Magnatone Starlett buzzing

[BigE]

I've got a old Magnatone Starlet 107 on the bench.  When you fire it up it buzzes like crazy.  The buzz follows the vol knob.  The old cord was roached so i put a 3 prong on it but I did NOT ground it.   I replaced the big filter caps on it and tried all new tubes but it still buzzes and the output is very low and sounds terrible.  I know this thing doesn't have a PT so I'm wondering if I'm missing something terribly obvious.  Heck there's not that many parts to this thing.

thanks in advance!

[BigE]

One thing to note is that my rectifier is a 35W4, not the 35Z5 like the schematic indicates.

Looks like I've got 3V AC on the + of the 50U Filter caps with about 98VDC on them.

[Slimtim]

isn't that one of those death trap lightning rod radios?why didn't you ground the 3 prong?lol there isn't much more to look at.either the resistors or the ot then right?

[sluckey]

why didn't you ground the 3 prong?

because chassis is hot.

[Slimtim]

ahh so it is a death trap

[Slimtim]

whoa thats a guitar amp.i saw the the tube combo and assumed it was a radio until i googled it.so since he 3 pronged it the risk of shock is lower even though it's not grounded to chassis?just keeps someone from plugging it in backwards right?bear with me folks,i'm learning here,lol.

[Willabe]

because chassis is hot.

ahh so it is a death trap

Yes!!!! Danger Will Robinson.

So before you go any further messing with this amp, YOU NEED TO BUY AN ISOLATION TRANSFORMER and USE IT if you don't already have one.

                      Brad      

[terminalgs]

Some of those ACDC amps have a schematic drawn with a "Common Return",   and actually made a distinction between chassis ground and circuit ground (this 'common return').   Usually the two are coupled with a parallel cap and resistor.  The 107 schematic seems to indicate they might have been doing this. 

See if the circuit ground is isolated from the chassis with a cap and resistor.  my first guess for the  hum/noise  is probably a ground loop somewhere.  Maybe probe it with a jumper to ground. (be safe with your jumper set up!).   Also, there are only 3 other caps in the entire amp, two .02's and two .01's.  the .01 to ground on the plate of the 50L6 is suspect if you have noise in  the chassis.

but since you said the output is low (low for 3W ACDC amp?)  maybe its the OT.   those cheap-o OTs are even cheaper than the ones they put in the $99 5W amps!

AC/DC amps are so annoying to work on because even once they are fixed, they really sound terrible.  That starlet 107 would be a perfect amp to convert to a SE 5W 6V6 Champ (two octals and you already have a 12Ax7!!).