Before anyone makes fun (and I see many doing so here sadly) I am a tube novice, grew up on germanium transistors and just never felt the need to understand tube circuits.
I have a circuit built (using an apparently well known and proven circuit) based on 6AU6 as a pre-amp. Its fed by +/- 30V, Mullard tubes and built on a PCB.
It works, driving it with 50-100mV 1kHz tone it's gain appears to be around 5, which is fine for my need. Now, without connecting any external source to the input (across 1R35 in the circuit below) the output is stable and hum/noise free. However connecting anything to the input (ground or signal side) creates a very high 50Hz hum. Even placing a direct short across the input the hum is present, though the 1kHz tone is killed. I tried shorting to signal ground and chassis ground and then bridging chassis and signal grounds, same.
I "feel" the input impedance is somehow compromised even though there is a 4k7 resistor, and that the actual impedance might be very high.
I cannot figure the circuit so if someone can assist in indicating the likely input impedance and what I need to change to increase/decrease the gain. Any pointers as to why this hum occurs will also be appreciated.
I have a circuit built (using an apparently well known and proven circuit) based on 6AU6 as a pre-amp. Its fed by +/- 30V, Mullard tubes and built on a PCB.
It works, driving it with 50-100mV 1kHz tone it's gain appears to be around 5, which is fine for my need. Now, without connecting any external source to the input (across 1R35 in the circuit below) the output is stable and hum/noise free. However connecting anything to the input (ground or signal side) creates a very high 50Hz hum. Even placing a direct short across the input the hum is present, though the 1kHz tone is killed. I tried shorting to signal ground and chassis ground and then bridging chassis and signal grounds, same.
I "feel" the input impedance is somehow compromised even though there is a 4k7 resistor, and that the actual impedance might be very high.
I cannot figure the circuit so if someone can assist in indicating the likely input impedance and what I need to change to increase/decrease the gain. Any pointers as to why this hum occurs will also be appreciated.
Suspect that the heater circuit ground is contaminating the audio ground.
Is the heater fed by DC or AC?
If the heaters are fed by AC, try hooking up a lab 12VDC supply instead of the filament supply,
and see if the hum is gone. If gone, try a better connection point for the 12V negative return.
If the heaters are fed by DC, there may be oscillation. Post photos of both sides of the pcb.
Is the heater fed by DC or AC?
If the heaters are fed by AC, try hooking up a lab 12VDC supply instead of the filament supply,
and see if the hum is gone. If gone, try a better connection point for the 12V negative return.
If the heaters are fed by DC, there may be oscillation. Post photos of both sides of the pcb.
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That is a very incorrect schematic . . .
The 6AU6 pin numbers are incorrectly documented on your schematic.
The correct 6AU6 pin numbers are:
# 1 control grid
# 2 suppressor grid
# 3 filament
# 4 filament
# 5 Plate
# 6 screen grid
# 7 cathode
If the circuit is really wired according to your schematic, then for a 6AU6 . . .
Signal is going to the suppressor grid, pin2.
Plate is negative, pin 5.
Cathode is positive, pin 7.
Control grid is floating / not connected, pin 1.
Screen grid is floating / not connected, pin 6.
All of the above connections do not allow the 6AU6 to work properly.
Perhaps this amplifier was wired for a completely different tube type, Not a 6AU6.
Just my observations.
The 6AU6 pin numbers are incorrectly documented on your schematic.
The correct 6AU6 pin numbers are:
# 1 control grid
# 2 suppressor grid
# 3 filament
# 4 filament
# 5 Plate
# 6 screen grid
# 7 cathode
If the circuit is really wired according to your schematic, then for a 6AU6 . . .
Signal is going to the suppressor grid, pin2.
Plate is negative, pin 5.
Cathode is positive, pin 7.
Control grid is floating / not connected, pin 1.
Screen grid is floating / not connected, pin 6.
All of the above connections do not allow the 6AU6 to work properly.
Perhaps this amplifier was wired for a completely different tube type, Not a 6AU6.
Just my observations.
John, I suppose the tube and the LM1875 share the power supply. It is better to have a dedicated power supply of about +150...200V for the tube. Negative voltage is not needed. The basic idea is good, but connect the input 1R36/1R39 to pin1 (g1) and tie pin2 (g2) and pin5 (anode) to the positive power supply through 220k resistors.
One more time, please . . .
The industry data sheet for the 6AU6 pin numbers says:
# 1 control grid
# 2 suppressor grid
# 3 filament
# 4 filament
# 5 Plate
# 6 screen grid
# 7 cathode
The schematics in Post # 1 and Post # 3 disagree with the 6AU6 data sheet.
1. That problem has to be fixed first.
2. Then fix the power supply polarity issues.
3. Then the leads of 1C29 and 1C30 need to be swapped too.
Is it possible that your schematic does not match the actual PCB traces?
As drawn, your schematics show a circuit that is next to impossible to work or amplify at all.
