Where to get a reasonably priced multimeter for biasing amplifier

shaunf

Hi Guys,

Please help me here. I have a Marshall JCM 2000 TSL combo. Now I must say up front that I'm not an electronics fundi, but I want to be able to do simple tasks like re-bias the amp when changing valves etc. I've googled how to bias this amp, and it seems to be a pretty easy process that I'm sure I could manage if I had a multimeter. There's a plug on the back of the amp that you connect the multimeter to and you bias to a setting of 80mV according to Marshall.

Therein lies the problem. I've been trying to find a multimeter to buy so that I can do this when I need to, which won't be very often, but the only multimeters I've managed to find that had 80mV within their measureable range cost over R3k, which just seems to be silly money to spend on something I'm otherwise hardly going to use.

Is this the norm, or do any of you know where I could get a multimeter that would be able to measure down to say 50mV that doesn't cost more than a couple of hundred rand?

Thanks in advance,

Shaun

Bob-Dubery

shaunf wrote: Therein lies the problem. I've been trying to find a multimeter to buy so that I can do this when I need to, which won't be very often, but the only multimeters I've managed to find that had 80mV within their measureable range cost over R3k, which just seems to be silly money to spend on something I'm otherwise hardly going to use.

Sorry to be dof, but that is that 80mV DC or AC?

If the former then you really shouldn't struggle. I got as no-name brand multimeter from Mica that has a 200mv DC scale on it. Was less than R120.

MikeM

Yea buy any cheap DMM and it'll do the job. If you spend more than R150, you're being unreasonable ?

Just don't go opening that amp up without knowing what you're doing.

Wizard

Isn't there a real risk of dying by mucking about inside valve amps?

shaunf

X-rated Bob wrote:
shaunf wrote: Therein lies the problem. I've been trying to find a multimeter to buy so that I can do this when I need to, which won't be very often, but the only multimeters I've managed to find that had 80mV within their measureable range cost over R3k, which just seems to be silly money to spend on something I'm otherwise hardly going to use.
Sorry to be dof, but that is that 80mV DC or AC?

If the former then you really shouldn't struggle. I got as no-name brand multimeter from Mica that has a 200mv DC scale on it. Was less than R120.

Hi Bob,

I'm not sure if it's AC or DC.

The thing is, I've seen plenty of cheap ones that have a 200mV setting on them for measuring voltage, but if you read the specifications of the meter, list ranges of say 200mV - 400V for example. Now the way I've been reading these specs is that the minimum voltage it will measure is say 200mV, and so it's no good to be able to set my 80mV. Maybe it's me that's been dof here ?

Regarding safety, yeah I know better than to go poking around inside the amp. The thing is, the JCM 2000 is apparently one of the easiest amps to bias. There is a little connector on the back panel, accessible without opening the amplifier, that you connect the multimeter to and set the bias by turning a little pot. I watched a couple videos on the Eurotubes website and it looks straightforward.

Gearhead

The ones that have a 200mV range will be fine - you are measuring at 40% of that scale which is almost perfect. It is going to work anyway even if the device is 20%off, if you measure your bias before removing the previous tube. Might in fact want to measure it before it becomes any older. The point is this: the bias that is right to your ears is the right one (unless it keeps frying expensive tubes). There are so many 'tube doctors' out there that listen to your complaint about the sound of the amp, charge R1100 and 're-bias' the amp away from its factory setting to be warmer or colder.

More important than the absolute bias is the relative one between the push and the pull tube in a pair (if the pair is not sharing its resistor). You are measuring DC unless you also play through the amp, then there will be a little AC involved to mess up your measurement. While measuring it would be clever to put a lead in the amp and put the tip somewhere on earth - gives more stable 0 signal.

nwalden

shaunf wrote: The thing is, I've seen plenty of cheap ones that have a 200mV setting on them for measuring voltage, but if you read the specifications of the meter, list ranges of say 200mV - 400V for example. Now the way I've been reading these specs is that the minimum voltage it will measure is say 200mV, and so it's no good to be able to set my 80mV. Maybe it's me that's been dof here ?

Usually that 200mV setting means that 200mV is the upper limit of what will be displayed on the given number of digits of a digital multimeter. If your meter has 3 digits on the display and you set it to the 200mV scale it should at least give you a reading from 0 to 200mV in 1mV increments.

aja

MikeM wrote: Yea buy any cheap DMM and it'll do the job. If you spend more than R150, you're being unreasonable ?

I disagree. If he buys a cheap multi I guarantee that he will end up using it for something else in the future, where a cheap multi is possibly dangerous to use.

Guess I am over cautious

MikeM

aja wrote: I disagree. If he buys a cheap multi I guarantee that he will end up using it for something else in the future, where a cheap multi is possibly dangerous to use.

Guess I am over cautious

Well if he's planning on doing some wild stuff, maybe a better one would be better suited. But my R100 Pick n Pay jobby worked just fine on my 135watt Twin and all other jobs I needed it for over 2 years. Maybe I am lucky.

Gearhead

I bought a very, very cheap one twenty years ago. It lasted nineteen. When it did die I immediately bought a new cheapie. Great, recommended. Few things you cannot do with cheapies.

Must mention I also have a Fluke, but I only use that at home and I only have two dmms to measure current and voltage at the same time.

MikeM

Also, the only reason I have bought a new one is so that I can have 2 as well and one that can measure capacitence.

nwalden

Gearhead wrote: I bought a very, very cheap one twenty years ago. It lasted nineteen. When it did die I immediately bought a new cheapie. Great, recommended. Few things you cannot do with cheapies.

Second! I started with a cheapie 10 years ago and when it eventually broke last year I bought a fluke. It can't do anything the cheapie couldn't, it's just better quality and "nicer" to use (will last a lifetime even if used every day as opposed to just a few years). It's also easy to get serviced and calibrated, all of which is irrelevant if you're only going to use yours once every couple of months. Rather spend the extra money on other bits and pieces ?

Bob-Dubery

MikeM wrote:
aja wrote: I disagree. If he buys a cheap multi I guarantee that he will end up using it for something else in the future, where a cheap multi is possibly dangerous to use.

Guess I am over cautious
Well if he's planning on doing some wild stuff, maybe a better one would be better suited. But my R100 Pick n Pay jobby worked just fine on my 135watt Twin and all other jobs I needed it for over 2 years. Maybe I am lucky.

But are you still alive?

shaunf

Hi guys,

Thanks for all the advice. It seems I was being a bit dof, thinking that the range in the specs meant that they wouldn't measure anything below the specified minimum in the range. One of the electricians at work lent me his one that has a 200mV setting, so I guess I should have just tried it before thinking it wouldn't work because the spec sheet specified the voltage range of 200mV - 400V. Just need to hold of some leads with crocodile clips at the end.

Thanks again, much appreciated.

Shaun

Heath

I use a multitech meter which cost just over 300 bucks and does everything you would ever need

Wizard

Here are some options.
http://www.toptronic.co.za/index.php?categoryID=5

I bought the T3601 from Yebo ...

Karel-Mars

I second the Toptronic choice. You can get a set of universal leads at Mantech that has croc clips on the ends (and a few other interchangeable connectors).