Possible to use 12sn7 to replace 12at7?

[peterq]

I have some good 12sn7 tubes, can I use them to replace 12at7? Which parts should I change? Thanks.

[Paul Joppa]

The 12AT7 has a tapped heater which can b used at either 12 volts or 6 volts. Stereomour II has only a 6 volt winding on the power transformer, and the 12SN7 has only a 12 volt heater. So you could use 6SN7s, but not 12SN7s. You could of course use a voltage doubler to get 12 volts DC, but you're on your own to design it - and figure out where to put it. edit: untrue

Even if you did use 6SN7s with AC heaters, you'd still have to cut a larger hole for the larger sockets, and figure out how to wire the 12SN7 6SN7s. The gain would drop to 1/3 of the original, in most cases requiring a preamp and thus rendering the volume/balance controls redundant. And of course many of the resistors in the driver and C4S modules would have to be replaced with different values to accommodate the different tube parameters. All in all, a fairly significant amount of work!

[peterq]

Hi Paul,
Thanks for the reply. From the schematic, I see 12.6v from PT, is it an error? I haven't measured the filament voltage but would like to check from the document first.
For 6sn7, I would like to buy two adapters so that I don't need to enlarge the hole on the panel.

Peter

[Paul Joppa]

Son of a gun! That's left over from the original Stereomour - when we went to the Stereomour II, we revised the transformer, and it looks like we failed to revise the circuit diagram. You tested this on page 32 of the manual.

And I apologize for an error in my earlier post; the 6v winding does NOT have enough current capacity to handle a DC supply for a pair of 12SN7s. I guess I was posting before I had my coffee that day. I'll correct the post.

[peterq]

Thanks Paul.