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Old 11th Aug 2018, 9:11 am   #1
'LIVEWIRE?'
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Question Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Could this be a Mains Transformer fault? (Hope Not!) The Primary DC resistance measures about 8 ohms on my DMM. Other than that I haven't yet carried out any tests, but I'm told that the (correct) 2A(T) fuse in the Mains TX primary circuit has blown more than once. The HT fuse (500mA(T) remains intact. There is no suppressor cap. across the mains i/p, which was my first thought. As noted, I've yet to try measuring the primary current, or taking any other measurements, besides noting that the HT Secondary DC resistance is about 40 ohms.
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Old 11th Aug 2018, 10:03 am   #2
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I would try removing the ht fuse and see whether the primary fuse still blows. If not I would suspect the output valves and check their anode current and biasing. If it still blows remove the rectifier, when they start to fail they often cause the primary fuse to blow. If it still blows it is looking like the transformer, although it would be worth removing all the valves and testing just in case there is a heater fault. If you have a lamp limiter you could use it to save fuses.
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Old 11th Aug 2018, 10:19 am   #3
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I have just measured the primary resistance of my laney 50 watt valve amp and it is 4.2 ohms ,
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Old 11th Aug 2018, 10:24 am   #4
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

In my experience usually the rectifier with these amps.
The modern Chinese AC30s do the same.
The Sovtek ones are the best replacement, in my opinion. The JJ ones are useless.
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Old 11th Aug 2018, 10:49 am   #5
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Thanks, everyone for your suggestions. B.t.w., the amp is a 1990 production version with the small components mounted on a fibreglass PCB. The rectifier is a GZ34, and the o/p valves are 'GT' KT66 x 2, although a maker's plate on the amp. chassis states that they should be 5881's, which should 'only be replaced with the same types' I won't be back in the workshop until Monday, but will follow the advice given. Martin, the message I got was something to do with the biassing, so maybe the o/p valves are at fault. Now to look for some 1.25" 2A (T) fuses!
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Old 11th Aug 2018, 10:54 am   #6
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Further to what omegaman has suggested, a shorted rectifier would be the prime suspect. If not then if the HT fuse is intact then it sounds like it is the mains tx to me? I have had to replace one with an identical fault on a 100 watter, turned out to be a shorted HT winding. Check the resistance of the those winding from the centre tap

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Old 11th Aug 2018, 11:15 am   #7
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I'll check the other parts as suggested, and also the HT winding. Looks like this could be an expensive repair, but as these amps. sell for 1000 or more on eBay, that shouuldn't be a problem.
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Old 11th Aug 2018, 11:28 am   #8
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Quote:
Originally Posted by 'LIVEWIRE?' View Post
............. I'm told that the (correct) 2A(T) fuse in the Mains TX primary circuit has blown more than once.
Yes, it is very common for the original primary fuse in this type of equipment to die of old age and for it to be replaced by the wrong type. It must be a time delay type.

However judging by the above you are already aware of that.

If you haven't got a schematic there is a link to one on this page.

https://drtube.com/en/library/schema...rshall-schemas

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Old 11th Aug 2018, 11:50 am   #9
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I'd already downloaded & printed the schematic diagram from that site, Colin. It looks fairly straightforward, but one thing puzzles me a little, which is what is the function of the semiconductor diode connected between one side of the HT secondary and the o/p valve grid bias resistors, via some r's & c's?
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Old 11th Aug 2018, 11:59 am   #10
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Just that....Grid bias...which has to be -ve WRT the cathode(s).

Lawrence.

Last edited by ms660; 11th Aug 2018 at 12:07 pm. Reason: clarification
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Old 11th Aug 2018, 5:02 pm   #11
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I've checked a few of these in the past and it's been the GZ34 rectifier. They don't seem to fare too well in these amps and I think it is because of the HT switch. When the HT switch is off (standby) and then switched on when the rectifier is fully warmed up I've seen them flash quite badly when they have to supply the current instantly to charge the reservoir cap. Valve rectifiers were never designed to operate that way. I did suggest to Marshall that if they insist on having an HT switch, they should consider fitting surge limiters in each anode of the rectifier. Whether they ever did I don't know.
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Old 11th Aug 2018, 7:45 pm   #12
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Hi , I've rewound a few of these so far. usually shorted turns on the HT sec.

