Grounding?

[mitch]

Hello- I built my 2nd 5F1 that had a bit more hum than the 1st one I built, which is almost dead quite, so I've been reading on here about reducing hum in a 5F1 circuit. The only difference in the builds was that this time I used a Hammond 125DSE OT and set it up for a 5000ohm primary to 8ohm speaker,as opposed to a MOJO 10watt @ 6000ohm primary to 8ohm.
Well a lot of what I read had to do with grounding so I went and re-did the grounds. I had built it with all the grounds going to a single star ground on the chassis (with the exception of the mains and HT CT grounds which had their own each). I switched the grounding so that now the pre-amp grounds go to their own chassis point,and the power grounds go to a separate chassis point. Am I getting the star grounding idea wrong?
Now the amp sounds like a machine gun whenever I turn the vol up past 8. Is this what they call motorboating? I should have left well enough alone as the hum was pretty close to acceptable.

[Bruce / Mission Amps]

Hello- I built my 2nd 5F1 that had a bit more hum than the 1st one I built, which is almost dead quite, so I've been reading on here about reducing hum in a 5F1 circuit. The only difference in the builds was that this time I used a Hammond 125DSE OT and set it up for a 5000ohm primary to 8ohm speaker,as opposed to a MOJO 10watt @ 6000ohm primary to 8ohm.
Well a lot of what I read had to do with grounding so I went and re-did the grounds. I had built it with all the grounds going to a single star ground on the chassis (with the exception of the mains and HT CT grounds which had their own each). I switched the grounding so that now the pre-amp grounds go to their own chassis point,and the power grounds go to a separate chassis point. Am I getting the star grounding idea wrong?
Now the amp sounds like a machine gun whenever I turn the vol up past 8. Is this what they call motorboating? I should have left well enough alone as the hum was pretty close to acceptable.

Motorboating... If the filter caps are all OK, the first thing to do is switch the OT primary wires around so the B+ from the rectifier and first filter cap is fed into the old plate wire and the old plate wire is now connected to the B+ rail.

[mitch]

Thanks for the reply Bruce - swapped the OT primary leads but now I'm getting squealing when the vol is 1/2 way unless I disconnect the NFB. This is only my 2nd build so I'm not positive if was motorboating or blocking distortion I was hearing (or are they the same thing?). I've never really paid attention to amp problems unless it was coming from 1 of my amps. The sound I was hearing when I hit the strings hard was kinda of a rat ta tat tat noise.
I changed my grounds back to the way I had them and the problem went away but the hum is still around too. Probably my lead dress or maybe this OT is just louder making the hum more noticeable.

[mitch]

OK - got the hum sorted out. Re-re-did my pre amp grounds and pushed my power and signal wires further apart. Hardly any hum at all now.
But I've developed a scratchy vol pot. I swapped out both .022uF coupling caps and no change. I've got about 100mVDC on furthest right pot lug (looking from the back) to chassis ground. Is that how you check for DC on a pot? Should I be concerned about this small amount of DC?
I've been pushing wires around all day and can't seem to clear the pot up. I believe my readings are in the ballpark but tell me if I'm wrong. All suggestions are highly appreciated.

B+ = 355v
6V6 pin3 = 350v
pin4 = 320v
pin8 = 21.1v
12AX7 pin1 = 169v
pin6 = 190v

[tubeswell]

I've developed a scratchy vol pot. I swapped out both .022uF coupling caps and no change. I've got about 100mVDC on furthest right pot lug (looking from the back) to chassis ground. Is that how you check for DC on a pot? Should I be concerned about this small amount of DC?
I've been pushing wires around all day and can't seem to clear the pot up. I believe my readings are in the ballpark but tell me if I'm wrong. All suggestions are highly appreciated.

B+ = 355v
6V6 pin3 = 350v
pin4 = 320v
pin8 = 21.1v
12AX7 pin1 = 169v
pin6 = 190v

Could be the pot itself. Did you let any solder or flux run into the pot when you were soldering it?

[mitch]

Did you let any solder or flux run into the pot

Not that I'm aware of, when I get the time I'm gonna get some fader cleaner and see if that helps. I was just going off the assumption that the pot is good and it's my wiring that sucks. But am I correct in thinking I shouldn't have any DC at all across the pot?
The pot also functions as the on/off switch and I don't have another but I guess I could just use a regular 1M pot and turn it on/off thru a plug strip till I know if I need a new one.

[Bruce / Mission Amps]

Frequently a bad preamp tube will leak DC to it's grid, creating a scratchy noise on the previous pot too.

[mitch]

That good info to know, but I've swapped tubes a couple of times looking for the right sound and the vol scratches with all of them. It only scratches when turning the pot. It wouldn't be a big deal except I was gonna give this one to my daughter for her B-Day.
I'll swap pots this weekend and see if that's the problem. Thanks a lot guys!