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Step 2So, does MY amp need one?
First, mains-rectified amps were generally small output, 1-5 watts. Manufacturers usually didn't skimp on the larger amps.
If your amp has only one transformer (the output transformer) the answer is YES, you need one. If your amp has two transformers, odds are you don't need an isolation transformer.
Power transformers, the type that's missing from these unfortunate amps, are the largest transformers. They also tend to get warm, so 19 out of 20 times they'll be mounted on the outside of the chassis. The lack of one will be obvious.
Output transformers (and no vintage tube amp can be without one) however are smaller, and might be mounted in various ways, some of which are hard to see. They could be on the outside of the chassis, yes--but also under the chassis, or on the speaker itself. But rest assured--there will be an output transformer somewhere.
But wait--it's not that simple. Some amps isolated the signal path from the mains, but not the filament voltage. If equipped with a three-prong cord, these amps are somewhat safer, as they do offer isolation in most cases.
One sure-fire way to know if your amp lacks isolation is to examine the tubes. American tubes are prefixed with the filament voltage (12ax7 has a 12V filament, 6V6 has a 6V filament, etc.) The AC/DC circuits were designed to run all the filaments in series on a 110V supply. They therefore have high prefixes:
One common set: 50C5, 35W4, 12AU6
...which together equals 97V, so a small resistor was also added in series to drop the 110V voltage an additional 12 to 15V. It should be immediately evident that this was a cheaper way to build an amp. And many were built.
So, from a safely perspective--does your amp need isolation? YES.
If your amp has only one transformer (the output transformer) the answer is YES, you need one. If your amp has two transformers, odds are you don't need an isolation transformer.
Power transformers, the type that's missing from these unfortunate amps, are the largest transformers. They also tend to get warm, so 19 out of 20 times they'll be mounted on the outside of the chassis. The lack of one will be obvious.
Output transformers (and no vintage tube amp can be without one) however are smaller, and might be mounted in various ways, some of which are hard to see. They could be on the outside of the chassis, yes--but also under the chassis, or on the speaker itself. But rest assured--there will be an output transformer somewhere.
But wait--it's not that simple. Some amps isolated the signal path from the mains, but not the filament voltage. If equipped with a three-prong cord, these amps are somewhat safer, as they do offer isolation in most cases.
One sure-fire way to know if your amp lacks isolation is to examine the tubes. American tubes are prefixed with the filament voltage (12ax7 has a 12V filament, 6V6 has a 6V filament, etc.) The AC/DC circuits were designed to run all the filaments in series on a 110V supply. They therefore have high prefixes:
One common set: 50C5, 35W4, 12AU6
...which together equals 97V, so a small resistor was also added in series to drop the 110V voltage an additional 12 to 15V. It should be immediately evident that this was a cheaper way to build an amp. And many were built.
So, from a safely perspective--does your amp need isolation? YES.
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