Little help repairing a Carvin Legacy ??

Tube or solid-state, this section goes to eleven!
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culturejam
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Post by culturejam »

I picked up a non-working Carvin Legacy 212 (chassis only) for what I thought was a great deal (couple pedals I didn't play plus some PCBs and parts). This is a beast of an amp, and not something I would normally consider, but the price was right. Anyway, it's in non-working condition, and the damage appears to localized to the power/output section.

I've built a couple of tube amps, but I'm no genius when it comes to electronics. So I'd appreciate some pointers on where to start testing to isolate the problems. Luckily, Carvin has made the schematic public, so I've attached it below.

I have posted some full-size photos of the trouble areas on my Flickr account:
http://www.flickr.com/photos/forrest_an ... 763828566/


And I'll post a few smaller ones here to get the ball rolling.

It's clear that someone has tried to repair the damage.


Any help you lads can offer will be great. Thanks in advance for your help.
Attachments
VL100 & VL212 Legacy Amp.pdf
VL100 / VL212 schematic
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legacy_01.jpg
legacy_02.jpg
legacy_03.jpg
legacy_04.jpg
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Nyquist5
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Post by Nyquist5 »

It appears that at least one of the power tubes has conducted current from cathode to heater as R79 has burned up. This could have been caused by improper bias, simple tube failure or a failing component in the bias circuit.

A burnt up PCB becomes conductive. This means that you need to replace it otherwise your amp will never work properly. Soldering components over the burnt surface like on the picture is pretty much useless. I would also get rid of the plastic sockets as they also become conductive when burnt, and yours could have suffered.

Given the low parts count, you could probably make a new board yourself using point-to-point technique. Once that is done, I would start the amp using a current limiter to make sure that no other part of the amp is the culprit of the power tube failure, or has been damaged by it.

This is not a simple repair but it isn't overly complicated either. If you already have built amplifiers, I am sure you can manage it (and that you know the dangers of working with valve amps).

Good luck!

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culturejam
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Post by culturejam »

Thanks for the input, Nyquist. :thumbsup
Nyquist5 wrote:Given the low parts count, you could probably make a new board yourself using point-to-point technique.
I was thinking about doing this, actually. I was thinking of putting new chassis-mount octal sockets in there, and then wiring up the rest with a small eyelet board.
Nyquist5 wrote:Once that is done, I would start the amp using a current limiter to make sure that no other part of the amp is the culprit of the power tube failure, or has been damaged by it.
How would you recommend I set up the current limiter?

Thanks again!!

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Post by kurtlives »

I hate amps with PCB mounted output tubes. Just way too many things to go wrong.

Those big cerment power resistors should be 1K if your using EL34s (which the amp was designed for :slap: ).

Personally if it was me I'd get some nice chassis mounted octal sockets and rewire/redone the entire output section. While your at it you might as well beef up the other resistors in the output section (grid lead, grid stopper etc) to higher power ratings. I usually do that to help reduce noise.


CJ here is a diagram and info on a current limiter, it is a very simple but highly effect tool for testing amps. I never power up a new build without it.
http://tubenewbie.com/light_bulb_current_limiter.html
I usually start with 25W, 50W and then 80W or around there.

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Post by culturejam »

Thanks so much, Chris!!

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Post by culturejam »

Actually, I was just thinking....

If I'm going to have to redo the power tube board anyway, why not just scale it down to two EL34s instead? The amp already has a 100w/50w switch, so I know the transformers are okay with that. Even 50w is overkill for me, but it's a lot closer to useful than 100w.

Time to do some thinking and planning. :hmmm:

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Post by kurtlives »

The difference between 50W and 100W is a mere 3dB.

The 100W will not be much louder, stick with it. The 100W will have increased bandwidth and will have more underlying grit, I'd definitely go for the 100W version. 100Ws are special amps!

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