1625 PP

Photos on construction progress

Tube characteristics using uTracer

Extracted models: 1625 Triode and 1625 Tetrode (description of the models can be found here and here)

Modeling using LTSpice and the extracted models

Notes on cathode resistors and bypassing in push-pull designs

Experiments with power supply

Examples of measurements at the 8Ohm outputs showing the switching diodes in the power supply, without snubbers on the secondary and with a 10nF+1kOhm snubber, respectively.

Using slugging 10nF capacitors parallel to UF4007 diodes created more ringing (which agrees with this, saying that fast-switching diodes should not be paralleled with capacitors).

A bit more about snubber can be found here. Some extra plots:

Testing the Lead-RC network

Testing the open loop connecting with a RC-lead network for different C, with R=39kOhm. Examples of Left and Right channels in open-loop configuration, and the frequency responses for (blue) no lead-network, (yellow) C=82pf, (green) C=100pf, (red) C=200pf:

THD measurements

Experiments with coupling caps (open-loop gain)

LTSpice model

Influence of lag-RC network (39kOhm+100pF) on the feedback