Measuring AC on the Heater Circuit

[James Knox]

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]

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]

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]

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.

 

[Lowerleftcoast]

^^^^
I agree.

Perhaps checking from each side to the CT would result in halfsies?

 

[James Knox]

^^^^
I agree.

Perhaps checking from each side to the CT would result in halfsies?

I’ll try that!

 

[Tele Slacker]

Post what you find. Interested to see the results.

 

[King Fan]

James, you may want to get involved with another thread running right now in this same forum:

https://www.tdpri.com/threads/filam...ent-voltages-very-high.1088053/#post-11085688

You may be able to help the OP there, or may find some advice that relates to your issue here.

 

[2L man]

Perhaps lamp has short to ground? This definitely would screw 6V6 bias when filament CT is there

 

[James Knox]

Perhaps checking from each side to the CT would result in halfsies?

Post what you find. Interested to see the results.

twas this... halfsies when checking from CT on Cathode pin to each side.

 

[Lowerleftcoast]

twas this... halfsies when checking from CT on Cathode pin to each side.

Good to know. I was shooting from the hip when I made the suggestion. Thanks JK!

 

[2L man]

What is the cathode DC voltage?

 

[James Knox]

What is the cathode DC voltage?

.004 vdc

 

[2L man]

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.

 

[James Knox]

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!