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fender concert 50w valve amp problem help please


guitar Dan

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ive just had 2 new valves fitted to my lads fender concert amp and its just gone pop and the red mains switch light has gone out ie the amps dead.

 

i had the same problem befor the new valves were fitted hence the valve re fit,just wondered if there is a standard amp fault that can cause this problem on the fender concert.

 

thanks Steve :wave:

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It's possible the bias on the power amp valves is set wrong, which would let way too much DC current through, making the plates glow red-orange for a while, then frying the output transformer or power transformer.

 

Did you happen to notice any dark foul-smelling smoke just before it started misbehaving? If not, you might be in luck; it's possible you blew out the rectifiers or something else in the power supply.

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Hi

no nasty smells or smoke basikly my young lad does a few gigs a month and the amp is well used,without any problems until last week the mains red light whent out and the amp whent off,we had a local chap take a look and he knowes his stuff,he fitted to new valves and checked the amp out,asked us to take it away and try it wich we did,monday the amp was on/ off 6 hours for practising no problem,my lad turned the amp on tuesday and it made a small pop sound after abought 10 mins and was dead,its gone back to the amp man for further investigation,but i thought i would ask on here,if there were any standard faults on the concert amp,just to save time.

 

thanks for your reply,all help is very welcome,were having a bad week ,our marshall 6100 combo has just gone down wich caps problems and now this,and we have 3 giggs this weekend.

 

thanks Steve

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Has he been adding extra speakers to the amp? Fender tube amps will blow tubes when the impediance is too high.

It can take a few weeks or so but the power tubes glow red, then the grid resistors blow. This kept happening to My

Fender blackface when I was a kid and didnt know better. I was running 16 ohms and the head was desihned for 4 ohms.

You can run it at 8 ohms OK but 16 was too much.

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he has a pedal board that he just connects into the return socket on the amp ,then we mic the amp up, no extra speakers .The amp is normaly run at around 40% volume,the only other issue we have now,is its summer here and the evening air temp is around 30deg plus (days 38 to 42 deg ) wich probably dosent help on a 2.5 hour performance.

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he has a pedal board that he just connects into the return socket on the amp ,then we mic the amp up, no extra speakers .The amp is normaly run at around 40% volume,the only other issue we have now,is its summer here and the evening air temp is around 30deg plus (days 38 to 42 deg ) wich probably dosent help on a 2.5 hour performance.

 

 

Temp and humidity have no bearing on it.

How old is the amp? I found a schematic here. http://www.freeinfosociety.com/electronics/schemview.php?id=873

Its got a near identical schematic as my bassman except the tube bias which is a bias balance circuit on mine.

These amps have to have a matched set of 6L6 tubes installed. If some guy stuck in an unmatched set it could be the problem

with it blowing tubes.

 

Next check the type of tubes installed. If theru are cheezy tubes that cant handel the voltage or mis matched

pair it could have been the cause of the problem.

 

If you open it up check the resistors on the bottom of the power tubes and see if one is charred. They should be 470 ohm

one watt resistors. These are the first things to go when the tubes go. Be careful not to touch anything in there. The power caps

retain voltage and can be leathal if you get zapped by them,

 

Also check the power fuse. It should be a 3 amp slow blow fuse. No substitute.

 

next remove the metal cover that has two screws on the top side of the chassis in back of the power tubes.

This contains you high voltage power supply caps. Look (dont touch without discharging and be sure the amps unplugged)

and see if the positive end has a bulge or electrolite leaking. Theres a dimple in the rubber on the end that allows the cap

to expel electrolyte if they go bad vs exploding. If the dimple is bulged out or leaking, you need new caps. They arent expensive

and arent all that hard to replace if yopu have basic soldering skills. if not you could buy them and have someone else install them.

 

If the amp bias runs hot you may need tubes that will handel the current. JJ tubes are built to handel high plate voltages. That may be an option.

Also check your speakers. That amp runs all four 16 ohm 10" speakers in parallel. you should have the same + and - wires going to all 4 speakers.

They should not be wired in series like christmas lights.

 

Thats about all I can suggest to check for a novice. If the amp is dead a power transformer could be out, a bad switch or any number of items

including shoddy technical work, cracked solder joints (combos cause allot of vibration and will shale loose solder joints. Temp changes with amps

heating and cooling and vibration may have caused a bad connection. All that needs to be checked out by a pro.

 

Good thing about those amps is the parts are available and can be bought faiely cheap. Unless the circuit has been modified like someone sticking high

wattage ceramic grid resistors in place of the carbon ones, the transformers rarely blow. That and the lack of a burnt smell is a good thing but you never

know. Tube amps can be very durable when maintained regularly. if not they can be your worst nightmare.

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Thanks for the above help & info ,ive asked the amp chap and he has fitted two matched USA 6L6G valves ,i will pass on the info and let you know how we get on ,our bass guitarist is lending us his ac30 for this weekend so hopefully we should get through the next few giggs.

 

My 16 year old tells me its all abought tone and only valve amps do it :cop:

 

thanks

 

Steve

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well the fault appears to have been a new faulty valve :mad: as one of the new valves fitted failed,our amp man tested the output transformer with a megga and fitted a new valve and we have done 5 3 hour giggs without problem :) ( Apart from a new battery issue in our jmp1 marshall pre amp :facepalm: that rock & roll ) so fingers crossed. :thu:

many thanks for the help advise to all.

Steve (dad ) & Dan :wave:

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