Measuring AC on the Heater Circuit

James Knox

Tele-Afflicted
Joined
Aug 13, 2003
Posts
1,278
Location
Phoenix
Trying to nail down the source of hum in a new build....

Measuring 6.7vac across the heater filaments at the Lamp. However, if I measure from one tab to ground, i get 6.7vac on one side and .005Vac on the other. Shouldn’t I get half of 6.7vac from either tab to ground?

The only other time I can remember something like this, the was an internal wiring problem with the PT.

The circuit is wired correctly (parallel).
 

dsutton24

Doctor of Teleocity
Joined
Dec 29, 2010
Posts
10,805
Location
Illinois
Well, what's the amp? Does the transformer have a center tapped filament winding? What did you do with said center tap? Or, are you using an artificial center tap, and how's it wired?
 

James Knox

Tele-Afflicted
Joined
Aug 13, 2003
Posts
1,278
Location
Phoenix
Well, what's the amp? Does the transformer have a center tapped filament winding? What did you do with said center tap? Or, are you using an artificial center tap, and how's it wired?

The amp is a Point-to-point 5F2A built in a Hammond Box using a Weber WRVBPT for the PT. It provides a Heater CT Which is elevated at the Cathode Pin on the 6V6.
 

dsutton24

Doctor of Teleocity
Joined
Dec 29, 2010
Posts
10,805
Location
Illinois
Since the center tap of the filament winding is tied to something other than ground, measuring the filament voltage to ground won't yield anything meaningful. Your measurement across the lugs of the lamp is in the ballpark, so I don't think that's part of your problem.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,313
Age
62
Location
Finland
Then cathode biased power tube bias point is not right. About 20VDC is what V6V should have there.

I think the filament elevation is not right. Perhaps other side of filament is grounded? Lamp assy might cause it? You wrote earlier that you see 6,7VAC against ground on other side but nothing on other side.
 
Last edited:

James Knox

Tele-Afflicted
Joined
Aug 13, 2003
Posts
1,278
Location
Phoenix
Then cathode biased power tube bias point is not right. About 20VDC is what V6V should have there.

I think the filament elevation is not right. Perhaps other side of filament is grounded? Lamp assy might cause it? You wrote earlier that you see 6,7VAC against ground on other side but nothing on other side.

Good insights - I’ll check this out tomorrow morning!
 




Top