Transformer problem

[mike60510]

So the 340V winding measures 109.6 ohm when non-powered but no voltage when powered?

Yes, when looking at where the leads for the 340 volt tap come out of the transformer, it looks discolored, like it was burnt.

The transformer also has an additional high voltage lead that powers the speaker field, I get about 57 ohms when I test that lead to the leads that feed the 5U4 tube.

[Chuck H]

Sometimes old transformers have a lot of oxide on the wire under the insulation. If you can read resistance across the HV winding I think you should be able read some voltage. Try again and keep in mind that good contact may be an issue. Don't get shocked.

[mike60510]

Sometimes old transformers have a lot of oxide on the wire under the insulation. If you can read resistance across the HV winding I think you should be able read some voltage. Try again and keep in mind that good contact may be an issue. Don't get shocked.

The insulation on the wires are not in good condition, would you recommend cutting back to near the transformer and splicing on new leads?

[Chuck H]

The insulation on the wires are not in good condition, would you recommend cutting back to near the transformer and splicing on new leads?

I've seen the old cloth covered wire on transformers surface oxidized all the way to the winding! Cutting it close to the windings might leave you in more of a pickle. It can be really hard to get flux and solder to deox and bite when the wire is like that. I managed it once by stripping about a half inch off the insulation and using a brass wire toothbrush in one direction. Spread and surface clean the braid like this until it looks shiny again. Then loosely twist the braid back together and apply flux and solder. A flux core solder should be fine. If it doesn't clearly wet all the strands apply more flux, solder and heat until it does wet the whole stripped end thoroughly. Maybe someone will chime in with a better solution. Solution might be considered a segue word because I think there are special fluxes available just for managing this oxide layer, but I've never looked into it further. Once you get the strands to wet for the area that will be connected you're good to go. Only a very small surface layer of the copper is oxidized so as long as you can make a reliable electrical connection it should work fine.

Did you measure resistances on the windings at the soldered connection before removing the transformer or were you able to take ohm readings from the stripped wire ends?

[mike60510]

I've seen the old cloth covered wire on transformers surface oxidized all the way to the winding! Cutting it close to the windings might leave you in more of a pickle. It can be really hard to get flux and solder to deox and bite when the wire is like that. I managed it once by stripping about a half inch off the insulation and using a brass wire toothbrush in one direction. Spread and surface clean the braid like this until it looks shiny again. Then loosely twist the braid back together and apply flux and solder. A flux core solder should be fine. If it doesn't clearly wet all the strands apply more flux, solder and heat until it does wet the whole stripped end thoroughly. Maybe someone will chime in with a better solution. Solution might be considered a segue word because I think there are special fluxes available just for managing this oxide layer, but I've never looked into it further. Once you get the strands to wet for the area that will be connected you're good to go. Only a very small surface layer of the copper is oxidized so as long as you can make a reliable electrical connection it should work fine.

Did you measure resistances on the windings at the soldered connection before removing the transformer or were you able to take ohm readings from the stripped wire ends?

Thanks for the all the recommendations!

[mike60510]

Do other electronic components such as the resistors and capacitors have a good track history for reuse?

[Chuck H]

Old carbon composition resistors (such as might be found in your amp, brown with sharp edges on the cylinder with color codes painted on), are prone to "drift" with age. The really old ones (such as might be found in your amp) are also known to absorb moisture in high humidity and make crackling noises if used in high voltage circuits (such as might be found in your amp). If there is no noise and they test within spec you can reuse them. Spec for old carbon comp resistors was usually +/- 20%!

Older paper and oil capacitors (such as might be found in your amp) are prone to leak DC with age. In tube circuits this can cause bias problems in capacitor coupled circuits (such as might be found in your amp). I would just replace any paper caps with modern film caps with no other consideration on the matter. Film caps last virtually forever and if they don't leak they're fine.

Electrolytic capacitors have a liquid electrolyte that does imply a limited working life. Any electrolytic cap over ten years old should be suspected, but older caps did seem to last longer than modern caps. Perhaps the electrolyte goo formula included some toxic carcinogen that is no longer used Sitting without use (such as might occur with an organ) is worse than if an amp is used regularly. Older electrolytic caps have been known to last for forty or fifty years in amps with ideal moderate, but regular use. This is rare. You should expect to replace any electrolytic caps.

You've already seen what the copper strands look like under the insulation so reusing wire isn't advised.

If the amp doesn't show heavy use wear you can probably reuse pots, jacks, switches, etc. They would certainly benefit from cleaning with a deox type product. For potentiometer cleaning use a deox/lubricating product.

If the tube sockets are Bakelite you should probably replace them. If they are ceramic and still hold the tubes snug then they could also be cleaned and reused. But don't use a lubricating cleaner for the tube sockets.

If the chassis isn't horribly rusty and suits your needs it can be reused, of course.

If the transformers show very mild rust they should be fine. More than mild rust and they should be painted to slow down the process. If they show heavy rust, well, that's just cause to suspect a lot of eddy currents in the laminations. Ideally the laminates have no electrical connection and rust spoils this. You would expect poor performance and overheating. OTOH I've seen moderate to badly rusted transformers that were working fine.

[mike60510]

I've watched several videos where they used a variac (hope the term is correct) to bring up the voltage on the amp. Is that a necessary piece of electrical equipment or a nice thing to have?

[Chuck H]

It depends... If you're a pro and need to be as efficient as possible it's a necessary piece of test equipment. If you're an amateur it'll probably save you a little time, but it's real value is that you can be a little smug about the cool stuff you have. I don't have one.

[g1]

sez Mr. Snarky Potato.

[Justin Thomas]

I was gifted one, but it only had a two-prong plug. Knowing I could mod it just like an amp, I think I hung onto it. (I kept it for the knob - soon to be the Volume control on my next amp...) Of course, it's now being stored back in the same attic it was gifted to me from...

Justin

[mike60510]

I cut back all the original wires close to the transformer and soldered on new leads, but still no voltage. Getting a closer look at the transformer, the section where the secondary high voltage leads exit the transformer the "paper" looks burnt and the wires are very fragile. Probably kind of dangerous to reuse. I've put the rest of the amp on ebay. Maybe someone can use the spare parts.

[Tom Phillips]

Something strange really is going on here. It still doesn't make sense that you have measured good resistance readings on the HV winding but it doesn't produce voltage even though the other windings do produce voltage.