ST120 power section oscillation using Pavel's AB-Qi ST70/120 autobias board

System: VTA ST-120, KT88's, Pavel's autobias board AB Qi ST70/120, VTA driver board version 16.

I'm experiencing an approximately 200 KHz oscillation, that runs away with increasing input. Both channels are affected. At onset the oscillation looks like this (measured at the output into 8 ohms.

With slightly more input it runs away:

Zoomed in it looks like this:

Has anyone else run into this? Voltage regulator instability?

To follow up: I've isolated the input/ driver circuitry and can run it up to clipping without any sign of the oscillation. I assume its occurring in the power section, including the AB board.

I had a similar problem some time ago but it was on my preamp. The standard way to chase this down is to understand the schematic and probe with your scope specific locations in the signal path. There are also some places in the power path as well. You have to be very careful and it can be a lengthy process on a workbench because you power down to change to many of the probe points. I have even soldered a short 16ga piece of wire on some location to be able to probe them. It is a good idea to have your scope powered on a lifted ground so to minimize risk.
ONE MORE WARNING: This is a lot of volts so do not move probes and grounds with the power on and make sure you take the time lt let the caps bleed down. I am seasoned with more than 40 years working with this stuff and I still turn off the power to move probes.

Some add 47Ω wrapped with bell wire which is effectively 0Ω at audio frequencies.



These create an additional pole that suppresses the lead / element RC pole

See Red Plate

I installed the auto bias kit some years ago (fantastic board) and started using KT88 drivers. To protect the auto bias board and my rectifier, I also added the micro-fuses on each driver. That saved me headaches twice now. Much easier to change out a socketed fuse than buying new rectifier tubes.

Now, I am considering the dual rectifier kit. Using solid state rectifiers on this design IMHO is foolish.
I have been doing fine with my Telefunken rectifier tube.

HDroller wrote:System: VTA ST-120, KT88's, Pavel's autobias board AB Qi ST70/120, VTA driver board version 16.

I'm experiencing an approximately 200 KHz oscillation, that runs away with increasing input. Both channels are affected. At onset the oscillation looks like this (measured at the output into 8 ohms.

With slightly more input it runs away:

Zoomed in it looks like this:

Has anyone else run into this? Voltage regulator instability?

I think it would help if you checked the ground connections.
The connection of the driver and AB to ground should be as short as possible and with thicker wire. Also, the earth point is very important - this needs to be tested and the occurrence of oscillations monitored.

Pavel

"Using solid state rectifiers on this design IMHO is foolish."

Tube rectifiers are great for musical instrument amps.
They have no place in a HiFi system.

HiFi power supplies should be as stiff as possible, not drooping when the going gets going.

AND since tube rectifiers wear just like audio tubes, amp performance is doubly compromised over time.

HDroller wrote:System: VTA ST-120, KT88's, Pavel's autobias board AB Qi ST70/120, VTA driver board version 16.

I'm experiencing an approximately 200 KHz oscillation, that runs away with increasing input. Both channels are affected. At onset the oscillation looks like this (measured at the output into 8 ohms.

a nice description of finding wobbling oscillations with a spectrum analyzer can be found at
https://www.diyaudio.com/community/threads/oscillation-in-tube-amps.145103/

Interesting information Seamus. Thank you. Those resistor chokes are in series with the OT Plate and screen taps? I'm trying to learn more about oscillation. The more I read it looks like there are many ways to produce oscillation. In the case with the resistor chokes, was it an interaction between the OT and the KT88? A resonance?

I need to look more at the ielogical site..

HDroller wrote:Interesting information Seamus. Thank you. Those resistor chokes are in series with the OT Plate and screen taps? I'm trying to learn more about oscillation. The more I read it looks like there are many ways to produce oscillation. In the case with the resistor chokes, was it an interaction between the OT and the KT88? A resonance?

I need to look more at the ielogical site..

Those resistor chokes are tend in series with the circuit heating  tube in series with the circuit heating  tube. Prevent signal transmission along the path of the heating.

The snubbers are in series with the plate and screen to swamp Miller Capacitance.

This is an excellent article @ Aiken Amplifiers on Miller Capacitance

I've not experienced any problems with heater contamination on tube amps, but that's not to say it can't happen with some valves and circuits.