*solved*: some questions about oscillation (Ampeg Reverbrocket Reissue)

[boogieWoogie]

Hi All,
I could use hints on debugging the cause of oscillation.  Questions at end.

I have an Ampeg Reverbrocket that is in permanent oscillation, loud.  When the power amp's input is grounded (using the line-in jack), the power amp is silent.  When the input is left floating, it howls.



The oscillation is @ about 12kHz, 700Vp-p.  Photo below show's the B+ oscillating at the plate of one power tube:



Full schematic: http://www.ampwares.com/schematics/ampeg/ampeg_r-12r_212r_50h_reverbrocket_ri.pdf

Questions

• If a power amp's input is floating, should I expect oscillations?
• Any advice how to continue to debug / home in on the source?

Thanks,
BW

[sluckey]

Does the amp play as normal or howl when nothing is plugged into the line input jack? Check the jack to be sure the switch is working properly.

[boogieWoogie]

That's it!! The normal'd switch for the tip wasn't making contact, so that input was floating & oscillating.  I started prodding the switch w/ a chopstick & the oscillation went away.  I cleaned the switch w/ contact cleaner & light sandpaper, reflowed the jack joints, and all is well.  Thanks for the quick idea!