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.