transformer position and oscillation

[alange5]

hello all.


I built a 5F1 a while ago and it's started having an oscillation problem.  Using a chopstick, I can get the oscillation to stop by moving around the blue OT wire (to 6V6 plate), but I can't get it to stop completely.  For instance, if I have oscillation with the volume dimed, I'll move the wire and it'll stop - then it might show up with the volume up halfway.  Position of the tone control also seems to trigger the oscillation in various positions.  I unbolted the OT (Classictone) and found that changing the physical position of the OT can also induce/reduce oscillation and overall noise.  The problem is I built the amp in an old radio chassis mounted in a tiny cabinet, and there's not much space to move things around without interfering with the speaker magnet.


My question:  Is the OT no good? Is it common for its physical position to induce noise? Or is this indicative of a larger internal problem?  My voltages are all good, and the amp was used regularly for about a year before the oscillation started.

[sluckey]

Does the oscillation go away if you break the NFB loop by disconnecting the 22K from the speaker jack?

Show us some pics.

[alange5]

Does the oscillation go away if you break the NFB loop by disconnecting the 22K from the speaker jack?

Show us some pics.

Nope, it's present with or without NFB, and OT primaries are oriented correctly.  I'll take some pics when I get home this evening.

[eleventeen]

Maybe your preamp tube went microphonic.


Small chassis > induce vibration in tube.


Got a swapper tube?

[alange5]

I just put a 1.5k resistor before the 6V6 grid.  Now I only get oscillation with the volume maxed and the tone control in a very specific spot, regardless of OT orientation.  For the heck of it, I installed a 12AY7 in V1 and it's gone completely.  Later, I'll try some other 12AX7's.

[PRR]

The plate-OT lead must be short and FAR away from low-level stages (including tone controls).

If your chassis can't do that, you have a problem.