I had a Silvertone 1485 (head only) come through my shop back in April. While this page is about the 1485, the information applies to the 1484 as well. The only circuit difference between these two amplifiers, is one or two output stages. The pre-amps are the same.
The owner mentioned that the reverb doesn’t work. This should not surprise anyone, as these are known to go bad all the time. I traced the issue down to a bad reverb tank. I also found an unrelated wiring error from a previous repair, but that’s besides the point.
The tank is nothing like what we normally see inside guitar amplifiers. Fender style tanks (and most other brands) were originally made by Accutronics, and use a tiny magnet suspended in a coil, to rotate the spring with the sound wave. The opposite end has a similar setup, except it changes the rotation back to electrical sound waves.
The Silvertone unit uses piezoelectric elements to convert electrical signals directly into vibration. The vibrations are transferred to the spring using 4 metal tabs. These tabs are supposed to be electrically isolated from the spring. To isolate the spring, Silvertone used thin layers of paper. Over time, this paper become brittle and start to crumble, shorting out the unit. This unit had one piece of paper replaced with electrical tape. However, at least two other pieces of paper had degraded beyond use.
Originally, I decided to repair the tabs the same way as someone had previously done, and I put electrical tape on the other tabs. Then I realized that the reverb is an electromechanical device. Electrical tape is an insulator, but it is spongy. This will change the sound of the reverb! I decided instead to replace the paper with more paper. I carefully cut a few pieces, and replaced the paper on the tabs. This restored the reverb back to original conditions.
While it was in my shop, I also replaced the two prong power cord with a proper three prong grounded plug. No more unexpected shocks! I turned it on to check my work, and to my surprise, the tremolo stopped working! It didn’t make sense at first. How could that break the tremolo? Then I noticed something peculiar about the tremolo tube in the schematic. A lot of times in schematics, they would publish typical voltage measurements at different points in the circuit. In this schematic, the cathode of the tremolo tube is grounded, but it says “0.05” by it. How could there be 0.05 volts there, when it is grounded?
When Silvertone built this amplifier (and other ones in this series), they used ALL SORTS of cute little tricks to save a few pennies in production costs. This was one of them. When this amplifier had a two prong plug, they needed a place to tie one prong to the chassis, through a capacitor. This capacitor is often known as the “death cap”, as any amplifier that used this had a 50% chance of putting 120 volts into your fretting hand! When I replaced the power cord, I removed this capacitor, and tied the chassis directly to ground. All was good, except it wasn’t.
When they put the capacitor in, it wasn’t DIRECTLY to ground, but it was at the head of a screw that was attached to the chassis. This same screw head was connected directly to the tremolo cathode. When the amplifier was powered on, there was a slight amount of voltage lost between this screw head and the chassis. How much? 0.05! They were counting on a small AC voltage drop across the screw to function as a start up trigger for the tremolo! It’s a wonder these amplifiers work at all!
So instead of going back to the wiring that had a 50% chance of killing the player, I wired a voltage divider from the AC line to ground, and tied the cathode to this divider. Voila, tremolo functions again! If you are curious, I chose 120k and 47 ohm, to arrive at a 0.047 Vrms AC voltage on the tremolo cathode. This is only 1mA current to ground, so it won’t trip a standard GFCI outlet.
Final note: The 1484 and 1485 actually used the same output transformers! The transformers have a 2.67 ohm tap and a 4 ohm tap. On the 1484, the 4 ohm tap was wired to a pair of 12″ 8 ohm speakers in parallel. On the 1485, the 2.67 ohm tap from each transformer was wired to three 10″ 8 ohm speakers in parallel. When converting to speaker jacks, you can wire these to: 1 @ 1.33, 1 @ 2, 1 @ 5.35, 1 @ 8, 2 @ 2.67 or 2 @ 4 ohms. Consider all the options, and pick what works best for you. Personally, I’d go with 1 @ 5.35 ohms (outputs wired in series) for the most flexibility. This allows for a 4 ohm or 8 ohm speaker with reasonably close impedance matching.
I hope this is helpful next time you have a 1484/1485 with tremolo or reverb problems.
JDP Amplifiers 