AA1164 Princeton Woes

[bigsbybender]

Hey Fellas...

For better or worse, I haven't been around... mainly because most of my repairs and builds have been going smoothly, until this week.

I have an early 1969 Silverface AA164 Princeton Reverb on the bench for a customer. This thing is in pristine looking condition, in and out.  The only things that have been replaced are the tubes, cap can, electrolytic bypass and a 3 prong grounded cord was added.

1. It is making an awful (and loud) electrical hum through the speaker.. I can pull every tube except the 6V6s and the Recto and the hum is still there. Tried new tubes, no luck. I've been ruling out the cap can because it's fairly new and it doesn't draw excess current or blow fuses.

2. When I play a guitar through it, I get an awful distorted tone that is an octave below the note that is being played on the guitar. This accompanied by an intermittent high pitched squeal. 

So I'm thinking oscillation..  I've tested almost all of the (dog turd) caps for DC leakage, and pulled them and put them on my capacitance meter. Nothing seems to show DC leak and the caps spec out.  So I've also been toying with lead dress, no luck there either... this is an early enough Silverface to have been wired nicely too.


This one has me stumped, I've never had an amp that sounded this bad that didn't at least have the courtesy to blow a fuse... 

As always, any advice and guidance is greatly appreciated!

j.

[sluckey]

I've been ruling out the cap can because it's fairly new and it doesn't draw excess current or blow fuses.

Filter caps has to be at the top of the list of suspects. I would not rule them out until I had subbed known good caps. An open filter cap will not draw excess current or blow fuses, but it will cause big hum.

Just bridge a cap across each of the caps in the can. Probably the first two since it hums with only the power tubes in.

[bigsbybender]

Well Steve,

Thanks for coercing me to do what I didn't want to do..  The 40uF section of the cap can was bad.

Most of the woes are gone, including the "Lower One Octave" sound.  There is still a slight squeal, but I think his old 5751 is going bad. I certainly don't have a "real" one to replace it, so that will be on the amp's owner.

This little thing kicks! I've always been a little weary of the lack of screen resistors and grid stoppers but this one has evidently been pretty reliable for 44 years.

j.

[eleventeen]

Or, if you can poke a scope probe onto the + terminals of the cap can (USE A SEPARATE ISOLATING CAP, at least 600 volt rating!! Something like a .01 or a .047 or a .1) and see big ripple, that's likely your problem.

Lacking a scope, you might check for AC volts on those caps with your DVM, as well.

And Steve's method (bridging in a known-good cap) is a classic one. All good.

[jjasilli]

Don't forget the bias cap.  Hum & noise can happen there too.  Actually if one cap is bad my preference is to rebuild the entire power supply including dropping resistors & plate resistors; all electrolytic caps in the amp including bypass caps; and rebuild the bias supply with higher wattage resistors.

[eleventeen]

"Actually if one cap is bad my preference is to rebuild the entire power supply including dropping resistors & plate resistors; all electrolytic caps in the amp including bypass caps; and rebuild the bias supply with higher wattage resistors."

Those are my leanings as well. And the 100K plate resistors while you're in there. Good for another 25 years.

[plexi50]

Could the transformer filament winding be bad and injecting this horrible hum?

[bigsbybender]

The customer got this Princeton back today. He was thrilled when I told him that it was built about the same time he graduated from High School.  The cap can in it was only maybe 2 years old tops but the 40uF section failed open (Whoever did that replaced the power supply resistors with heavier metal films). I soldered on 2, 22uF caps in parallel right to the leg of the open cap section of the can... good as new.  The bias cap and all the e-lytics on the circuit card were all replaced within the last few years as well. He just ended up with a bad cap can...

He also had me fix a ~20 year old 5E3 clone... It just had a couple cold solder joints and needed the tube sockets cleaned and retensioned.

j.