Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boogieWoogie on January 26, 2020, 10:42:03 am

Title: *solved*: some questions about oscillation (Ampeg Reverbrocket Reissue)
Post by: boogieWoogie on January 26, 2020, 10:42:03 am

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.

(https://i.imgur.com/KKH1Qks.jpg)

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

(https://i.imgur.com/jEKOdTK.jpg)

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

Title: Re: some questions about oscillation (Ampeg Reverbrocket Reissue)
Post by: sluckey on January 26, 2020, 11:04:51 am

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.

Title: Re: some questions about oscillation (Ampeg Reverbrocket Reissue)
Post by: boogieWoogie on January 26, 2020, 01:14:13 pm

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!