old PT: is repair possible, or should I replace?

[f13ndus]

I've got an old Rauland PA amp (5U4, 2x6V6, two octal preamp tubes I can't remember off the top of my head) that I picked up to rebuild into a guitar amp - so I want to reuse the chassis, power transformer, and sockets, maybe more parts (resistors, etc) if they test out ok. The amp powered on and worked when I got it, although it was very tinny and had low volume, the usual problems with tubes being shot, old caps, etc etc.

Anyway, the problem I've come across is the leads coming from the PT are in terrible shape. The insulation is falling apart, and since they've been in the same layout for maybe 50-60 years the wire itself isn't really flexible any more. What I want to do, if I can, is snip off all the leads close to the transformer, and attach new leads, solder and heatshrink the connections, and go ahead and use it in the amp. In theory this should be simple, but will it cause any problems? would the connections be liable to fail if soldered?

If not possible to reuse the old transfomer, should I just ditch it and try to find another one that'll fit the chassis? Or would it be worth my money to have the original PT rewound with new leads (I assume replacing the leads as part of a rewind shouldn't be an issue)?

Anyway, just looking for advice, thanks!

[olddawg]

Most transformers are made of wound lacquered copper wire. The leads are attached pretty much like you say in the first place. Just make sure that you remove any lacquer from the winding wire or it will not solder. Many power transformers have a thermister, a zero ohm resistor, or a fuse buried in the first couple of windings of the primary side. I have fixed a great many open primaries by carefully cutting the paper and unwinding the primay back a few turns to replace or jump the offensive device. This situation is more common in modern equipment though. I always add a pigtail fuse to the lead if I do it and replace the paper.

[52 Bill]

I've done as Olddawg has done, and there is no reason you can't fix it.

Is the transformer potted? Are the leads free inside the end bell? If there is no permafil locking in the leads, I'd suggest taking off the end bell and replace the wires inside the case. Then re-assemble the case for a cleaner look.

Be sure to use wire that is color coded correctly and of a large enough size, as to not cause problems with current draw.

[f13ndus]

The transformer is bare, no end bells, I forgot to take a look at it since I posted this, been pretty busy. It mounts through the chassis like a Bassman power transformer... (actually, I should measure the mounting holes and see if I could use one of those... might be fun to build one of the early tweeds with octal preamp tubes... but that's off the subject!)

Anyway, if I go ahead and do the replacement leads, I'll practice with a junk transformer first, I haven't really done any work with transformers besides swapping them... I've got a couple of dead OTs around, they're smaller but it'll be the same concept to work with the leads, right?

And when I replace the leads, I should use these colors, correct?

Black - Primary AC winding
Yellow - 5V winding
Green - 6.3V Heater winding (green/yellow for a center tap, but don't think it had one)
Red - High voltage winding

Thanks for the advice!

[f13ndus]

Just an update: I've found out that the power transformer from a Fender Deluxe Reverb RI will fit my chassis perfectly, and so I've decided to use one of those for my build. Since I'm building a 5C3, it should work out perfectly... now to sort out an OT, got to measure the original to figure out the ratios. It seems to have 3 leads on the primary side (I'm assuming 2 plate, 1 center tap) and 4 leads on the secondary (3 taps and a ground? or 4 taps? we'll see)

I'll be saving the original PT as I want to repair it and will probably use it for a project down the line, but for now I figure I'll concentrate more on getting my circuit building perfect without worrying about transformer issues.

Here's one more question: After looking at the original PT it doesn't seem to have a center tap for the high voltage leads - looks like apart from the primary AC leads, it's got six secondary leads: 2 for 5V, 2 for 6.3V, and 2 for HV. I can't find the pictures of the circuit I took before I removed everything from the chassis... how would that work out? Remember this amp had 2 6V6GT, a 5U4, and two octal preamp tubes (I want to say 6SN7 and 6SF5?)

I haven't looked at too many circuits, guess I need more education...

[Steve Conner]

If it had a tube rectifier, the HT must have a center tap. Maybe it's connected to the core, or maybe the wire fell off.

I'd be kind of wary of 1950s transformers with crumbling insulation, you really don't want it to catch fire while you're out of the room.

[f13ndus]

Yeah, that's why I decided to find a modern transformer that would fit. I'm sure the iron would be good for a rebuild (vintage = better, right? ) but other than that it's a bit dodgy, in my mind at least.