Discharging Filter Caps, the Gerald Weber way

[the_crowing]

Hey guys, reading Gerald Weber's "A Desktop Reference of Hip Vintage Amplifiers", I read about his method to discharge filter caps, but I haven't seen it recommended or even talked about anywhere else. He says: While the standby switch is in the play mode, use a simple jumper wire from the plate of any preamp tube to the chassis, then from the plate of any power tube to chassis, waiting a few seconds for each. The electricity should be drained.

Anybody ever used that method? Is it simpler/safer than the "probing method"?

 

[Zaphod B]

Nope, I've always gone by the "unplug the amp before I go to bed and mess with it in the morning" technique.

 

[boingoman]

It works fine. You start with the preamp tube because they have plate resistors that will drain the caps at a less-than explosive rate. If you hooked a plain wire from the caps to the chassis when the caps were fully-charged, it could be pretty exciting!

"A few seconds" seems scarily vague. Meter the caps before you move to the power tubes.

 

[dementedchord]

sustain a chord and switch it off... IF there's any left just short it...

 

[ggunn]

sustain a chord and switch it off... IF there's any left just short it...

I don't think that playing as the amp goes off is a reliable method. When the heaters cool down, the tubes stop conducting even if there is charge left on the caps.

 

[stevieg318]

I have a couple of 25 0hm 10 watt cement resistors in series. I have an alligator clip soldered on one end and hook that end to the chassis. The other end I touch to the filter side or the standby switch or the + side of the filter caps. It will drain the caps in a couple of seconds. I always measure them before working in the amp to make sure they are fully drained.

 

[dementedchord]

I don't think that playing as the amp goes off is a reliable method. When the heaters cool down, the tubes stop conducting even if there is charge left on the caps.

they dont cool off that quickly!!! and there's not that much current either... and i also suggested shorting what was left... i worked on them professionally for like 18 yrs and ive been bit a few times but never from a guitar amp when i did this.... i did get thrown across a room one time working on an old hammond amp that was live... 600V.... yee haaa.....

 

[timthetortoise]

A small bleed resistor between cap and ground has never failed me. Don't know about grounding the plate, especially in a grounded-plate design. Even with a "fail-proof" method, it's always best to check and make sure.

 

[JCH]

This is what I use. There's a 100k ohm resistor on the end of the chopstick.

 

[gbdweller]

This is what I use. There's a 100k ohm resistor on the end of the chopstick.

That's a pretty slick little tool. I'm gonna go make one!

 

[brianbicknell]

Does the alligator clip just connect to chassis? Newb question! Also, has anyone done an isolation transformer install on a Supro Bantam?

 

[drumslinger]

Yes, the alligator clip connects to the chassis. I like to use test leads with alligator clips on the end. I cut the test lead in half and solder a 100k or 150k resistor between and cover it up with heat shrink. One side clips to the filter caps and the other clips on to the chassis. Drains in seconds. **Word of caution, DO NOT FORGET TO REMOVE THE CLIPS WHEN POWERING UP THE AMP**. I like the chopstick one, gonna have to make one of those.

 

[brianbicknell]

Thanks! I've seen the screwdriver method, but sparks scare me. I think your design and the chopstick one are neat, will definitely try. Drumslinger: any ideas re: supro bantam? lots of the web stuff is contradictory and I was just quoted 60 dollars to install the iso transformer....seems steep!

Yes, the alligator clip connects to the chassis. I like to use test leads with alligator clips on the end. I cut the test lead in half and solder a 100k or 150k resistor between and cover it up with heat shrink. One side clips to the filter caps and the other clips on to the chassis. Drains in seconds. **Word of caution, DO NOT FORGET TO REMOVE THE CLIPS WHEN POWERING UP THE AMP**. I like the chopstick one, gonna have to make one of those.

 

[drumslinger]

brian,

Never modded the bantam but i do know of them. Transformerless amp, two prong plug, death cap and voltage going straight from the wall to the tubes. The amp needs modding to make it safe hence the 1:1 iso transformer but you will also need to have a 3 prong (grounded plug), death cap removed and fuse wired to the transformer for extra safety. **Don't trying playing the amp barefoot on concrete unless you have a death wish**. If it's $60 for the whole modding with parts then it's a decent price AND he can give it a good look over for any potential hazards.