I’ve been restoring a 1959 Magnatone 260A amplifier. The amplifier circuit is mounted on a sub-chassis. Many on the internet have described this sub-chassis as a “shock mount”.
I can assure you the internet lore is incorrect.
The sub-chassis is mounted on four screws, with aluminum spacers into vinyl grommets on the main chassis. This mounting scheme offers no vibration isolation.
However, this mounting electrically isolates the preamp section from the main chassis. In fact, the entire sub-chassis is electrically grounded to the main chassis at exactly one place. The sub-chassis has its own filter capacitors for all of the low-level circuits. The return current from the pair of 6L6GB are returned directly to the main reservoir filter capacitors near the rectifier on the main chassis.
Very interesting and clever grounding scheme all around. The amp has surprisingly low hum as well.
Here’s a photo of an identical, but much cleaner 260A chassis than the one I’m working on. (note the 3-wire plug is not correctly wired, I did mine correctly).
The 3-section capacitor near the rectifier is the main reservoir filter and also has a capacitor section for the cathode bypass of the 6L6GB output tubes. All the rectifier and heavy currents for plates and screens are returned to the main filter grounding point.
The floating pre-amp chassis has a 1k dropping resistor off the screen supply and a 2-section electrolytic capacitor. The entire pre-amp chassis is electrically isolated from the main chassis by the grommet mounts.
The input jacks are electrically grounded at the front panel, but no direct ground connections other than the single-point ground.
Magnatone, single-point grounding, not a shock mount!
Moderators: pompeiisneaks, Colossal
Magnatone, single-point grounding, not a shock mount!
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Re: Magnatone, single-point grounding, not a shock mount!
Someone there understood ground currents. An apparent rare occurrence in amp manufacture.
Interesting supports for those 2 pairs of Ajax caps. What material are those discs made from?
Are they hard, to support their mass, or soft to dampen vibration also?
Interesting supports for those 2 pairs of Ajax caps. What material are those discs made from?
Are they hard, to support their mass, or soft to dampen vibration also?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Magnatone, single-point grounding, not a shock mount!
The two pairs of 0.1uf "blue molded" Ajax caps are in the vibrato circuit. The attaching hardware is just a plain old machine screw and the extra-large washer is just punched out sheet metal (steel) probably made in house from scrap.
They are screwed to the sub-chassis, and Magnatone was generous enough to leave a hole in the main chassis for the head of each screw so you can service the caps if needed.
The other interesting trick is that the heater center-tap is connected to the cathode of the 6L6GB tubes, which biases the heater supply about +25v above ground.
It's pretty low in hum, despite the lack of any shielding for the chassis or enclosure.