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Post subject: Vibro Champ XD Tube Resistors Burned
Posted: Thu Oct 31, 2019 1:28 pm
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A neighbor gave me a Vibro Champ XD that had been dropped (case cracked, tolex peeling) with no tubes, because they had no idea whether it worked or not, and didn't want it sitting around.

I fixed the tolex and tidied everything up, then, with no tubes installed, tested the power, and the jewel light lit up, no sound from the speaker. I did not try to get anything out of the amp, at that time.

I put in an EH 12AX7 and a JJ 6V6 and again hit the power switch. This time the speaker let out a loud hum/buzz immediately, while both tubes warmed up and started glowing what looked like correctly.

Also, smoke started coming from the base of the power tube, so I shut it down, unplugged everything, and removed and inspected the tubes. They both seem fine. The fuse was also fine. However, definitely something had burned, given the tell-tale smell.

I pulled the amp out of the box and noted:

1) There was a light brown singe mark at the center of the 6V6 mount, on the top side of the circuit board, and

2) R11 and R12 were completely toasted ... burned and melted. There were even soot marks on the wire bundle hovering above them.

Could this have been caused by a bad out-of-the-box power tube?
Or, maybe something that had been damaged when the amp was dropped?

Any thoughts are welcomed ... I'm basically junking it, at this point, but if it was caused by a bad power tube, I'd like to have a chat with the distributor that sent it to me.

Thanks, in advance. Lots of great advice, in these forums.


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Post subject: Re: Vibro Champ XD Tube Resistors Burned
Posted: Fri Nov 01, 2019 9:04 pm
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stupidscript wrote:
Could this have been caused by a bad out-of-the-box power tube?
Or, maybe something that had been damaged when the amp was dropped?


Either or both could be possible.

Do you have the schematic for the amp?

If so, start by powering up the chassis sans tubes and performing some fundamental voltage checks at the critical points, chiefly the power supply and tube sockets. Compare your findings with what the drawing specifies.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Vibro Champ XD Tube Resistors Burned
Posted: Fri Nov 01, 2019 9:34 pm
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Thanks for your reply!

With no tubes installed, the amp seems to power up ... the jewel lamp lights up, and no noise or other symptoms.

When I put in any tubes, either the 12AX7 and/or the 6V6, it makes a big humming noise and the smoke starts to pour out of it, as soon as the power switch is flipped. As I noted, the R11 and R12 resistors are now completely fried ... please see the schematic in the following link (see page 2):

https://www.thetubestore.com/lib/thetub ... ematic.pdf

It's something broken in the tube circuit, I'm betting ... but my real question is whether the fried resistors could be the result of a bad tube (either the 12 AX7 or the 6V6), or if there are other possibilities? I note that the cabinet is broken from an earlier incident ... probably from being dropped ... but I don't know what kinds of problems that might have introduced that could cause R11 and R12 to get fried and melted, like they are.

Is there a troubleshooting process I can follow, to figure this out?

Thanks, again.


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