Peavey Alphabass round two-troubleshooting

[g1]

If it's power tubes, it should do it with the PI tube removed. Is that true? Are the power tubes balanced?
In the other thread you said it was coming from the first stage, which is it?

[Enzo]

IIf the first stage is making the hum, then the volume control should be able to turn the hum up and down.

[catalin gramada]

No, no, I was wrong. The hum is not affected by any settings, including engaged eq. It is perhaps from power stage. The tubes are not matched but are close, the offset cannot be horrible. I made a new variable bias supply for this amp but have not balance control. I will make another dual bias tomorrow to get the tubes perfect in balance. Thanks for helping!

[Enzo]

The tubes may be matched, but are ALL of them conducting? That is why I suggested pulling the power tubes and checking pin 4 of each for B+.

Also, plugging a shorting plug into the FX return will tell you if the power amp is the area to explore.

In post #16, g1 suggested pulling the PI tube, did you do that?

We suggest these tests for a reason, we are not just guessing.

[catalin gramada]

Yes, all tubes work. 20-25mA iddling at 510V , 85-86mA onset clipping at 465V. I pulled off 12ax7 inverter (v3) the hum it stopped.

[catalin gramada]

What should I try to do please ? Electromagnetic screening ? Recheck grounding contact points? Current balancing ?...I.m over...

[catalin gramada]

As much as can follow the power grounding circuit look like this. I don't see nothing wrong...

[glebert]

Push a shorted plug into the FX return. Does that stop the hum?

Did you try this one?

Does this schematic cover two different versions of this amp? Seems like there are two different preamp schematics. I find it hard to follow where the signal goes when it hits ribbon cables when the schematics aren't very easy to read and I don't have the amp in front of me.

One other thing you can try is using something like a chopstick or piece of dowel to move the cable around and see if your hum gets better or worse as you do.

[catalin gramada]

Hey,
No, in the pdf the first two pages in the top shows other device, a standalone preamp called also alphabass, nothing related with alphabass amp. The document related to the amp start with schemo in pag 3 and the next

[catalin gramada]

With shorted jack inserted in fx return the hum is present in same amount like no jack inserted

[Enzo]

OK, so power amp in jack shorted has no effect, but pulling the PI tube does. SO the hum is coming in between those two points.

You said the hum is 100Hz, correct? SO it is power supply ripple related. Shielding wouldn't help that.

The FX return feeds V2b, so pull V2, does that kill the hum or does hum remain. V2b is between the FX return and the PI tube.

[catalin gramada]

With pulled V2 hum is still here, it disapear just pulled off V3(PI)

[catalin gramada]

Not sure if 50 0r 100cps. It sound like 100 to me...I.ll put the scope to outputs In a minute...

[Enzo]

So issue seems between V2 and V3. Nore on schematic the ground symbols used for V3 circuits. Now look at ground symbols for V2 circuits. They are different. Grounds are in groups. If I read it right, the FX loop jacks board grounds that squared symbol ground to chassis. Poor contact there might cause this issue. At this point my radar is looking for poor integrity among the grounds between those to circuit areas.

[Enzo]

OH my, it is VERY important to know the difference, they come from entirely different sources.