I would like to live to hear the change...

T

TheRip

New member
So I recently aquired a cheap JCM 900 Model 4500 (I'd like to keep it, so dont tell me to sell it), and I've read about a couple simple mods that can be done to the lead channel. But, for that, and to bias it, I'd have to open it up and get to the electronics...here comes my question: I've seen a video on how to open a JCM 900, and the guy in the video pulled the chassis out by holding it on the transformers, and he only adjusted the bias, so no cap discharge. Though, he had a mercury transformer built into it, and it looked insulated or at least painted...mine are bare metal. Is it safe to grab the transformers with your bare hand to get it out to discharge the caps, even when they are bare metal? And when the caps are discharged, is it safe to touch anything in the circuit/solder on it? I've read about confirming the discharge via a multimeter, how would I measure it? Volt setting and then black to - and red to + on the caps and check for 0V?
I plan on taking the discharge method of connecting pin 1 of a preamptube to ground, is it heavily discouraged, or somewhat safe?

So, this ended up being a novel, but hey, I'd like to live to hear the changes! I'm totally fine with a yes on everything, or pointing out no-gos, you dont have to answer all of my questions seperatly :)

Also, I'd like to mention that I'm fairly confident in my aquired electronic skills, but that's just my first tube amp, so I don't have experience where I can touch it while the caps are charged...

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Re: I would like to live to hear the change...

Yep. The only things you really dont want to touch are the filter caps and the tube pins.
Also...don't mod it. Keep it stock and choose some excellent speakers for it to run thru.
Just put in some strong new tubes and bias it.
You will have a lot of fun.
 
Re: I would like to live to hear the change...

Ok, thanks for the confirmation! Well, new tubes for sure, and I should probably do this first, but I cant wait for the beginning of next month as always^^
But, I'm at least gonna disable the hardwired second tonestack for the B channel and bridge the C27 to get channel B sound more like channel A, which is a MUST due to the shared EQ...
Also, since we're on it, the powertubes have already been changed to TAD 5881s which sound fantastic compared to the ToneBone TAD EL84 which I got with the amp for free...but the three preamp tubes are Marshall tubes, and I dont want to know how old those are :S
Is mixing different tubes/manufacturers a good idea? Due to money and availability over here in germany, I though about a TAD 080 in V1, a EHX 12AX7 in V2 for the FX loop and a TAD RT008 in V3 for the phase inverter stage...or should I go ask in a seperate post?

At the moment I cant consider changing speakers, though that WILL be necessary soon, this crate box seems to be the cheapest junk you can find on the internet, but hey, I got a mildly used 95' JCM 900 with a 4x12 and wierd tonebone stuff for 400 european bucks, so he...
But I'll try my laney combo's celestion G70-whatever at some point when I get the amp back to my house.
 
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