Carlsbro 50 top OT-problem?

[cowboy]

Hi! My friend got hold of a vintage Carlsbro 50(think it's from '70-something) with a broken OT. Has anybody got any idea what would be a good replacement? Can't find to much info on the amp...
Anyway, here's a schematic of the amp if someone's interested!

http://www.chambonino.com/carlsbro/50-top.jpg

[Alex R]

- something from Mojo or somewhere for a 50W Marshall with EL34s will do it. Don't worry about the speaker impedances - those are 4, 8 and 16 ohms in the real world.

When you've done it, try lifting the negative feedback loop - the wire goes from the impedance selector to a resistor on the board. It makes an interesting difference to these amps.

[cowboy]

Thanks, me and my friend have been looking around for awhile now, and we kinda started to suspect that we should go for a marshall 50w, but it's great to hear it from someone else too!

[cowboy]

OTs

Btw, which one would you recommend out of these two?


Mag 50w Output Transformer for Marshall JCM800 JMP Amps - eBay (item 380191783015 end time Jan-25-10 15:57:59 PST)

or

Mojo Transformers Marshall Style Plexi/JCM800 50 Watt Output Transformer

I haven't got any exp. with these things, so it would be really nice if you could recommend somethin'

[Alex R]

I've used Mojo plenty of times and they work ok, I haven't had a failure. The other is so much cheaper though! I don't have any experience of them so I will leave it to someone else to comment.

[billyz]

You said broken? I recently replaced the feet on a Sunn OT. It's originals had broken off. If bad then replace. Broken = fix.

[cowboy]

You said broken? I recently replaced the feet on a Sunn OT. It's originals had broken off. If bad then replace. Broken = fix.

Ok, it's bad! If the amp's in standby, everything is OK, all the tubes are glowing etc. But when you get it out of standby, the OT becomes overloaded and shoots sparks. When me and my friend opened it up it was just black!
We suspect that the guy who had the amp before us let it stay on without a speaker connected.

[Steve Conner]

Sounds like it has plenty wrong with it besides the OT! Better make sure that it doesn't blow the replacement one too.

[cowboy]

Sounds like it has plenty wrong with it besides the OT! Better make sure that it doesn't blow the replacement one too.

Really? Do you have any idea of what it might be? As I said, it powers up and looks good until you hit that standby button, and on the inside everything looks fine, (caps etc.) btw thanks for telling me!

[billyz]

Well it does appear to be shorted . Most likely the primary side. As Steve said, I would check for other possible problems too. Like Shorted Power supply caps. Shorted arcing tube sockets, Shorted Power tubes. An amp that old should have all the power supply and bias circuit Electrolytics changed .

This is not unusual for an amp that old. I would use a 50 watt Marshall or equivalent OT. Marstran, Heyboer, Would be my choices. Even a Dynaco clone would be nice. Mercury Mags are fine and too expensive. Or an off the shelf model from Antique Radio supply or Mojo. Antique radio will have the electrolytics as well. F&T is good, JJ too.

I have had problems with Trannys from Triode and would not do business with them. Go for a Plexi or even JTM 45/50 model. The ones from Mojo look fine.

[cowboy]

Thanks for the tips! Don't know if this matters, but when I got the amp, i noticed that a powertube was quite loose, and that it would fit the socket in multiple positions.
That powertube was missing the piece of plastic on the bottom that should keep it in position. Also, on the OT, its the primary-side that goes sparky-sparky when you turn the amp on, so it's quite likely that it's the primary side that's shorting.

[Steve Conner]

Well there's your problem... If the peg is broken off one of the power tubes, then it can be inserted 8 different ways. Only one of those ways is right, and the other 7 are an electronic version of Russian Roulette. (that's why the peg is there!) It could easily short the OT to ground, explaining your symptoms. Or, a previous wrong insertion could have blown the tube, and now it won't work no matter which way you put it in.

So, add a new power tube to your shopping list. In fact, might as well get a new pair. The OT will be shot if it's sparking, but you might as well try removing the power tubes and see if that stops the sparking.

[cowboy]

Well there's your problem... If the peg is broken off one of the power tubes, then it can be inserted 8 different ways. Only one of those ways is right, and the other 7 are an electronic version of Russian Roulette. (that's why the peg is there!) It could easily short the OT to ground, explaining your symptoms. Or, a previous wrong insertion could have blown the tube, and now it won't work no matter which way you put it in.

So, add a new power tube to your shopping list. In fact, might as well get a new pair. The OT will be shot if it's sparking, but you might as well try removing the power tubes and see if that stops the sparking.

That's kinda what I've been thinking, now I got it confirmed, thanks. Power tubes are already added to my shopping list, haha! Thank you so much for the help, obviously I'm no expert at amps, but I'm learning!

[cowboy]

Tested the amp today with the tubes placed right, the sparking from the OT is gone, but it still needs to be replaced, so I bought one off ebay today, think it's gonna be just fine! Thanks for all the help I got, I will update with a few soundclips when the amp's up and running!