AO28 Filter Cap Tale

[Tim_at_Jonas]

Humm/Buzz. One or more of the can cap sections were dead (determined with an ESR meter). So lets go with separate caps. Three Terminal strips. Piece of cake. Wrong.

Still had some buzz ;-( I won't waste time with the troubleshooting story. The issue was in using the same ground point for all seven can cap sections, two of which are cathode bypass caps. We ended up using the original ground points of each can. Now it's as it should be. Would a third ground point for the cathode bypass caps be even better? Maybe - I wasn't up for any more fiddling around.

Here's a pic. The leads are a bit wonky, as the caps went in and out and back in. They are so far apart because I kept the same + strip for the left cap, and put the new ground to the left. Next time, we won't have so much space in between.

[Sweet Pete]

Thanks for posting Tim! You do tidy work!


Just tearing down my home 147,#83745,1966 it looks like,thought it might be a '64 because of #?
Whoever recapped it did a neat job using an 'above' 2-X-.33/500, an individual .33/450 below, along with a 10/350.

It hummed from the get go in 2000 when I got it so I never turned it up that loud; being in the living room.
It has always hummed. Neat work done by someone who sourced 'close enough' components in 1974 or so if the components were fresh.
Don't you love finding neat work? Too bad these replacement caps don't last like the originals.
I located a Cinch FP can socket and will be returning it back to the stock field replaceable can.
Reports of bad CE replacements have happened,not to myself yet.

I was tempted to just redo this beautiful work,but whoever is next will have an easier time replacing the can.
Both recapped Leslie tube chassis here hum more than the can cap ones, so that's my reasoning.
Don Leslie meant those to be away from the other components. That's why Mallory made them in the first place.

Same thing with this recapped 'below' AO28 B3 preamp, circa 1988. The one with 1963 cans is quiet like brand new.
I'll stay with the stock cans here too.
I have a 30/30/30/10/475 CE can ten years old that checks out fine and works as it should in my gig 147.

Hard to beat that L133 recap I opened up to find though.
Regardless of component choice,some people are way in over their heads!
Funny thing is the chassis is quiet......of course I redid the sloppy workmanship.

[Jaim]

I always install the discrete bypass caps on the tube sockets.

Jim

[Sweet Pete]

I always install the discrete bypass caps on the tube sockets.

Jim

Below the deck? Or do you make up your own 'can'?

[David Anderson]

Yes, sometimes changing the grounding system makes no difference, and sometimes bad things happen. If all grounds were equal and all ground paths had zero resistance, it wouldn't matter, but, in real life, they do.

It can be particularly bad in some solid state amps with large power supply capacitors that result in large peak charging transients. I once had to replace power supply filter caps in an old Vox SS amp, and I moved one ground connection. The amp buzzed horribly. When I moved that one ground connection back to its original location, everything was quiet again. I also had to play with the grounding connections in my replica of the Farfisa F/AR that I built without an original for reference. Grounds are shown on schematics, but schematics typically don't show the actual layout of the grounding system. There are several well-known grounding methods for audio circuits (star, buss), but it can be hard to predict which one will actually work best in a given application.

In particular you have to be careful about the ground connections of the first B+ filter stage since that has the most AC ripple on it.

[Jaim]

Originally Posted by Jaim

I always install the discrete bypass caps on the tube sockets.

Jim

Below the deck? Or do you make up your own 'can'?

Under the hood. I was speaking of the 2 cathode bypass caps on the common cathode of V1,2 &3 and the 12BH7. On the former there is room on the 12AX7 socket pin 3 or 8 for the cap. Pickup a ground from the socket. On the 12BH7 pin 8 to socket ground.
Disconnect the wires running to the cans.

Jim

[Sweet Pete]

Thanks Jim.
Those two caps can cause thunder and rumbling and other unwanted noise and are critical to the tone.
On the AO29/M3 the cathode cap on the 12AU7 exhibits the same character when it fails.
First time I attempted a refresh on an AO29 I didn't use enough solder on that cap + terminal.
Now I know how though....