Last years I've been fooling with 60s radios and radio recorders I've been buying. BTW, a cheap source of germs :-)
I ripped transformers and germs out of some, modded others.
A year ago, while playing with a transformer coupled push pull amplifier on the breadboard, much like the deprecated Mullard schematic used to build a Deacy, I got mad with weird noises, chord artifacts and unpleasant note decay.
I traced back the problem to the driver transformer and its biasing.
I tried a lot of things like replacing the driver, the transistor, a filter cap here and there, a resistor across the iron, NFB, a John Deacon photo below the breadboard ... no luck ... until I thought "what if I put a FET in there?".
Suddenly all noises were almost gone. Cool.
In my first attempt I put a pot at the source to bias the fet, 2SK246, 2SK117 or BF245A. I began lowering the source resistance until I hit zero which was the sweet spot.
Not very familiar with FETs characteristics so I'll appreciate if you explain me why a few different FETs bias at zero resistance with different driver transformers.
That is, Vgs=0, Id= Idss IIRC.
More, there is a wide range above zero where the sound is on focus, unlike BJTs.
Since then I'm using FETs at the preamp and driver.
I just put a 10 ohm resistor at the source to be safe. Besides a bit of NFB from the output to the source-10R junction helps.
Yeah, I know I'm 70 years late, but a Crown CRC 530 SW radiorecorder was my first amp, and maybe my last one. It's been with me for 55 years so I have a kind of platonic love with these old things.
mac