1Hz oscillation with CCS biased output valves

[bigwill]

I seem to be the victim of all kinds of weird problems with amps. My latest one is that when I use a constant current cathode biasing system based on LM317s as opposed to the usual cathode bias resistor / cap combo, the amp develops a low level oscillation, who's severity changes depending on which output tap I use, 4 ohms being the worst and progressively better from 8 to 16 ohms.

It's at a low level - in the fraction of a volt range, if aggravated it doesn't get much beyond that as the output transformer saturates at such a low frequency. On the 4 ohm setting, it tries to develop into a full power oscillation, whereas on the others it settles at a maximum. With a woofer connected you can push it in time with the oscillation and make it worse, until the movement is visible on the cone, which then sustains itself until the amp is turned off.

With the input valve pulled, there is no oscillation, it certainly seems to be a feedback issue.

My theory is that the resistance of a normal cathode resistor acts as some kind of damping for an LF resonance or phase shift around the OPT, and the (near) infinite impedance of the CCS down around DC exacerbates this.

When I get around to it (hopefully tomorrow) I will try smaller coupling capacitors to the output valves to roll off excessive LF gain (it's a mullard topology so the first stage is DC coupled) and HOPEFULLY this will solve it, although I'm not getting my hopes up

Does anyone have any insight into this before I go chopping things up?

[gootee]

Can you post a schematic of the LM317 portion at least?

[atmasphere]

Originally Posted by bigwill

With the input valve pulled, there is no oscillation, it certainly seems to be a feedback issue.

Is your house built over a cemetery?

By any chance have you reversed the phase of the output transformer in such a way that the feedback is now positive?

[Wavebourn]

Does you amp have a choke loaded rectifier?

The schemo of your amp would definitely help me to help you.

[Sch3mat1c]

Too many coupling capacitors. Williamson topology makes an especially excellent phase shift oscillator.

Might I suggest DC coupling? ;D

Tim

[Enzo]

And is your 317 properly stabilized so it doesn't spend its time trying to correct itself?

[bigwill]

Here is the circuit

[bigwill]

Originally Posted by Wavebourn

Does you amp have a choke loaded rectifier?

The schemo of your amp would definitely help me to help you.

The PSU is a simple CLC afair, I will post the circuit

Originally Posted by Sch3mat1c

Too many coupling capacitors. Williamson topology makes an especially excellent phase shift oscillator.

Might I suggest DC coupling? ;D

Tim

It's a Mullard topology

Originally Posted by atmasphere

Is your house built over a cemetery?

By any chance have you reversed the phase of the output transformer in such a way that the feedback is now positive?

I'm 99.9% sure the feedback is correct, the amp works fine with resistor based cathode bias

Originally Posted by Enzo

And is your 317 properly stabilized so it doesn't spend its time trying to correct itself?

I think so. By itself powered with a PSU it seems very stable, and it is loaded with 100nF caps on the output of each chip because before I've had problems with HF oscillation

[bigwill]

Amplifier circuit

[DF96]

This circuit is prone to LF instability - you can see this in the original Mullard frequency response plots. There are three LF poles in the gain path: phase splitter (R6,C6), output grid drive (C8/9, R10/17), and the output transformer. Some people forget the PS one because they think that DC coupling avoids this - not true.

In addition, there are two LF lead-lag networks: C7, R13 and the cathode arrangements for the EL84s. I suspect your problem, as you say, is at the EL84 cathode. Try reducing the value of the 470uF decoupler. Alternatively, try changing (up or down) the values of C6 or C8/C9. The CCS turns the lead-lag into a pole so it changes the LF phase shift. Maybe you will have to deliberately degrade the CCS with a parallel resistor.

Another LF problem is that the forward path can have some gain down to very low frequencies which are not filtered by the HT decoupling. Mains voltage variations can enter the amp at the anode of V1.