Oscillating Audio Innovations S500 Amplifier

[juancho]

Hi,
I've got an Audio Innovations S500 in the workshop, one of many of these I've worked on. Nothing unusual in the layout therefore but this one has an oscillation /motorboating when the volume pot is turned up towards max. It only occurs when no source is connected and stops immediately a source is connected. The only unusual feature of this amp is that it was converted to triode mode at some point.

The design uses an ECC83 valve as the line input gain stage and was built without grid stopper resistors. The original circuit showed a 50pf cap between the grid and ground which this amp and no other example I've seen had. I've fitted the 50pf caps and 1k grid stopper resistors with no result whatsoever. If I remove the line input ECC83 the oscillation stops.

All electrolytics have been replaced which has also had no effect on this behaviour.

Anybody any suggestions before I get back to investigations next week?
Thanks
David

 

[6A3sUMMER]

Is the 50pF connected directly to the ECC83 grid, or connected before the grid stopper resistor? I would not connect the grid directly to a cap to ground, that defeats the purpose of a grid stopper.

You may have a ground loop of some sort. "ground" is not always ground, things like 'lead inductance' gets in the way.

What was the original input tube, a pentode? Which one? In the original amplifier, was there a dominant pole (series RC) from the pentode plate to ground? For the modified amplifier (now triode input), if you did not re-adjust the dominant pole RC network, or took it out, the negative feedback may now be marginally be unstable.

 

[JonSnell Electronic]

I used to fit a 220p with a 4k7 in series, sat across the 1st anode load resistor, when I has problems like your describing. It is HF and the power supply sags, causing the instability to recur.
Dependent on serial number, that will cure the issue.

 

[juancho]

Thanks for the replies. The output valves are EL34s or 5881s. I'll try the cap /resistor on the first anode load. Otherwise maybe reverse out the triode mode to see if that makes any difference
David

 

[JonSnell Electronic]

5881s have the wrong bias and will not work properly and don't like 400v on the screen grids!

 

[juancho]

I'll check the bias. The mod was done by David Wright who modded many for 5881s. I seem to recall David found the Sovtek mill spec were happy with 400v on the screen grids but again I'll check it out.
Thanks

 

[juancho]

Interestingly David Wright says on his Website "It may be 'good practice' to change the cathode resistors to a higher value when fitting Sovtek 5881/6L6GC WXT but it is not essential and it will cost money if done by a repair shop. Plugging and playing is perfectly fine”