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Old 14th Aug 2018, 10:18 pm   #41
'LIVEWIRE?'
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Default 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!
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Old 14th Aug 2018, 10:35 pm   #42
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Default 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!
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Old 14th Aug 2018, 10:47 pm   #43
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Default 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.
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Old 14th Aug 2018, 10:48 pm   #44
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Default 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!!
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Old 14th Aug 2018, 11:32 pm   #45
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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Originally Posted by Boater Sam View Post
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.

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.

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Old 15th Aug 2018, 5:39 am   #46
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Default 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.

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Old 15th Aug 2018, 9:14 am   #47
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Default 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.
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Old 15th Aug 2018, 9:22 am   #48
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Default 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.
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Old 15th Aug 2018, 9:33 am   #49
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Didn't think about it that way, Tony!
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Old 15th Aug 2018, 1:21 pm   #50
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Default 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.
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Old 15th Aug 2018, 1:44 pm   #51
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Also BY127 and BY133 for increased PIV over 1N4007.
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Old 15th Aug 2018, 1:56 pm   #52
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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Originally Posted by 'LIVEWIRE?' View Post
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.
Now to be completely radical, why not do both, in series with and parallel with the GZ34, big value resistors on the parallel ones to severely limit the current passed?

Is there ever a reason for putting valve rectifiers in series?
Parallel was used in some TVs.
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Old 15th Aug 2018, 3:22 pm   #53
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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So far, I haven't found a source for 1N4008's, but I have a number of 4007's.
I have seen 4007s used and also the fatter 1N5408s and faster UF5408s.
A url with diagram:-
http://dynacotubeaudio.forumotion.co...fier-diode-mod
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Old 15th Aug 2018, 6:40 pm   #54
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I'll bear that in mind, and maybe use UF5408s
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Old 15th Aug 2018, 10:14 pm   #55
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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Originally Posted by turretslug View Post
Also BY127 and BY133 for increased PIV over 1N4007.
Also, if I remember correctly these diodes are much faster than the 1N4XXX series. Also they don't create as much noise as the 1N4XXX series

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Old 16th Aug 2018, 8:52 am   #56
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I'd forgotten about BY127's & BY133's, which, IIRC, were/are basically the successors to the BY100 'top hat' rectifiers. In a circuit diagram for a ST-70P PSU 4 x MUR4100 diodes are used, two in series in each 'leg' of the transformer secondary, ahead of the GZ34 anodes. Never heard of the MUR4100, but it presumably has a lower PIV than 1N4007, etc., hence the need for two in series. No doubt I'll opt for the UF series or the BY1xx.
Incidentally, as an 'old hand', I always use the term P.I.V. What's the difference between that and V.R.R.M., which has now been in use for some years?
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Old 16th Aug 2018, 11:18 am   #57
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I haven't yet found a circuit diagram of the JTM45 showing which, if any, of the suggested mods were fitted by Marshall themselves. Anyone know??
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Old 21st Aug 2018, 1:11 pm   #58
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Having found some GP30M 3 amp diodes, I may use those, but have some UF5408s on order from CPC, so, for the moment I'm keeping my options open.
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Old 23rd Aug 2018, 8:45 am   #59
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I now have some UF5408 diodes, so will fit these. The 2k2 wire-ended 10watt resistors are quite long (about 1" + wire lead-outs, so fitting them might be a bit problematical, given that they will run at least warm. There isn't a lot of space between the mains Tx. and the rectifier valve base, besides which the primary circuit fuse-holder is fitted directly underneath the GZ34's socket . I still haven't found a Circuit diagram showing which, if any, of the suggested mods. were ever factory fitted, b.t.w.
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Old 23rd Aug 2018, 12:38 pm   #60
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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I still haven't found a Circuit diagram showing which, if any, of the suggested mods. were ever factory fitted
The advice given regarding modification to the rectifier, though sound, isn't necessary in my opinion. Personally I'd fit the new GZ34, bench test it for a few hours, monitoring the HT current and so on, then button it up and give it back.

I can attest that Marshall's (and indeed other brands) do occasionally pop their mains transformers, but this is the exception rather than the rule in my experience. I know some on here will say better safe than sorry, but on the other hand manufactures also know a thing or two about design and if they dont deem a change necessary then I'd go with that in most cases
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