arced output transformer

[Doug H]

I'm getting really suspicous aboiut my output tranny on an 83 jcm800.

It just sounds "assless" for lack of a better term, especially when I hear it beside other killer amps. I play a lot at low volume with a MV where there's not as much nfb which thickens it up a bit, but it gets all ratty and top end when it get's cranked.

Ya, I've totally rebuilt this so there always a possibility of, ahem, human error , but I'm really starting to think I've been dogged by a bum tranny for a while now.

To start with, Ive booted it up a few times without a load, my bad, even strummed the guitar a few times. Mostly the master would have been low, but it might have got cranked before I realized there was no sopeaker, my really bad L(.

Coming off standby it makes a scratchy noise. IT used to be worse. Replacing tube sockets, tubes etc... may have helped, but it's still there somewhat. It used to be pretty bad.

Any insights? This sound like a tranny problem?

Thanks
Doug

[Doug H]

I'm thinking of swapping in the OT from my 45/100 kit just to see how it sounds. The primary impedences should be close. Anyone think that's fry my amp?

[Structo]

I believe a 50 watt OT is double the primary impedance of a 100 watt OT.

I suppose you could try it just to see if it diagnoses your problem but, I believe you have to change the impedance selector to 16 ohm for an 8 ohm speaker load.

[Doug H]

ya, I'm going to go ahead. Cause the 45/100 is for kt66s, it's like 3.4k primary where the one I'm replacing is 4k, shouldn't be a problem as I understand things.

[Structo]

No, that's close enough. Worth a try to see where the problem is.

[Doug H]

bwahahaha this guy's bad-ass. About twice as heavy as the one it's replacing and as big as my pt. Too cool.

[Doug H]

bloody hell. It's in, no volume, scratchy squacky sounding. Underneath it all I can hear some good tone trying to come through...<sigh>

I fired it up without power tubes in it, hit a couple strings at a lower volume setting, I hope to hell I haven't fried it right out of the box.

[Doug H]

wired the old tranny up, volume level is back, wired the new one up again, no volume, so its the OT. Can you really fry a OT by running it without power tubes for 5 seconds?

[Doug H]

Now that I think about it, there's no way simply not having any PT in could hurt anything, there isn't even a signal going anywhere. Guess my power tranny jsut can't drive the 100w OT

[Doug H]

It's a user problem lol. I had the 4ohm and 100v leads ust lying around so I tied them up, taped off the end and tucked them away for the time being, stupidly taping the ends together in the process. The volume is back, I haven't really played it yet. Here's hoping I didn't damage the OT.

[Structo]

Without power tubes in the amp, there is no signal going to the OT, so no fear there.

So you had the secondaries you weren't using laying there and hey shorted?

The old rule is, tubes in speaker connected, tubes out no problem.

Hope you fixed it.

[Doug H]

Right, I think I might be good. I didn't actually short the unused ends out to the chassis or anything, I freaken' taped the stripped leads together bundling them up lol.

Cranked her up today, I think all is well. There's about 10 times more bass than I had with the old OT, of course I'm not sending a lot of nfb through it right now, but I'm convinced. This tranny just might end up staying in this amp permanently.

I can really hear how it doesn't saturate like the 50 watter.

[Doug H]

So I'm redoing some connections on the output jacks, little do I know my too hot iron has melted the plastic and one of the metal tabs has slid down so the tip of the speaker jack won't make contact. There I am again twisting knobs, strumming the guitar with no sound coming out, you think I'd learn...but I knew damn well I had everything connected properly, and I did, just that one of the cliff jacks was bad.

I think I may have toasted another one <sigh> I'll have to swap in my new OT when it gets here to tell.

www. Doug's custom boat anchors for the Marshall enthusiast opening soon...

[Structo]

Man dude, you are not having good luck at all.

What kind of soldering iron are you using?

Does it have variable temperature?

If you are melting plastic then your iron is either too hot or you are staying on the solder joint too long or both.

[Doug H]

My ad999 delay pedal went the same day and my keyboard stopped working right.

Ya, iron is too hot and I've probably got a heavy hand making sure the joint isn't cold. Sometimes I put it up high to fix my flasses which broke a couple weeks ago or to undo joints quickly, or just to work fast. it was set pretty hot.

Took the bells off my old tranny, have no idea how to get the wraps off of the core, some kind of solvent? There are black diagnal powdery lines on the side of the wrapping that look like they could be carbon traces. I've always fancied winding my own tranny, so I"m going to at least try and tear the old one down.

Too bad you can't just open it up, find a bad wire and wrap some tape around it.