The industry data sheet for the 6AU6 pin numbers says:
# 1 control grid
# 2 suppressor grid
# 3 filament
# 4 filament
# 5 Plate
# 6 screen grid
# 7 cathode
The schematics in Post # 1 and Post # 3 disagree with the 6AU6 data sheet.
1. That problem has to be fixed first.
2. Then fix the power supply polarity issues.
3. Then the leads of 1C29 and 1C30 need to be swapped too.
Is it possible that your schematic does not match the actual PCB traces?
As drawn, your schematics show a circuit that is next to impossible to work or amplify at all.
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Any ripple on the NEGATIVE 28V rail (mis-labeled +28V) will be grossly magnified.
Did it ever work? New build? And show the power supplies in considerable detail.
And show the DC voltage on the pins! This may quell the complaint of "wrong pin numbers", if you show OK voltages. (And not if not.)
Did it ever work? New build? And show the power supplies in considerable detail.
And show the DC voltage on the pins! This may quell the complaint of "wrong pin numbers", if you show OK voltages. (And not if not.)
The schematics show the signal is applied to Pin # 2, the suppressor grid (the only grid that is connected in the schematics).
Pin # 1 control grid. Not shown, not connected, It is floating.
Pin # 6 screen grid. Not connected, It is floating.
It can not work that way.
Suppose the circuit is working. Then that means the schematic is wrong.
Fixing a problem with an incorrect schematic is just like blindfolding an archer, and asking him to hit the bullseye on the target.
Pin # 1 control grid. Not shown, not connected, It is floating.
Pin # 6 screen grid. Not connected, It is floating.
It can not work that way.
Suppose the circuit is working. Then that means the schematic is wrong.
Fixing a problem with an incorrect schematic is just like blindfolding an archer, and asking him to hit the bullseye on the target.
Besides a correct schematic of your build, could you also provide (a link to) this well known and proven circuit?
About the schematic in post #13.
Assuming for now that no control grid current is flowing. The control grid voltage is than fixed at -2.98 V through the 470K + 56K potential divider between -28 V and ground. In order for the tube to operate with negative bias, the cathode voltage would have to be higher than this -2.98 V. The current needed through the 220 Ohm cathode resistor to get the cathode higher than -2.98 V would have to be higher than 113.7 mA (!), which ofcourse is impossible given the 4K7 anode resistor and the 6J1 not being able to pass this current anyway.
So I would think that control grid current has to flow in this circuit (through the 56K resistor to ground, which is at a positive potential with respect to the control grid) in order to get the control grid a lot more negative. That current can't be more than 0.5 mA, but I don't think it will be much less than that.
Assuming for now that no control grid current is flowing. The control grid voltage is than fixed at -2.98 V through the 470K + 56K potential divider between -28 V and ground. In order for the tube to operate with negative bias, the cathode voltage would have to be higher than this -2.98 V. The current needed through the 220 Ohm cathode resistor to get the cathode higher than -2.98 V would have to be higher than 113.7 mA (!), which ofcourse is impossible given the 4K7 anode resistor and the 6J1 not being able to pass this current anyway.
So I would think that control grid current has to flow in this circuit (through the 56K resistor to ground, which is at a positive potential with respect to the control grid) in order to get the control grid a lot more negative. That current can't be more than 0.5 mA, but I don't think it will be much less than that.
In the schematic in post #13 the electrolytic coupling capacitors between the 6J1's and the opamps have their negative sides connected to the anode while their positive sides are at ground potential. At start-up the 6J1's don't pass current yet, so the negative sides of the coupling capacitors start at +28 V. I would think this is bad for those capacitors.
huggygood and johne999
Is johne999's 6AU6 amplifier on a PCB?
That schematic in Post # 13 shows a 6J1 tube, correct?
The tube on that schematic is drawn as if it is a twin triode with parallel cathodes, and parallel plates.
But a 6J1 is a pentode, not a twin triode.
A 6J1 pentode pin-out is different than a 6AU6 pentode pin-out.
We need a true and correct schematic of the actual 6AU6 amplifier that johne999 has.
All other troubleshooting is just based on conjecture.
I suppose, you suppose, everyone supposes, and so we all get it wrong.
This is the 20th posting, and we are no closer to solving the problem.
There is lots more work and delay to the solution when troubleshooting is done without a correct schematic.
Is johne999's 6AU6 amplifier on a PCB?
That schematic in Post # 13 shows a 6J1 tube, correct?
The tube on that schematic is drawn as if it is a twin triode with parallel cathodes, and parallel plates.
But a 6J1 is a pentode, not a twin triode.
A 6J1 pentode pin-out is different than a 6AU6 pentode pin-out.
We need a true and correct schematic of the actual 6AU6 amplifier that johne999 has.
All other troubleshooting is just based on conjecture.
I suppose, you suppose, everyone supposes, and so we all get it wrong.
This is the 20th posting, and we are no closer to solving the problem.
There is lots more work and delay to the solution when troubleshooting is done without a correct schematic.