figuring out pt voltages.... a safe smart way?

[yalesmith]

Hello. After a year or so I have started to build again. I didn't realize how Manny projects I had open..... from a completed amp that needed feet installed to half completed turret boards. One of the projects I have open is determining the voltages on a couple old transformers I have laying around. My question is how would you go about doing it? Any guesses at what the wires might be? I have my suspensions but still like to think what you guys think

Thanks

[M Fowler]

My favorite amp video guy is Uncle doug here is a video he has on the subject.

https://www.youtube.com/watch?v=QqWM3pB ... ozLxwPzEjQ

[joeboo88]

Thanks for the link Mark...Uncle Doug is awesome, and Rusty as well

[Smokebreak]

Radio shack still sells wall warts for answering machines. I have one that steps down to 8,10,12,15VAC or something like that. I rig it up to the primaries and then do the math.

[tubeswell]

The way I test PTs that aren't connected up to a load, is to;
1) securely ground the transformer casing to the mains earth
2) Hook up a 1A-2A mains fuse in series with the active/phase mains wire and the primary winding.
3) Keep the secondaries separate. For modern PTs with grain oriented steel laminations where fuses tend to blow more easily with in-rush current (when there's no load on the secondaries), you can put something like 10k-100k between each end of each secondary to give it a bit of load to stop the mains fuse from blowing. Where the secondary has a CT, use two same-value resistors, one from each end of the secondary to the CT.
4) Use insulated gator clips on your VAC-meter and hook these up with the PT power off.

[yalesmith]

Thanks that video made things alot easier. Ok so this is what I got the red wires are hv and are 375v measured at center tap. Greens are filaments and those measure at a high 6.9 or 3.45 at the center taps. Black wires are your 120. I am assuming the yellow wire (bottom second from right) is the bias as it measured at 55v to the CT of the hv. The only issue I am having it the blue wire (top second from right) no clue what that could be..... any suggestions? I'm a bit sad because there seems to be no rectifier tap, this would have been perfect for bassman project I am almost done with.

[tubeswell]

Is there any VAC between the blue wire and the ground return? (for your own sake remember that you need to have your PT casing grounded to the earth mains to do this measurement)

re: the highish vAc on the heater winding - this will drop under a proper load, (as will the VAC on the HT winding)

[Phil_S]

The result suggests the HT is about 350-0-350, maybe 360-0-360 under load. This is a guess. If it was actually built for 120V mains, expect 6.9 to drop near 6.3 and that suggests the HT is possibly 340-0-340, but I don't think so.

[RWood]

Thanks for the link Mark...Uncle Doug is awesome, and Rusty as well

I agree...Uncle Doug rules! His simplistic style makes it easy to follow on complex issues. Great teacher.
I also had a healer like Rusty once. Great dogs.

[hans-jörg]

Thanks that video made things alot easier. Ok so this is what I got the red wires are hv and are 375v measured at center tap. Greens are filaments and those measure at a high 6.9 or 3.45 at the center taps. Black wires are your 120. I am assuming the yellow wire (bottom second from right) is the bias as it measured at 55v to the CT of the hv. The only issue I am having it the blue wire (top second from right) no clue what that could be..... any suggestions? I'm a bit sad because there seems to be no rectifier tap, this would have been perfect for bassman project I am almost done with.

Hi,
which colour is the HV CT?
The only blue one I see is between the two red (HV) - CT ? The top second from left!
I see two yellow - what is what?
And what do you mean with "your 120" ?
For me it looks like there could be a second filament.
Does the 6.3 filament have CT?

Hans-Jörg

[yalesmith]

The HV CT is the yellow and red wire (bottom first from right) and the other yellow (bottom second from right) is the HV Bias. The filaments have a CT its the yellow green (Top first from left).

Sorry for the misunderstanding but the 120 is the primary, I forget that we have a bunch of non-American members on this board!

The blue wire is a HV voltages because of the resistance and voltages that its running. The only odd thing is there is only one wire, maybe one got snip off?

Lastly, anyone have information about replacing leads on a transformer? I don't want to start a new thread......I have another transformer that the leads are all mangled. A video is would be great!