bad ot

[tsutt]

Anybody ever unwind and repair and rewind a tranny.

[kwijabo]

i'd love to hear if anyone did this, too.

i have a bushted triode a470s that is sitting on a shelf. i have it on my list of projects to disassemble it carefully, not with the intention of putting it back together but to take pictures and and post them here. wouldn't mind hearing if anyone hearing if anyone has ever rewound and how.

[JazzGuitarGimp]

katopan just finished designing and building a 30W PT & OT set. Here is the thread:

https://tubeamparchive.com/viewtopic.ph ... ansformers

It is very enlightening.

CHeers,
Lou

[tsutt]

ya i followed that for the most part. I'm not talking about putting in new wire. im talking about unwinding it to find bad spot and fixing it and wind it back up.

Not sure if this is the problem. So let me ask another what would cause a pair of tubes to draw way more current than they all of a sudden. pt is fine cause the other pair of power tubes is working fine (stereo). everything else seems to be ok. the resistance of the ot primary measures the same as the other one but under load maybe a different story.

Is there any way to test the ot under load?

[TUBEDUDE]

I hate to be captain obvious here, but have you checked the bias of that pair?

[tsutt]

Of coarse i cant adjust it down its gone out of control the other side is exactly were i set a couple weeks ago. this is an old amp that i rebuilt with the old iron it did have a problem before but it was long before i knew anything about amps. i have checked everything i can think in this thing that would be involved its the only unkown.

[JazzGuitarGimp]

Since you are measuring the same DC resistance from one OT to the other, I would be inclined to look other places before the OT. Is the amp fixed- or cathode-bias?

If it's fixed-bias, I'd start by pulling all of the output tubes, power it up, and measure the bias voltage at the control grid pin at all the output tube sockets to see if they are all within 10% or so of one another.

If it's cathode-bias, I'd start by measuring the resistance of the cathode resistors and compare left and right channel measurements. If the cathode resistors are bypassed by electrolytic caps, you'll also want to make sure they (the caps) are in good shape and in fact, you may need to lift one end of the cathode resistor so you can measure it (the resistor) without the influence of the cap.

Are the output tubes in the defunct channel red-plating? What is the B+ voltage, the current through the left channel tubes, the current through the right channel tubes, and the output tube type?

The DC voltage drop across the transformer primary is typically just a few volts, so I think the bias voltage will play a more significant role in how much current the tube pulls at idle - what I am saying is that I _think_ the OT could be a dead short, and the current through the tube wouldn't increase much.

Is the output stage wired as ultra-linear? Are the output tubes triode-strapped?

Is it possible the output tubes experienced a flash-over and now a plate to cathode short?

[tsutt]

The channel still works, but it just keeps getting hotter. so i shut it down. its fixed bias. it was working flawless for a couple weeks. then started this. B+ is right where its supposed to be All power tube sections on both channels have been replaced. not ultra linear. all screen v's are less than 1 volt difference. I cleaned and checked bias pots all functioning properly. FYI all ecaps and couplings caps were changed also on the rebuild.

[Phil_S]

Anybody ever unwind and repair and rewind a tranny.

At least two times. One time it was a broken lead on an otherwise very nice PP OT, maybe a 40W model. I tried hard to fish out the end from the paper and only managed to break off some turns. I took it apart, opened the paper, removed about 10 broken secondary turns and put it back together. It is still on the shelf, but it tests good.

To do this, the lams have to be split apart one at a time and removed from the stack, all the while keeping track of the E's and I's for orientation so you can put it back exactly as it was. This is done with a razor blade and maybe a bit of modest pounding with a rubber mallet. Then you remove the core, carefully open the paper and pray that you can find your problem without going too deep. Going too deep is a formula for not being able to fix it.

Then you put the lams back the way you found them. Bake it some for about 250F to get the lams hot. Spray with electrical varnish before it cools off. Hope it works. The heat is needed to allow the lams to suck the varnish in. That seals the OT and holds it together.

The next time I did it was with a cheap little SE. It fell apart when I removed the bracket from the outside. I rewound the secondary. I didn't work right.

In a PP OT, the lams are alternated so the E's rotate 180 degrees in pairs. The I's keep changing ends. In a SE OT, all the E's face one direction so there is provision for the air gap (a bit of cardboard) between the E's and the I's. It is much harder to get the SE OT right without professional equipment.

See if you can find that something else is the problem. It will take you hours to split off the lams. Avoid this if you can.

[tsutt]

Ok got it solved bad tube . So i got a bit of an education. brand news tested tubes arent supposed to just go bad right?. So i pulled out the tube tester checked all the tubes but for the 2 on the bad channel, one is down on mc of match quad the other one show no mutual conductance. doesnt show a short however. so now that i dont have a matched quad i pulled out 2 of the original tubes 1964 and they actually test within about 6 % on mc of the new ones. and everything come into spec. Amazing. like i said lesson learned. only been messing around with tube stuff for a short time. Thank for the input. And no money to buy new ones.

[katopan]

Glad you found the cause. About rewinding, it's generally a bad idea to reuse the old wire. Unless you just wound it as new, unwound and then rewound because you made a mistake (like I did in my 30W thread). But old wire work hardens from the vibration and heat inside the windings. If you unwind and then rewind the same wire you have straightened and rebent it all. A lot more likely to have a future failure and I've always been warned not to do it.

[tsutt]

thanks katopan. that is one of the points i was wondering being a metal guy. just played 2 albums with working good hope it stays that way.