Weird howl--Marshall

[Chuck H]

Obviously it wasn't the actual speaker wire that wasn't connected (or there wouldn't be any sound at all). So if the OT secondary end wasn't connected, but you got that hum through the speaker then you clearly have A SHIT TON of ripple on the chassis. That's got to be dealt with. With all metal output jacks your speaker was terminated at the chassis. This is usually the case actually anyway. Even if you're using the standard cliff jacks you must have had a ground connection to the jack of some kind or there would have been no sound from the speaker.

[Dave H]

I'm guessing the speaker and OT were connected together correctly but there wasn't a ground connection and it needs one because of the NFB otherwise the floating speaker and OT secondary would be acting like an antenna hanging off the NFB input.

[Chuck H]

I'm guessing the speaker and OT were connected together correctly but there wasn't a ground connection and it needs one because of the NFB otherwise the floating speaker and OT secondary would be acting like an antenna hanging off the NFB input.

Could be part of it. But that wouldn't explain the greater noise with the preamp tubes installed.

[doombass]

Only the PI tube installed increased the noise and that makes sense.

[Chuck H]

Right! Good call. We left off regarding the peramp tubes with this:

does it with all pre tubes pulled...but at lower volume

But in the original post he DID say this:

The preamp and MV dont affect it ...

So neither should the preamp tubes being in or out.

I Hope Dave called it then. I truly never would have guessed the speaker and OT coils were acting like antennae feeding the NFB input. Maybe if it happened to me before, but not as a blind consideration.

[Helmholtz]

A picture of the "speaker wire" in question would help to clarify.

[Valvehead]

the wire is from the plastic cliff speaker jacks negative to chassis ground

the OT wires are , as far as i can find out , Brown - ground , black 4 ohm, yel 8 , green 16

[Helmholtz]

That would at least have disconnected the feedback loop and increased the power amp gain.

[Valvehead]

Probably another stupid question----when measuring my heaters to ground...they are 0 ohms--so both ends going straight to chassis ground....the CT of that 6.3v tap is physically grounded ....is that correct ??

[The Dude]

Yep. You're reading through the center-tapped filament winding to ground, which will be very low resistance.

[Enzo]

This^^^^


They DO NOT go straight to ground. You are measuring through the 6v winding. That winding is a smallish number of turns of heavy wire and so has VERY low resistance.

[Valvehead]

Poking around with magnefiers on....i found something.....it looks like someone had put in shielded cables in the preamp area . At least 4 to 5 in there..but, the shields are cut flush. Meaning the shield isnt connected to ground on either end. Could this cause the wacky noise ?? Looked like they are at grids and plates of the first 2 12ax7s

[Chuck H]

Poking around with magnefiers on....i found something.....it looks like someone had put in shielded cables in the preamp area . At least 4 to 5 in there..but, the shields are cut flush. Meaning the shield isnt connected to ground on either end. Could this cause the wacky noise ?? Looked like they are at grids and plates of the first 2 12ax7s

A shield that isn't connected to ground or an anti phase potential to surrounding influence at either end could act like a larger surface area to collect surrounding radiant fields and transfer them to the conducting lead above the frequency knee of the cables capacitance. But that's just my more mechanical understanding of the matter. With that said I HAVE used un-grounded copper plate as an effective shield against EMF. Copper can do that for you in the right circumstances. I think you should probably keep the shield for the first and maybe second cascade stages in the "channels" and ground the shields. Lose the rest in favor of plain lead wire (routed such that like phase circuits are avoided, of course).

[nosaj]

A shield that isn't connected to ground or an anti phase potential to surrounding influence at either end could act like a larger surface area to collect surrounding radiant fields and transfer them to the conducting lead above the frequency knee of the cables capacitance. But that's just my more mechanical understanding of the matter. With that said I HAVE used un-grounded copper plate as an effective shield against EMF. Copper can do that for you in the right circumstances. I think you should probably keep the shield for the first and maybe second cascade stages in the "channels" and ground the shields. Lose the rest in favor of plain lead wire (routed such that like phase circuits are avoided, of course).

Thats a very technical way to say it acts like an antenna

nosaj

[Chuck H]

Thats a very technical way to say it acts like an antenna

nosaj

Well... I see an antenna as connected at one end.?. I guess this would be more like a dish. A reflector with more surface area in close proximity to the sensitive lead which is the antenna.?.