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300B Single-Ended SRPP


This amp uses a SRPP as input stage and a 300B final stage but at a relatively low power, around 4.5W, and a plate dissipation around 20W, far lower its maximum specs (40W).
I converted it from a VT52 amp, that's the reason for such a low output power. Anyway, its sonic qualities are very good.
Actually used as a full-band amp, it will be soon modified as a medium-high amp and further as a medium amp in a tri-amplification scheme.
As a full-band amp and with high efficiency speakers, the hum level is a bit high and a test with a continuous heater voltage for the 300B is advised.

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Power Supply

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Printed Circuit & components Insertion

This printed circuit groups the supply for the driver heaters and discharging circuit.

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Construction & Tests

The amp without its protecting grid:

The base is a multiply board which has been treated with a lead foil and a layer of asphalt to reduce the board resonances. The lead foil is grounded to serve also as an electromagnetic screen.
To reduce microphony, the tubes sockets are mounted on silicone joints. These are molded using carton tubes from ... Tampax tampons, then cutted in slices and drilled.
I used Sovtek tubes (300B/12AX7WXT+ platinum matched pairs) in this prototype, not bad at all, especially considering their low price. The sound improves after a few weeks.
I'll probably try Svetlanas and, if I find the money, genuine Western Electric tubes.

Full-power square waves, measured at the output on a resistive 8.2Ohms load:




10KHz, the small ringing is due to the Tango U808 100KHz resonance


40KHz, the square wave is progressively rounded to a sine wave.
Roll-off starts around 45KHz.

And some square waves at the SRPP output and 300B plate:

100Hz, SRPP output

100Hz, 300B plate

20KHz, SRPP output

20KHz, 300B plate. Very small 100KHz ringing
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