ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Re: replacing components on old amps

:

8/15/1999 4:33 AM

Re: replacing components on old amps

Mook:

With pliers (after draining filters)...

I just wanted to add that when you are replacing the caps in the power supply their charge can build back up if there is no load on them (as there would be if they were still connected to the rest of the amp). If they are already discharged you can clip a regular jumper between the positive and negative leads to keep them from charging back up...

Steve Ahola

8/20/1999 7:20 PM
Mook	Yes, I always use jumpers. I use 1 jumper (with a 7.5K 10 watt resistor) for each filter. Mook

8/20/1999 9:54 PM
SpeedRacer	fwiw, I leave the amp "on" (unplugged of course) and I've never used those draining jumpers.. also never had a cap (even brand new spragues) build up more than 20 or so VDC on me during mods and repairs. Anyhow have caps really come back at them? Just curious.

8/21/1999 4:55 AM

Speed:

Anyhow have caps really come back at them?

I've just gotten some sparks when touching the tip of a grounded soldering iron to a filter cap that had been partially disconnected from a circuit. They weren't big snaps but I was glad it wasn't me that they were going through...

As for just leaving the power switch and standby switch on for a minute or two (with the amp off) don't tell the newbies, but I think that is good enough at least 95% of the time... (unless you are working with the power supply). There should be enough of a load in there to drain off the typical filter caps. But I'd hate to see someone get shocked because they didn't use a resistor/jumper to drain off the charge.

Steve Ahola

P.S. I think it was Andy Ruhl who got the shock of his life just retightening the tube pins on an amp that hadn't been drained sufficiently...

8/21/1999 7:42 PM
R.G.	Electric power linemen have a saying that keeps them alive - "If it's not grounded, it's not dead."

8/21/1999 8:09 PM

R.G.

For large power lines, even if the power source is disconnected, they can pick up voltage just by running parallel to the live lines. When my father was growing up on a chicken ranch in Petaluma (no, not the Mesa Engineering facilities!) they ran a wire parallel to the nearby utility lines, which picked up enough voltage to power the lights in the henhouses...

When I've gotten lazy and did not properly ground out the power supply to one of my amps, it's only because I've worked on them enough to know which wires and components to avoid. But even then I was taking a risk that wasn't necessary ("why tempt fate?"). I got the shock of my life when I was 15 and decided that I wanted to make a guitar amp out of an old tube hi-fi: "But it wasn't plugged in- how could I get a shock?" After that experience I put that old hi-fi in the garbage because I knew I was dealing with forces that mortal man was never meant to handle... (Back then the only "net" I knew about was tied to the badminton poles in the backyard so I couldn't log on to AMPAGE to read about the proper procedures for discharging power supply caps.)

Steve Ahola

8/22/1999 9:16 PM
Bob	I've always wondered why this BBS lacks the mean spirited personal attacks you see on some other boards, everyone seems so congenial, now I know the answer... self administered electroshock therapy ;^)

Page 2 of 2