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Old 27th August 2013, 08:42 PM   #1
kkcinc is offline kkcinc  United States
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Default Making a simple se crank

I pulled out the simple se after a while. Been listening to a commercial pp kt88. I really missed the se amp. Sound quality is so much more magical. I want to really push it though. I tried 3k opt and the big edcor 5k. I also tried pentode. I like it in pentode but the bass is too boomy even with cathode feedback. I saw a thread on another simple way to get feedback but I lost it. Can someone explain that to me and tell me if it will help the boom. Also what about connecting to a different tap? Now I'm on the 8 ohm with 8 ohm speakers. 15'' threeway I might add.
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Old 3rd September 2013, 12:56 AM   #2
kkcinc is offline kkcinc  United States
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I would love to see the other chat on other feedback option to tame the boom in pentode.
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Old 6th September 2013, 03:16 PM   #3
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There are two methods of applying feedback to the SSE. The traditional method is GNFB.

Lift the ground end of the cathode bypass cap on the 12AT7 and insert a resistor in series to ground. I used something small like 22 ohms to avoid gain loss. Then connect another resistor between the speaker lead and the junction of the cap and the 22 ohm resistor. The value of this resistor controls the amount of feedback. Use a pot to find the amount of feedback needed for your speakers. About 390 ohms worked best in one of my experiments. The phasing of the OPT secondary should be such that the amp's gain is reduced as the resistor value is reduced.

The other method is local output stage feedback, often called Schade. Wire a large resistor (300K and up) from the plate of the output tube to the plate of the 12AT7. Many experts will tell you that you can not do this with a triode driver, often this is accepted as a fact without understanding why it is true. The plate resistance of a triode varies with the applied signal, so the amount of feedback is constantly changing often increasing the distortion. This is especially true with a CCS loaded triode. However, I have seen cases where it works. Understand that the feedback resistor will see nearly twice the B+ voltage across it under full power so it needs to be rated for 600V or more. Use several 1 watt resistors in series.

NOTE: These are experiments that I have done on my boards. I don't particularly care if I blow something up, sometimes it's good fun. YMMV
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