Help before I lose my sanity

What people are working on at the moment
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#16 Re: Help before I lose my sanity

Post by RhythMick »

I'm still intrigued by the original fault. The missing earth connection can't have caused that can it?
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#17 Re: Help before I lose my sanity

Post by RhythMick »

In this page...http://www.valvewizard.co.uk/heater.html

Merlin Blencowe states...

Rule number one is that the heater supply must have a DC connection to audio ground. This may be a direct connection or an elevating circuit (see shortly). This is equally true for AC or DC supplies. Leaving the heater supply floating will result in almighty hum due to primary-to-secondary transformer leakage current, and is a common beginner’s error.

How does this transformer leakage current work and how does a ground reference stop it? Could this be the cause of my fuse blowing when I had missed the ground connection?
User avatar
Nick
Site Admin
Posts: 15707
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#18 Re: Help before I lose my sanity

Post by Nick »

How does this transformer leakage current work
Remember there are hidden capacitors between the primary and secondary, and if the secondary is isolated then there is noting stopping the DC level of the secondary floating anywhere between + and - 320v, the range of the 240v sine wave on the primary by the capacitors charging up with static voltage.
and how does a ground reference stop it
Being grounded means that the other end of those capacitors are connected to ground, so any leakage current through them cant charge up with static voltage.

If the heater supply is floating it can assume any voltage WRT signal ground that it wants. That voltage will be expressed across the insulation between the heater and the cathode, coupling to the cathode via the heater/cathode capacitance. So in effect you have a capacitor coupling the incoming AC mains to your cathode. Hence hum.
Could this be the cause of my fuse blowing when I had missed the ground connection
No.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Post Reply