montgomery Wards gvc-9058A squealing

[Mars Amp Repair]

This is an informational post;
I had a Monkey Wards GVC-9058A head in for service. With new 6l6gc's it would squeal like a pig (not a sign wave..more like a pulse). Older Fender type 6l6gc's worked just fine. Perhaps some of you have seen this issue before. This is only the 2nd one of these I have had experience with.

Not having the nos to spare for this amp I had to figure out why it was doing this, besides it bugged me that it wouldn't work with any 6l6.

If I moved the B+ wires around (yes, just the B+ wires) for the 6l6's, the squeal would change. Bridging the electrolytics had absolutely no effect.

Removing all tubes but the driver & outputs still produced the squeal, so it was between the driver & the outputs.

Adding snubber caps to the plate load resistors in the inverter stage had a negative effect.

Removing the negative feedback had no effect.

I finally added output screen limiting resistors of 470 ohms to the 6l6's & voila...no more squeal. Stock this amp apparently does not have the limiting resistors. Apparently some interaction with the output screens & the rest of the circuit...I have theories about it, but if someone can come up with the technical reason this worked...I'm all ears.

I thought this might be helpful for others. glen - Mars Amp Repair