Triple sided triodes have been causing people grief for years and we are here to help. You will find these tubes in Ampeg guitar amplifiers and various audio applications, especially tuners. The trouble this these tubes is that they were never in mass production and there are no companies producing them anymore. They are so scarce that not only are they expensive, sometimes they cannot be found and you'll have no choice but to use a substitute or equivalent tube.
Please note, all of the available substitute tubes are significantly different from the originals but will work in place of them without causing any damage to your amplifier. However, the amplifier may sound different. The largest difference between the tubes is gain factor so your amp may have higher or lower gain depending on the substitute you use.
6AC10: Higher Ip, Medium Gain (67/67/67 vs 100/100/100)8AC10: Same as above but with an 8 volt heater which means even lower Ip and Gain6AK10: Higher Ip, Medium Gain (53/53/53 vs 100/100/100)6D10: Higher Ip, Medium Gain (57/57/57 vs 100/100/100)6BK11: Higher Ip, High Gain (70/100/100 vs 100/100/100) - Best Choice6K11 Substitutes: 6Q11: Much higher Ip, Low Gain on first section (18/100/100 vs 70/100/100)
There are plenty of other potential substitutes but these are the most common and the most likely to work.