Using 12AU7 in 30 volt Sopht Amp

[Yazoo]

A while ago I built just the power amp stage of the Sopht All Tube 30V amp. I recently decided I'd redo it with the pre-amp but substituted a 12AU7 for the 12U7 in the original because I had one handy and it did seem to work. It must be a classic line "It worked when I switched it on..." However, when I came back to use it all I got was a low hum but no actual volume. I took it to bits and checked my work and it worked again but - you can guess what happened when I again came to use it. I knew I was taking a risk when I substituted the 12AU7. Is it going to be that the 12AU7 needs more than the circa 28 volts provided by the design? What is the minimum voltage a 12AU7 expects for the plate voltage?

[slacker]

I've used them with a 30 volt supply and they work fine. I'm pretty sure people have used them in valvecasters as well and they run at 9 or 12 volts.

[GibsonGM]

+1.
Sounds like something is loose!  Try chopsticking things to see if you can find a bad wire, etc...

[PRR]

12AU7 is only "sorta" similar to 12U7.

And I have never understood the R6 R8 cathode bias values in this plan.

Whatever. In any "don't work" case, get evidence. On tube triodes, plate cathode and grid voltages.

In a high-volt stage, we normally expect plate to be about-half (1/4 to 3/4) of supply voltage. We expect cathode to be very much less, in proportion to tube Mu. At super low voltages, we may arrive at other values, but for 30V I'd expect Plate to be 10V to 25V and cathode at a volt or so. In this curcuit, Grid must of course be zero.

And I'm sure you know that 12AU7 heater must be fed 10V-15V? At 28V, it may "work" but burn-up in an hour or a few minutes. When heater is blown the tube is less useful than an empty beer-bottle.

[El Heisenberg]

PRR

Im planning on trying this.

You drew up my 12AU7/LM3886 hybrid amps tube preamp section.

I have 2 12U7 on the way.

Whats so bad bout the cathode values? Should they be lower? The schematic is not much different now.