How to measure voltage (safely) in a tube amp?

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How to measure voltage (safely) in a tube amp?

Postby Entrant_21 » 15 May 2009, 19:39

Hi everyone, I'm looking for a few points on how to check the voltages inside a tube amp, a friend of mine was checking what output a transformer had in his old radio (valve) and he put the MM+ on the live and MM- on neutral and nearly blew himself up..

I'm looking for a few pointers on how to be safe when measuring voltages in a tube amp, I'm confident in working with the caps drained etc. but now its a step up!

Thanks :)
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Re: How to measure voltage (safely) in a tube amp?

Postby lolbou » 15 May 2009, 19:58

First, use a DMM instad of any analog one, since it has a higher input impedance. Your friend might have done some mistake somewhere. But though harmful, AC is less lethal than DC somehow.

I use Metrix probes rated 1000V DC. Most of the time I use a crocodile clamp to have a safe ground, using only one hand at one time in the amp to check voltages with the red probe.

Always keep your multimeter on its higher range when beginning to measure, taking it down step by step. Be sure you don't have overrange high voltages since your multimeter won't appreciate it that much.

Use a well earthed equippement and take extreme care with any high voltage rated capacitor, big or not.

Hope this helps a bit... :)
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.
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Re: How to measure voltage (safely) in a tube amp?

Postby Entrant_21 » 15 May 2009, 21:31

Thanks for the tips :) how would I go about measuring current? I have a DMM which has current measuring on it too but it says (10ADC) is that enough?
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Re: How to measure voltage (safely) in a tube amp?

Postby JiM » 15 May 2009, 23:11

Measuring currents imply opening the circuit to put the multimeter in series. It can be quite unpractical, and you'd want better connection than crocodile clips ...
In a tube amp, it's better (easier, safer, smarter ...) to stick with voltage to ground measurements, and compute currents from voltages across resistors.

Anyway, 10 A will be enough for the high-voltage supply (if the DMM can handle that voltage on its current sensing input !), but it might be a bit short for the heater supply of a big amp with many tubes.

lolbou wrote:Most of the time I use a crocodile clamp to have a safe ground, using only one hand at one time in the amp to check voltages with the red probe.

Totally agreed ! On trick is to keep the left hand* in the back pocket of your jeans.
The worst damages occur when the current travels from hand to hand through the heart.

* It's not about the hand you don't write with, but the one that has the shortest, hence less resistive, path to the heart.
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Re: How to measure voltage (safely) in a tube amp?

Postby tictac » 16 May 2009, 04:32

I like the clip-on leads rated at 600v. I clip the black to ground and use the red to read the voltages I want to measure.

Get a Fluke DMM, Fluke, Fluke, Fluke...

Don't mess with the cheap ones, it's not worth it!

A variac with a current meter is nice, I've got one with a built-in volt and current meter, nice to monitor the AC volts & current...
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Re: How to measure voltage (safely) in a tube amp?

Postby Entrant_21 » 16 May 2009, 22:08

Thanks for the tips guys! successfully biased my Peavey Windsor (converted to 800 specs) today!

I used my cheap but great DMM, I had a look at those fluke ones...very expensive :P I'm doing it myself to gain knowledge and to save money lol!

If I was doing it on a regular basis I would likely get a bias probe, or like you suggested a good quality fluke MM.
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