Hi, long time no see…

Guitar Amplifier using center tapped mains transformer
0 – 110v – 230v

Manufacturer wired the Aussie models mains socket to the 110v
Customer powered it up and blew mains fuse (1A)
Customer returns amp.
Manufacturer told supplier to swap the primary tap to the 230v winding, and replace fuse.
Amps then sold as a ‘customer return’ at a discount price (30% off)
New customer (me) receives working amp, tests it for an hour then switches it off, upon power up several hours later, amp lights up for 5 seconds then blows fuse.

110v winding reads 5 ohm
230v winding reads 14 ohms
No apparent shorts on the secondary side

What caused the fuse to blow?

Aquila said:

Hi, long time no see…

Guitar Amplifier using center tapped mains transformer
0 – 110v – 230v

Manufacturer wired the Aussie models mains socket to the 110v
Customer powered it up and blew mains fuse (1A)
Customer returns amp.
Manufacturer told supplier to swap the primary tap to the 230v winding, and replace fuse.
Amps then sold as a ‘customer return’ at a discount price (30% off)
New customer (me) receives working amp, tests it for an hour then switches it off, upon power up several hours later, amp lights up for 5 seconds then blows fuse.

110v winding reads 5 ohm
230v winding reads 14 ohms
No apparent shorts on the secondary side

What caused the fuse to blow?

Just a guess, um, electricity? in fact too much electricity!

>What caused the fuse to blow?

when fired up initially (with 110V tap primary – 240V supply) the amplifier rail voltages would have been double perhaps, lot more anyway.

transition said:

>What caused the fuse to blow?

when fired up initially (with 110V tap primary – 240V supply) the amplifier rail voltages would have been double perhaps, lot more anyway.

Yes. It’s like the only button on a computer which will really break it. The 110 or 240V slide switch.

Yeah I know transition
The inrush current to saturate the primary would have been double, doubling the secondary output voltage

The Tx has a built in thermal fuse. All windings measure low resistance.

valve job?

otherwise, ignoring the mains transformer, assuming a simple amplifer, the filter capacitors have a maximum voltage, then going straight to the output devices that drive the speakers (assuming unregulated rails), these might have had double voltage across C-E (or D-S if mos) junctions, (exceeding max safe V), then there’s the rest of the amplifer.

If any output devices are shorted it’ll show by placing a meter low-ohms across C-E junctions

>The Tx has a built in thermal fuse.

if that was open T primary’d measure OC

shorted turns it’d probably stink’f‘s been cooked, anyway you can disconnect the amplifier from (filter rectifier, capacitors etc and run’t with no load attached – open leads on secondary), see’f‘t blows fuse, gets hot or whatever.

A good trick in these situation is a lead with suitable plugs slipped into power lead with a series ~100Watt bulb, makes a good resetting fuse. Cold resistance usually low enough. It lights up instead of blowing the fuse.

Transformer primary (DC r) vary a lot depending on power to deliver. Probably in the range of less than 1 ohm to greater than 2K ohms typically of stuff connected into a power point, i’d guess?

Generally raising V across supply rails that much will not be good. It may blow the fuse fairly quickly, and even work afterward, but as is often the case components fail properly later. Damaged semi junctions, capacitors too perhaps.

This is relevant of you’ve been using the right fuses, not too low value, slow blow or whatever as required.

only thing i can offer is wrong fuse, fast blow instead of slow blow. i believe some amps use slow blow.

JudgeMental said:

only thing i can offer is wrong fuse, fast blow instead of slow blow. i believe some amps use slow blow.

I much prefer a “slow blow” until just before the “vinegar stroke” and then a fast and erratic speed up!

bob(from black rock) said:

JudgeMental said:

only thing i can offer is wrong fuse, fast blow instead of slow blow. i believe some amps use slow blow.

I much prefer a “slow blow” until just before the “vinegar stroke” and then a fast and erratic speed up!

Bob, stow this shit.

JudgeMental said:

bob(from black rock) said:

JudgeMental said:

only thing i can offer is wrong fuse, fast blow instead of slow blow. i believe some amps use slow blow.

I much prefer a “slow blow” until just before the “vinegar stroke” and then a fast and erratic speed up!

Bob, stow this shit.

i laughed.

I hate agreeing with Boris, so I won’t, but will give him qualified support. A slow blow fuse is normally a good idea at the input to a line transformer. Saying that, if the amplifier powered up for five seconds the second time it may point to another concern; although it may have been that the fuse was already stressed and just required a bit of thermal jiggling to let go.

I’d replace with 1 amp slow blow and retest.

sibeen said:

I hate agreeing with Boris, so I won’t, but will give him qualified support. A slow blow fuse is normally a good idea at the input to a line transformer. Saying that, if the amplifier powered up for five seconds the second time it may point to another concern; although it may have been that the fuse was already stressed and just required a bit of thermal jiggling to let go.

I’d replace with 1 amp slow blow and retest.

adds post to file “Sibeen agrees with Boris”.

well actually it just has this one entry atm. but i hope to add to it in the coming years.

