1974X croaked......

Buc McMaster

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It's #7 of 40 built in 2004 to commemorate GC's 40th anniversary......pretty much the hand wired model with salt & pepper grill cloth. Been playing it without issue for about two weeks, put a full set of new tubes in it about a week ago, but yesterday it just quit......completely silent. Fuses good. Replaced the rectifier and it fired up once more....for about two minutes and it went silent again. So it's off to the shop to be healed. I'm wondering if anyone here has experienced a similar failure. Logic tells me the amp would not have come back to life with a new rectifier if a transformer had blown.......yes? Any thoughts?

 

Spanngitter

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A ton of possibilitites, ranging from a faulty tube (I would target the power tubes first since problem started with new set) to a failed capacitor in the powersupply or a failed Voltage Dropper resistor. This amp has some birth defect from day #1 as the standby is before the 1st reservoir cap so everytime you turn on standby there is a huge inrush current hitting the EZ81, stressing it out. I'll recommend on this kind of amps (any many more, mainly cathode biased) to just disregard the standby and turn on the amp with Standby already off and use it only for a quick mute e.g. when changing guitar. This will allow the capacitors charge slowly when tubes are warming up and since the EZ81 also takes it time to generate HV there is no stress on the power tubes as well.
 

neikeel

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@Spanngitter nailed it.
This causes premature rectifier failure and quite a few early ones killed their PTs too.
Question: Why did you replace the tubes?
 

Buc McMaster

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Question: Why did you replace the tubes?
When first landed, the preamp tubes had been removed for shipment.....the power tubes and rectifier were still in the cage. The tremolo did not work well.....intermittent and weak. Swapped V1 with V3 and the tremolo was better but still weak. And so, being the electronic idiot I am, I figured a new set of tubes might be a good call for an 18 year old amp. Perhaps this is what caused the failure, perhaps not......I dunno. It has been delivered to a respected amp tech for evaluation. Rookie or not, I know that what's been broken can be fixed.......just time for a bit of patience.

Thanks for your input, gentlemen!
 

Buc McMaster

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I'll recommend on this kind of amps (any many more, mainly cathode biased) to just disregard the standby and turn on the amp with Standby already off and use it only for a quick mute e.g. when changing guitar. This will allow the capacitors charge slowly when tubes are warming up and since the EZ81 also takes it time to generate HV there is no stress on the power tubes as well.
Was the original 1974 wired this way or is this only in the 1974X reissues? Just curious........
 

Spanngitter

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Was the original 1974 wired this way or is this only in the 1974X reissues? Just curious........
I haven't checked all Schematics but the original 1974 was this way, also a ton of 18W Clones (Tremolo as well as TMBs) and I doubt that Marshall changed it for the X.
As a quick fix you can bridge the Standby with a 68k/2W Resistor which will allow the reservoir cap charge slowly already with Standby ON.
Howeve,r this will also cause the Standby not being useable at all as a complete Mute but there is always 2 sides to the medal...
 
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Pete Farrington

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Just rearrange the standby switch so that it doesn’t interrupt the connection between rectifier and reservoir.
Marshall have a long track record in screwing up standby.
 


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