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Doing an ST-70 overhaul.

The owner of the amp said I could use his if I fixed it. He brought it home, fired it up, it warmed up and a white flash, no output tubes.

He figured as I did, the grid coupling caps were old and leaky.

So after much effort, the rebuild was ready to go tonight... so I thought.

Well, I found the problem why it blew up before... not capacitor...
flakey output transformers!!!!!

Last night, I had the supply done and HV tested it for 20 minutes. That
held OK.

Tonight I popped in the EL34's and biased up the left channel, great!
Biased up the right channel and as soon as the tubes began conducting,
*BZZZZRT!* and the fuse blew.

The good news, the tubes live... 50% of their screens are gone on that
pair, but they still live.

Bad news is I popped the right OPT apart and it's black with
carbon traces everywhere. Looks like she's been breaking down for a long
time. That iron looked practically new on the outside!

Other bad news is that means the left OPT is a good chance will do the
same eventually.


Is this common, or did I get the short straw?

Cheers!

Transformers and humid environments don't mix. If they are stored
in a garage or basement, etc ... they soak up humidity and tend to
burn up when voltage is applied. As far as I know, the Dynaco
transformers are no more prone to the problem as anything else.

I would think baking them in an oven BEFORE applying power might
be a way to drive out the moisture ... but I'm sure no one ever does it.

Thanks!

Closer look at the trasformer shows the carbon traces in HV lightning "Tesla effect" patterns.

Methinks someone ran her up with no load

Cheers!

Sounds pretty likely unfortunately.. In 50 or so ST-70 that I modified or repaired I never encountered one with a bad OPT. IMO unless abused they are pretty rugged.

Wondering what kind of load this thing might have been driving or whether it was located in a house that was struck by lightning?

I have tested OPTs to destruction in the past and generally just saw smoke, and upon disassembly no or not much carbon tracking.. That is a bit odd.

I have run into a couple of bad power transformers over the years, but again not many.

Maybe this is why people pot their OPTs in epoxy.

Min

Wouldn't have helped mine... when I unwound it, I found it was a winding flaw that finally gave out.

Cheers!

I haven't yet seen any ST70s with blown OPTs, but asnything is possible, I'd pin the most likely bnlame on running it into no load, the inducde voltages inside the OPT windings could arc over, burn the insulation between the windings and cause a catastrophic failure.

I have seen a couple of bad Z-565s, as mentioned in another post, whether thy had electrical failures or just the results of bad storage, or marginal production quality, is unknown.

I was this very thing happen to a friend's VTL MB450s several years ago.
The OPTs arced over internally, punctured the insulation, which then carbonized, and developed a short between the primary and secondary (I did a basic resistance check on one of them, and found the OPT's shorted condition) though as far as I know, he never ran them into "no load" conditions.

Fortunately no other damage was caused, the short just blew the B+ fuse, before anything else could happen.

Perhaps it was Devo "whip it good" being played at near max volume that finally did the evil deed

However, the amp's speaker connections were made by means of ring lugs screwed onto the speaker terminals, and there could have been a flakey connection. I much prefer soldered connections, though I have also used ring lugs in some of my builds.

VTL replaced the OPTs in both amps, and added some upgrades, for no charge, other than shipping the heavy beasts. These were the "signature" series of OPTs and I'd SWAG that they would cost well over a grand apiece...

/ed B in NC