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Re: All Lundahl Transformers - 417a Preamp

64.12.96.9

I presume that you mean to use the LL1660 in connection alternative "Q", meaning 4.5:1 stepdown? Be careful with the 417A. Being an RF triode designed to operate differently than we commonly use it in audio, care must be taken to prevent signal dependent oscillation and RF excitation. A "grid stopper" resistor is a really good idea. The value should be chosen by listening, but start at 100 ohms and work up. I would also advise ferrite beads on the heater wires right at the socket and a low inductance ceramic 0.1uF 50v or 63v bypass across the heater pins at the socket. You may also find that a lower value volume control (like 10K) is sonically beneficial, too.

Have fun,

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