Originally Posted by manofmathematics
I tried out my Telefunken 12AU7 and it worked just fine. Of course, the gain was rather low. I had to turn the pot to 50% to obtain a decent level. No distortion or clipping was noticeable.
Is there any chance that having my volume set so high could cause any harm to the power tubes?
No .... the high volume setting is just a consequence of the low gain of the tube. If you need to turn up the volume even louder there will be a point where the tube can not meet the voltage demands, with distortion and eventually clipping as the result. Tubes clip very gracefully, unless severly overdriven, so you may not even detect the clipping until the effect is blatant.
Since this amp seems to be forgiving of a wide range of 12ax7 pinout tubes .... there are number of inexpensive 12ax7 pinout tubes, like the 12av7, that are very good sounding. Better yet, the 12av7 and 12av7 subs have a higher mu/ gain of 43-48 that should be more comparable to the mu/ gain of 70, of a 5751. These tubes have a heater current draw of 0.45 amps vs the 0.30 amps of a 12ax7/5751 .... which should be insignifigant. But, it would not hurt to contact ASL and ask. These tubes do draw a good bit more current.
What are these tubes? For the 12av7 you have the 5965/6414/6829 and 7062. You can also use the 12at7 and the 12az7 that were already mentioned.
Here is a list of all the 12ax7 pinout compatible tubes ....
NJ7P Tube Database Search
Tubes with base 9A
6AU7 6AX7 7AU7 9AU7 12AD7 12AE7
12AT7 12AU7 12AU7A 12AV7 12AX7 12AX7A
12AY7 12AZ7 12AZ7A 12BH7 12BH7A 12BZ7
12DF7 12DM7 12DT7 12DW7 12FV7 5751
5814 5814A 5963 5965 5965A 6057
6060 6067 6072 6072A 6189 6201
6201 6211 6211A 6414 6679 6680
6681 6829 6851 6913 7025 7247
ECC81 ECC82 ECC83