Wednesday, August 28, 2019

Tube amp to drive a speaker using power transformer for output transformer

 The Designer In Training gave me this circuit to consider. She intends to build one. Hopefully we will see a report on it.

This one is really cool in that it uses a power transformer which can be bought at any well stocked electronics supply. It serves in a communication receiver quite well. If you are and audiophile you might want to pass this one up. I priced the transformer at Mouser for about $10.

The transformer has 4 windings 2 - 115Vac primary and 2 - 6Vac secondary. I measured a filament transformer for the sim. The transformer used in the amp is a better choice. To simulate a coil we need the inductance and resistance. The resistance of the dual primary and secondary transformer is a good bit higher. Anywho, I used the data I had and expect the build to perform better than the sim.Notice the input is fed through an attenuator. R2, R3 and R4 drop 50% of the input voltage level. If R2 is raised to 100k the output will double. The amp is part of a receiver that I don't have the complete schematic for. The designer placed the attenuator there for a reason that is yet to be seen.
Driving a 4 Ohm speaker.
It could drive a headphone too.
Driving an 8Ohm speaker.

NOTE: In the sims I am using a green LED. The bias is effected by the LED voltage which varies with the color.

EDIT: oops! DIT pointed out I used 2 transformers. She intends to use 1 with 2 primaries and 2 secondaries.

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