Hum only at max

[audiosalvage]

I've got a tweed princeton build here and it functions well and with quiet floor noise. Then when volume or tone pot is from 9 to 10, it suddenly gives off a hum. Ideas?

[R.G.]

Is it AC line hum or motorboating?

[audiosalvage]

sounds like AC, doesn't oscillate. just a sudden hum. I switched to a 12at7 and this does not happen.

[TUBEDUDE]

Ground integrity of associated pots and caps? New build? Is it a 5E2?

[pdf64]

Maybe the hum is an artifact of ultrasonic oscillation?

[Stevem]

Working on amps without the aid of an Oscilloscope is like signing loan papers without knowing the interest rate!

[R.G.]

sounds like AC, doesn't oscillate. just a sudden hum. I switched to a 12at7 and this does not happen.

Lower gain tube = no hum? That's a vote for going into ultrasonic oscillation.

[Littlewyan]

Any photos you can post of the amp? It's such a low gain amp is oscillation really that likely? Unless there is actually positive feedback on the output stage.

When you say tone or volume between 9 and 10. Can you stop the hum if volume is on 10 but tone is turned down? And vice versa? Or do they both have to be at 10 to cause this?

[audiosalvage]

Ok, I forgot to mention that it only does this when negative feedback is removed and when either knob is maxed.

[R.G.]

Ok, I forgot to mention that it only does this when negative feedback is removed and when either knob is maxed.

That's another vote for suspecting ultrasonic oscillation, and possibly poor grounding technique.

It needs to be 'scoped.

[Stevem]

Well the good news is the gain you have when you open the feedback loop.

Where is the coupling cap to the output tube?

If it’s closer to the 12ax7 then the 6V6 I would swap that.

Solder the cap right to pin 5 of the 6v6.

When you do this you’re shortening the length of grid wire that you had from the 6v6 which could have been making that issue.

In general with any amp hood things happen when you have the least amount of grid wire length in amp.

Also are you running shielded wire from union of the input jack resistors to the coupling cap on the input of that 1st gain stage?

[TUBEDUDE]

The old mantra for defeating oscillation is "short wires on the grids, long wires on the plates".

[audiosalvage]

all sensitive wires are shielded at one end. coupling cap could probably be closer. I did notice while chopsticking, my NFB switch wires make hum louder when touched, do these need to be moved or shielded as well?

[audiosalvage]

pics

[Stevem]

That 5 watt 470 ohm is going to fry those two caps it’s stuffed in between.