Here is a classic old micro-power amp that I restored to safe and toneful service. Sold originally by Sears, the Silvertone 1430 harkens back to a time when line isolation transformers were considered an expensive luxury. The tone of such single-ended amps is still revered by many harmonica and guitar players who frequently risk electrocution by using one in its original hot-chassis form. Good tone isn't very useful to dead musicians, so among other modifications I added a line isolation transformer to this amp.
The Amp Before Modification. I drew the following schematic from this amp before its modification:
Pre-Modification Silvertone 1430 Schematic; please return using your browser's "back" button.
Notice that the user's sweaty hands are separated from one side of the 120 VAC line merely by a 68K-ohm resistor and a parallel 0.05-microfarad capacitor. A short in either of these components could be fatal to a grounded user. The only component transformer-isolated from the power line was the heater of the 12AU6 preamp tube. This was necessary to avoid exceeding the 12AU6's specified maximum average heater-to-cathode potential of 100 V. (Either orientation of the non-polarized plug is possible--and users could experiment to find the lowest-hum orientation. One orientation would give a heater-to-cathode potential of 120 V without this small transformer).
Aside from the 12AU6 preamp tube, the only other active device in the model 1430's audio signal path is a 50C5 pentode used as a single-ended (class A) power amplifier. The output transformer was physically attached to the original 6-inch speaker, old radio-style. On disconnecting the secondary and testing the amp into a 4-ohm resistive dummy load, I rated this amp at about 1.25 W maximum power before soft-clipping. The speaker itself was in poor condition due mainly to a rotted surround, but it still "worked." There was quite audible continuous background hum, but I could make most of that go away by clipping extra filter capacitors in parallel with the old filter capacitors. However, low-level signals were audibly modulated at 60 Hz, which made the decay envelope of notes sound bad. (Adjusting the trimmer capacitor at the second grid of the 12AU6 had no effect. If anyone out there knows what this trimmer was supposed to do, please email me and I'll post it here so we can all learn something.)
The Modified Amp. Here is a schematic of the modified amp:
Post-Modification Silvertone 1430 Schematic; please return using your browser's "back" button.
The bottom part of this schematic represents the "isolation module," which includes two separate power transformers: a 50 V-A 1:1 line isolation transformer for the high voltage circuit and a 25 V, 2A filament transformer for the heater circuit. I used a voltage-multiplying rectifier and wired the heaters in series-parallel, with 47.5 VDC across the heater network under load. I simplified the audio circuit, basing it on the Harmony H-303A, which uses the same pentodes as the Silvertone 1430 but has an isolated power supply. I replaced all three tube sockets with ceramic types and included a shield on the preamp tube. I detached the output transformer from the original 6-inch speaker and re-installed it inside the chassis where the 12AU6 heater isolation transformer used to be (it is the same physical size). I replaced the speaker with a new 6" 4-ohm unit from Jensen's "Mod" series.
The amp now has very little background hum, and a bright responsive tone with beautiful clean ring-outs. Here is a link to more photos of the modified Silvertone 1430.
More photos of the modified Silvertone 1430
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