Ghost in the 18 watt

[doctorrobert]

Hello forumites,

I've got an annoying ghost note in my GDS kit-derived 18 watt. I hear it as a faint tone about 120 Hz below what I'm actually playing, mainly on notes played on the 2nd and 3rd strings and with the amp dimed.

Because of the ~120 Hz difference in the notes, I suspect power supply ripple is getting through in high gain situations.

Things I have tried:

Changing to a different speaker cab (it's not cone cry)
Removing the attenuator from the equation
swapping out preamp, PI, and power tubes
Replacing all power caps
Upping the 16 uF (last) power cap to 22 uF, then 44 uF
Changing the EL84 cathode cap from 100 uF down to 50 uF

None of these has exorcised the ghost note.

http://www.broad.mit.edu/~robriley/music/ghostnote1.mp3

I'm thinking about changing the rectifier next.

Ideas?

[pdf64]

If it's not the ez81, I'm thinking this maybe caused by ripple at the screen grid supply node.
Have you confirmed that the first B+ dropper resistor really is 1k5 (ie not 150 ohms or something)?
If ok you could try increasing the values of the first 2 power supply caps. 50uF is the most a ez81 can take, but the second could go higher as the 1k5 dropper resistor isolates the cap from the ez81 - maybe 100uF.
Or try adding a choke to the 1k5 resistor.
Or it maybe something weird like the OT primary windings have a significant imbalance, thereby losing the common mode rejection. You could check for that by inputing 1Vac to the secondary, and comparing the voltages between one primary leg and the CT, and the other side.
Good luck - Peter.

[doctorrobert]

Thanks, I'll check these things and post back.

[BackwardsBoB]

Oscilloscopes are affordable and loveable.

[doctorrobert]

Less affordable than an ez81, more affordable than new transformers, so I'm thinking a 'scope is the next stop if changing the rectifier doesn't help.

Nothing in the amp realm is as lovable as good transformers.

[doctorrobert]

No luck with the ghost note upon changing the EZ81.