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EL 36 SE amplifier

Low activity on Tubes/Valves site this summer. As a teacher I have seven weeks off this summer... I have an EL36 Se project going on. One 6SL7 for one channel..
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Beautiful!

Check the phase of your output transformer primary.
Otherwise you will have Positive Cathode feedback, instead of the Negative Cathode feedback you want.

The problem might be, if you connect the primary so you have Negative Cathode feedback, the amplifier might be Inverting from input to output.
That is OK for 2 identical mono-block or 1 stereo amplifier. But it will not be correct with another independent amplifier that is Non-Inverting.
In that case, just reverse your amp to speaker leads (swap the common and 8, or common and 16 as the case may be).