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Old 2007-05-02, 17:25
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Jopop Jopop is offline
Senior Metalhead
Join Date: Mar 2006
Location: Norway
Posts: 398
I have lots of tone mods and the like available to amps. Thing is, an amplifier is seriously dangerous business - you don't want to poke around in one unless you know what you are doing.

This is not a mod - this is required reading before you unscrew the first chassis screw.


Rule 1: There are no exceptions to any rules described in this document.

By no exceptions, I mean no exceptions. Even in the case of "oh damnit i forgot to solder on that little point, It'll just be a brief second" the rules apply. No matter what, you need to obey all the rules described in this document.

Rule two: Think of the power switch as faulty.

The power switch is not a good enough barrier between you and the AC mains, unplug the cord going into the wall or into the amp.

Rule three: Unplugging your amplifier from the wall does not mean it cannot kill you.

An amplifier's power supply contains fairly large capacitors with pretty high voltage and max current storage capabilities. You need to properly discharge these before you can start working inside your amp.

Rule four - Put one hand in your pocket when working on an amp that might carry high voltages.

If you should get burned on your one hand the worst thing that might happen are minor burns. However, if the current flows from one hand, through your heart, and out your other hand, there's a possibility you might die or you blood might go acidic. The chassis of your amp is actually connected to one end of the power supply (reference, usually 0 volts) or ground (or in most cases both). Having one hand touching the chassis while the other reaches in to measure some voltage while the amp is "live" or discharging capacitors with one hand on the chassis and the other shorting the caps can be lethal.

Rule five - Don't work on an amp together with other people that might distract you.

People who ask "What're you doing? What does that do?" might break your concentration just enough to slip your tools somewhere they shouldn't be, causing damage to you or your amplifier. Don't talk on the phone either. Stay focused.

Rule six - Things might blow up, protect your eyes and face.

Capacitors, diodes, resistors et cetera might blow up if you use the wrong value component or wire it wrong. This is just like setting off a small hand grenade, it might make you blind. Close the chassis or wear glasses that cover your eye when you power up your amp. Don't sit with your face unprotected over it.

Rule seven - Solder might drip.

Wear pants long enough to cover your socks, and wear socks. And, at least wear a t-shirt. Solder dripping on your knee or feet not only hurts, but leaves permanent marks as well.

Rule eight - Don't wear metallic objects.

Off with jewelry, rings and watches and other metal stuff on your hand. Also, don't wear a necklace that might fall down into the amp shorting stuff. Piercings in your face, ears, or on your body other than your hands is okay.

Rule nine - Use your common sense, don't do anything stupid.

This rule explains itself. Don't put your coke bottle on top of your chassis while working in there, stuff like that.

Rule ten - If in doubt, ask.

Don't assume something will be okay, maybe it won't. No question is stupid when it comes to safety.

Well that should be a pretty damn comprehensive safety rules list.

How to decharge capacitors

1. Make sure you followed the safety list from top to bottom.

2. Open up the amp. Wait 10 minutes or so before doing anything else.

3. Locate the power supply capacitors. They are cylindric in shape, and have two or more ("multi-section" capacitor) leads going out from the "shell". They're generally one color with a black, grey or white stripe or arrow pointing towards one of the leads. Looks like this:

As you can see there might be one lead in each end or several coming out from one end.

4. Drain the caps. There are several ways to do this. You could either short the capacitor, by deinsulating a wire a little bit in each end, and connecting one lead on the capacitor to the other lead (REMEMBER RULE FOUR). There'll be a small pop and a flash, but it's nothing to worry about. Leave it there for 10 seconds or so, then go on to the next one.

The other way (which i prefer) is connecting a switch and a 50K resistor between a preamp tube's plate and ground. Solder a wire onto pin 1 on a preamp tube socket.

Here's the pinout for 12A*7, ECC 8* and so on:

Solder the other end of the wire to one leg of a 50K 5 watt resistor. Solder the other resistor leg to one lug of a simple SPST switch. Remember rule four. These are like $0.1. Now solder a wire to the other lug on the SPST switch. Solder the other end of that wire to a ground point, or solder it / screw it down to the chassis (but not a painted area, paint is not a good conductor). And remember rule four. Now anytime you'll be working on your amp, just turn the switch on, wait 5 minutes, measure the voltage over the 50K resistor, and if it's under 10 volts or so you're good to go. If not, wait a little while longer.

Remember to turn the switch off again before testing out any modifications. Leaving it on won't harm anything, but it'll decrease the plate voltage on one triode (creating a voltage divider), meaning shitloads of distortion and (in most cases) bad sound. Leave the switch engaged while working inside the amp.

Make sure you get a five-watt resistor, they're quite a bit sturdier and won't break as easily. A two-watt or even less should also be fine in all cases but the 5-watt is best anyway, and is not that much more.

Now start modding

List the mods you want explained, i.e. more/less gain, more/less bass, more/less treble, less buzzy tone, darker / brighter tone, more low mids, more high mids, tighter bass, more clarity or similar, if i know how to do it I'll try to explain.

Proud member of the "$20000 worth of pro gear but can't play worth shit" squad
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