How to read B+ current on my '69 Vox AC30?

[jhanly]

Hi All, I'm a newbie here - and I'm still fairly new to amp building/repair.

I'm wanting to measure the current where the HT secondaries enter the 4-diode bridge rectifier on my 1969 Vox AC30.

I followed a post in this thread https://el34world.com/Forum/index.php?topic=15297.0 which said you can measure the current at this point by effectively inserting DMM probes where the secondary HT wires enter the input of the rectifier section.

So I've unsoldered one of the secondary HT wires where it enters the bridge rectifier and I used probes to complete the circuit at this point. DMM set to Amps AC. Rinse and repeat with the other secondary HT wire.

With the power on (and 276V reaching the el84s' plates) I'm barely measuring 1mA of (idle) current at this pre-rectifier point in the circuit. Specifically, on one of the secondary's I'm reading 0.8mA and the other just makes 1mA.

I haven't played the amp with DMM attached like this because I thought the amp was class A & so no real point - but have just read it's debatable AC30's are class A. So maybe the current does fluctuate (ie, increase) significantly in use?

I'm measuring ~130VAC per HT wire entering the rectifier section. So ~260VAC.

The amp is playing fine so I'm guessing either this very low current reading is expected at the front-most end of the B+ line - or else I'm using my DMM incorrectly.

The reason I'm wanting to know the current at this point in the circuit is I'm considering installing a pair of fuses (as per the above-linked thread). Happy to explain why if helpful. But the current goal is to determine what value fuses to install if I do go this route. The typical rule of thumb (i.e., double the measured amps to get fuse value) doesn't seem to be making sense here. There are no 2mA fuses - and that value just doesn't seem right for a fuse at this point in the circuit anyway - to my untrained eye.

TIA for your input
J

[sluckey]

So I've unsoldered one of the secondary HT wires where it enters the bridge rectifier and I used probes to complete the circuit at this point. DMM set to Amps AC. Rinse and repeat with the other secondary HT wire.

With the power on (and 276V reaching the el84s' plates) I'm barely measuring 1mA of (idle) current at this pre-rectifier point in the circuit. Specifically, on one of the secondary's I'm reading 0.8mA and the other just makes 1mA.

Set your meter to measure DC current. Any better?

[jhanly]

Thanks for the reply, to be honest I'm assuming the DMM setting I'm using is good to read AC, but the setting's symbol is actually ambiguous - it's a straight line above an AC wave symbol, indicating it's actually agnostic to differences in DC vs AC. But that's just my uneducated guess of what that symbol means. There are 3 ampere settings; uA, mA and A. They all have the line-over-wave symbol. So just magnitude options. Sorry if that's clear as mud!!

[sluckey]

What brand and model number is your meter? I'm asking because the reading you got is totally wrong.

[pdf64]

A photo of your test set, showing the meter and leads, may be beneficial.

Rest assured that 2mA or 20mA fuses would blow rather quickly 
At a guess around T250mA may be ok.

[jhanly]

Thanks all, I've been trying to upload a pic of my meter. It's a Mastech MS 8268. Hopefully pic attached...

[shooter]

as a "general rule" I typically start with the probe in the 10A position, the dial would be set for A~-.  That way unexpected stuff don't blow the fuse, meter....  Once you're sure the actual reading, say .002A, the move the probe to where you have it and set the knob one turn CW to mA~-

[jhanly]

Thanks Shooter, the 10A setting shows no reading (ie, just 00.00), the mA setting reads ~ 01.00 & ~00.80 respectively and the uA setting moves the decimal point to the right on these readings as expected. I'll post a pic of the full test set up tomorrow - I must be hooking things up incorrectly. Thanks again all!

[sluckey]

Thanks for the reply, to be honest I'm assuming the DMM setting I'm using is good to read AC, but the setting's symbol is actually ambiguous - it's a straight line above an AC wave symbol, indicating it's actually agnostic to differences in DC vs AC. But that's just my uneducated guess of what that symbol means. There are 3 ampere settings; uA, mA and A. They all have the line-over-wave symbol. So just magnitude options. Sorry if that's clear as mud!!

You must push the "SELECT" button to tell your meter to read AC or DC current.

[jhanly]

Times like these I feel a bit like a goose. Thanks muchly Sluckey. And thanks everyone. Will try and make my next contribution to this board a little more auspicious!

[PRR]

Glad you found the AC/DC function.

The meter expects you to be reading sine-like waves. The waveform in a rectifier&capacitor supply is very NON-sine. May be significant error. Fuses respond to the RMS of a wave. There are "True RMS" meter but they may not have the "crest factor" to properly compute big spikes.

You know that preampery and a happy pair EL84 will suck less than 100mA. Two pair, 200mA. DC. The AC RMS via a cap-in supply is 1.6X to 1.8X the DC. It is conservative to call this "double". So 0.4A fuse.

Note that fuses also have a voltage rating, typically 250V (if not 50V). You are right up there. The danger is that an arc may never self-quench. (The 20,000V fuse on my main line street-wire is a foot long.) On AC, inside an amplifier, after a transformer, I would not fret too much.

[jhanly]

Makes sense PRR, and helpful info. I kept blowing the dropping resistor (DR) that sits just after the rectifier section - before I realised I had a bad el84 (I assume drawing too much current). The original DR burnt a hole in the tag board when it went down - which made me nervous so close to the (orig) PT. But I guess that proves the general point above - the PT was unscathed through all that drama. I did hear that if a diode fails in the bridge it can zap a PT - which again made me nervous given things got a bit pyro so close by. But a pair of fuses probably doesn't offer much more than false hope. Many thanks