Oscillation in tube amps

[Mr Moscode]

Unhook the negative feedback resistor network. If it is an output tube it will probably happen with the nfb unhooked. And they can show up at any point in the wave but usually halfway or higher up to the peak.

Grid resistors usually tame the beast. A snubber across the primary of the opt can help but that is usually feedback related.

Sometimes it takes some capacitance between tube elements to stop it or an r-c series network. Small 100pf 1kv caps are a way to start.

These gremlins are usually layout related or due to lack of grid resistors. You can also use ferrite beads on a screen grid or plate lead.

Trial and error. Don't ask me how I know.

[nigelwright7557]

I have found grid stopper in SS and valve amplifiers work a treat.

[Alan0354]

Yes, I know what you mean even though I don't think it's called a grid stopper. The idea is the same. The resistor has to be right on the base/gate of the device for it to work. Particular MOSFET, they die a sudden death when oscillates. Die quietly, with no pop, heat, just die stone cold with source shorted to drain.

[nigelwright7557]

I have designed numerous class d and class AB mosfet amps successfully.
I recently designed my first bipolar transistor amplifier.
It oscillated badly. I hadn't noticed at first and the first I knew the amplifier had fried and blown a PNP transistor.
I tried my old mosfet amp tricks like increasing VAS and feedback capacitor values to fix it. Neither worked. In the end I resorted to the mosfet gate resistor trick and added 10ohms to each base connection and that fixed it.

[Alan0354]

Yeh, those oscillation has nothing to do with the amp global NFB and all. They just oscillate on it's own as an independent stage.

It seems like the tubes are much easier to layout and wire as they are not very prone to oscillation. It seems like their frequency response is a lot lower than SS. You can get away with sloppier layout and wiring with tubes than transistors.

[TonyTecson]

Originally Posted by PB2

I've been thinking recently (the past few years) that it makes a lot of sense to put an RC damper on the secondary of the OT. Just to provide some reasonable HF load rather than the pure (more or less) inductance from the tweeter or whatever driver.

Have not tried it. Surprised that this is not seen more in tube amps. I have only seen a few commercial designs where it is done.

Pete B.

i have seen this done on many japanese tube amps...i also put them in my amps...
an RC network across the OPT primary also, but this is harder to do, but i have seen this done....

[TheGimp]

I'm currently chasing Snivets in a 6P41S amp that is laid out on a breadboard similar to the way George builds this prototypes.

The oscillations take place during zero crossings, and only with a speaker attached as a load. They do not show up with a resistive dummy load.

The oscillation is more likely when cranking up the plate current (makes sense as gm increases with current).

Oscillation frequency is 166KHz, and is audible as harshness on S and F, particularly on female voices.

I have ferrite beads at all control socket terminals as well as Grid-Stop (4.7K) and Screen-Stop (270R) resistors.

A temporary fix of a 500pF cap in series with a 1K resistor from anode to cathode seems to quench the oscillation.

[Rod Coleman]

Could be a resonance in the effective network formed by the OT and speaker reactances.

Moving the series R-C network to across the OT primary may spoil its Q enough to suppress oscillation.

Maybe 500pF (1600V MKP) + 3.3K 3W: this should do very little harm to the sound.

[TheGimp]

That was pretty much my thought Rod. I could not get the oscillation with a resistive load, but when connected to the FF125WK ( single Full range speaker) it would.

I'll try increasing the resistor value to see how large I can go before it starts to oscillate.

There is a good chance that it is in part because of the breadboard layout. Once I do a proper layout in a chassis it may not be an issue, however I will leave room for Zobels in case they are needed.

This begs the question of where the Zobel is best applied. (1) across the output tube anode to cathode, or (2) across the reactive load (speaker terminals).

In either case the reflected impedance is seen by the output of the tube, dampening the oscillation response. So it seems reasonable to me to apply it at the anode of the tube.

[lordearl]

Originally Posted by JamesG

In my experience, oscillation is usually a function of layout and proper placement of grid resistors, which should always be soldered directly to the tube socket.

When wiring from the rca jack hot signal to the grid, if I am not mistaken it is best to have the grid resistor directly wired to the tube socket, then on the other side of the resistor, run the signal cable to the tip of the jack with a shielded cable.

Where is the best place to solder the shield of the signal cable? I'm assuming the closest ground point nearest to the socket?