Plates glowing on my Fender Concert after blowing fuse - diyAudio
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Old 29th July 2011, 02:43 PM   #1
nooel is offline nooel  United States
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Default Plates glowing on my Fender Concert after blowing fuse

So I'm wondering if anyone might have some insight as to why my plates are glowing on my tubes. The fuse blew on the amp some time ago when a friend of mine accidentally dropped an empty pint glass on the amp. After replacing the fuse, the amp worked for a little while, only to blow another fuse after emitting an orangish flare from the back of the amp that I saw reflected off the wall. Well, after checking out the tubes, I noticed that the plate on one had completely burned through and then burnt a hole in the tube. I bought some cheap chinese tubes on eBay to start figuring this out, and their plates are glowing too. When standby's switched off, there is no glowing, but as soon as you switch it on the creases (for lack of a better term) of the plates are glowing. Any ideas?
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Old 29th July 2011, 02:59 PM   #2
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I am not expert but I would say do not plug it in again until it is looked over for the "Flame" issue is taken care of. There seems to be extra currant getting to the tubes, Please have a Tech look it over. Sounds like a fire hazard.

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Old 29th July 2011, 03:03 PM   #3
nooel is offline nooel  United States
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I agree there is extra current, but I'm too poor to have a tech look her over. I have done a bit of soldering in my day so I'm wondering if there just a few components I could replace.
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Old 29th July 2011, 03:56 PM   #4
nooel is offline nooel  United States
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http://ampwares.com/schematics/concert_6g12.pdf

here's the schematic.
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Old 29th July 2011, 04:04 PM   #5
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If you put a known good tubes in an amp and it starts red-plating right away, it's conducting full blast like WildOne says.

I've seen more than my fair share of tube circuitry gone bad due to problems like cracked solder joint or opened resistors in bias circuitry. Fixing is easy once you know what it is, but you will have to troubleshoot and identify the source of the problem somehow. Big part of what you are paying the repair techs for is the time to identify these problem.

If you don't have the expertise to do this, it's really best to have a tech with those skills look over it.
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Old 29th July 2011, 04:06 PM   #6
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There you go, I was just going to get it from schematic heaven and upload it, but the file size was to much. Anyway, are you up to making some voltage measurments? Let's start with the B+ voltage from ground to the top of the 20 ufd main filter caps- you can leave the standby switch off to prevent the nasties. I have to inject here that there is a considerable safety hazzzard with this measuement!
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Old 29th July 2011, 04:08 PM   #7
nooel is offline nooel  United States
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what safety precautions do i need to make?
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Old 29th July 2011, 04:13 PM   #8
nooel is offline nooel  United States
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I should add that its only the power tubes, and both of the power tubes.
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Old 29th July 2011, 05:02 PM   #9
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Quote:
Originally Posted by nooel View Post
what safety precautions do i need to make?
I agree with the previous recommendation to take this to an experienced repair person.
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Old 29th July 2011, 05:06 PM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by nooel View Post
what safety precautions do i need to make?
If you have to ask then maybe yo need to hire a pro. The voltages inside are lethal. If it can burn through a steel plate it will do even worse on your nervous system. You don't get a second chance. Also this is a very complex amp and you will not be able to fix it unless you can pass this simple test: Look at the schematic, point to the bias supply and tell us specs on it's filter cap. What kind of rectifier does it use? what is the expected voltage?" Those are dead easy questions, right up there with "how many beers in a six pack". If the answers are not obvious hire some help.

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(0) Remove the tubes. For this level of debugging it's best to not have them in.

(1) Do NOT place both hands in the amp. There is high voltage even with the amp unplugged from the wall. Filter caps can store power for a good long time. So with one hand in back pocket, connect a meter to the B+ and ground, then apply power and wait for volts on meter to stabilize. Leave this meter in place. It wil be the "safety meter" when it reads over about 30 volts keep clear. Meters cost only $5 at harbor Freight and there way-cheap meters are good enough. Buy four of them.

(2) you can make a "discharge stick" for about $0.50. Get a (say) 100K 1W (or 3W or 5W) resistor and clip one lead kind of short. Tape the resistor to the end of a wood or plastic chop stick so that the short lead points out from the end. put a short length of wire on the other lead and solder a clip to the wire. clip the wire to the chassis. Poke anything with the pointed tip before you touch it. notice the "safety meter" quickly goes to zero if you connect the stick tip to the B+. This stick will save you a lot of time, otherwise you'd have to wait a while for the amp to be safe after powering down. The safety meter will verify the stick is doing the job.

I suspect a broken bias supply. What you need to do is look at each part. is the diode shorted, filter cap OK. The common problem is the adjustment pot fails but I don't see one. Seems to be non adjustable. So maybe a fix resisor has failed.

Last edited by ChrisA; 29th July 2011 at 05:21 PM.
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