This amp uses a SRPP as input stage and a VT52 heater voltage of 5V (Hytron are
normally 6.3V and Western Electric 7V). It is advised to adapt the heater voltage to the
tubes used.
I converted this amp to a low power 300B amp and will rebuild
it as a treble amp avoiding completely electrolytic caps in the supply.
You can replace the ECC83 by an ECC81, the gain and output impedance of the SRPP stage is lower.
This printed circuit groups the supply for the driver heaters and discharging circuit.
The amp during the tests:
From left to right: the generator, oscilloscope, calculator, multimeter, amp with, in the back, the 8.2Ohms/50W resistors mounted on large radiators, some ECC83 tubes, a 2KV probe and a pair of probe clips.
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've used Hytron VT52 tubes (US Army production dated september and november 1943 !) and
Sovtek 12AX7WXT+ for the prototype and keep a pair of Western Electric as spares.
Full-power square waves, measured at the output with a 8.2 Ohms resistor as load:
 40Hz |
 100Hz |
 1KHz |
 10KHz, the small ringing is due to the Tango U808 100KHz resonance |
 20KHz |
 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 VT52 plate:
 100Hz, SRPP output |
 100Hz, VT52 plate |
 20KHz, SRPP output |
 20KHz, VT52 plate. Ringing due to the OPT at 100KHz |