I'm following it. Just got very busy and this is the first chance I had to get back here since soon after the initial post. I do appreciate all the comments so far. I love this site, and hope one day to join the DIYers!
If you are a newbie, just turn off the amp and wait 15 minutes, unplug, check the voltage across the caps before working. Really if you do everything right the first time, you never need to re-work. Also I have had bad near death experiences with cheap DMM's, I believe if you are building a tube amp a FLUKE DMM is the only way to go. If you don't believe me check 350+V with a $10 DMM set to resistance, the thing will explode!
Last anything your doc prescribes you like muscle relaxant, etc, don't take while working on the amp.
Last anything your doc prescribes you like muscle relaxant, etc, don't take while working on the amp.
I have actually modified a mains plug so it has only the earth pin left, and only one wire out. I bolt this earth wire to the chassis. That plug stays in the wall socket all the time; AC power can then be removed at leisure without losing chassis earth.
Can a typical 100uf-1000uf SET PS cap hold enough charge to turn a chassis into a HV electrode if you don't do this?
Hi
Another good rule when working with HV is to NEVER use rings in your fingers! can be dangerous!
Yeah, they pop pretty good when your wife ohms the wall outlet.
Cheap multimeter $4
New wall outlet $1
The look on Sherri's face when she sticks the probes in the outlet, then switches on the meter (you must pass ohms to get to AC V) PRICELESS
Mentioned in the safety thread on my web page. In my case the engineer lost his finger and the voltage was only 5 volts!
Yes, and they can still keep the caps charged up and make the rest of the circuitry potential HV Electordes if the caps haven't been discharged.
Some power supplys incorporate safety resistors across the cap terminals to discharge the cap when power is removed. It takes 30 seconds to several minutes depending on the RC time constant.
Some power supplys do NOT incorporate this feature. Add it if if yours does not.
Yes I understand that but I thought you were saying that an unplugged tube amp could have the entire chassis brought up to HV from caps discharging to the star ground. I have never seen this as being an issue.
The chassis will be at negative B+ potential (e.g. -400V) if the filter caps are charged (e.g. no bleeders) *and* there is no connection from chassis to safety ground (AC cable removed) *and*the B+ terminal inside the amp is then shorted to safety ground on your bench.
This creates a partial circuit allowing the chassis to be "back-fed" in reverse polarity from the (potentially) charged filter caps. 10uF would be enough to hurt you.
It's only an issue if these 3 things occur, but I have had similar experiences.
My points are that one should always use bleeder resistors, and that one should also think about using a separate safety ground lead from amp chassis to bench ground when an amp is on the bench with power being applied.
A general comment on the original question:
It's probably not possible to remove all the risk, but there are procedures and methods you can use to reduce the risk to an acceptable level (for you alone to decide if it's worth taking). The HV safety thread is helpful as well as common sense. Relatively lower voltages (300V instead of 600V) might be less dangerous but 80 volts can still kill you under the "wrong" circumstances. Your brain is your most effective safety device; think first.
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