This is an adjunct thread to the thread on modding the 3 channel Triple Rec parallel loop to series. There is a simple mod I call the "FX Mix Pot Mod" suggested by 94Tremoverb and others. Essentially, you remove the wire connecting the FX Mix Pot to the Send Pot to convert the loop to serial. Some people, including myself, notice there still appears to be sound "bleeding" through. Also, there is a bit of confusion on the function of the FX footswitch. I think I know what the problem is, and I would like to see if it sounds reasonable to people with more knowledge than myself. Here is the different behavior of the FX Mix Pot Mod in my Tremoverb and 3 channel Triple Rec:
Modded Tremoverb: I performed the FX Mix Pot Mod on my Tremoverb. It works perfectly, and the footswitch still bypasses the loop, and the mix pot does nothing. With the loop engaged, if you disconnect the return cable from the amp, you get dead silence, no bleed through at all. If I press the footswitch to bypass the loop, the effects are taken out of the signal and you can disconnect the FX unit from the amp and still get the normal sound.
Modded 3 Channel Triple: The FX Mix Pot Mod works partially as it should. The FX mix pot is no longer operable. However, with the loop engaged and effects in the loop, if you remove the return cable from the amp, you still get some dry signal. Bypassing the FX loop with the footswitch results in the same faint dry signal you get when removing the return cable from the FX loop.
I do not think the dry signal is bleeding from the FX pot (or tube for the FX loop as the Mesa tech suggested). As I said, I think I know why the FX Mix Pot Mod produces different behavior in each amp.
Here is the serial loop from the schematic of the Rev 1F DR:
When the loop is engaged with the foot switch, LDRs 11 and 12 run the signal to the send and return jacks, and LDR 13 opens so the complete signal runs through the loop. Of course, bypassing the loop reverses the behavior of the LDRs.
Here is the FX loop from the schematic of the Tremoverb:
Notice that it is the same as the Dual Rec Rev 1F, with the addition of send and FX mix pots and an additional connection has been made from the FX send pot to the FX mix pot. The FX Mix Pot Mod simply removes the bit of dry signal that is not going the send and return jacks. LDR 16 is the FX bypass. I think this is why the footswitch still works and bypasses the loop on the Tremoverb.
The Tremoverb circuit is the same as the Rev G dual:
The 3 channel Dual and Triple seem to have TWO paths on which the dry signal can travel. The first is the usual connection from the send pot to the mix pot, and breaking that connection terminate 90% of the dry signal. However, the connection between the transistors opening and closing the connections to the send and return jacks seems to be a path that this "extra" dry signal bypasses the FX Mix Pot Mod. I think this is why the original mod in the thread has these transistors removed (I do not think it is necessary to remove the third mute transistor). Also notice that this circuit does not have the FX bypass route that the Dual and Tremoverb have. Therefore, when bypassing the loop with the footswitch, the amp should go dead silent. If the connection between the transistors is carrying the extra signal, shouldn't there be some way of getting rid of it without removing the transistors? Wouldn't the mod that someone posted from Mesa essentially bypass the transistors and take the second path for the dry signal via the transistors out of the circuit? That mod has the send and FX mix pots connected directly to the 0 ohm resistors next to the send and return jacks. Question is to all you techs out there that are more knowledgable then I am, does this seem reasonable?
Modded Tremoverb: I performed the FX Mix Pot Mod on my Tremoverb. It works perfectly, and the footswitch still bypasses the loop, and the mix pot does nothing. With the loop engaged, if you disconnect the return cable from the amp, you get dead silence, no bleed through at all. If I press the footswitch to bypass the loop, the effects are taken out of the signal and you can disconnect the FX unit from the amp and still get the normal sound.
Modded 3 Channel Triple: The FX Mix Pot Mod works partially as it should. The FX mix pot is no longer operable. However, with the loop engaged and effects in the loop, if you remove the return cable from the amp, you still get some dry signal. Bypassing the FX loop with the footswitch results in the same faint dry signal you get when removing the return cable from the FX loop.
I do not think the dry signal is bleeding from the FX pot (or tube for the FX loop as the Mesa tech suggested). As I said, I think I know why the FX Mix Pot Mod produces different behavior in each amp.
Here is the serial loop from the schematic of the Rev 1F DR:
When the loop is engaged with the foot switch, LDRs 11 and 12 run the signal to the send and return jacks, and LDR 13 opens so the complete signal runs through the loop. Of course, bypassing the loop reverses the behavior of the LDRs.
Here is the FX loop from the schematic of the Tremoverb:
Notice that it is the same as the Dual Rec Rev 1F, with the addition of send and FX mix pots and an additional connection has been made from the FX send pot to the FX mix pot. The FX Mix Pot Mod simply removes the bit of dry signal that is not going the send and return jacks. LDR 16 is the FX bypass. I think this is why the footswitch still works and bypasses the loop on the Tremoverb.
The Tremoverb circuit is the same as the Rev G dual:
The 3 channel Dual and Triple seem to have TWO paths on which the dry signal can travel. The first is the usual connection from the send pot to the mix pot, and breaking that connection terminate 90% of the dry signal. However, the connection between the transistors opening and closing the connections to the send and return jacks seems to be a path that this "extra" dry signal bypasses the FX Mix Pot Mod. I think this is why the original mod in the thread has these transistors removed (I do not think it is necessary to remove the third mute transistor). Also notice that this circuit does not have the FX bypass route that the Dual and Tremoverb have. Therefore, when bypassing the loop with the footswitch, the amp should go dead silent. If the connection between the transistors is carrying the extra signal, shouldn't there be some way of getting rid of it without removing the transistors? Wouldn't the mod that someone posted from Mesa essentially bypass the transistors and take the second path for the dry signal via the transistors out of the circuit? That mod has the send and FX mix pots connected directly to the 0 ohm resistors next to the send and return jacks. Question is to all you techs out there that are more knowledgable then I am, does this seem reasonable?
