12AU7 as tamer reverb driver in BF amps

[tubeswell]

From what I can see, according to Tung Sol, the 12AU7 has a typical plate resistance of 7k7 with a plate voltage of 250, and a max plate dissipation rating of 5.5W for both plates, whereas a 12AT7 has a plate resistance of 10k9 at the same voltage and a max dissipation rating of 2.5W per triode. So is there anything likely to be particularly troublesome about subbing the 12AU7 into the BF reverb driver circuit (with the SE RT giving a 20k reflected load at a plate voltage of around 400) in order to tame the reverb a bit?

[MWJB]

Plate voltage may drop some as the 12AU7 will draw significantly more plate current, so that 400v maybe less. It's typical to run a tube into an OT with 2 to 4 times the plate impedance so for the 12AU7, both triodes, that's a primary Z of 7K7 up to 15K4. It won't hurt to run the 12AU7 into 20K (or 25K, RTs vary from 14K500 to 25K) they live perfectly happily in circuits with 100K or higher plate resistors.

12BH7 might also be an option, bigger heater current draw at 600mA.

In either case you might want to measure current draw & adjust cathode resistor value if you are greatly exceeding 100% PD?

Personally, I might be looking at a dwell control to tame reverb, if you don't have one already?

[greekie]

How about using a Dwell-control to tame it? I do that on most BF amps where the owner wants to have more control over the reverb. If you replace the hum-control with 2x100ohm resistors, then you have a free spot on the back where you can install it. Remember to use shielded cable!

Jake

[Amp Kat]

I agree that the big thing you get with the AT is more stability. The plates are running usually high coming off of the reverb tranny and the AT does well in that spot. Although the AU has a slightly higher wattage rating the AT seems to hold up better. You might could lower the plate voltage to tame it easier than doing it with the tube.

[Diablo]

12BH7 might also be an option, bigger heater current draw at 600mA.

Yes, the 12BH7 works very well to tame the reverb. I'm using one in a BF Deluxe Reverb clone. No issues so far.

[Jazz P Bass]

You could also try an 12AT7 in the "mixer" position.