are your thoughts on scaling everything to use 100K pots instead?
Scott17,
I built the 2 band tone control from the Angelfire website, and like you, fitted all the tone control components onto the bass and treble potentiometers. Also like you, I had lots of hum, as I used normal wire to connect the tone control section to the to the grid of the 12AX7. In the end I used coax cable that was grounded only at the the grid end, and cabled direct to the grid pin of the socket, so that as much of the cable was screened. That got rid of the hum for me.
In your case you'll have three coax cables to ground in this way.
I also had a slight buzzing which required connecting a ground wire to the back of the potentiometers (fitted under one of the tabs) as they were mounted on a wood section of the enclosure.
Hope this helps
I built the 2 band tone control from the Angelfire website, and like you, fitted all the tone control components onto the bass and treble potentiometers. Also like you, I had lots of hum, as I used normal wire to connect the tone control section to the to the grid of the 12AX7. In the end I used coax cable that was grounded only at the the grid end, and cabled direct to the grid pin of the socket, so that as much of the cable was screened. That got rid of the hum for me.
In your case you'll have three coax cables to ground in this way.
I also had a slight buzzing which required connecting a ground wire to the back of the potentiometers (fitted under one of the tabs) as they were mounted on a wood section of the enclosure.
Hope this helps
The effect of the transformer line field can show 60 hz by scope.( 50 hz in my country).If see120 Hz hum in the scope, it is usually not a field issue.. Hum can be affected by the tube filaments. To test, ...When the preamp is on... disconnect the filimants from Voltage lines. The tube is hot and for a while It continues to work for a while. If the hum is fixed... the power supply of the filaments must be changed.
polycell: thanks for the information. Did you use 500k pots or 100k? Yes, the cable from the tone control section to the 12AX7 grid is shielded with the shield attached to the ground bus at the tube end. I have the shielded cable connected to a tag strip "near" the grid, but from there I have a 1K stopper going from the tag strip to the 12AX7 grid, with the resistor body close to the socket pin. There is maybe 25mm of resistor lead that is not shielded. I should try to get it closer.
hooman: thank you. I'm using a ground bus that you should be able to see in the overall photo. It is connected only once to the power supply board, and the power supply board ground is connected to the chassis a the IEC power inlet. I had a loop breaker (10R//100n) in between the power supply ground and chassis ground, but removed it. No difference. The potentiometer cases are grounded to the chassis through the mounting. The RCA in & out jacks are isolated from the chassis and connected to circuit ground via the I/O signal cable shields to the ground bus. I tried disconnecting the heaters while monitoring the hum signal. It did reduce somewhat, but not as much as bypassing the 12AX7 portion of the tone circuit as I described in the original post. The hum sine wave frequency is 50kHz, which seems odd. Edit: If you look at the 2msec grid wave as a long sine wave, it is 16msec long which is right about 60Hz.
hooman: thank you. I'm using a ground bus that you should be able to see in the overall photo. It is connected only once to the power supply board, and the power supply board ground is connected to the chassis a the IEC power inlet. I had a loop breaker (10R//100n) in between the power supply ground and chassis ground, but removed it. No difference. The potentiometer cases are grounded to the chassis through the mounting. The RCA in & out jacks are isolated from the chassis and connected to circuit ground via the I/O signal cable shields to the ground bus. I tried disconnecting the heaters while monitoring the hum signal. It did reduce somewhat, but not as much as bypassing the 12AX7 portion of the tone circuit as I described in the original post. The hum sine wave frequency is 50kHz, which seems odd. Edit: If you look at the 2msec grid wave as a long sine wave, it is 16msec long which is right about 60Hz.
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