Stereo Amp Transformers

Power and output transformers for tube amps are getting hard to find and expensive.  For power, I used a widely available type of transformer designed for use in powering solid state circuits in international applications.  This type has two primaries which can be wired in series for use in European countries where the line voltage is 240 Volts or in parallel in the US where the line voltage is 120 Volts.  They also have two secondaries which in this case are rated at 10 Vrms.  For this circuit, one primary is used as the primary and the other is used as a high voltage secondary.  Using a full wave voltage doubler it produces 300Volts DC for the output tube plates.

One of the secondaries provides 10 Vrms to the filaments of the 10BQ5’s and the other, through another doubler, provides –26 Volts DC.  Regulator U3 drops this to –20 Volts to supply the filament voltage for the 20EZ7 as well as bias for the 10BQ5’s.  DC filament suppy to low level stages keeps AC hum low.

I found some very nice output transformers at a company called Triode Electronics.  They specialize in tubes for tube amps and have designed some reproduction output transformers for replacement in older amps or for new designs.  They are excellent performers with a very wide frequency response.  The ones used in this amp are rated for 25 Watts but used at only 15 Watts.  They “sing” just a bit at high output, but I never saw a transformer that did not.  Even the potted ones do it, and you will never hear it with speakers attached.

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