Hi Everyone! I'm looking to see if I can't diagnose noise occurring in a Don Sachs vacuum tube preamplifier, which is based on an Aikido preamplifier circuit also used by Tubes4Hifi in its SP14 preamplifier.
Here's the scoop.
The preamplifier uses 4 6SN7 octal tubes, has dual mono regulated power supplies for each channel, for both B+ and filaments.
The Issue:
I emailed Don but he's doing a new thing now (no longer building these) and I did buy this preamp used. I know he said once that he recommends putting carbon film caps on the grids and cathodes for all 6sn7s, but I haven't inspected it that closely to know.
Here's a link to the SP14 schematic:
https://www.icloud.com/iclouddrive/0c1iv9KCt0l7X80KfV0UNxpEw#Public
I will take a photo tonight of the actual PCB in Don's amp to show where there are differences.
I opened the unit just to see if there was a cold solder or disconnected ground, etc. Everything is sturdy and sound (from observation with the eye). I haven't tested anything with my multimeter and I don't have other testing equipment.
My sense is there's some DC leaking in a capacitor or a failed resistor, but I'm not sure where to start. Maybe there's a ground loop, but I'm not seeing it with a naked eye.
Any help is super appreciated. If you're in the Chicagoland, NW Indiana, or SE WI area I'd pay you to consult!
Thank you!!!!
Here's the scoop.
The preamplifier uses 4 6SN7 octal tubes, has dual mono regulated power supplies for each channel, for both B+ and filaments.
The Issue:
- Hum/buzz on left channel only; Right channel is nice and quiet.
- On startup, the hum is barely there if at all. Within a few minutes the hum comes on and grows in volume over time.
- Hum is not affected by: moving to different system, house, swapping new/different tubes, changing cables, removing other equipment, etc..
- Hum is present even with no source connected.
- Hum is present even if using power conditioners or not using them.
- I've disconnected everything and the hum is present (so long as an amplification source and loudspeaker are present on the left channel.
- The sound of the hum/noise oscillates a bit and sounds like a motor running and changing speed due to changes in power sent to it.
- It has grown over time (last month) to now it has to be addressed.
I emailed Don but he's doing a new thing now (no longer building these) and I did buy this preamp used. I know he said once that he recommends putting carbon film caps on the grids and cathodes for all 6sn7s, but I haven't inspected it that closely to know.
Here's a link to the SP14 schematic:
https://www.icloud.com/iclouddrive/0c1iv9KCt0l7X80KfV0UNxpEw#Public
I will take a photo tonight of the actual PCB in Don's amp to show where there are differences.
I opened the unit just to see if there was a cold solder or disconnected ground, etc. Everything is sturdy and sound (from observation with the eye). I haven't tested anything with my multimeter and I don't have other testing equipment.
My sense is there's some DC leaking in a capacitor or a failed resistor, but I'm not sure where to start. Maybe there's a ground loop, but I'm not seeing it with a naked eye.
Any help is super appreciated. If you're in the Chicagoland, NW Indiana, or SE WI area I'd pay you to consult!
Thank you!!!!
Later today, I'll post an actual photo of the inside of the preamplifier chassis and the actual pcb. I'm stuck at work now.
If noise is only in the left channel, we can rule out the power supply.
Is the noise still present with shorting plugs inserted into both inputs?
Leave the plugs in during all testing.
If you have swapped tubes around, we can rule tubes out.
So remaining are solder joints, resistors, tube sockets, and capacitors.
Clean all tube sockets with a tube with pins moistened with deOxit D5.
Clean any PCB areas with visible solder flux deposits, esp between socket pins.
Try tapping with an insulated tool on various areas of the left channel pcb and parts.
If no luck, try freeze spray on one part at a time while listening to the noise.
Is the noise still present with shorting plugs inserted into both inputs?
Leave the plugs in during all testing.
If you have swapped tubes around, we can rule tubes out.
So remaining are solder joints, resistors, tube sockets, and capacitors.
Clean all tube sockets with a tube with pins moistened with deOxit D5.
Clean any PCB areas with visible solder flux deposits, esp between socket pins.
Try tapping with an insulated tool on various areas of the left channel pcb and parts.
If no luck, try freeze spray on one part at a time while listening to the noise.
What does C6 do? It has that look about it (on the diagram) as something that could fail and introduce ripple from the rail into the signal.
It might be worth seeing if the DC conditions at the output (before C3) vary at all on the faulty channel.
It might be worth seeing if the DC conditions at the output (before C3) vary at all on the faulty channel.
Thank you Rayma! You seem incredibly knowledgeable.
I will do this later today and also upload photos.
I will do this later today and also upload photos.
Solder joints are statistically the most likely cause of noise, but resistors can get more noisy than normal.
However, one thing to try in this circuit is to remove the hum cancellation capacitor C6 and see if that fixes the problem.
In some cases that supply feedback can actually cause noise rather than decreasing it.
However, one thing to try in this circuit is to remove the hum cancellation capacitor C6 and see if that fixes the problem.
In some cases that supply feedback can actually cause noise rather than decreasing it.