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Heater lead to the Cathode
#1
Guys:

Just studying the circuit for the SEL and it shows the heater lead from the transformer going to the cathode of the KT88.  What is the technical reason for this?  Is it to effectively alter the heater voltage?
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#2
simple answer is yes. the technical answer is that it adds DC to the AC heater which may be needed by some cathode follower circuits and it also helps reduce hum. try a search for heater elevation.
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#3
Stack...thanks so much. I did find "heater elevation" info on the valve wizard and now that makes sense. The reason I was trying to find out some foundational info on this is because I am building an AX84 for my dad and he plays straight ahead jazz. As such I want to employ an effective NFB strategy on this build to get a super clean Ed Bickert kind of sound. My initial thought was to feed back to the KT88 cathode with a resistor off one of the unused taps. However the introduction of the "heater elevation" using the KT88 cathode gives me pause. At this point I am thinking of introducing the NFB between R15 and the cathode of V3A (pre-amp stage 3). Please let me know your thoughts.
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#4
Well, the SEL may not be suited for Jazz... It is based of a Marshall and will get pretty distorted. A better start point may be the "Clean" or "Simple" preamp and the 20W SE power amp for the building blocks projects. https://ax84.com/archive/ax84.com/buildingblocks.html
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#5
Ok. thanks for this advice.  Looks like the PS transformer has a center tap on the heater coil.  Guessing this is the "heater elevation" function.  So going back to NFB, it appears I would then be able to implement NFB from the 16 ohm tap of the output transformer to the junction of R11 and C7.  I totally owe you a beer!
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#6
One more thought. If you go with the building block approach, you can use an Edcor XSE25-2.5K or a CXSE25-2.5K for the output transformer and that will give you a screen tap that you can use to run in ULTRA LINEAR configuration on the PA. I think going that route you can skip the negative feedback loop. Also reduces the parts count some... https://www.edcorusa.com/xse25-25k
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