Is it Remotely Feasible to Modify the Crack to use 12SN7s?

[williaty]

Title pretty much says it all. Is it remotely feasible to alter the Crack+Speedball to run 12SN7s rather than 6SN7s, or would that basically be a ground-up redesign?

[Lee Hankins]

Space is a concern.  One would need a 12.6 xfmr, small ac to dc regulated heater board, and octal socket.  The easiest would be to just use a 12au7 to 6sn7 adapter, and forget about using 12sn7 tubes.  A stock Crack would be the easiest to modify, gets extremely crowded with speedball, large caps, and CX-7 choke.   There definitely is space for a 12.6V xfmr and board in the rear above the power transformer, it would be mounted on the rear of the case, I have the choke and a large 52mf cap located in this area so I know for sure that 12.6V xfmr and board would fit, and then mount the choke on the top, or leave it out all together.  I do not believe that the choke improved the sound at all, but my ears are extremely old and abused.

[Paul Joppa]

You could do a voltage doubler from the 6vAC winding to get 12 volts DC, without adding a 12-v transformer.

[williaty]

You could do a voltage doubler from the 6vAC winding to get 12 volts DC, without adding a 12-v transformer.

Is that something the tranny will put up with in the long term?

[Paul Joppa]

You could do a voltage doubler from the 6vAC winding to get 12 volts DC, without adding a 12-v transformer.

Is that something the tranny will put up with in the long term?

Yes it is. Use a resistor before the doubler to adjust the voltage, leave one transformer terminal grounded, and feed 6.3vAC to the 6080.

[Lee Hankins]

Sorry that I didn't mention using the voltage doubler, completely forgot about using it.  I have gone to using a separate filament Xfmr in most of my newer builds, this makes it alot easier with the correct switch to turn on the filaments before the B+.  Like youself I have gone to using 12SN7/12SL7/12SX7 in place of the much more expensive 6 volt versions, but some how I now have a drawer full of 12SN7/SL7 tubes so in my case I probably didn't save a lot of money.

I mentioned using an 12AU7 to 6SN7 adapter, I have found that the adapter can and does increase the microphonics of the installed tube.  This is the only downside that I have found using the adapter, excepts for its looks.