Magnatone 480 Stereo Amp...TWO bad OT's???

[jrfrond]

I got a Magnatone 480 in for service today. Same one I rebuilt for drummer/producer Steve Jordan two years ago. BOTH channels were seriously compressed and oversaturated. The power tubes (6973) looked overheated, and the getter flash was flaking off on two of them. After looking at the most common thread (power supply), which was rock-solid as well as swapping power tubes (which needed it anyway), I 'scoped the test signal all the way up to the power tube plates, where I had plenty of drive. Turns out BOTH OT's had partially shorted secondaries, as the signal output signal was clamped at 5VAC into 8 ohms. To verify, I clipped-in a couple of Blues Jr. OT's, and BANG.....there was the output, loud and clean, as I had remembered it. Well, at least I found a home for those BJr transformers I had in stock!

Moral of the story: expect the unexpected, especially with vintage budget amps.

[Enzo]

Wow, that is really sad.

But it is a great way to point out that one should never rule something out just because it seems unlikely.

And we don't run into those 6973s much. I see them mostly in old Seeberg jukebox amps, but maybe some Leslie amps?

[jrfrond]

Yeah, the fact that BOTH of them toasted kinda freaked me out, because it was SO unlikely, but I then chalked it up to age. Both transformers made at the same time had aged at similar rates. Most likely breakdown of the shellac used on the windings back then.

Those 6973's are rare, and aside from this Magnatone, I've seen them in a Univox amp or two. Couldn't believe I actually had six of the EH version in stock. The specs are very similar to the EL84, so I just went with the BJr transformers.

It's Miller Time!!!

[Gtr_tech]

Wow, that is really sad.

But it is a great way to point out that one should never rule something out just because it seems unlikely.

And we don't run into those 6973s much. I see them mostly in old Seeberg jukebox amps, but maybe some Leslie amps?

Valco/Supro used them in a couple models.

[Enzo]

Yep, seen a few in this game, but not often.

[capnjuan]

Those 6973's are rare, and aside from this Magnatone, I've seen them in a Univox amp or two.

Hi; they were used in some Univox U305R models while other 305Rs were fitted with 7591s. I have a U305R/6973 schematic if anyone wants it. John

[RybCage]

I realize this is an old, perhaps long forgotten topic but if you still have the U305R schem I would love to see it. I am trying to find a schem for the Univox U202R which also uses the 6973 power tubes & have not had any luck so far. I am grasping at threads trying to find other models that may be pretty close. Of course if you happen to have the U202R that would be even better!

[capnjuan]

I realize this is an old, perhaps long forgotten topic but if you still have the U305R schem I would love to see it.

If you want an easier-to-use jpg copy, send me a message with your email addie. J

[olddawg]

Well, I'm not as experienced in servicing tube amps as some of you guys, but it seems like to me somebody overdrove paralleled the inputs of that old stereo Magnatone and supersaturated it for hours on end. That would wear out the finals and cook the OTs in the same way at the same time. Those OT weren't exactly robust anyway. I used to have an old Magnatone that sounded the best when it was just about to melt. Evidently that owner was of a like mind. Hopefully the replacements don't change the tone.