I have sold my Single Ended EL156 monoblock amplifiers to a friend of mine. However he has a number of output tubes that he would prefer to use over the EL156. With that in mind we made some changes to his amplifiers so he could safely run the 6550/KT88/EL34/6L6GC/etc.
The self bias adjustment resistor was changed to a 500 ohm / 5W potentiometer. This allows a large range of bias, allowing the use of several different types of output tubes. I also knocked down the B+ so tubes with a smaller plate dissipation than the EL156 could be used with a bigger margin of safety. Note that I used a 200mA meter to measure the amount of current flowing through the output tube - a 100mA meter would also be suitable for most tube types.
The original 1.5uF power supply capacitor was changed to a .68uF unit. The 82k EF86 dropping resistor should also be changed to 50k.
These minor changes have allowed the amplifier to be used with a wide variety of output tubes. Power output has dropped versus the EL156, but with the right speakers and listening space this amplifier continues to shine. With a pair of original Tungsol 6550s we listened to a few songs on a pair of KEF C-75 speakers and this amplifier sounds very fast with a delicate sweet treble. I'm in love with the sound of this amplifier all over again!
One could also roll EL38s (with plate caps), 6AR6s (with wiring changes), TT21s (with plate caps), and a whole bevy of other output tubes. Have at it.