Measuring amperage draw

[pbman1953]

I need to measure the amps drawn from a filament transformer. I need the abc's


I verified the voltage is 6.8 ac. I want to know the amps being drawn.


Thanks

[sluckey]

To measure current, you must break the circuit under test, then place the Meter in series with the circuit. See your owner's manual.

[pbman1953]

Thanks but there's no owners manual for this Hammond PT-166q6-


https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166Q6?qs=zAx27f%252BWkoElcPca8RQ65g%3D%3D


I know I have to interrupt the circuit with the meter. Does it matter which green wire I pick?

[sluckey]

Thanks but there's no owners manual for this Hammond PT-166q6-


https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166Q6?qs=zAx27f%252BWkoElcPca8RQ65g%3D%3D


I know I have to interrupt the circuit with the meter. Does it matter which green wire I pick?

Well then, look in the owner's manual for the ***METER***!   

Doesn't matter which green wire.

[pbman1953]

Gotcha, I'm all set with that.


Questions:


To get the best reading does it matter if the amp is on or ok just in standby


How long should it stay on for a reading?


Should I play through the amp to push?

[sluckey]

With amp in standby, leave it on until your eyes get blurry from looking at the meter. Don't play through the amp.

What meter do you have? Will it read 5 amps AC?

You're the first person I've ever run across that wanted to actually "measure" filament current. Most just get the filament current from the tube data sheets and add them up. Good,nuf.

[shaun]

Yes, as Sluckey says.

I read the specs on each tube in the circuit. The manufacturers' tube specs list the filament draw for each tube, so you can then add up all the tubes for the overall current draw on the tranny. Then I err on the side of caution when I'm sourcing the tranny and get one that's rated at 20-30mA more than I think I need. In your case, that's probably what the amp manufacturer did. Although they were probably more accurate than me:).

That is the main concern regarding current-draw expectations of the power tranny.

The other main issue concerns the voltage supplied on the secondary side, which will effect the voltages throughout the circuit. It would be typical to have a reading of around 250-350vac on the secondary windings of the PT, with 120vac on the primary. And those legs would go to the plates of the tube rectifier.

You mention 6.8v, probably referring to the voltage for the filament winding secondaries. That's typical. There may also be a pair at 5vac windings that also go to the rectifier, if the rec is a 5Y3 or 5U4 or similar 5 volt tube.

Of course, it can be dangerous to test live circuits, so I'm sure you're taking great care.

Hope that helps.

[pbman1953]

Well my eyes are a blur and here's the readings-


1 Pr- 3.98 amps


2 Pr- 6.43 


3 prs-  8.3


I spoke to an old tech of mine recently who worked on a SVT, years ago. This is the test he does in order to see really what's happening in order to select the right FT. I tried out a Hammond that had a 6 amp rating and now I see that I need the next one up that will be 10 amps. The goal, suggested by the tech, was that the KT120's are power hungry and the goal was to take the high heat away from the PT. The Hammond did but it's underrated for this use. In turn the PT was just warm instead of untouchable. Plus the 10 amp version is only slightly larger that the 6 amp. There is a 12 amp also. I have the area luckily to fit


Plus for safety I always use clips.


Thanks

[pbman1953]

I got the 10A FT and it's connected and is running at 9.17 amps. It's slightly warm and the PT is cooler. Win Win


Next up is to add one to my 2000s also to use KT120's. I tested the draw of the 2000s and it's at 6.17 amps. I wrote to Hammond because I still have the 166q6 a 6.3 volt @6 amps. I wanted their opinion of using the 6A Ft.




Here's what they said-
 The transformer will run the extra current but the temperature rise of the transformer will increase from 45deg C to 50deg C. The secondary voltage will be very slightly lower.  From 6.35V to 6.28V. 
If you are OK with this then it’s OK with us.


My question- what happens to overall operation and sound with a reduction of  .07 volts, if anything


Thanks


Anyone?

[sluckey]

After 50 or 60 years your hearing loss will be such that the amp sounds shitty.