I was asked to help a friend repair a small cheap guitar amplifier today which had a burnt out mains transformer. The original transformer was simply 230v primary, 18 volt @ 1amp secondary and had expired because A, the cabinet was fully enclosed with no ventilation, and B, the amp had been run flat out for long periods.
My friend had replaced the transformer with an off the shelf 9-0-9 volt transformer rated at 1 amp and not used the center tap taking the output across the two 9 volt leads where he expected 18 volts.
He had noted that it was smaller than the original, but the supplier had told him that as it was the same current rating, it would be fine. On testing the amp, it failed to produce the original output power and the transformer got red hot and also failed. He then assumed that there was something else that was breaking down causing an overload so he brought it to me to investigate further.
After doing the usual visual and smell checks, I hooked the bridge rectifier up to a decent 18v transformer and wound it up on a variac. I played my Les Paul through the amp for an hour flat out and it was perfectly ok.
Then I explained to my friend, his and the suppliers mistake. The original transformer was able to supply 18volts @ 1amp to a bridge rectifier, but the 9-0-9 volt one was designed to supply 9 volts at 1amp. The type of rectification it was designed for is where only one half of the winding is conducting in any half cycle. By feeding the 18 volts to a bridge rectifier where current is drawn on each half cycle, the transformer VA rating (which wasn't specified) was exceeded causing it to overheat.
It is a shame that there are so many suppliers of components that don't have the full spec of these (and many other) components available. Just a brief primary- secondary voltage, and an ambiguous current rating. It also appears that some only refer to the voltage after rectification and smoothing at a specific current. I have also noticed that they have terrible regulation. The manufacturers (probably Chinese) try to use as little core material that they can get away with.
My friend had replaced the transformer with an off the shelf 9-0-9 volt transformer rated at 1 amp and not used the center tap taking the output across the two 9 volt leads where he expected 18 volts.
He had noted that it was smaller than the original, but the supplier had told him that as it was the same current rating, it would be fine. On testing the amp, it failed to produce the original output power and the transformer got red hot and also failed. He then assumed that there was something else that was breaking down causing an overload so he brought it to me to investigate further.
After doing the usual visual and smell checks, I hooked the bridge rectifier up to a decent 18v transformer and wound it up on a variac. I played my Les Paul through the amp for an hour flat out and it was perfectly ok.
Then I explained to my friend, his and the suppliers mistake. The original transformer was able to supply 18volts @ 1amp to a bridge rectifier, but the 9-0-9 volt one was designed to supply 9 volts at 1amp. The type of rectification it was designed for is where only one half of the winding is conducting in any half cycle. By feeding the 18 volts to a bridge rectifier where current is drawn on each half cycle, the transformer VA rating (which wasn't specified) was exceeded causing it to overheat.
It is a shame that there are so many suppliers of components that don't have the full spec of these (and many other) components available. Just a brief primary- secondary voltage, and an ambiguous current rating. It also appears that some only refer to the voltage after rectification and smoothing at a specific current. I have also noticed that they have terrible regulation. The manufacturers (probably Chinese) try to use as little core material that they can get away with.
Last edited: