Hello all, I’m new to the forum and still learning amp repair. I have a long way to go. My question is, I’ve read the thread on the Magnatone 107 and making it safe. I have a Magnatone 107 B! I don’t believe it’s a hot chassis. But I wanted to post the schematic and see what you all think. Any help would be appreciated Thanks
That old circuit met the very poor safety regulations of the day.
Early on, the plug was not even polarized, and the power outlet was not polarized.
With non polarized system, you had a 50 / 50 % chance that the amplifier "return" circuitry was connected to the mains power Hot wire!
Later, at least Hot and Neutral had different sized lugs.
But even with properly connected polarized 2 wire plugs, and properly connected 2 wire polarized power outlets, there is another problem . . .
In the US, Neutral is a different voltage than ground. Yes, true.
Only all the way back at the mains power panel does Neutral Volts = Ground volts.
I have seen as much as 8VAC difference between Neutral and Ground (in a company's work center).
Try connecting Neutral to Ground, and get sparks, hot wiring, etc.
Safety First!
Prevent the "Surviving Spouse Syndrome".
I am glad that you have an isolation transformer on the way.
Even with an isolation transformer, there can be some leakage current from the primary to the secondary (typically caused by the capacitance of the windings either to each other, or capacitance of both windings to the laminations.
Early on, the plug was not even polarized, and the power outlet was not polarized.
With non polarized system, you had a 50 / 50 % chance that the amplifier "return" circuitry was connected to the mains power Hot wire!
Later, at least Hot and Neutral had different sized lugs.
But even with properly connected polarized 2 wire plugs, and properly connected 2 wire polarized power outlets, there is another problem . . .
In the US, Neutral is a different voltage than ground. Yes, true.
Only all the way back at the mains power panel does Neutral Volts = Ground volts.
I have seen as much as 8VAC difference between Neutral and Ground (in a company's work center).
Try connecting Neutral to Ground, and get sparks, hot wiring, etc.
Safety First!
Prevent the "Surviving Spouse Syndrome".
I am glad that you have an isolation transformer on the way.
Even with an isolation transformer, there can be some leakage current from the primary to the secondary (typically caused by the capacitance of the windings either to each other, or capacitance of both windings to the laminations.
Last edited:
You need a power transformer.
From experience you can get away with 2 of the old isolation transformers for bathroom wall sockets. You need one for th eB+ and one for the filiments.
This one is similar, but uses just a 50EH5, https://www.t-linespeakers.org/tubes/SEP_50EH5.html
I only used 1 of them for teh build, i burned it out. Realizing my mistake i wired the filaments directly to minimize futzing, but it really should have had a second transformer.
dave
From experience you can get away with 2 of the old isolation transformers for bathroom wall sockets. You need one for th eB+ and one for the filiments.
This one is similar, but uses just a 50EH5, https://www.t-linespeakers.org/tubes/SEP_50EH5.html
I only used 1 of them for teh build, i burned it out. Realizing my mistake i wired the filaments directly to minimize futzing, but it really should have had a second transformer.
dave
You also should add a fuse on the primary side of the transformer. On the original circuit the rectifier tube does have in a sense the role of safety fuse if the high voltage supply shorts to ground, you need a fuse to protect the added transformer and to save the rectifier tube. Route the wires on the primary side of the insulation transformer to keep them separate from the rest of the circuit. The transformer should be placed at some distance from the output transformer and input tube, and mounted with a different orientation to avoid magnetic coupling (hum). One of the speaker leads should be tied to the chassis, if this has not been done already.
Actually that tech was half right. If you have no mains xformer, you have no 6.3V for the heaters, you only have the mains. So, you put all heaters in series, hoping to get the sum up to the mains voltage or make up the shortfall with a heat-wasting pwer resistor in series. That's what you see in your amp. So that tech should have said: "if the filaments are wired in series, you do have a hot chassis" .
Jan
The filaments in series only tells you one thing . . . there is no power transformer.
The reason that the chassis is hot, is not because of the series string filament connections . . .
It is because the tubes plates and cathodes are powered Directly off of the power mains with no power transformer.
The DC high voltage return (-) normally goes to the amplifier tube's cathode circuits, and the cathode circuits connect to the chassis.
That means that the chassis is either connected to the power mains Neutral (bad); or much worse to the power mains Hot!!!
. . . Now which way did I plug that 2 wire plug into the wall socket?
The reason that the chassis is hot, is not because of the series string filament connections . . .
It is because the tubes plates and cathodes are powered Directly off of the power mains with no power transformer.
The DC high voltage return (-) normally goes to the amplifier tube's cathode circuits, and the cathode circuits connect to the chassis.
That means that the chassis is either connected to the power mains Neutral (bad); or much worse to the power mains Hot!!!
. . . Now which way did I plug that 2 wire plug into the wall socket?
I guess the ground symbol at the jacks denotes chassis and not ground?
I'm trying to figure out what happens to chassis when the unit is switched off (caps charged, valves warm) mains plug in either orientation.
Is this just a US thing?
I am aware European stuff was made without transformers but AFAIK nothing metal was supposed to be touchable: bakelite knobs, wood cabinet, no external connections to the chassis.
I'm trying to figure out what happens to chassis when the unit is switched off (caps charged, valves warm) mains plug in either orientation.
Is this just a US thing?
I am aware European stuff was made without transformers but AFAIK nothing metal was supposed to be touchable: bakelite knobs, wood cabinet, no external connections to the chassis.
According to schematic, chassis is connected to the live ground trough the 0.05 uF capacitor with a 270K resistor in parallel. Current flowing from mains to earth trough the user body is limited to levels that were deemed to be safe by this capacitor, the 270K resistor, and the internal mica supports and cathode to filament insulation of the 12AX7. I've seen designs like this on european hot chassis devices. Fun fact: on 230V hot chassis devices with this circuit, if you short the 47K input resistor and touch the signal input jack, sparks may be seen inside the tube, from the cathode to the grid, with matching sound from the speaker. Operating a hot chassis device without a insulation transformer is a bad idea.