Rivera Thirty Twelve (R30) super Hum!

[Indyryder02]

Ok. This one is driving me nuts. Time to get my head out of the trees and get some other points of view.
This amp has lots of nasties originally. Whooshing sounds, motor boat ticking etc. Many cleanings of switches and pots, reflowing many solder joints and addressing some lead dress eventually took care of all that. However there is a consistent 120hz hum. I’ve been able to trace it with the scope along the high lighted part of the attached schematic. I’d had replaced all of the electrolytic caps as a few were out of spec and it’s over due anyway by time. But to no help with the hum. I haven’t found any ground issues.. I’ve tested all the parts along the highlighted area for failures etc. But can’t find anything to cause this noise. Hopefully someone will see something I’m missing here.

thanks in advance!!!

[Helmholtz]

Does the hum stop when you pull V2 or V1?

[Indyryder02]

Yes. Probing V2 pin 1. The noise goes away when pulling either V1 or V2. A possible hint. It goes away immediately when pulling V2. But slowly fades away when pulling V1

[Jon Snell]

120HZ or in the UK 100HZ hum is poor smoothing of the DC high voltage supply.
I would check/change C118 and the rest of them in the circuit.

[The Dude]

A defective tube can sometimes cause hum. Have you tried swapping out V1 and/or V2?

[Indyryder02]

That’s what I would have guessed as well.
C118 along with all other electrolytic replaced with same type and brand Nichicon. No change in hum.
have swapped all tubes. (Which were all new, checking for cathode to Heater shorts etc. ) nothing found so far.

[Helmholtz]

Check especially C101 and C123. Not every new cap is a good cap (might have been stored too long). You can try paralling a good cap of similar value across the existing ones.

Tube or heater hum would not be 120Hz.

[g1]

Tube or heater hum would not be 120Hz.

Looks like at least the first preamp heaters are DC.

[Helmholtz]

Looks like at least the first preamp heaters are DC.

So you suspect a bad heater filter? Might be a possibility.

[g1]

Maybe. I was thinking about the thing he said about the hum slowly going away after V1 is pulled.

[Enzo]

It is indeed from 120Hz ripple currents in the filter network, but doesn't have to be a bad cap. I don't know how you tested for ground issues, but certainly a poor ground path can cause this. Do we know that ALL of this model don't have this hum? I lose confidence in the design when I see things like the heater supply simply grounded on one side.

But they did do the same thing PV does in the preamp, they wired the three 12AX7 in series as 12v, and the three in a string are powered from +/-18v rails, so 36v for three 12v tubes. So if you pull V1 or V2 or V3, all three of them go dark. Well, no, the other two go dark. Actually while it is across all 36v, they do ground the pin 9 of V2, so they are really on two separate 18v rails.

When you pull V2, you are killing all three tubes. When you pull V1, you kill V1 but only half of V2. I suspect that explains the difference in how fast it fades.

Heater hum on an AC heated tube would be 60Hz, but it is conceivable if the +18 rail has lost filtration (which would then pass 120Hz ripple) we MIGHT get some heater hum. Not my favorite theory of the crime, but...

I note the ground (common) for V1 and V2 is isolated and gets connection to system ground through a connection to the input jack board. It appears to get grounded via the input jack sleeve connection to chassis.

[Jon Snell]

Try this;
Lift C111, has the hum gone? If yes now you know which area it is coming from. If not, lift C114 result?
If not remove R128, hum gone?
That would be the simplest way to isolate the problem.

Check the ground on C118, it connects the signal ground to actual ground. Adding links will not improve but may worsen your symptoms.

[Indyryder02]

Ok. Lifted the above components and checked ground of C118 (ok).
no change when probing pin 1 of V2 until finally lifting R113.
Still have major hum though. Mostly seems affected by channel 1 master volume. Unplugging the connection to the patch loop causes constant hum with no control via master.
probing from output section back again.. I’m finding noise now all around V4.. very strange. I’m sure this will wind up being something dumb after all this!
attached a highlighted area of noise currently with R113 removed. Losing my mind. Need to take a break from it for a few. Ugh! Kicking my butt!

[Indyryder02]

Think I’m in the weeds again. Going to back and retest for noise up front again. Getting different results.

[Indyryder02]

Removed all tubes but V1 and V2 to stay focused.
With C111, C114 R28 removed. I still have noise on pin 1 of V2. This noise is switched on with Ch 1 and affected by volume of Ch1 .
If R113 is lifted. The noise goes away. Yet persists in the output audibly. Maybe there’s more than one source?
the smallest ripple is seen from c111 to R130 but nothing like what’s seen at V2 pin 1 with R113 in place. It’s also not affected by ch 1 controls. This one really has me stumped.
mostly because I’ve been on it too long now and wish I could just Chuck it! Haha