PPIMV LOUD HUM

[Gaz]

Off topic, but I didn't know there was a proper way to wire the output heaters - I've always observed the hum cancels regardless.

[rockstah]

welp put it back together without the PPIMV. Something is for sure wrong.

i guess the PPIMV was just making whatever is going on more obvious and that makes sense because again i install those masters all the time.

the amp makes a slight hum in standby and is real week sounding with ugly highs going on.

i have changed everything is this amp except the tube sockets.. brand new OT, new PT,new chock... stumped...

[Jana]

A couple more thoughts before my 2nd brain cell gives out.

What kind of coupling caps are you using? If they are orange drops, although this can happen with any cap, they are kind of finicky with the leads. Sometimes the lead will detach internally from the foil. It appears like the lead is still connected but it only makes a partial connection that sometimes does strange things.

Where is your speaker output grounded? I find it important, especially in Marshalls, to use isolated jacks and run the ground wire for the speaker jacks over to a place near the tail of the PI. The actual ground from the OT is connected to the ground lug on the jack but the whole thing is "grounded" to the circuit at the tail of the PI. Does that make sense?

A hum in standby tends to point to transformer orientation issues. Is this a proven layout?

[Jana]

Oh, If all else fails, put a Slim Whitman record on the stereo, take off all your clothes and run around the yard yodeling. It might not fix the amp but the ensuing chaos and flashing lights will make the amp problem pale in comparison.

[rockstah]

A couple more thoughts before my 2nd brain cell gives out.

What kind of coupling caps are you using? If they are orange drops, although this can happen with any cap, they are kind of finicky with the leads. Sometimes the lead will detach internally from the foil. It appears like the lead is still connected but it only makes a partial connection that sometimes does strange things.

Where is your speaker output grounded? I find it important, especially in Marshalls, to use isolated jacks and run the ground wire for the speaker jacks over to a place near the tail of the PI. The actual ground from the OT is connected to the ground lug on the jack but the whole thing is "grounded" to the circuit at the tail of the PI. Does that make sense?

A hum in standby tends to point to transformer orientation issues. Is this a proven layout?

Thanks!

CDE polyester film caps - they would show voltage where there shouldn't be... which they don't.

its a 68 plexi clone with a lay down power transformer.

its been a while since i built a lay down chassis style clone but never had the issue of transformer hum.

i usually ground the output jack to one of the output tube ground lugs.
by tail you mean where the PI cap is grounded?

the issue i'm having is more than pt orientation though. the amp sounds harsh and thin...just doesn't right.

another thing that's odd is usually when i take voltages, with the amp volume all the way off i never hear audible pops through the speakers when i touch the plate resistors with the meter. this amp makes noise when i touch the plate resistors.

[Jana]

Yes, by tail I mean the same area that the 4k7 resistor is grounded. The one that goes to the .1 cap, 10K, etc.

Even with the volumes off in a plexi, you should still get pops from probing the plate resistors on all except the first stage.

"CDE polyester film caps - they would show voltage where there shouldn't be... which they don't."

Not necessarily. The caps aren't shorting out and conducting DC, they are making a finicky contact to the lead.

It is starting to sound more and more like a grounding issue of some sort. Is this a new build? Do you have good grounds? Ie.--plating sanded off if the chassis was plated or anodized?

I'm off for the night--there is a pretty extensive Slim Whitman collection on Youtube if you get too frustrated.

[rockstah]

Yes, by tail I mean the same area that the 4k7 resistor is grounded. The one that goes to the .1 cap, 10K, etc.

Even with the volumes off in a plexi, you should still get pops from probing the plate resistors on all except the first stage.

"CDE polyester film caps - they would show voltage where there shouldn't be... which they don't."

Not necessarily. The caps aren't shorting out and conducting DC, they are making a finicky contact to the lead.

It is starting to sound more and more like a grounding issue of some sort. Is this a new build? Do you have good grounds? Ie.--plating sanded off if the chassis was plated or anodized?

I'm off for the night--there is a pretty extensive Slim Whitman collection on Youtube if you get too frustrated.

ill recheck my grounds - thanks!

[roberto]

rockstah,
have you found the problem?

[RB]

It seems that you have addressed most if not all the issues you could have with the master dual gang pot. Which brings me to you original post

With the bright and normal volumes on zero and the PPIMV on zero I get a loud hum. With the PPIMV ON 10 the hum goes away.

Nothing in the thread tells me you have investigated the other condition for hum - that being the volume pots on zero.

I haven't had this exact problem but I have built amps that have exhibited similar hum issues with the tone pots and or various interstage volume/drive pots.

In every instance it was fixed by replacing one or more pots. I say "or more" because I just started replacing pots till the issue went away.

In one case I could not get the correct value for the replacement pot. That situation was curred by replacing the ground wire on the pot with a 100R resistor.

I am assuming that keep what ever condition with the pot and its internal shielding issues from seeing absolute ground.

Is it when both volume controls are at zero or can you get it with just one?

Also does moving the ground on the master to another location change anything?

Just some thoughts

Regards
Randy

[rockstah]

rockstah,
have you found the problem?

roberto, i decided to check all the ground points. I ended up removing ground lugs and taking a Dremel to the chassis as to make sure the contacts were good and no more hum.

Thanks Jana for the food for thought!

Mark