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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
 Posted - 01/31/2010 : 18:25:49
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Hi all
Probably a bit OT as this is mostly pedal orintated but my Marshall is acting a bit strange on start up...after i flick the standby to on she is sounding weak and and nasty sounding..i tried changing the output valves to a full,brand new set EL34's and she wouldn't work at all...!! so i put the valve adapters with the EL84's back in and she was still not great but worked and after about 5 minutes of playing she was as good as new working fine... ...i thought if it was a capacitor i would hear some DC crackling but nothing like that...i read a story on the net about a guy with a failing output tranny in an old marshall who got the amp to work by using an attenuator..he reckoned it took some of the pressure off the transformer..i'm wonder if my valve output adapters are doing the same ..?... any thoughts..?......
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Edited by - FRANZONI on 01/31/2010 18:26:47 |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/01/2010 : 04:27:29
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This is just my 2¢, so remember what you paid for this wonderful advice. Your hearing gradually gets worse as you get older, but you compensate a little each year. Your eyesight gets worse little by little, but you learn to squint until you cave in and get glasses. Your amplifier transformer does not deteriorate little by little. It works, or it doesn't. There's many reasons your amplifier is acting the way it is. Bad wiring, a bad output tube arced across the socket, so now any new tube won't work 100%, improper bias setting, bad preamp tube, and the list goes on. The weirdest 'problem' I have personally seen is when the voltage or impedance selector was set wrong!
I hate to say this............ well, not really, but......... you need a qualified technician to do a 10,000 mile check up. You may not have any choice.
And an attenuator runs the amplifier full tilt, but disipates some of the power internally and sends a much smaller signal to the speaker, so it absolutely, 100%, no boudt adout it, DOES NOT take any pressure off of anything. Just forget that right now! |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 02/02/2010 : 11:07:24
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There lies the problem bro.... there are only maybe two techs in Dublin and the dealings i've had with them in the past were less than satisfactory...i just found it strange that the amp runs on a full compliment of EL84's with the tonebone adapters at around 35 watts but won't run with full compliment of the original EL34's that marshalls are famous for using...possibly you have something with a failing component theory,possibly a resistor or something heat related but instead of going dodgy when warm it seems to be the other way around.........i tried changing the preamp valves with some spares and i wired the cab myself to 8 ohms so i'm pretty sure the ohm selector is ok...i thought trannys could go gradually i didn't realise they were either working or dead... thanks for the 2 cents... |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/03/2010 : 04:11:09
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Hmmm, maybe a bad connection somewhere on one of the tube sockets? If the 'adaptors' (do you mean Yellow Jackets?) work after five minutes, I'd lean more towards poor pin contacts. Sometimes I take a small jeweller's screwdriver and 'pinch' the tube pins together and help make solid contact. Then I'll spray clean the sockets with a spray that emphatically states that it leaves no residue (usually a silicon based). Some socket types cannot be tightened up, but most can, so check your sockets.
If EL34's won't work at all, something isn't right. Do you mean the amplifier makes no sound at all? Weird. I've got theories, but before I reveal how wrong I can be, I have to make sure I understand the problem  . |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 02/03/2010 : 15:12:42
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Thanks for the reply Ramjam....
Yep..TAD tonebone adapters basically the same as yellowjackets
I didn't try the jewellers screwdriver just yet.. what i did do over the christmas was i cleaned everything with servsol electricial switch cleaner including the tube sockets..i did what the guy in eurotubes recommends at that was after sprayng some cleaner ,working a old tube in and out of the sockets to help make better contact...
The EL34's wouldn't work as such..all i got a was a broken,horrible,thin/scratchy distorted sound...with the tonebones its a not as bad and after about 3 to 5 mins of coming off standby into playing mode it seems to sort itself out and the full sound with the bass end reappears...
The amp originally came with a 'no name' cab of dubious ohmage that i got rid of but i'm not sure how long the amp was played through it brefore i got the amp..it did have these signs with the EL34's before i got the tonebones a few years back but not as bad and since i got and used the tonebones i've never had any real problems,what i noticed just before christmas was that the 'treble' channel stopped working but after cleaning everything it came back working...i've always been led to believe that of all the tube amps Marshalls don't like mismatch with the ohms the most and it can damage the transformer...
BTW my Marshall is the superbass mkII head with the metal front and the rocker switches and the 4 inputs but pre master volume era...i use it for guitar on 8 ohms into an 8 ohm cab and shes a great amp..i just want her to sound her best.... ..... |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/04/2010 : 01:47:57
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| You'd think it was a bum tube. If this is a 100-watt head, try just the middle two tubes. If it still sounds bad, try the other pair in the middle sockets. If that still sounds bad, try the first two tubes in socket #1 amd socket #4. Lastly, try the other two tubes in socket #1 and socket #4. Eliminate any possibilities. Resolder anything on the output tube sockets that can't outrun you. Slowly, this will all make sense, and answers will appear. It just may take some time. |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 02/04/2010 : 10:50:29
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| Yep ..it's a bit of a headscratcher alright...i've changed pots,did a whole cap job on another amp i have and other bits and bobs over the years.. but i've never come across anything like this before..usually things start going wrong as the amp heats up not the other way around...!!!..i'm wondering if it's a resistor or a cap going bad somewhere.... |
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svengali
Bronze Member
Australia
111 Posts |
Posted - 02/08/2010 : 12:39:51
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HI,
it could actually be the o/t.
the el34's are going to send a bigger ( much, much bigger ) charge through the windings of the o/t.
over time, and especially with voltage doubling power transformers ( theyre called flyback voltage spikes - double the plate voltage going through the windings in the o/t ...100 watt marshalls etc ) what can happen is that these voltage spikes weaken the insulation covering these windings and there is arcing between windings. Sounds awful. Sounds like the symptoms you describe.
running el84s is possibly reducing or eliminating this phenomena.
you can prevent damage in these voltage doubling amps by running diodes to ground from pin 3 of output tubes... ( better check that before you do it )
you dont need an amp tech, what you need is a soldering iron and a spare transformer. dont even remove the old one.
just take the two primary leads off pins three of the middle two output tubes, and the third wire from the primary which should go to one of the filter caps,...... and the secondaries which go to the speaker impedance selector switch.
position the new/used whatever transformer on the bench next to the amp and use aligator clips to connect the primaries, the b plus and then the speaker wires.
fire up the amp and crank the bastard.
discharge those filter caps first.- before you touch anything else.
thats what an amp tech would do to check an o/t.
if you go ahead and want to do this, email me at taskaras@gmail and ill talk you through it.
you can die trying this so be careful mate.
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Topic |
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Sign of failing output transformer in guitar amp..
