6SN7 hum

[Mucker]

Hi all,

I know this has been discussed to death but I am still getting slight hum from most of my 6SN7 tubes (using the Garage adapter). Once I switch over to 12AU7, the amp is silent.

I read about trying a 6V AC to DC wall wart to use for the 9 pin socket heather wiring. I do know how to hook it up from a prior post Paul provided and also know about the Jameco wall wart he suggested.

There is no buzzing, or hissing, only hum. Would I be wasting my time trying this. The hum is livable but it is slightly annoying and it would be nice if most of it was eliminated.

If I do end up trying this, do I plug the wall wart in the same time as powering on the amp? Thanks.

Greg

[Paul Birkeland]

You could wire the wall wart to be powered up from the power switch in the Crack.  You might even be able to find one that would mount inside the chassis since the power demands are minimal.

Why not just rewire the amp for the 6CG7, which is the 9 pin equivalent of the 6SN7?

[Mucker]

Thanks Paul, I guess I didn't catch up far enough to know about the 6CG7. I just found your old post describing how to do this. Awesome, looks like it's worthwhile to try!

Does the 6CG7 not have the heater issues of the 6SN7? I understand this is an AC vs DC thing from reading. Is that correct? Thank you!

[Paul Birkeland]

The 6SN7 should be silent on AC heating, but with an adapter in place that may not be the case.

[Mucker]

Ah, got it, thanks

[Natural Sound]

FWIW I have been using a 6SN7 in my amp for 11 years and there is no hum using AC heaters. As PB mentioned it's probably your adapter that's causing the trouble.

I have a set of electricians punches so I was able to enlarge the hole in the chassis to install an octal socket quite easily. I then had to figure out a way to mount the bias LED's since there is no center tab on the octal. I used a small terminal strip. A simple rewire of the 9 pin miniature socket for the 6CG7 would be MUCH easier. If you have a large collection of 6SN7's that you'd like to use then maybe an adapter for a different vendor might be worth a shot.

[Larpy]

Years ago, I converted my Foreplay III to 6SN7s, and I used a step drill bit to enlarge the holes.  Less than $10 at Freight Harbor.  You'll need to install octal sockets and reroute the wiring, but that's no big deal.

Ideally, you'd want to adjust the resistor between the rectifier diodes and the 6 volt heater supply:  the 6SN7 draws a little bit more current than the 12AU7 does, so to get the 6SN7 tubes precisely at 6.3 volts you'll need to allow for that.

[Natural Sound]

Ideally, you'd want to adjust the resistor between the rectifier diodes and the 6 volt heater supply:  the 6SN7 draws a little bit more current than the 12AU7 does, so to get the 6SN7 tubes precisely at 6.3 volts you'll need to allow for that.

The FPIII circuit had a DC PS powering the filaments so it was a bit different. Crack runs the transformer secondary (AC) directly to both tubes. There is no "Rheater" used in the crack amp.

[Mucker]

FWIW I have been using a 6SN7 in my amp for 11 years and there is no hum using AC heaters. As PB mentioned it's probably your adapter that's causing the trouble.

I have a set of electricians punches so I was able to enlarge the hole in the chassis to install an octal socket quite easily. I then had to figure out a way to mount the bias LED's since there is no center tab on the octal. I used a small terminal strip. A simple rewire of the 9 pin miniature socket for the 6CG7 would be MUCH easier. If you have a large collection of 6SN7's that you'd like to use then maybe an adapter for a different vendor might be worth a shot.

That's great info thank you! I do have quite a few 6SN7 and they work well in my Woo Audio WA2. I think I will just try the rewire for now. Kind of interested to give the 6CG7 a go. Tubes are cheap enough. Really chomping at the bit for my S3X to arrive. Hopefully soon.