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.

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.

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 ,

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.

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!

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

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.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

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

/

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?

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

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

Lawrence.

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.

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

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.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

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

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

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

No problem, the -ve supply is due to the diode (-ve out) that you were unsure about.

Lawrence.

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.

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??

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!

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

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.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Thanks for the info, SB. In the meantime I have found, by following the good advice given by various forum members, that the fuse blows due to the GZ34 flashing over after 30 seconds to 1 minute after switching on the amp, with the HT fuse removed and the 'standby switch 'off'. I now have to get a new GZ34. I note that Howard (omegaman) recommends Sovtek rectifiers, whilst Joebog1 says he only uses genuine Mullard, RCA, or GE ones. Which to get? I've left a message that a new GZ34 can be had for around £20, but have also seen them at prices up to and even over £100. A firm in Kidlington, not far from where I live is selling Golden Dragon GZ34's for £18 + P&P so, maybe I'll get one of those. Needs thinking about!!

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

You could take a look at Watford Valves (watfordvalves.com), they also do capacitors and other amp components.
Langres.co.uk also worth a look.
Cheers
John

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

I'll have a look in my stash later. I may have one.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Sorry it's a 5V4 that I have.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Thanks for looking, anyway, SB. John, I've checked out Watford Valves, Langrex, and Cricklewood Electronics, all of whom stock GZ34's. Watford stock a Cyrogenically trated version, which is claimed to offer better performance. In a way I'm spoiled for choice, and the highest price of any I've checked is £25., so I'll have to make up my mind.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

It's your call of course, but I wouldn't use a cryo-treated one. There are people who believe that they sound better, and belief is a very powerful thing. But it's hard to see how, objectively, this process can make any positive difference. And if the very unusual thermal stresses should happen to micro-crack the glass-to-metal seals that will weaken a valve which needs to be at the top of its game, so to speak.

My 'test' GZ34 is a Golden Dragon one which is cheap enough that I can afford to risk it (unlike the Mullards) yet robust enough that it's run for quite long periods in quite a few different amps, some of which had faults which might well have overloaded it. Of course I could just have been lucky with this one.

Cheers,

GJ

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Incidentally I think the data for GZ34 recommends 100 ohm limiters in each anode.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

I have just downloaded (from the Marshall secure website) the JTM45 schematic, dated 8/9/99.

According to this, F1, the primary mains fuse should be T3A (not 2A).

The plot thickens - how quickly does the 2A fuse blow?


Kevin

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

All I can say is that the fuse in the amp I'm in process of repairing was a 2A(T) one, and in the circuit diagram I downloaded it iis shown as being 2 Amp. Maybe it was changed during the production run, which, if I understand correctly lasted almost 30 years. I didn't time it precisely, but the rectifier flashed over, blowing the fuse in no more than a minute

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Is there a mains filter capacitor connected across the mains?
If so check that has not gone S/C. You can simply clip it out for now.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

EL34/6CA7 from eastern countries are the same as their rectifiers. In Australia we made quite a few amps using EL34/6CA7, and even the origional Mullards fried when used at "maximum" ratings. Thats 775 volts on the anodes, and 400 volts on screens, 11,000 ohms plate to plate load. 100 watts was the supposed output from a single pair. The amps like this fried regularly, usually taking out the output transformer with it.

As far as using silicon rectifiers, the latest Fender AND Marshall amps I have worked on, have valve rectifiers with silicon diodes strapped across them as well. WHY ??? I dont know. They do appear to be a bit more reliable though. Sovtek and Svetlana I put into the same bag as my first post. Golden DRagon are Chinese and I havent used any. Groove Tubes are also in the same boat, i.e. Chinese and have a name for being very noisy. Their rectifiers in particular giving the amp various popping and clicking noises which can be very annoying in the studio.

Joe

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

stick a new GZ34 in,you'll be ok I'm sure. if there's any doubt, Marshall will happily talk to you about the amp, they're that sort of company.

Guitar players a re a funny bunch, so replace the valve rectifier with diodes at your peril! Apparently, a sagging rectifier adds some sort of creamy distortion or other rubbish.

Wonder if you could fit a surgegard in the ht line to prevent rectifier stress. Even a good one will flash if you operate the standby switch. Silly circuit.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

That clears the fuse of any culpability. If the rectifier flashes over, you want the fuse to blow to save the mains transformer.

The question switches to why did the rectifier flash over?

1) Fault in the rectifier valve

2) Too much current demanded of it, EG HT line short or demand from amp beyond its capabilities.

3) Too much voltage on rectifier - especially with HT switch off.

4) Mains transient. Transformer saturation acts to limit these, either way the fuse gets it.

David

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Partly related to this topic, when I built my Argus TV's I initially fitted it with a Russian made 5U4. When I initially powered it, the tube flashed over and I thought I might have had a circuit or component fault. But after comparing it to an RCA 5U4 and GEC U52, I found it was the Russian 5u4 that was prone to it. So I tried another, the same. The internal construction of the rectifier looked good otherwise with springs to support the filament.
I have always found GZ34's to be a very good rectifier and they drop less forward voltage than a 5U4 and run cooler for the same current.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

It's true that the GZ34 is a very good rectifier. However it took Philips/Mullard a while to achieve that. The datasheet http://www.r-type.org/pdfs/gz34.pdf is very telling. The first two graphs show the forward voltage drop as a function of current through a single diode. "Hang on" you say "Why are there two graphs for one parameter ?". The answer is in the bottom left hand corner of the page - it's the date. At the start of 1954 if you drew 100mA through a GZ34 you'd expect a forward voltage of more than 15V. By the middle of 1958 that had dropped to just 8.5V. The change at 400mA is nearly as impressive - from more than 35V in 1954 to less than 22V in 1958.