JudgeMental said:

bob(from black rock) said:

JudgeMental said:

only thing i can offer is wrong fuse, fast blow instead of slow blow. i believe some amps use slow blow.

I much prefer a “slow blow” until just before the “vinegar stroke” and then a fast and erratic speed up!

Bob, stow this shit.

thank you, jm

yes regular fast fuse can get fuse fatigue, or blow straight off, powering up the transfomer, but importantly filter caps after the rectifier/bridge (if old school PS) and further on charging up to running voltage. Filter caps have very low impedance (to operational frequencies, then some) particularly to fast risetime pulses too like when ya flick the big switch.

if the power supply’s probably alright and it’s got discrete transistors for output devices i’d swing a meter across the legs of them all, particularly E-C looking for a short.

did one in the beast a while back, using it in bridge mode and a strand of the 6mm wire shorted the regular speaker posts (protection apparently doesn’t work in that situation).

it’s got four lots of seven devices in parallel, was easy enough to meter off and find the shorted one, I just cut it out, figured it’s got 28, how many does it need. Got a new one here’ll slip it in when put new fans in.

transition said:

yes regular fast fuse can get fuse fatigue, or blow straight off, powering up the transfomer, but importantly filter caps after the rectifier/bridge (if old school PS) and further on charging up to running voltage. Filter caps have very low impedance (to operational frequencies, then some) particularly to fast risetime pulses too like when ya flick the big switch.

if the power supply’s probably alright and it’s got discrete transistors for output devices i’d swing a meter across the legs of them all, particularly E-C looking for a short.

did one in the beast a while back, using it in bridge mode and a strand of the 6mm wire shorted the regular speaker posts (protection apparently doesn’t work in that situation).

it’s got four lots of seven devices in parallel, was easy enough to meter off and find the shorted one, I just cut it out, figured it’s got 28, how many does it need. Got a new one here’ll slip it in when put new fans in.

Damn,

That looks like one heavy-duty piece o’ gear.

It was a new fuse, as per my OP
Yes it was T1A as per manufacturer specs

Was tested before resale, it worked for over an hour when I bought it, then on second power up it blew fuse after five seconds

What could be the detrimental effects of applying 240V to a 110V primary winding of a Tx connected to a power supply and other circuitry designed for 110V before the mains fuse blows from over current?

….maybe it was 3 or 4 seconds, but I saw the preamp valve light up

Aquila said:

What could be the detrimental effects of applying 240V to a 110V primary winding of a Tx connected to a power supply and other circuitry designed for 110V before the mains fuse blows from over current?

Scratch that
I think it is multi country/ multi-voltage design, the factory just incorrectly connected the wrong primary wire tap or it was shipped to the wrong country.

Aquila said:

It was a new fuse, as per my OP
Yes it was T1A as per manufacturer specs

Was tested before resale, it worked for over an hour when I bought it, then on second power up it blew fuse after five seconds

What could be the detrimental effects of applying 240V to a 110V primary winding of a Tx connected to a power supply and other circuitry designed for 110V before the mains fuse blows from over current?

few thoughts

firstly, if it’s just simple dynamic regulation (think it’s called), which is basically the T output then rectifier + filter caps (the unloaded voltage is highest) it’ll be way up (let’s say the T secondary V is double what is suppose to be using the 110V tap primary with 240V applied), then a lot of components will have x 2 their usual V applied. Something might burn out, ideally a fuse, but if it doesn’t (quick enough) something will break down, something that usually has insulating or somewhat linear resistance qualities up to some voltage, then above that damage is done.

Ignoring the power supply…..like rectifier or caps are shorted…..and i’ll assume it’s a transistor amplifier

Take a simple old well known transistor, say a TIP31, you can use this to drive a speaker. The max collector current is 3Amps, maximum power dissipation is 40Watts, maximum voltage between collector-emitter is 40Volts (between C and E is also where most of the current flows – it’s the junction that does the brute work).

You could have a break down of the transistor from overvoltage.

Or if the bias went pear shaped (no input signal assume) it might turn on hard or partially, + have a higher voltage across it. With a TIP 31 (assuming it’s got a good heatsink) various V x I combinations arrive at the max power dissipation. But until it gets too hot to do damage I (current is the problem). Trying to move too many electrons through the tiny thing.

Of temperature from dissipation, this is mostly I x Vd across cellector-emitter. Max temp for TIP31 is 150C. 20V across it with 2Amps = 40Watts. That currents alright, it’s good to 3A. The power dissipation’s at the limit though, inadequate heatsinking/or cooling or high ambient temperature and it might have a melt down. It’ll short and tie your power rail down permanently, most likely.

Just a simple example above, it applies to other parts of and parts in the amplifer too though.

….yeah…nah…. all good

was just a dodgy electro tri-modulator

..chuckle

Aquila said:

….yeah…nah…. all good

was just a dodgy electro tri-modulator

..chuckle

I think mono-modulators are more reliable.