Marshall tube ampilfiers

[robbo1172]

Hey all, there's some smart cookies on this forum so thought I'd ask if anyone has a knowledge of the inner workings of Marshall JCM900 amplifiers. I bought one about 15 years ago and hardly used it. About 5 years ago I modded it to use EL34 power tubes instead of the 5881 ones and removed the LED clipping diodes. I can't recall where I got the info from to do it, but I did it and got it working. First gig with it was Duck Creek Races and it sounded great for the first 3 songs, then it sounded silent... (all good, was able to plug my effects board direct to the FOH which saved the day).
Anyway, haven't touched it since until last weekend when I cracked it open to see what was going on. The output fuse had popped so I'm now trying to work out what I have done.

So if anyone has a knowledge of these things, I'd love to hear from you...

Robbo

[hoodoo]

Hi mate, a bit bizarre that my first post on a camper trailer forum has nothing to do with camper trailers  but..........DANGER Will Robinson DANGER. Amps store charge that can kill you even when unplugged and turned off. Capacitors need to be drained before touching the insides of any amp. I don't think anyone could/should, give you advice,  on a forum, especially a camper trailer forum, on how to fix something you have modded , without sighting the amp, and if they do, be wary. Take it to a pro mate.

[speewa158]

1st Welcome hoodoo to the Maddness of MySwag , Strap in for the ride of your life       
robbo1172  Whats this rave on about Marshalls Portable Music  Machine   , positively  absolutely the best you've  ever seen  lts been done  before                     

[GeoffA]

......Marshalls Portable Music  Machine   , positively  absolutely the best you've  ever seen......

You're really old......

[robbo1172]

Hi mate, a bit bizarre that my first post on a camper trailer forum has nothing to do with camper trailers  but..........DANGER Will Robinson DANGER. Amps store charge that can kill you even when unplugged and turned off. Capacitors need to be drained before touching the insides of any amp. I don't think anyone could/should, give you advice,  on a forum, especially a camper trailer forum, on how to fix something you have modded , without sighting the amp, and if they do, be wary. Take it to a pro mate.

Welcome about Hoodoo. Thanks for the heads up but as an electrician turned electrical / electronics engineer I'm OK with this stuff. It's the tube stuff that I'm just learning about... I do appreciate the reminder though.

I'm slowly getting to the bottom of it all, I think the most glaring mistake was putting in 2.2ohm resistors in the screen grid instead of 2.2kohm... (how embarrassing)...

Speewa "Whats this rave on about Marshalls Portable Music  Machine   , positively  absolutely the best you've  ever seen"

What a cracking song, released the same year I was born, hmm, that means I'm turning 43 this year...

[dales133]

Hey robbo on a side note you may be just the man ive been looking for.
In singapore i brought some realy nice japanese retro bakerlite nobs that i want to wire up for an electrical pannel in my camper.
My question is i dont know anything much at all about switch ratings and stuff but i need some 12v on off rotary switches with 7mm spindles.
I could also maybe use push on off dimmer type or just straight push on off that fit these nobs.
Got any clues?

[hoodoo]

Thanks for the welcome, great site. Bought a customline a couple of months ago and loving it. Sorry I assumed you didn't know what you were doing mate 

[hoodoo]

A good site wth lots of info is amp garage mate.

[robbo1172]

Hey robbo on a side note you may be just the man ive been looking for.
In singapore i brought some realy nice japanese retro bakerlite nobs that i want to wire up for an electrical pannel in my camper.
My question is i dont know anything much at all about switch ratings and stuff but i need some 12v on off rotary switches with 7mm spindles.
I could also maybe use push on off dimmer type or just straight push on off that fit these nobs.
Got any clues?

To be honest, all of my work is in industrial stuff, mines, cotton gins, water treatment / pumping etc, so I wouldn't even know where to start.

I take it you're the same dales133 from woodworking forum?

Robbo

[dales133]

Yes i am,
I just saw the electrician  turnened electrical engineer and thought you may.
Nice to make your aquaintance somewhere else

[Symon]

If you sign up to the overclockers forum, there are some very knowledgable people in the electronics and electrics subforum.  Worth a try - http://forums.overclockers.com.au/

[robbo1172]

Thanks Symon

I joined up to Marshallforum and got some advice.

I replaced the screen resistors with the correct size ones, biased it up and it now lives.

Pretty happy with the result but also enjoying learning more about tubes and tube amplifiers.

Robbo

[Troopy_03]

Pity you hadn't posted something about 2 years ago before I gave away approx 500 to 750 assorted valves that I'd been keeping for when I got into valve amps. KT66's, KT88's, EL34's and stacks of various other stuff.. I did my radio trade training in valve theory many years ago.

[robbo1172]

Pity you hadn't posted something about 2 years ago before I gave away approx 500 to 750 assorted valves that I'd been keeping for when I got into valve amps. KT66's, KT88's, EL34's and stacks of various other stuff.. I did my radio trade training in valve theory many years ago.

It's always the way, isn't it

I probably won't find time for a few years yet, but you never know.

Robbo...

[HKB Electronics]

Hi Robbo1172,

Without looking at it would be hard to say could be anything, when you say output fuse where is
the fuse in circuit, is it in the speaker path?

Do you have a schematic for the unit?

Might be easier to take it to a TV repair or alternatively the local music store as they would have
a repairer contact for their amps etc.

[robbo1172]

Thanks HKBE. When I changed from 5881 output tubes to EL34, I was supposed to change the screen resistors from 470 ohm to 2.2k ohm. Unfortunately I missed the k and put in 2.2 ohm. This was too much current in the screen circuit and the fuse failed. Good thing it did. I gave it s bias and away it went. Sounds alright to me too

Robbo...


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