Not long ago I lost the reverb in the Mark V.
At first I thought it was tube related (may have been caused by it). I was currently using vintage Mesa tubes that I purchased back in the 90's. Turns out the tubes are just fine, issue was a JFET (m2) decided to drop the gait and not turn off. 98 ohm shunt to ground on the signal will render the reverb extremely hard to hear on V4. I had removed the JFET and the reverb returned but much louder than I expected. It may not be the JFET but one of the relay drivers may have failed. Shortly there after I installed a pull down resistor of 220 k ohms, that helped but the tone of the amp was like it was out of the box, ice pick. Cured that with a low pass filter 560pF ceramic 1kV capacitor and what a big improvement. I still wanted to return the amp back to its original form so I ordered some J175 FETs. I had J174 but the gate to source voltage was much lower (1v compared to 10v) so I did not use them. With a New J175 in M2, sounded no different than not having the JFET in place. I put back my resistor and cap network connected in the S and D pads where the JFET was. Sounds much better with the mod. Not something I wanted to do but it sounds really good now. Also have more reverb level control for each channel. (reason why I believe the relay is not switching since it uses different value shunt resistors for each channel, better term to use is the lower portion of a voltage divider for the reverb signal, there is no change in reverb level due to change in channel selection. ) I am not recommending anyone to open the amp up and modify it. Also this is not a task for a novice tech to perform.
At first I thought it was tube related (may have been caused by it). I was currently using vintage Mesa tubes that I purchased back in the 90's. Turns out the tubes are just fine, issue was a JFET (m2) decided to drop the gait and not turn off. 98 ohm shunt to ground on the signal will render the reverb extremely hard to hear on V4. I had removed the JFET and the reverb returned but much louder than I expected. It may not be the JFET but one of the relay drivers may have failed. Shortly there after I installed a pull down resistor of 220 k ohms, that helped but the tone of the amp was like it was out of the box, ice pick. Cured that with a low pass filter 560pF ceramic 1kV capacitor and what a big improvement. I still wanted to return the amp back to its original form so I ordered some J175 FETs. I had J174 but the gate to source voltage was much lower (1v compared to 10v) so I did not use them. With a New J175 in M2, sounded no different than not having the JFET in place. I put back my resistor and cap network connected in the S and D pads where the JFET was. Sounds much better with the mod. Not something I wanted to do but it sounds really good now. Also have more reverb level control for each channel. (reason why I believe the relay is not switching since it uses different value shunt resistors for each channel, better term to use is the lower portion of a voltage divider for the reverb signal, there is no change in reverb level due to change in channel selection. ) I am not recommending anyone to open the amp up and modify it. Also this is not a task for a novice tech to perform.
