quiet when grounded

[Tdale]

When I ground the grid of my preamp tubes, the hum quiets down... That's quite natural I think, but how do I interpret the results from a test like that.

When the amp quiets down when ground a grid, a wiper of the master pot etc, how do I use that to find the problem? Does it mean that the problem is before or after the master pot?

One more thing. If I ground the 230V input ground to the positive side of the bias cap (which then is grouned through a cable), could that cause a problem?

Tommy

[Bob-I]

When I ground the grid of my preamp tubes, the hum quiets down... That's quite natural I think, but how do I interpret the results from a test like that.

This means that the hum is coming from BEFORE the location that you're grounding. So keep moving towards the input.

Fro example, if you gound V1b and the hum goes away, the hum may originate from V1a or V1b. Test next at V1a and see if the hum goes away.

When the amp quiets down when ground a grid, a wiper of the master pot etc, how do I use that to find the problem? Does it mean that the problem is before or after the master pot?

Before.

I'd start by unplugging V2, the OD tube. Troubleshoot the clean side. If grounding the master wiper kills the hum, then the problem must be before that location, I.E. V1 or the wiring around V1.

One more thing. If I ground the 230V input ground to the positive side of the bias cap (which then is grouned through a cable), could that cause a problem?

It shouldn't, but .....

Remember that the bias is directly connected to the grids of the power tubes. I'd ground that 230V ground directly to the chassis at the PT bolt.

More likely the problem is somewhere in the clean side gain stages. Don't forget to try another tube.

[Tdale]

Ok.

The wiper of the master kills the hum completely, while grounding the input side of the master, the grid of V1b (second stage) and wiper of volume pot, lowers the hum alot, but not 100%

Is that normal?

Tommy

[Bob-I]

Ok.

The wiper of the master kills the hum completely, while grounding the input side of the master, the grid of V1b (second stage) and wiper of volume pot, lowers the hum alot, but not 100%

Is that normal?

Tommy

Let me understand this.....

Grounding the input side of the master lowers the hum, but doesn't kill it?

Grounding all 3, input of the master, grid of V1b and wiper of the volume lowers the hum?

So the wiper of the volume is close to the grid, just the 220k/360pF between them so that didn't tell you much, but the grid and master input is tell you that the hum is most likely coming from V1b.

Try disconnecting the input of the master and grounding it. That should have the same effect as grounding the wiper.

BTW, I'm on AOL instant messenger as bobi49 and msn messenger as bobingram3@hotmail.com if you want to talk more directly.

[Tdale]

You mean disconnect the input of the master, and grounding the input lug, not the wire that comes from the switch, right?

I'll try that.

I added you on msn messenger by the way!

Tommy

[VacuumVoodoo]

230vac ground must be secured to the chassie separately with its own lug and screw. Place to close to the mains socket / cable inlet.