I have been working on the EA-72 and was wondering about the 2 on positions on the rotary switch. I'm guessing polarity but looking at the schematic I don't see how they are achieving that. Anyone know?
I’m looking at this schematic: http://schematicheaven.net/gibsonamps/ea_72-constellation-bass.pdf
The switch seems to have 4 positions:
Position 1: PT energized, ‘neutral’ grounded via small cap (.022), red light on
Position 2: PT energized, ‘live’ grounded via small cap, red light on
Position 3: PT energized, small cap out of use, green light on
Position 4: PT off, no lights on.
Position 4 is clearly ‘OFF’, but as for the rest – doesn’t make any sense to me! Hope someone can explain.
The switch seems to have 4 positions:
Position 1: PT energized, ‘neutral’ grounded via small cap (.022), red light on
Position 2: PT energized, ‘live’ grounded via small cap, red light on
Position 3: PT energized, small cap out of use, green light on
Position 4: PT off, no lights on.
Position 4 is clearly ‘OFF’, but as for the rest – doesn’t make any sense to me! Hope someone can explain.
Just to add to it, I've now noticed a fourth gang to the switch (not mentioned in the legend) which seems to mute the signal in positions 3 and 4.
This would be 'standby' for position 3.
But I still don't get the two 'ON' positions (position 1 and position 2).
This would be 'standby' for position 3.
But I still don't get the two 'ON' positions (position 1 and position 2).
Probably the .022 cap is to shunt EMI that might be on the mains. It would have to be a special 'Y' type to be safe. Still seems strange to have the option to connect it to either live or neutral, though.
Perhaps you are supposed to switch between 'position 1' and 'position 2' to try to reduce any audible mains-borne noise.
Perhaps you are supposed to switch between 'position 1' and 'position 2' to try to reduce any audible mains-borne noise.
Actually it is pretty straightforward. The drawing shows the switch in the OFF position. The S1a deck thus completes the mains circuit through the primary, so position 1 is off, and 2,3,4 are all ON. The S1b deck is the ground reference. Position 2 doesn't matter because the amp is in standby there. Position 3 connects the 0.022 cap to the mains hot side, while position 4 connects it to the neutral side. So this deck is the equivalent of the old ground switch on the back of a zillion old Fenders. yes, it is there to chose the position with the least hum, and potentially reduce any voltage on the chassis. There is a three wire grounded cord on the mains, but outlets are not always wired correctly or completely, especially when this amp was made. This is the cap often referred to as the "death cap."
The S1c deck controls the pilot lights. Position 2 is standby and lights the green standby light. Positions 3 and 4 light the red run light.
The S1d deck shorts points A and B together in positions 1 and 2, and opens that short in positions 3 and 4. A and B are the opposing power tube grids, and shorting them together silences the amp. Thus the amp is in standby.
Oh, sorry Malcolm, I seem to have numbered them in reverse order of your description. I didn't mean to do that.
So position 1 is off, position 2 is standby and positions 3 and 4 are run with choice of ground reference cap.
The S1c deck controls the pilot lights. Position 2 is standby and lights the green standby light. Positions 3 and 4 light the red run light.
The S1d deck shorts points A and B together in positions 1 and 2, and opens that short in positions 3 and 4. A and B are the opposing power tube grids, and shorting them together silences the amp. Thus the amp is in standby.
Oh, sorry Malcolm, I seem to have numbered them in reverse order of your description. I didn't mean to do that.
So position 1 is off, position 2 is standby and positions 3 and 4 are run with choice of ground reference cap.
Thanks guys for all the help. So then when I switch between the 2 ON positions one is noticeably more quite than the other. This would be normal operation according to the above explanation.
- Status
- Not open for further replies.