Ed
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Old 11th Aug 2018, 9:30 pm   #13
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

After I've had a chance to check the amp. further, sometime on Monday, I should know what is at fault. In the meantime, Ed, what would you charge to rewind the mains transformer? New ones are around the 100 mark on different sites I've checked.

B.t.w., Lawrence, I only mentioned Bias, because I'd been given a message from the owner about the Bias, but didn't know exactly what he meant. After almost 50 years repairing electronic gear I know that grids should be negative w.r.t. cathodes. Perhaps the owner, to whom I haven't spoken personally, suspects there may be a fault in that part of the circuit.
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Old 11th Aug 2018, 11:03 pm   #14
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Quote:
Originally Posted by Omegaman View Post
In my experience usually the rectifier with these amps.
The modern Chinese AC30s do the same.
The Sovtek ones are the best replacement, in my opinion. The JJ ones are useless.
In my experience ALL Russian and Chinese rectifiers are dodgy. When I am given an amp like this to repair I insist on replacing the rectifier with a REAL Mullard, GE or RCA rectifier. If my advice isn't taken I won't honour any warranty.

Ed Dinning has it as well. Modern transformers are not as good as the origional Marshall types, but ALL Marshall's have been known to cook mains transformers. I think this is mostly due to hard thrash rock that gets played through them though.

Joe
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Old 12th Aug 2018, 6:05 am   #15
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Players like to drive them into rectifier sag rather a lot, it's why they want thermionic rectifiers in the first place. This puts an awful lot of heat into the bottle, stresses seals, outgasses everything, vaporises metal and generally takes them to the point where you can get an anode-to-anode flashover. Bye bye transformer.

The valves which survive are those which can take excursions right off the datasheet and onto the next desk.

David
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Old 12th Aug 2018, 9:49 am   #16
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Quote:
Originally Posted by 'LIVEWIRE?' View Post
B.t.w., Lawrence, I only mentioned Bias, because I'd been given a message from the owner about the Bias, but didn't know exactly what he meant. After almost 50 years repairing electronic gear I know that grids should be negative w.r.t. cathodes. Perhaps the owner, to whom I haven't spoken personally, suspects there may be a fault in that part of the circuit.
No problem, the -ve supply is due to the diode (-ve out) that you were unsure about.

Lawrence.
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Old 12th Aug 2018, 12:15 pm   #17
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Thanks, Lawrence, for explaining the function of that semiconductor diode. The diagram, t.b.h., isn't that clearly drawn in some ways. Maybe that's why I had difficulty working out what the diode (4000x?) was there for. Speaking of diodes, a note on the diagram states that, in some cases, 2 x 1N4007s are used instead of the GZ34, but only in amps fitted with EL34's. Having looked at the data for '34s,, 5881's, and KT66's I can see no reason why 4007's (or other semiconductor diodes) shouldn't be used with any of the 3 o/p bottle types. If necessary 3Amp diodes (1N540x) could be used, with appropriate series resistors, of course.
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Old 13th Aug 2018, 11:38 am   #18
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Referring to Sideband's comments in post #11, what value & wattage of surge limiter resistors would he, or any other member, recommend? 100, 150, or 220 ohm 5 watt Maybe??
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Old 13th Aug 2018, 12:17 pm   #19
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

The EL34 was apparently designed to withstand unusually high HT voltages for its class, there's even circumstantial talk of its having been a spin-off from line output valve design- perhaps it's better able to withstand the initial over-voltage from solid-state rectifiers before significant HT current is drawn. Part of the problem with the whole 6L6/5881/KT66 family of valves is that they were so popular that different variants appeared over the years with increasing anode dissipation and HT voltage ratings. 6L6 variants in particular gave range to lots of suffixes that need to be followed carefully, limiting 5881 specs might also have been a moveable feast between US industrial spec. originals and inexpensive Russian substitutes with similar caveats applicable between GEC and Far Eastern KT66s- Marshall might have been safeguarding against lower-rated versions being substituted, given that musicians' amps worked their bottles hard anyway and were bought to be thrashed!
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Old 13th Aug 2018, 2:24 pm   #20
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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Originally Posted by 'LIVEWIRE?' View Post
Referring to Sideband's comments in post #11, what value & wattage of surge limiter resistors would he, or any other member, recommend? 100, 150, or 220 ohm 5 watt Maybe??

You might find that 50 ohms (47 ohms preferred value) is sufficient. I wouldn't go higher than 100 ohms or the HT might be reduced too much and limit the output...and I'd use 10 watt types.
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