So what you are saying is that with the inputs shorted you are still hearing something out of both channels hooked up to some solid state amp, yes?
I don’t know if your voltmeter is an RMS voltmeter (hopefully it is as most Flukes are), but 1.5-1.6mV RMS AC is going to be audible from a preamp standpoint. Preamps should be about 0.1mV AC RMS typically or less.
If your preamp has 1.5mV RMS AC on the output and it is amplified 20 times (for example from a solid state amplifier, which is about 26dB), then the hum out of your solid state amplifier would be 30mV AC RMS.
As a test, hook it up to your solid state amp, but do not hook up the speakers. Just measure the output binding posts with your Fluke set to AC. What is the value now?
Best,
Anand.
I don’t know if your voltmeter is an RMS voltmeter (hopefully it is as most Flukes are), but 1.5-1.6mV RMS AC is going to be audible from a preamp standpoint. Preamps should be about 0.1mV AC RMS typically or less.
If your preamp has 1.5mV RMS AC on the output and it is amplified 20 times (for example from a solid state amplifier, which is about 26dB), then the hum out of your solid state amplifier would be 30mV AC RMS.
As a test, hook it up to your solid state amp, but do not hook up the speakers. Just measure the output binding posts with your Fluke set to AC. What is the value now?
Best,
Anand.
So both of these results above you have listed are from the amp output and you have given me AC values with the right channel much louder than the left channel. Max from the amplifier should really be at 1mV RMS AC or lower, especially with sensitive speakers. Your DC values look fine, not unsafe for your speakers although if there is a way to adjust your DC offset on your solid state amp, that can be done to dial down the DC offset closer to zero volts.
Congratulations on having a nice Fluke 87, great meter, excellent quality.
I want you to check what your AC RMS reading is on the output of your regulated B+ supply. We need to check if your regulator is operational or not or if there is AC hum arising from there. Look through the schematic and start tracing backwards while eliminating ancillaries in your circuit. For me, selector switches and volume controls are ancillaries but they are not a main part of the circuit, only the filament supply, B+ supply and amplification parts of the circuit are. So a systematic sleuthing will be required (which is why I requested shorting the inputs). I would start with the 1st capacitor right after rectification has occurred or basically C11a on your schematic you published on page 1 and then move forward. After the first CRC supply, AC rms values should decrease but may not be eliminated. Then the HV regulator comes. Is everything working there? Hard to say. Check AC RMS at the cathode of D13, then anode of D13, there should be a marked difference. If things look odd, we may need to resolder, recheck parts to make sure the proper ones were used, etc…a cold joint can make a major difference so get a loupe and start looking! A scope would be very useful here but we’ll try it with the Fluke 87!
I would do a similar study of your heater supply. Set your Fluke to DC and have your red lead connected to the junction of R25 and R25 and your black lead to ground. What is the value? This part of the circuit is part of AC hum reduction and to protect your main signal tubes. Also check what the actual heater voltage is themselves. You can just place both leads across each of the respective heaters on the tubes themselves or across H+/H-. Finally, switch your Fluke to AC and check what the AC value is for your 6X5GT heater since that is only supplied by your toroid directly (pins 2 and 7 on the 6X5 per your schematic).
Please list your current values of C11a, C11b, C17, C35, C36, and C39. They can all affect AC hum. What are those wires coming from the mounting of your IEC terminal? Where do they go? And why? I’m looking to see a clear view of your chassis ground, and I don’t see one with all the wood. I do see that your main pcb is mounted to a piece of metal, hopefully aluminum? Electrically, your back panel and whatever the metal you have used to mount the main pcb need to be connected. This becomes very important once you introduce non metallic materials like wood into your circuit. The front panel will need to have this connection too, particularly the main VC hub/bushing…too bad there are so many VC’s on this design, perhaps for adding a ‘balance’ control.
I’ll be honest, I’ve looked at this pcb a few times, from a layout perspective and I’m not entirely enthused as grover has mentioned there are parts where a ground loop may occur. As such, one of the things I will mention if all else fails is to try a ground lift circuit. You can design one of these or just purchase it from Broskie. You would install it between the final ground from the main circuit (going to a star ground) and the chassis ground. From the chassis ground you really should have only one wire going to your IEC ground terminal. That’s it. On the 1st page, you have a drawing showing a purple wire going from the ground of the main pcb directly to the IEC ground terminal and that to me is what I would not do. I would connect it to where the purple wires from the Antek toroids attach but immediately before, place a ground lift. That way you are ‘lifting’ the main part of the circuit from the chassis ground.
Here is the ground lift circuit link with pcb. And here is the DC Blocker circuit link with pcb. Both are good (if you purchase the DC blocker, there is no need for the ground lift circuit board as it is included all on one pcb, see links). The DC blocker is an additional circuit to help with buzzing from toroidal transformers (and you have 2!). It might be highly beneficial in environments where there is DC resting on your MAINS line which many houses have due to the multitude of switcher power supply devices, refrigerators, etc…that dump that back into your mains, which can make mains toroidal transformers buzz. EI Core transformers will do it a lot less due to their construction.
