6SN7 tubes, adapters, and hum. Please help.

[thatguy]

I have a frustrating mystery hum at ~120Hz, and I'm hoping somebody can help.

I decided to give 6SN7 tubes a try, so I ordered a 12AU7-6SN7 adapter from Garage 1217 and some tubes.

The first 6SN&GTB I popped in sounded great for a few days, until a background hum appeared. This coincided with a similar failure of one of my 12AU7s, so I figured my wiring was the problem. I gave my amp the once over and reflowed most of the connections. It still hummed.

I tried my remaining 6SN7GTBs. It still hummed.

I finally tried another 12AU7, just in case, and it was fine. Dead silent background.

I did notice something in the pinout of the adapter that might be relevant. Compared with an adapter pinout I found elsewhere in these forums, the heater pins are reversed. In my adapter, 12AU7 pins 4 and 9 correspond to 6SN7 pins 8 and 7, respectively, rather than the other way around. If the problem really is the tubes, are they just trash? or might it help to burn them in or rig a Faraday cage around them, or something?

Thanks for reading through all that.

[mcandmar]

I have encountered that hum before with 6SN7's and in my experience its almost always the tube.  I've tried different grounding, center tapping, hum pots etc and the only way to get them to run silent is to switch to DC heating.  I have no idea why as 12AU7 variants and 6CG7's always run quiet on AC.

[attmci]

Try to use a 12au7 socket saver with the adapter. 

I have good 6sn7gts and they are all works very well.

PS I don't use 6SN7GTA or 6SN7GTB.

There are a lot of very nice 12au7s you should try before rolling the 6sn7s. The 6sn7s are generally more expensive and need a good adapter.

you probably should follow this path:

Get a good 12au7 driver tube--> a TS 5998, Bendix 6080wb, GEC 6080,GEC 6as7g, WE421a--> 12BH7 etc. --> 6SN7s--> 7N7, 6BL7, 6BX7 etc.

[attmci]

I have encountered that hum before with 6SN7's and in my experience its almost always the tube.  I've tried different grounding, center tapping, hum pots etc and the only way to get them to run silent is to switch to DC heating.  I have no idea why as 12AU7 variants and 6CG7's always run quiet on AC.

Magnetostricition for sure (if all your tubes behalf like that).

[thatguy]

McMandar,

Thank you for the advice. I'll try hooking the heaters up to DC.



attmci,

Do you mean I should plug my adapter into a socket saver, and plug the socket saver into my amp? Would that help somehow?

[attmci]

McMandar,

Thank you for the advice. I'll try hooking the heaters up to DC.



attmci,

Do you mean I should plug my adapter into a socket saver, and plug the socket saver into my amp? Would that help somehow?

Give it a try.  Sometimes it definitely helps (my 6sn7-to-12au7 adapter cannot sit appropriately/tightly  to the amp socket)

Let us know if that works.

[thatguy]

Wiring DC to the heater was a qualified success, sort of.

It got rid of the volume knob-independent hum on most of the tubes. However, it introduced a new, more jagged sounding hum. The new one comes in when I turn up the volume and is audible at listening levels. Not entirely sure what to make of that.

I returned the wiring to stock while I try to figure out my next move. Hopefully I can work something out before I need to return the tubes. They sound awesome, for the part that sucks.



attmci,

Your advice makes sense, but I don't think it applies in my case. My adapter is really snug.