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Power valve question ?


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Hi there, for home use I have a Fender Champ 12 valve amp. Thing is I think I blew either the output transformer or the power valve.

To change the valve (it is a one power valve model) will I need to bias it too ? Or does that only apply when you have more than one valve ?

I need to fix this myself as this is my first valve amp and I want to get to know the ins and outs as it were.

Funny thing is. Years ago valve amps were so out of fasion you couldn't give them away. I actually turned the offer of a free vox ac30 because the cloth on the front was ripped. Pffffff !

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With valves, you can just plug in a replacement - the bias, especially with a one valve (A- class) o/p is taken care of by the circuitry. Incidentally, you've got to go some to blow an O/P valve or even the transformer. Valves are virtually indestructable electrically. Internal short circuits are one of the rare possibilities, but this usually shows up as a rather un-healthy red glow from the anode and is the result of heavy mechanical shocks.
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daif - I did a crazy thing. I turned on my solid state HH 100watt combo and put a jack lead from the Fender champ 1volt line out to one of the combo inputs. It sounded really good for 10 minutes. Then it all went a bit crackly and crunchy. Thats how its stayed now. The controls have no effect at all on volume, bass, gain. I think I've fried it.
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I'd get it repaired!


Firstly valva amps have some really nasty voltages, Secondly you don't really know what you've blown and you could be hunting down a fault for a lot longer than a skilled repairer.


Read a couple of books on valve amps first. Then get a circuit diagram.

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daif - I did a crazy thing. I turned on my solid state HH 100watt combo and put a jack lead from the Fender champ 1volt line out to one of the combo inputs.


I fear you may very well have done a crazy thing.


Does the champ really have a dedicated 'line out'? What exact model do you have?


Is the speaker on the Champ on a plug lead? Did you disconnect it when you plugged it into your combo?


If so, you've probably managed to fry the output transformer and potentially quite a few other bits.


Valve amps generally really really don't like being run without a load on the output transformer,or into a high-impedance load like the input of another guitar amp.


You may have wound up destroying the output valve and/or output transformer through arcing and subsequent overload/short circuit.


Apparently this is a quite effective way to turn most higher powered valve amps into a very small combined smoke & light show.


It's perfectly possible to get spare output transformers for current Fender valve models from the manufacturer, and plenty of manufacturers make imitation vintage transformers for various ages of Fender/Marshall/Vox amps.


However, if you really have managed to induce arcing in the output valve it's quite possible that the valve itself, its socket, and virtually all capacitors in the anode power supply chain are compromised by the high voltages induced and arc-tracking.


You may have fun fixing this completely.


I suspect that unless you feel very confident working on 350V DC supplies (live or not) and have some way of testing the reliabilty of capacitors at working voltage you may want to get a more experienced technician involved at this stage.


Remember the anode voltages in even quite small and innocent-looking valve amps can kill instantly, and the capacitors can give you quite an astonishing belt some time after power is disconnected, depending on whether sensible bleed resistors are fitted.


They are distincly not 'fuer gewerken bei das dummkopfen', even if you do keep one hand in your pocket at all times.


I would recommend you buy a basic book on valve amp repair and modification before you attempt this, espcially if you were not aware of the no-load issue with output transformers. I can't really recommend a book for you, but there's plenty of forums on t'internet with relevant information and links.


As stated previously, something like a Fender champ shouldn't need biasing if it's one of the models with a single 6V6 output stage.


I'm sorry to be dampening your enthusiasm in this way, but although wonderfully simple and forgiving in terms of circuit design and modification, they are not toys for beginners in electronics simply because of the voltages involved.

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daqrkenstein - thanks for a full and frank reply. I feel such a burke. I know a lot of the rules to do with linking amps. Yet in my excitement to hear my new valve amp through extra speakers my caution was non-existent. Thing is my solid state combo with 2x12" speakers has a socket in the back that I should have plugged my "champ" into.

I am also well aware of the high voltages present inside valve amps. I wouldn't try any more repair than simply renewing valves.

I really think this is a job for a technician.


Thanks everyone for your input in to my output problem, Much appreciated.

I'll re-post when I've had someone look at the little fella.

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daqrkenstein - thanks for a full and frank reply. I feel such a burke.


If your amp actually corresponds to the one in the schematic Jivemaster posted, then you may not actually be a berk at all. The design looks like:


a) You can't unplug the speaker, except by really messing with it.


b) The headphone socket is switched and has a load resistor built to disconnect both the speaker and prevent the amp frying itself.


c) The line out doesn't disconnect the speaker.


However, if it really is a standard champ 12 from the early '90s, the jack sockets are soldered directly to the main circuit board, known to be of quite poor quality and may have failed.


This one has a 6L6 valve as standard, not a 6V6. Sorry. That makes the 480V anode voltage (B+) at the output stage plausible, which is deep in ouchie territory.


If you did fry the output transformer, I'd love to know how you actually managed it without disconnecting the speaker.


Admittedly, if it wasn't yours from new it may have simply been modded at one stage to have a speaker socket or adjustable bias- I think mod kits have been available for these for years, and they have often been badly fitted.

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Actually, I'd seriously look at all the jack connectors and all their associated switching contacts, carefully applied switch cleaner may be the answer. -every contact, then wait til it's dry before powering up.


If you unroll that link you can go back to the whole fender section and look at schematics for various amps.

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