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no reverb in GA19rvt Crestline with 6C4 on V3

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  • #1

    no reverb in GA19rvt Crestline with 6C4 on V3

    I have a GA19rvt Falcon Crestline on the bench. it uses 3-6EU6's a 6C4, 2-6V6's and a 5Y3. I have worked on several of these amps and also a Epiphone EA28rvt that has the same circuit.
    This amp is different because it has an aluminum chassis! It also came to me in pieces and on inspection it was pretty much untouched inside, but the reverb transformer had the wires cut on top of the chassis before they went thru inside. The transformer tested open and I'm not surprised as the wires in these are like hairs and most break if they are pulled on at all.
    I've gone thru the amp and replaced the electrolytic caps and power resistors and such and it sounds great. I had an extra fender reverb transformer that I installed and still can't get reverb to work. I can connect speaker to the transformer, pull the power tubes and play a guitar thru the speaker, controlling the volume using the reverb control. I've replaced the rca cord from the recovery side with a new cord but no go. NOTE: I am using a fender reverb tank to go with the new transformer.
    I'm hoping someone can point me in a direction to troubleshoot this issue.
    I've got my working Falcon chassis on the bench next to this one and, everything seems connected properly. I've subbed my working Falcon tubes in place of the others with out any affect.
    Any help would be great. The attached schematic is for a Pathfinder but it is exactly the same circuit.
    Dave

    epiphone-ea-28rvt-pathfinder-amplifier-schematic.pdf
    Tags: None
  • #2
    Have you confirmed that signal is coming out of the pan? If yes then is there signal at the recovery grid? if yes then is there signal from the recovery plate? if yes is there signal on the downstream side of the reverb coupling cap? Have you checked to see that the switch is functioning correctly?

    Basically, anything downstream of what you've already checked. And FWIW there's still a possibility of a wiring error. I build mostly custom, on off's and I can't count the number of times I didn't see my error after many passes to confirm that I had things correct.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Originally posted by davohilts View Post
      ...I'm hoping someone can point me in a direction to troubleshoot this issue...
      You have shown that the reverb drive stage is working. Is the recovery stage (V2 second triode half) working at all? Start by checking the DC voltage levels on the recovery triode pins, check the recovery cable continuity. Do you hear a pop or buzz when you probe the grid pin? (V2 pin 5). Just treat the reverb recovery circuit as a pre-amp stage, inject a signal and signal trace it through the stage. Then on through C23, make sure the foot switch isn't closed (Either by accident or a bad shorted switch) then follow the signal through R41 which sums the wet signal with the dry and feeds the other V2 triode grid.

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      • #4
        thanks for the help Chuck and Tom!
        I have 175V at the V2P6, 0V at V2P5 and 1.45V at V2P4. If I inject a tone I can get it at V2P8, at R41, at C23 and V2P6. I get a strong signal at V3P5, weaker at V3P6, stronger at V3P7.

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        • #5
          Originally posted by davohilts View Post
          ...I have 175V at the V2P6, 0V at V2P5 and 1.45V at V2P4. If I inject a tone I can get it at V2P8, at R41, at C23 and V2P6.
          Are you injecting the test tone at the reverb recovery cable with the pan disconnected. It appears that the reverb recovery electronics are working.


          Originally posted by davohilts View Post
          ...I get a strong signal at V3P5, weaker at V3P6, stronger at V3P7.
          OK. That's the drive circuit which you already demonstrated is working. Where are you injecting the tome for that test and is the pan connected for that test?

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          • #6
            I injected the tone into the input jack and followed it thru the circuit with the tank installed.

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            • #7
              It appears that a lot of the signal chain is working OK. The wet signal is making it all the way to the junction of C23 and R41. There isn't much left after that. The wet signal going through R41 is summed with the dry signal going through R20 and that point connects to grid pin 8 of V2. That pin is not labeled on the schematic. If you are hearing the dry signal then we know that the V2 triode that feeds the phase inverter is working. Therefore, it seems that the problem is R41 itself or the connection between R41 and V2 pin 8.

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              • #8
                Originally posted by Tom Phillips View Post
                It appears that a lot of the signal chain is working OK. The wet signal is making it all the way to the junction of C23 and R41. There isn't much left after that. The wet signal going through R41 is summed with the dry signal going through R20 and that point connects to grid pin 8 of V2. That pin is not labeled on the schematic. If you are hearing the dry signal then we know that the V2 triode that feeds the phase inverter is working. Therefore, it seems that the problem is R41 itself or the connection between R41 and V2 pin 8.
                Wow Tom! I'm going to get back to you with what I find out. I've been banging my head against a wall trying to figure out why the verb won't work. It is a hard amp to work on because the board has very little clearance in the back so I've been poking around with my chopstick hoping to find a loose connection without any result. I really appreciate you knowledge on how to troubleshoot even over the internet! It is hard for me to explain things because of my limited understanding and I really value your insight!
                I'm too tired to be much good tonight but will get back on this maybe tomorrow night or saturday!
                I hope this will do the trick.
                Dave

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                • #9
                  Well Tom, you hit the nail on the head. When I decided to desolder R41 and test its value the resistor came apart. I soldered a new one in and the amp now works perfectly!!
                  I really appreciate your help on this one. For a guy like me, trying to trouble shoot the issue was like looking in a haystack for a needle.
                  The amp sounds fantastic and if I pop in a GZ34 in place of the 5Y3 I still get 5.2 volts across pins 2 and 8 on the rectifier and get a big boost in tone to boot! I'm thinking about keeping the GZ34 in place unless you think that is ill advised.
                  Thanks for your help on this, I really appreciate it.
                  Dave

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                  • #10
                    Cool that you were able to fix the amp. It was very helpful that you provided the schematic and you did a good job tracing the signal through the amp. It's nice to do organized trouble shooting rather than shotgun parts replacement.

                    It's probably OK to use the GZ34 in place of the 5Y3 in that amp as long as your B+ voltages don't exceed the filter cap voltage ratings. It will be a little more stress on the amp because the GZ34 will give higher B+ than a 5Y3 but that's common for guitar amps. We are used to do that if it makes for better sound.

                    Cheers,
                    Tom

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