I wanted to make up a 'test box' with an ampmeter and voltmeter in it so that I can test both voltage and amps at the same time - I have seen a couple of videos on Youtube but they don't use the earth wire which I would like to. I want to be able to plug the box into my variac and the radio into the box.

Any suggestions on how to achieve this and a possible basic schematic would be appreciated (including the type of meters I should buy - I see there are several ones available on Ebay that are not expensive).

Could the box have switch on it? Would the switch go on the neutral or live side of the mains?

Regards
Angelo

If you unhide your email address in your member profile, I'll send you a basic schematic that suits your purposes.

There are a couple of issues here. I am assuming you are working mains side?

The Variac I have actually has a voltmeter on it. I do have ammetters but the issue is how many amps you want to measure. My variac is only good for 3A, which is good enough for most radios.

240 Watts is one amp and very few radios, get anywhere near that. Rarely do I find a need to check the current draw on the mains side.

The current fixer on the bench is 110V so there is another current guzzling transformer (220/110) on the variac (saves major overload accidents) that renders readings misleading.

The more significant is the voltage & current draw of the HT Rail of the set: The voltage of which I always monitor on a newly repaired radio.

Switches always go on active however, In that position I would prefer DPST switch, which breaks both rails.

Often the phases get inverted and this is the only guarantee. You can buy plug in testers to ensure everything is wired the prefferd way, and if there are issues you get an electrician to sort them out ,

For the rare occassions I have a specially made up cable that I can use safely with a clamp meter, to measure mains current. The 0-10A meter is not sensitive enough for a radio.

If you use mechanical panel meters on AC make sure that they are AC capable eg Moving iron.

Circuit is easy,ammeter goes after the voltmeter,not before.

Marc

Thanks GTC. I will unhide the email address. Yes Marc I want to test the current draw of the HT rail of the set.

Will a 0-3 amp ac amp meter be sufficient for my needs? They seem to be plentiful on ebay for around $8.00. I have seen that most of the radios I have and worked on draw about 50Watts. Am I correct in saying that this converts to approx 0.208 amps (208 milliamps) assuming a 240V mains?

Is a DPST switch easy to wire up in what I am trying to do? Would that work with the schematic you had in mind GTC?

Regards
Angelo

I sent Fendertweed pictures of my Dad's test box, anyone wanting photos let me know or we can ask Brad to put it up on the forum.
It is in PDF format.
Peter

Is a DPST switch easy to wire up in what I am trying to do? Would that work with the schematic you had in mind GTC?

Yes, you can use a DPDT if you want to.

Thankyou to both GTC and Airzone for the pics and schematics. As a newbie this information is invaluable - maybe a section on the website for test equipment would be a good idea.

DPST switch is simple. It is actually two switches in mechanical parallel. (isolated from each other electrically)

Circuit is easy if required.

The issue will be the physical size of a fixed meter movement as most radio's will draw less than half an amp mains side.

You may have to shunt a small current meter or use the voltage drop method. Which I have seen on a DC movement I bought for a neighbourd battery charger recenly.

This includes the sum total of the secondary windings and power used by the transformer itself.

Measuring the secondary HT (valve sets) is an entirely different issue.

"The current of significant interest" is mainly the DC and its only sets like Midwest with 18 valves, four of which are in Push Pull amplifying or Large Amps line amps. that are going to get near 200mA on B+.

In most sets that have back bias or like the Bandmaster which has a predictable resistance in B+ or B- (it has both) the current can be deduced from the drop across the resistor, reasonably accurately. In most cases due to voltage fluctuations, manufacturers quote voltages plus or minus 20%.

Marc