Replace 12AU7 with 6SN7 ?

I could be wrong but I thought the 12 and 6 mean the voltage of those tubes!

Yogi, that is correct as I understand it also. I can't see how you could use a 6 volt tube in a 12 volt position even with an adapter, unless that adapter also steps down the voltage?

12AU7 and related 9 pin triodes have a center tapped filament that can run on 12 or 6 volts depending on whether the segments are wired in series or parallel. In most amps they run on 6 volts as that saves having a special 12 volt secondary winding on the power transformer.

6SN7 draws 600 ma of filament current. A 12AU7 draws 300 ma filament current in parallel mode (6 volt operation).

Simply replacing 12AU7s by 6SN7s would double the filament current drawn (3.6 amps vs 1.8 amps for 6 tubes). That could easily burn out the power transformer if its rating is marginal.

I would follow the manufacturer's recommendation as mentioned above.

Yogi and Mofi, yes, the 12 and the 6 are the filament voltages that the tubes are designed to be operated with. Obviously, applying 12 volts to a 6 volt filament would result in pyrotechnics.

Looking at a tube manual I have, though, I see that the 12AU7 apparently has a center-tapped filament, and its filament has two ratings, 12.6 volts at 0.15 amps, or 6.3 volts at 0.3 amps. The 6SN7 rating is 6.3 volts at 0.6 amps.

I would surmise, therefore, that SOME designs operate the 12AU7 by applying 6.3 volts between the center tap of the filament and both of its end points, thereby operating the two halves of the filament in parallel.

Other specs on the two tubes are generally in the same approximate ballpark, except that the 6SN7 can handle higher power levels, especially in its GTA and GTB variants, which are also rated to be able to handle considerably higher voltage levels.

So IF the adapter steps down 12 volts to 6 volts, OR IF the component is designed to operate the 12AU7 at 6.3 volts (with the two halves of the filament in parallel), AND IF the power transformer can comfortably supply the doubled filament current that would be required by the SIX tubes, it may be within reason to make this substitution. However, unless one is familiar with the details of the design of both the specific component and the adapter, and unless one knows exactly what one is doing, disaster is being invited.

Regards,
-- Al

Ghostrider, just saw your post. Great minds think alike!

Regards,
-- Al