C12a mod to remove cathode follower?

[Spencerleehorton]

moved the toroidals out the case and no difference? hum still there

 

[Spencerleehorton]

the hum is there whether the PSU is on or off, if the PSU if off the hum gets louder if the PSU is on its quieter?
must be transformer then right??

 

[rock soderstrom]

this schematic I guess, what kind of hum do you have? 50Hz or 100Hz? Does it change with the microphone position? What happens when you switch off the PSU? Is the hum there from the beginning?

 

[rock soderstrom]

must be transformer then right??

power transformer? Why?

 

[Spencerleehorton]

no transformer inside mic i meant, gonna make a recording of the hum, gimme a few minutes

 

[Spencerleehorton]

here is the hum!!

 

[Khron]

What capacitors are in the power supply?

 

[Spencerleehorton]

it makes the hum without the PSU on so thats why i think its the transformer in the mic?

within the PSU it has1000uf/63v on the H+ side and 47uf/250v on the B+ side

 

[Khron]

Is the headbasket well fastened to the rest of the mic body?

 

[Spencerleehorton]

yes headbasket is connected well, i'll take a look at the transformer and make sure its ok, sounds to me like around 100hz frequency wise?

 

[rock soderstrom]

i'll take a look at the transformer and make sure its ok, sounds to me like around 100hz frequency wise?

I'm on the train, but that also sounds like 100hz to me, there's an offset current flowing between two loads, ground loop!

 

[Khron]

Might wanna consider disconnecting the 3pin XLR's pin 1 from the mic psu earth.

Do your mic cables have both the XLR shell and pin 1 connected to the cable shield?

 

[Spencerleehorton]

no cable shield is just cable shield, but i will double and triple check

XLR chassis and pin 1 on XLR are connected together, do you mean disconnected XLR pin 1 from XLR socket chassis?

 

[Spencerleehorton]

ive just changed the transformer in one mic for another i have lying around, its not the correct ratio but now when i plug the mic in the hum goes, so it must be the transformers.
If there was a winding sheet for a similar spec transformer i will need to rewind these transformers i think!!

 

[MagnetoSound]

I think Khron is suggesting an earth loop between mic PSU and the mic preamp. I would concur that there is at least a chance of this being the culprit.

Lifting the earth at the XLR-3 is a good way to eliminate this.

 

[Mr. Moscode]

And it can only possibly be the fault of the type of circuit, not grounding or wiring or power supply filtering, or anything else..

No, it could filtering in filament supply or lack of it, a h-k short in the tube. If the tube is cold and you are getting hum on turn on then it's not the tube. Pull the tube and see what you get.

The follower drives the transformer and keeps the load off the plate.

 

[Spencerleehorton]

sorry for the confusion here, do you mean removing pin 1 on XLR from gnd on psu pcb or do you mean the chassis gnd on the XLR tab take that off Pin 1 on XLR?

 

[Spencerleehorton]

as i said the hum is there before the PSU is powered, it just gets less when the valve powers up

 

[Spencerleehorton]

hum gone, a stay wire had come off in the cable!! was touching pin 1, rewired it no hum all good!!

thanks again guys!! got there in the end, as ushal i went round the houses!!

Khron gets all the points today!!

 

[Khron]

Assumption is the mother of all f-ups (in this case, assuming the cable was intact)

Have i won the internet yet? :O