Twin Amp current

[JC@]

I am working on a new Twin Amp here that blew it's mains fuse. I replaced it with a new one.
It is a 4 x 6L6 amp, 230 volts export version, brand new.
The fuse is a T2A. With 230 volts from the mains and 360W (these specs printed on the back), that T2A fuse should be ok.

I see it iddles at 0.9 Amps, the whole amp.

Now my question is if 0.9 Amps is a little bit too much, and I should suspect a faulty power tube or something.

Thanks,
JC

[Enzo]

How hot is the bias running?

Add it up. Add up all the heater currents times voltage - heater watts. Now with th e plate voltage check the idle current in the power tubes. I'd expect maybe 80 watts for them in dissipation. Now add in the small tubes. I'd figure maybe 3-4ma per 12AX7 and a few more for the reverb tube. A few watts for losses in the PT. What do you get, how many watts?

0.9A at 230v means over 200 watts to me. That seems high. If the power tubes are not way underbiased, try removing them one at a time and see if any one makes a much larger drop in mains current than the others. What does the amp draw in standby? is most of the excess coming from having the B+ on or not? For that matter, disconnect the secondaries and see what the PT draws all by itself. A shorted turn would let it still work, but it would be working a lot harder.

[Steve Conner]

Sorry to sound like an ass, but are you actually running the amp off 120V or 230V when you do these tests? You never said whether you're in the US or abroad, and amps for the international market have a voltage selector, so you could be doing either. If you set the voltage selector to 120V, you need to change the fuse to 4A, too.

And is it really 230, not 240 or 250? If you had the voltage selector set to 220 and were running off a high 240V line, that could explain the excessive draw. The UK line voltage is nominally 230V but I've seen it get over 250V in my workshop late at night.

[JC@]

Oh no... I have a bad power transformer over here. Over 0.6A with all secondaries disconnected. The amp is under warranty and they told the owner to repair and send the invoice.

Steve, this amp is specifically fitted and wired for 230 volt mains. No voltage selector. The transformer has a sticker with a big 230 hand written on it. But with someone like me, that keep asking the same things a hundred times, your comments and warnings are not a waste at all. In fact this is the third transformer I found bad in a couple of months, so expect me to ask again the same thing next time.

[Steve Conner]

I wouldn't be so quick to condemn it. If your line voltage was on the high side of 230 (and if you're in the UK it might well be!) it might draw a lot of current just from saturation. As an extreme example, the cheap transformers in microwave ovens draw around 5 amps from the line unloaded.

I'd suggest you leave it running and see if it smells bad, bubbles or smokes before trashing it. Also measure your actual line voltage, and listen for buzzing from the transformer.

[JC@]

I wouldn't be so quick to condemn it. If your line voltage was on the high side of 230 (and if you're in the UK it might well be!) it might draw a lot of current just from saturation. As an extreme example, the cheap transformers in microwave ovens draw around 5 amps from the line unloaded.

I'd suggest you leave it running and see if it smells bad, bubbles or smokes before trashing it. Also measure your actual line voltage, and listen for buzzing from the transformer.

Why not. I hate it. This transformer just stinks so it's going straight away to the recycling container
Seriously now, it buzzes like mad. And I plug everything I'm working in to a huge dial 0-5 Amp ammeter in series with a 12Amp variac.
It doesn't get hot, but the transformer is definitely faulty. It may work more or less ok, but 'they' will pay for the new one so... Plus when I turn the amp on, the ammeter climbs to almost 2 Amps for a moment, so it's too close to the border line. Plus, I tend to take 220 volts as the mains reference voltage and so I set the variac.

I don't live in the UK but my wife is from Leeds.

[Steve Conner]

What'll you do if the replacement does exactly the same thing, then? This buzzing, high idle current and switch-on surge all just say to me a cheaply designed transformer that doesn't have enough iron in it.

The "cure" for that is just a bigger fuse...

[JC@]

I'm glad science is in accordance with my first impression, in fact the amp was "repaired" with a 2.5 amp fuse, then closed, but at that point is where I decided to post about it. This is an expensive amp and the owner didn't pay for an achy transformer.
I will post the readings as soon as I get the new one.

[Enzo]

In my experience the transformers Fender uses in stuff like this don't tend to be cheapies.