Causes of ghost notes and a stubborn Magnatone 421

[Evergreen]

hello all, I've got a Magnatone 421 that came in pretty rough (no sound, just hum) which I have operational after a full cap job minus ceramics, and a few reflows. also replaced a few drifty Carbon comps. it now functions relatively as it should, but I'm getting ghost noting, even at low volumes, which is strange, given that I just replace the filter caps. stock values were 40uf/20uf/10uf, I replaced with 47/20/20. even tried 40uf in the second stage just to see if it helped. what else could cause ghost note problems? output transformer maybe? using a known good speaker, the only other issue is hum, but I'm assuming that could be attributed to the heater wiring being the "one side grounded" arrangement. thanks in advance.

[shooter]

may help, may not;  with no signal in, put your meter to VAC, black to *appropriate* ground (pre, pwr) and the red on the cathode, typically you'll have 1.3ish VDC in the preamp, set back and watch..0 0 0 .4, 0, 0, .5, 0.  Scope is even better.  If there's a slow moving *modulation* there's your ghost, finding him, that's the hard part.  You can do the same at the plate, BUT, the volts are way bigger.

[Evergreen]

Didn't see any movement on the needle on the cathodes, at least not enough to register on my meter. here's a link to the schem i've got for it, a couple values vary slightly but it's close.

http://www.bustedgear.com/images/schematics/schematic_Magnatone_421.gif

It's a consistent octave down type sound, I tried swapping the OT primaries with no change.

[shooter]

consistent octave down type sound

it happens with the switch in either, tremolo, or vibrato?

[Evergreen]

it happens with the switch in either, tremolo, or vibrato?

yes... I've also been testing with the trem "footswitched" off. it's almost a ring modulation type sound. I even hooked the original cap can back up to get the exact original lead dress, no change. Could it be the minimal two diode rectifier just be insufficient? I really doubt the amp would be produced with the potential to sound this crappy.

[Paul1453]

A 2 diode rectifier should be completely fine.

It is just the 1st process in converting AC to DC,
and should have very little noticeable effect on the resulting sound.
As long as the diodes are working, that is. 

[jjasilli]

geofex.com > tube amp debug page > Unintentional and ugly distortion :


Faint out of tune sound on every note:  Excessive power supply ripple, usually indicating that the power filter capacitors are going bad. This can also be caused by speaker cones and voice coils with problems so they just rub slightly, too.


[Evergreen]

jjasilli, thanks for the reply. my first post might have been confusing. the first thing I tried was replacing the filter caps, as well as increase the values, which didn't correct the issue. and I've been running it through my test bench speaker cab which works fine with any other amp.