Safety for Tube Amp Nu-B

[David Vinson]

Hey All,

I’m starting on some maintenance tweaks on my SF DR, and I’d like to make sure I don’t kill myself. Here’s questions that have come up:

A) If I unplug, turn off, then discharge the filter caps in the PS, does that insure I’ve removed all lethal voltages anywhere else in the amp?

B) Another source said to short the plate voltages in the preamp stages to ground. Is that necessary once the PS caps are discharged?

C) I read somewhere else that negative bias circuits can also store ~50vdc; and the (-) side of the electrolytic needs to be shorted to ground. Is that true?

I have some background with electronics, tubes, and 480VAC systems that would inject modulated data signals into industrial building wiring. So I have a healthy respect for lethal volts, just need to make sure I’m safely discharging these amp circuits so I can work on them.

Looking forward to any input - Thanks very much!

David Vinson

[black_labb]

i havent done any amp stuff, but i read that its best to keep the different caps and the such grounded while you work on it as they can somehow build the current back up, dont ask me how.

[Don Symes]

Your best starting point is R.G.Keen's Tube Amp Debug page - read and understand the safety warnings first.

My personal rule for working in a powered tube amp:
1) ALWAYS keep at least one hand in a pocket (never touch with both hands).
2) Keep the guitar far enough away that it's impossible to touch both amp and guitar (guitar strings are grounded - you touch 'em and you're grounded).

[David Vinson]

Good source

Thanks Don

I think this was the source I had read earlier about shorting a preamp's plate to ground. (Lotsa reading in the last 48 hrs ;-)

I think shorting the positive side of the power supply's filter caps is the same thing: The plates of the preamp's first gain stage tie to B+ through 100k 1-watt resistors.

Mebbe I'm being too detailed here, but is it better to short the plate-side of the resistor (and let a 1-watt resistor handle that surge) or just short the filter caps (+) side? Maybe it depends: if the filter cap pan is off and the caps exposed, this is a more direct method; if the pan is on, then short a plate?

Also, I notice that as I power down, the voltage across my filter caps drops from ~480 to ~8 in less than 20 seconds. So I don't get that "arc welder" spark when I short the caps. Is that normal? The stuff I've read so far says the juice can last for "hours, even days".

Thanks for any suggestions!

[Don Symes]

You don't want a spark when you drain your caps - they don't like having to source all that current all at once. Installing a bleeder resistor of 390k-500k (2 watss or so) across your main filter cap will take care of it for you - in fact it sounds like you already have a bleeder installed.

If you want to go ahead and drain 'em, use a 100k 2W resistor with an alligator clip on one end and a wire and 'gator on the other. clip the short end to the chassis GND point and the wire end to the + side of the main filter. Leaving the drain in place until you're ready to power back up is not a bad idea.

Hope this helps!

[David Vinson]

It helps a lot, Don!

Since the drainage appears to be happening, and I'm not getting arcing, I'll keep doing the jumper thang for now, & build the resistor-jumper later

Thx!