I messed up!

[tubedood]

Hi Guys.
I was tinkering around on a little 5f1 build and I wasn't liking the 6.8v on the heaters so I was going to switch my primary taps to see what the voltages would be. I hosed up and put the two taps together and didn't bring the common to the party. There was a pop and a lil smell. Is there any chance this transformer is salvageable? I am not sure how to test things at this point and I don't want to plug it in again.
Chalk this up to complacency as I should have used the light bulb limiter. I am so upset.

Dale

[tubedood]

Never mind I kinda found a way to check and the prognosis isn't good... One primary tap to the common is 5.4 ohms.... the other is open.

[J M Fahey]

You donīt tell us (the eternal "these guys see everything even if blindfolded" mentality ) so we must wildly guess

So I guess:

1) one tap was, say, 115V , the other 125V (fill in approppriate values)

2) 115V tap was originally connected

3) you connected mains wires from 115V tap to 125V tap so 120V mains saw a 10V winding

instant:



4) mmmmaaaaayyyyybbbbbeeee the exploding winding did not cause widespread damage because the properly rated fuse blew instantly.

5) IF so (lucky b*st*rd if so) the 115 to 125 winding opened, but the 115V to common one is still alive.
Try it , now through a series bulb, preferrably a smallish 25W to 40W one and of course with Ļ*all* secondaries disconnected and safely taped.

If it behaves normally, then you can try reusingb it.
Of course, you will still have that slight filament overvoltage that used to annoy you.

Just in case, I would further test it for health:
a)leave it connected to mains, for at least a couple hours, with a somewhat larger bulb (100/150W) and touch it every half hour: does it unduly overheat it into untouchability? ... then itīs damaged beyond salvation, you have shorted turns somewhere.
b) if not, increase the bet: repeat the experiment but no lightbulb, just the proper rated mains fuse.

If it survives ... lucky guy !!!!

[mozz]

If it's still alive, here is a online calculator to figure what value and size dropping resistor you need. There are also other tube related calculators on his website.

http://www.mh-audio.nl/DroppingResistor.asp

[tubedood]

mozz thanks for that dropping resistor calculator! Slowly I am making quite a nice list of bookmarks to help with almost any questions I have about this fine hobby!

Yep Juan that was the exact scenario... I put 120v to the two primary tap options (120v OR 125v) with the common not connected. I will try today to power the transformer up via a current limiter and see if the tap that has the 5.4 ohms to the common is still useable. I sure hope so.

[tubedood]

Bleahhh

25 watt bulb limiter glows fully when plugged in. Lessons learned here the hard way.

[tubedood]

On a side note I am thrilled we have this site to tell our triumphs and our tribulations. I learn from you all. A 50 dollar transformer is a little costly... but I rushed and was excited to power up. This could have gone much worse.
As always thank you all.

Dale

[eschertron]

Bleahhh

25 watt bulb limiter glows fully when plugged in. Lessons learned here the hard way.

Same result with a 100W light bulb?

[eschertron]

On a side note I am thrilled we have this site to tell our triumphs and our tribulations. I learn from you all. A 50 dollar transformer is a little costly... but I rushed and was excited to power up. This could have gone much worse.
As always thank you all.

Dale

I, too, recently learned the hard way https://music-electronics-forum.com/...ad.php?t=46712 from an experience with a power transformer. And I agree, small price to pay for the education gained here at the MEF.

[tubedood]

Should I step up to the 100W? My ohm readings show primary taps to chassis as over 400Mohm on the two selectable taps and the common. One primary (120v tap) to common shows 5.2-5.2 ohms. The other primary tap (125v) shows open to common. I have a few different wattage bulbs to try if this seems like a good idea.

[tubedood]

I am not trusting my measuring skills here. Now if I re-read my ohm reading to chassis I get 12Mohm roughly on each primary and common to the chassis. Maybe there is a coating on the chassis?

[nevetslab]

Should I step up to the 100W? My ohm readings show primary taps to chassis as over 400Mohm on the two selectable taps and the common. One primary (120v tap) to common shows 5.2-5.2 ohms. The other primary tap (125v) shows open to common. I have a few different wattage bulbs to try if this seems like a good idea.

I wouldn't, based on what you've seen with the 25W bulb. Power Transformers of that size, unloaded (nothing connected to the secondaries) only draw a few watts from the excitation current flowing thru the primary. You have a substantial amount of current flowing to light the 25W bulb to normal intensity. If you left the 25W bulb in and let it run like that for a while, it will no doubt get warm to the touch if not hot after a while longer. At least you're not showing low resistance to the core/mounting to your chassis.

[tubedood]

I tried the current limit power-up one more time... 25W bulb but I pulled the pilot lamp in case that provides a draw that the limiting bulb sees. Same result, glows brightly. I will order another transformer and chalk this up to being impatient.

STUPID TAX.

Thanks to all for your input and advice. I hope one day to be able to offer the advice and not be asking all the questions.

[eschertron]

The only reason I suggested a 100W bulb was that I wasn't sure how close to "full" brightness you actually had on the 25W bulb. If you've seen the 25W bulb on the PT while it was OK, then you would know... and I would not. Carry on!