soapfoot
Well-known member
For years people have enjoyed converting the crude Magnasync Moviola URS film editing amp ("squawk box") into a guitar amp. I love it; I've been using them for the better part of a decade now (when I can get away with the noise floor).
And that's the catch--they're all noisy. Very noisy. Re-capping them sometimes helps a little bit, but it's always been clear that there are other problems.
All the best-sounding ones I've heard have been implemented in the most mercenary, minimal way: just add a quarter-inch jack that couples directly to the grid of the first stage (it's probably "wrong" to not to use a grid stopper of any kind, but I don't argue with results).
About a year ago, I decided to modify mine for balanced heaters with a pair of 100R resistors as an artificial center tap, thinking this might help make it quiet. The amp just laughed at me--any improvement was so marginal as to seem insignificant to the noise that still remained.
Last night I got so fed up that I decided to blow it apart again, and I think I've finally solved the issue.
The original had two Aerovox-style multi-section can capacitors--one serving as the first two power supply filters (plus 6AQ5 cathode bypass), the other serving as the last stage of filtering plus preamp stage cathode bypass. So far so good. The cans were both isolated from the chassis to give control over how the grounds were routed. Again, so far so good.
But then there was one grounding bus connecting the two cans, and almost all grounds were connected to this--the volume control, the 12AX7 grid's ground reference, the main power supply filters... everything. This whole bus was connected to the chassis near the 12AX7.
The aftermarket input jack (on mine) was connected to chassis near its physical location. Power transformer HT center tap was connected to ground at a PT bolt (my artificial center tap for the heaters was connected there, too). This was far away from the aforementioned ground bus. Safety ground connected to another PT bolt. Output transformer ground clear across the chassis, at an OT mounting bolt.
Additionally, there was a bit of other stuff near the input, designed to interface with other Magnasync equipment. It's not part of the amp and can be deleted.
Here's how I made it quiet:
This was all a bit rushed... it was done in a moment of frustration, kinda, so it's not necessarily the prettiest job (I just yanked original resistors over to a better place, rather than replacing them). But functionally, I'm happy with the results. At full volume the hiss now predominates (not the hum, as was egregiously the case before).
I'm not sure whether every single thing I listed above was necessary/helpful, and someone might spot something I could've done differently/better, but the end result was good and a vast improvement.
Here's a hacked-together schematic of what I did:
Here's what it looked like before (balanced heaters, but few other mods)
And after I hacked on it (could still be tidier, but at least it's quiet!)
Hopefully this helps someone in a similar situation in the future.
And that's the catch--they're all noisy. Very noisy. Re-capping them sometimes helps a little bit, but it's always been clear that there are other problems.
All the best-sounding ones I've heard have been implemented in the most mercenary, minimal way: just add a quarter-inch jack that couples directly to the grid of the first stage (it's probably "wrong" to not to use a grid stopper of any kind, but I don't argue with results).
About a year ago, I decided to modify mine for balanced heaters with a pair of 100R resistors as an artificial center tap, thinking this might help make it quiet. The amp just laughed at me--any improvement was so marginal as to seem insignificant to the noise that still remained.
Last night I got so fed up that I decided to blow it apart again, and I think I've finally solved the issue.
The original had two Aerovox-style multi-section can capacitors--one serving as the first two power supply filters (plus 6AQ5 cathode bypass), the other serving as the last stage of filtering plus preamp stage cathode bypass. So far so good. The cans were both isolated from the chassis to give control over how the grounds were routed. Again, so far so good.
But then there was one grounding bus connecting the two cans, and almost all grounds were connected to this--the volume control, the 12AX7 grid's ground reference, the main power supply filters... everything. This whole bus was connected to the chassis near the 12AX7.
The aftermarket input jack (on mine) was connected to chassis near its physical location. Power transformer HT center tap was connected to ground at a PT bolt (my artificial center tap for the heaters was connected there, too). This was far away from the aforementioned ground bus. Safety ground connected to another PT bolt. Output transformer ground clear across the chassis, at an OT mounting bolt.
Additionally, there was a bit of other stuff near the input, designed to interface with other Magnasync equipment. It's not part of the amp and can be deleted.
Here's how I made it quiet:
- Input jack isolated from chassis using rubber grommet (I was out of nylon shoulder washers)
- Stock ground bus removed
- All components circled on schematic below deleted; their 4-pin connector repurposed as a new star ground point
- All grounds marked with a "1" on schematic below rerouted here (this could be done just by repositioning original components in nearly every case)
- This star ground point connected to chassis near the end (I used the 4-pin's original ground point), far from the second star point (below)
- All grounds marked with a "2" on schematic below rerouted to a separate star point at PT mounting bolt (with the center tap grounds).
- OT ground left in its factory location (relocating it didn't seem to change much)
- Very-sloppy factory heater wiring touched up slightly, routing over the top of 12AX7 socket (rather than forming a loop around)
- 120VAC wiring twisted together, with lead dress improved
- Wiring to headphone/speaker out relocated away from the main heater run (the two were side-by-side for virtually the whole length of the chassis)
This was all a bit rushed... it was done in a moment of frustration, kinda, so it's not necessarily the prettiest job (I just yanked original resistors over to a better place, rather than replacing them). But functionally, I'm happy with the results. At full volume the hiss now predominates (not the hum, as was egregiously the case before).
I'm not sure whether every single thing I listed above was necessary/helpful, and someone might spot something I could've done differently/better, but the end result was good and a vast improvement.
Here's a hacked-together schematic of what I did:
Here's what it looked like before (balanced heaters, but few other mods)
And after I hacked on it (could still be tidier, but at least it's quiet!)
Hopefully this helps someone in a similar situation in the future.
Last edited:
