RE: Q: UL PP Instability - Anode Grid2 Snubber - LinuxGuru

Hi, folks !

Thanks a LOT to all who replied with helpful ideas, tips and suggestions!

Schematic is slightly modified Williamson (available in a bunch of sources), switchable between penthode, UL, CFB, UL+CFB, and triode mode, adopted to KT88 / KT120 / 6550, fixed BIAS and 460-490V B+.
I tested it with 4 different custom built 60W / 3.3K - 3.9K transformers.

Since it is draft / test assembly, everything laying across the table, wires are long as road to Siberia (B+/GND - 80 cm, filament - 60 cm, transformer leads - 28 cm, input signal wire - 1 m).
DC supply rails are decoupled with electrolytes and 1000 pF ceramic caps.
Filament common for all 4 tubes, false center tap (made with 2 x 100 Ohm resistors) lifted to 40V with resistor divider from B+.

One transformer worked fine out of the box, another caused oscillation of UL even with GNFB disconnected, third only at high signal level (with GNFB on).
Williamson is very well known to be very picky when it comes to output transformers. However, I managed to get ALL of them to work, with up to 60W in UL mode. Since leakage inductance is in range 8 - 18 mH (depending upon particular transformer), it appears stray capacitance (all iron is quite big !) played major role in stability issues.

The cause of high-frequency roll off (described in my 1st post) was the conjunction of high-frequency RC filter (10K + 500pF) in pre-driver which was installed to cut off everything above audio band, and anode-grid#2 snubbers (seprately they don't have such side effect). Removing RC filter from pre-driver solved this problem.
Anode / grid#2 snubbers were suggested in a book published by G.E.C. "An approach to audio frequency amplifier design". In my case they are 4.7K + 1000 pF.

I didn't tried Zobel at secondary, it was not necessary at this time, additionally, it could "mask" high frequency oscillation if its level is small (once, with another amp, I had 8mV - 20mV at 8 Ohm output terminals).

THD/IMD analysis was not made yet at this moment, I just put away soldering iron, but picture looks promising. There is no stability issues or oscillation with any trafo I have (only slight tremble of picture on oscilloscope screen with output signal at very low level, e.g. 100mV), even at heavy overload / clipping.
Just look at present 2 KHz square wave with worst transformer, which at the beginning didn't worked at all (oscillated in UL mode with GFNB off).

RE: Q: UL PP Instability - Anode Grid2 Snubber

Post a Message!