Radford Low Distortion Oscillator Series 2

[thaumaturge]

I'd switch your meter to DC and take a look at offset. Both with the AC output turned up just below clipping and down to almost no output. Might point you into correct section. If it was Solid state I'd look for a soft -V cap. But with tubes I couldn't say. An output transformer should negate any offset.
Doc

[audiomik]

Thanks, the circuit is a 'Bridged T' in the feedback loop with additional bootstrapped feedback to the first Valve (EF184) in a pair with the second one (EL822) doubling as the Output in Cathode follower mode without a transformer.
Thinking something may have gone 'high' in the cathode follower output stage to reduce negative half cycle current availability.....

No full schematic yet so doing this the 'hard way' at present from the generalised cct below
Mik

[DRC]

A cathode follower would explain the -ve rail clipping - output valve is not passing any current !!

dc

[audiomik]

Yup, need to spend some more time circuit tracing and component checking!

With date stamps on the Caps of April '67; things will be expected to have drifted in value which is the most likely cause.

Anyway interesting to do as haven't seriously worked with Valves since the '60's

Mik

[DRC]

I was trying to point out that a cathode follower (or even emitter follower) would not be biased for symmetric clipping if you want to get low distortion. I would assume a capacitor coupled output so I would check that cap for leakage.

Is the O/P voltage, at clipping, 3.2V RMS ?
What's the max indicated O/P voltage ?

dc

[audiomik]

At the output of the cathode follower, there should be a bit over 10volts of signal available with ability to drive the attenuator.

In the test I am running with 10dB of attenuation; it isn't quite achieving the 10volts without the asymetric clipping happening. The attenuator loading causes the clipping which appears just below full output. Have so far checked the output cap for ESR and it appears to be OK.

Haven't had time to investigate further yet, but biasing of the output cathode follower could well be one issue, in which case the anode or cathode resistor(s) of V1 might be high.....

Mik

[audiomik]

Since my last post, I've ordered a pair of new Valves for the Oscillator circuitry and when they arrive in a couple of weeks will have had the opportunity to properly check the associated passive components and acquire replacements as necessary.

So hopefully some further results to post in a couple of weeks or so

Mik