Higher than expected current draw - Fender Pro Reverb Amp

[Mick Bailey]

This is the amp from 2002 (completely unrelated to the original amp of that name).

Just checking it over and noticed current draw higher than expected. No shorted turns show on the PT when tested off my ring tester, but it could be breaking down under full primary voltage. With a completely disconnected secondary it draws 230mA and hums a lot more than usual. Output voltages spot-on and it's wired correctly for 240v UK mains, so it's pulling 55w unloaded. No leakage, but something isn't right. I'm wondering if it's on the point of failure.

[pdf64]

'it could be breaking down under full primary voltage'
How about monitoring the current whilst winding up the voltage on a variac, to see if it rises unexpectedly at any point?

My impression of the PT from the (very few) pro tube series amps I've seen is that the seemed to run hotter than expected, with crude thick laminations.
Pete

[Enzo]

55 watts might be reasonable for the complete circuit at idle, but 55 watts for a bare transformer is not a good part.

I have not encountered any problems with Pro Tube series amps eating transformers, for that matter I see very few Fender tube amps needing PTs. The ones I do see tend to be more the solid state ones with the transformer bell hanging out the rear panel.

[Mick Bailey]

I comes up smoothly on the variac. I notice that the laminations are quite battered underneath the varnish. It looks like it was mishandled at some point during manufacture and they're shorted out. Could explain the losses in the absence of a winding fault.

http://music-electronics-forum.com/i...attach/jpg.gif

Edit: Just checked across the laminations with some hardened needle probes and all of them are shorted together.

It's been running like that for over 10 years, though!

[Steve Conner]

Don't get your VA and your watts confused. Just because a transformer draws 230mA at 240V, doesn't mean that 55 watts of heat are being dissipated inside it. The current is mostly reactive and can be quite harmless.

I would also expect all the laminations to read shorted. The voltages induced in them are tiny, so even a little bit of resistance that would read as "0" on an ordinary ohmmeter does a good job of squashing eddy currents. Some really cheap transformers have the laminations welded together and it doesn't seem to do much harm.

[Mick Bailey]

Excellent point about the VA. I wonder what one of those plug in power meters would register? They're about £7 - worth a try just to see.

I was expecting something like 10 ohms or so across the stack. Now you mention it though, many Peavey transformers are welded right across, or have the stack welded to a steel base.

[Steve Conner]

I've got one of those power meters bought from Maplin. Very useful, can highly recommend it.

[pdf64]

Do those things actually derive the VA though?
I've got a couple and the voltage is just a user preset, it's not measured or phase related to the current. Maybe they assume a standard power factor of 0.9 or something?
Pete

[Tom Phillips]

The temperature rise could also be a clue. How warm / hot does the transformer get after an hour running with just the primary connected? I would expect it to get warm but not so warm that it is uncomfortable to leave your hand on it for a long time. From my experience the temperature rise of an unloaded PT of the quality used in guitar amps would be ~20°F ( ~11°C) above the ambient temperature. This is after it has been on for 1 to 2 hours and the temperature has stabilized.

No matter how the power factor affects a power calculation the temperature rise is a directly proportional indication of the power being dissipated within the transformer.

[Steve Conner]

The Maplin one (ProDigit 2000MU-UK) certainly did. It read out the actual voltage, current, frequency, watts, VA, power factor and kilowatt-hours since you last remembered to reset it.

I checked it against an old-fashioned mechanical dynamometer wattmeter on some really nasty loads and the agreement was good.

I think it's also sold as the Kill-A-Watt P4400.

[Mick Bailey]

The temperature rise could also be a clue. How warm / hot does the transformer get after an hour running with just the primary connected? I would expect it to get warm but not so warm that it is uncomfortable to leave your hand on it for a long time. From my experience the temperature rise of an unloaded PT of the quality used in guitar amps would be ~20°F ( ~11°C) above the ambient temperature. This is after it has been on for 1 to 2 hours and the temperature has stabilized.

No matter how the power factor affects a power calculation the temperature rise is a directly proportional indication of the power being dissipated within the transformer.

After 15 minutes it's over 70°C and then starts to level out, but I haven't run it for longer than that. The hum level also rises but the output voltages remain fairly constant. OK, I missed the PF (basic schoolboy error) but this is still turning a lot of power into heat - it's a large transformer.

[Steve Conner]

If it reaches 70 degrees unloaded, sounds like a pretty crap transformer.

It may have been designed more on the 230v, 60Hz side of things. In the UK we can be scraping 250 at times. Did you measure the mains voltage while doing the tests? One of the things they had us do in EE labs was run up a transformer on a variac and plot how the magnetising current varied with voltage. Above a certain voltage it just takes off as the core starts to saturate. Transformers are usually designed to run right at this saturation point to save money, so a small change in voltage or frequency can make a big difference to the magnetising current.

Then again, if the core was made of thick laminations of poor quality iron installed with a sledge hammer, that wouldn't help either! I don't have any hard evidence, but I wouldn't be surprised if the magnetic properties of transformer iron were hurt by beating with a hammer. (I once used a Sound City 120 OPT as a hammer in an emergency so I hope it is OK. )

[Mick Bailey]

I measured 243v.

The first thing I check with Fender amps is whether they're wired for 240v. It's a strange annoyance that Fender supplies amps to the UK wired for 230v and stickered "UK Mains - 230v", when internally there's always an unused 240v tap that brings the voltages down in line with their schematics when reconfigured. This amp was wired the same when I got it, though the schematic shows two wiring schemes; "Primary wiring for 230v EUR only" and "Primary wiring for 230v UK and 240v Aust. units"

It's no secret that we're on 240v, though the regs include us in the 230v -6% +10% bracket so that Europe can have what appears to be a harmonised supply. Would hate to be getting 206.8v at the lower end of this range - presumably this would still fall within acceptable limits.

Cor blimey! they're 'aving a larf, aint they Guvnor?

I've ordered a plug-in energy meter from CPC. Would make a useful addition to my Variac.

Here's an interesting review of the LIDL unit - I saw them earlier this year for £6.99. Should have got one but I thought they'd be inaccurate;

Review: LIDL Silvercrest energy monitor, Energie kosten meetapparaat, Energiekosten Messgeät - YouTube

[J M Fahey]

Those laminations *look* fine.
I sometimes hammer mine more than that with no ill effects.
Your lamination may be real crappy though.
70ºC idle is gross.
Touch alternatively laminations and copper .
Hot laminations, not so hot copper: poor laminations.
Hot copper : shorted turn.
Not necessarily shiny copper to shiny copper contact, you may have carbonized enamel jumping adjacent turns.

In any case , that transformer has a bright future at some recycling plant

[km6xz]

You can get a good estimation of shorted plates by measuring the capacitance, all those plates constitute a large array of series connected capacitors. The capacitance will increase is some are shorted. Eddy currents are causing those losses and heat so time for new transformer. With them all shorted, if that is the case, you might as well have just a solid block of iron for all the good the shorted plates do for you. Inductance goes down, losses increase, heat increases, core easily saturates, resulting in even higher primary currents flowing...no positive outcome with that transformer installed. It will get worse.