GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: Center-tap and optionsPosted by floobydust on: 5/10/2009

I like having a center-tapped filament winding for flexibility. First, you can use a traditional approach on hum balance (what you have now) or use a slightly different approach. I use the center-tap as a fixed DC balance (cathode bias resistor solo from c.t. to ground) and use the balance pot with the bypass cap only to ground for adjustable AC balance. Regardless if you try this approach or not, I always recommend padding the hum balance pot to reduce the effective DC resistance that the pot adds into the circuit. As an example, using a 100-ohm balance pot, your effective (series) resistance with the bypass cap is 25 ohms. Using a pair of 12 ohm resistors (wiper to each end) drops this to ~5 ohms. It helps... and there's no such thing as too quiet.

Regards, KM

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