Finally, we’ll get back to the upgraded caps and such but we need to find out if the main parts of the circuit are properly operational.
I hope this helps you.
Best,
Anand.
Congratulations on having a nice Fluke 87, great meter, excellent quality.
I want you to check what your AC RMS reading is on the output of your regulated B+ supply. We need to check if your regulator is operational or not or if there is AC hum arising from there. Look through the schematic and start tracing backwards while eliminating ancillaries in your circuit. For me, selector switches and volume controls are ancillaries but they are not a main part of the circuit, only the filament supply, B+ supply and amplification parts of the circuit are. So a systematic sleuthing will be required (which is why I requested shorting the inputs). I would start with the 1st capacitor right after rectification has occurred or basically C11a on your schematic you published on page 1 and then move forward. After the first CRC supply, AC rms values should decrease but may not be eliminated. Then the HV regulator comes. Is everything working there? Hard to say. Check AC RMS at the cathode of D13, then anode of D13, there should be a marked difference. If things look odd, we may need to resolder, recheck parts to make sure the proper ones were used, etc…a cold joint can make a major difference so get a loupe and start looking! A scope would be very useful here but we’ll try it with the Fluke 87!
I would do a similar study of your heater supply. Set your Fluke to DC and have your red lead connected to the junction of R25 and R25 and your black lead to ground. What is the value? This part of the circuit is part of AC hum reduction and to protect your main signal tubes. Also check what the actual heater voltage is themselves. You can just place both leads across each of the respective heaters on the tubes themselves or across H+/H-. Finally, switch your Fluke to AC and check what the AC value is for your 6X5GT heater since that is only supplied by your toroid directly (pins 2 and 7 on the 6X5 per your schematic).
Please list your current values of C11a, C11b, C17, C35, C36, and C39. They can all affect AC hum. What are those wires coming from the mounting of your IEC terminal? Where do they go? And why? I’m looking to see a clear view of your chassis ground, and I don’t see one with all the wood. I do see that your main pcb is mounted to a piece of metal, hopefully aluminum? Electrically, your back panel and whatever the metal you have used to mount the main pcb need to be connected. This becomes very important once you introduce non metallic materials like wood into your circuit. The front panel will need to have this connection too, particularly the main VC hub/bushing…too bad there are so many VC’s on this design, perhaps for adding a ‘balance’ control.
I’ll be honest, I’ve looked at this pcb a few times, from a layout perspective and I’m not entirely enthused as grover has mentioned there are parts where a ground loop may occur. As such, one of the things I will mention if all else fails is to try a ground lift circuit. You can design one of these or just purchase it from Broskie. You would install it between the final ground from the main circuit (going to a star ground) and the chassis ground. From the chassis ground you really should have only one wire going to your IEC ground terminal. That’s it. On the 1st page, you have a drawing showing a purple wire going from the ground of the main pcb directly to the IEC ground terminal and that to me is what I would not do. I would connect it to where the purple wires from the Antek toroids attach but immediately before, place a ground lift. That way you are ‘lifting’ the main part of the circuit from the chassis ground.
Here is the ground lift circuit link with pcb. And here is the DC Blocker circuit link with pcb. Both are good (if you purchase the DC blocker, there is no need for the ground lift circuit board as it is included all on one pcb, see links). The DC blocker is an additional circuit to help with buzzing from toroidal transformers (and you have 2!). It might be highly beneficial in environments where there is DC resting on your MAINS line which many houses have due to the multitude of switcher power supply devices, refrigerators, etc…that dump that back into your mains, which can make mains toroidal transformers buzz. EI Core transformers will do it a lot less due to their construction.
Finally, we’ll get back to the upgraded caps and such but we need to find out if the main parts of the circuit are properly operational.
I hope this helps you.
Best,
Anand.
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One more thought…please make sure that your outputs are well isolated. There shouldn’t be any continuity (check in ohms), between your RCA hot and ground/tab. The RCA grounds can have continuity with each other. We need to ensure that your RCA’s are well isolated from the metal back panel. I think visual inspection looks good judging from the 1st page but ensure it also makes sense electrically.
Best,
Anand.
Best,
Anand.
Thanks for all that! I need my coffee before I consider having at that list
Plus, I have to prep for some work this week, which I will get out of the way this morning so I can have a clear mind for this!
look at the wire from selector out to input, this wire should be wired in parallel with the wires going to the selector it is going very close to your capacitor and other stuff. It is close also to the power on light wire
Other than that what you did looks more professional.
The biggest hum is in my power mono amps the feedback wire catch power supply ripple and hum, I have to completely redesign this amp and do it all again, ptp
You can also try using an inductor in series with the ground of the power outlet
Other than that what you did looks more professional.
The biggest hum is in my power mono amps the feedback wire catch power supply ripple and hum, I have to completely redesign this amp and do it all again, ptp
You can also try using an inductor in series with the ground of the power outlet