Interestingly guitarists will pay a king's ransom for 'metal base' GZ34s - ones like this http://www.jogis-roehrenbude.de/Roeh...by-Amperex.jpg. I believe these were only made in the very early days and perhaps it's the poorer performance (=increased sag) that they're looking for ?

Cheers,

GJ

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

The flashing over a short while after switch on seems to be common.
In my experience, the Sovtek GZ34 is the best value for money and has a reasonable longevity. True, a genuine Mullard or GE type will last and work the best, but your average guitar player won't pay the eye watering prices usually charged for one! The JJ ones are poor and I've had several that flash over straight out of the box! Swapped for a Sovtek and no problem. The Chinese ones are actually very good. These can be often branded TAD (Tube Amp Doctor) but are actually Shuguan valves.
Best price for Sovtek ones I've found is Hotrox in Nottingham (No connection).

I'd stick with a T2A mains fuse.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Agreed it's not desirable. However there are many who think that the valves should be heated fully before HT is applied hence the 'standby' switch and 'the musicians want it' according to Marshall. The poor GZ34 pays the price for this 'desirable' feature.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

A more desirable implementation of "standby" might be for a double-pole switch that simultaneously biases output valves to cut-off and applies compensatory dummy-loading to PSU. (Taking it as granted that heaters need to be left alight for quick resumption of play.) That would introduce a fair bit of extra complication and expense, not to mention switchery rated for keeping +500VDC and -100VDC apart without creepage. It seems that the standby is a "must have" feature in the market, so the rectifiers are stuck with the "oh cripes" hit every now and then, it's difficult to break the "we've always done it like this" syndrome.

I expect that the GZ34 underwent the same evolutionary improvements to reduce forward voltage that marked the contemporary PY32-PY33 development. I gave away quite a lot of NOS CV1377 to folk in years gone by. Oh well, hope they were appreciated.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Further to the mains fuse rating topic. On the amp. I'm repairing it states 2A(T) with 220-240v supply, and 3A(T) if run from a 110vAC supply.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Risking ridicule, a suggestion.

Put a silicon diode 1N4007 in series with a resistor, say 2K @10w across each anode to the cathode of the GZ34 to advance the HT voltage rise before the GZ34 conducts the surge.

If that is heresy or stupid, just say.

It would be ideal if we could come up with a simple HT delay circuit when using silicon diode rectifiers. Using time delay and relays seems a bit archaic.
Sam.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

I've taken the plunge and ordered a PM GZ34 from 'That's Audio' in Kidlington. For a little over £20, including P&P, it seemed a good buy. It should arrive by the coming weekend, so next week I should be able to get the JTM45 up and running again. I may try as you suggest, Sam, if there is room under the chassis to fit 2 x 2k 10watt resistors as well as 2 x 100 ohm 10 watt surge limiters, assuming both mods. would be needed. I don't consider such mods. heresy, but then I'm not a guitarist!

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

When I rebuilt my Linear Conchord amplifier I followed a hint and added a 1N4008 in series with each GZ34 anode. The idea is that in the event of flash-over in the GZ34, AC does not reach the reservoir cap so no fuse/cap/transformer problem.
This technique is also used in a Fender amp. You still get the desired "sag" but of course the HT is reduced by 0.7 volt!

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Referring to Tim's comment in post #30, I did say at the start that there is no mains filter cap. in this amplifier. This was the first thing I looked for, though from my, admittedly limited knowledge of Marshall Amps. they don't fit mains filter caps.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

George, I don't think 0.7v. is going to be missed from a 350v HT Line! After all it's only 0.2% of the total HT!!

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

As Sam has said, modern Fender and Marshall amps have those diodes there already, BUT without the resistor in series. That was what I was talking about in my previous post. It does make the amps more reliable, I guess because the GZ34 doesn't really do anything except keep the amp cabinet warm.

Joe

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

The GZ34 does do something. It's an awful lot faster turning off than a 1N4007/1N4008 diode. So the GZ34 will get all the voltage across it for a number of microseconds when the 1N4007 allows through current in its reverse direction. This won't be enough time for the reverse voltage on the GZ34 to build up to anywhere near its peak value.

The silicon diode takes the worst of the stress off of the thermionic rectifier, and the thermionic rectifier covers for the problems of these very slow silicon diodes. Symbiosis!

I normally try to talk people out of using the higher voltage members of the 1N400x family because of the RF hash they can generate, but in this case they should be OK.

David

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

George(buggles) says 'put a 1n4008 in series with each anode of the GZ34, whilst Boater Sam says fit 1N400x diode in series with a 2k resistor across the rectifier valve, i.e. from each anode to the valve's Cathode I can se what the latter would do, but am wondering why dropping 0.7v out of 350v or more would make any difference.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

I think the idea of the series diodes is that if the GZ34 flashes over, they will prevent a dead short across the HT secondary winding of the mains transformer. And keeping the GZ34 in-circuit presumably allows the HT to sag under load which some users like the sound of.

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

Didn't think about it that way, Tony!

Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
 

So far, I haven't found a source for 1N4008's, but I have a number of 4007's.