Tripple Triode Substitutions 29 May 2011 mikeday@backwater.com With the shortage of 6C10 and 6K11 compactron triodes, many people have been attempting to substitute other triodes that are still available. Unfortuantley, the substitutions seldom work. There are 9 compactron triodes that are interchangable physically, but their characteristics are quite different as shown below. * These tubes have an internal shield -------------------------------------------------------------------------------------------------------------- Tube : Heater Plate Watts Max Plate V Max Plate mA Gain M. Conductance Plate Resistance 6AC10 : 6.3V 0.6A 2/2/2W 330/330/330V 9/9/9mA 62/62/62x 5800/5800/5800Gm 10700/10700/10700rp 6AK10 : 6.3V 0.9A 2/2/2W 330/330/330V 10/10/10mA 53/53/53x 7000/7000/7000Gm 7500/7500/7500rp 6AV11 :* 6.3V 0.6A 2/2/2W 330/330/330V 10.5/10.5/10.5mA 17/17/17x 2200/2200/2200Gm 7700/7700/7700rp 6BK11 :* 6.3V 0.6A 0.4/0.4/0.4W 330/330/330V 1.6/1.2/1.2mA 70/100/100x 1550/1600/1600Gm 45000/62500/62500rp 6C10 :* 6.3V 0.6A 1/1/1W 330/330/330V 1.2/1.2/1.2mA 100/100/100x 1600/1600/1600Gm 62500/62500/62500rp 6D10 :* 6.3V 0.45A 2/2/2W 330/330/330V 4.2/4.2/4.2mA 57/57/57x 4200/4200/4200Gm 13600/13600/13600rp 6K11 :* 6.3V 0.6A 0.275/0.3/0.3W 330/330/330V 10.5//1.2/1.2mA 17/100/100x 2200/1600/1600Gm 7700/62500/62500rp 6Q11 :* 6.3V 0.6A 3/1.2/1.2W 330/330/330V 22/22/1.2mA 18/100/100x 2500/1600/1600Gm 7000/62500/62500rp 6U10 : 6.3V 0.6A 2/2/1W 330/330/330V 9.6/9.6/1.2mA 18/18/100x 2300/2300/1600Gm 7700/7700/61000rp -------------------------------------------------------------------------------------------------------------- Tube : Heater Other specs 8AC10 : 8.4V 0.45A --- same as 6AC10 --- 12AC10 : 12.6V 0.3A --- same as 6AC10 --- 9AK10 : 9.5V 0.6A --- same as 6AK10 --- Possible substitutions those marked with * may not work 6AC10, 6AK10*, 6D10* 6C10, 6AC10*, 6BK11* 6K11, 6Q11 Alternatives: An alternate solution to the lack of available tubes is to rewire the circuit to accept more readily available miniature tubes. The most difficult tube to find is the 6C10. this tube is essentially a tripple section 12AX7 tube. So one possible solution is to replace it with two 6AX7 tubes or two 12AX7 tubes wired for six volt operation. However this will double the heater current which could potentially cause trouble, plus 6AX7/12AX7 tubes tend to be expensive. There are however other tubes that have the same characteristics as the 12AX7 that can be used instead. The 6AV6 is a single section of the 12AX7 with an added dual signal diode section. Just ignore the diode section, or connect the plates to the cathode. This will still cause the heater current to be slightly higher (by 0.3A), but the 6AV6 is commonly available and low cost. Alternately a 6BK6 could be used instead of the 6AV6. They are not as commonly available as the 6AV6, but are still low cost and have the same triode characteristics as the 6C10. If heater current is a problem, consider using the 6DR4 which has the same triode characteristics of the 6C10, but only 0.15mA of current, so three of them in parallel could draw 0.45mA. If you need to have exactly 0.6mA of heater current, then use one 6AV6 and two 6DR4 tubes. A little known 6V tube that has the same characteristics as the 12AX7 but with a 6V heater is the 6EU7. You could use a pair of these, or one of them and a 6AV6, 6BK6 or 6DR4 tube. Or if you want to get wierd, you could use three 6SF5 tubes. The 6SF5 is a single section of the 12AX7 tube inside a metal tube with an octal base, or the 6F5-GT which is the same thing in a glass octal tube, or even the 7B4 which is the same thing in a loktal tube. There are also some other direct subtitutes for the 12AX7 that can be wired for 6 volts; 12AD7, 12DF7, 12DM7, 12DT7, 6681, 7025, 7729 however these are rare so you will probably not find them. There is also the 7689 which is a special version of the 6AX7 tube. If you are willing to rewire for an different filament voltage, that opens up the tube selection a bit more. That would allow the use of the 12AV6, 12BK6, 7058, 7689 tubes as well as the 12F5 and 12SF5 tubes. And one of the odder ones being the 20EZ7 which is a 20 volt version of the 12AX7 tube.