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  • Deluxe Reverb problem

    Hi, I've built a deluxe reverb clone and have hit a snag. My screen and plate voltages on the 6v6s are both only 153v. I get a zero reading on pin 8 (through 1 ohm resistors), both mA and mV. I measured and set the negative bias voltage at -35. It's not a tube problem- already swapped another pair with same results.

    B+ is 440, then the second node drops all the way down to the 153v. The remaining power caps measure about as they should.

    Obviously there's no output, unless I dime channel 1 and get faint crackling notes. Vibrato channel is silent all the way up.

    Does this seem like a power cap or maybe transformer problem? I've checked the wiring and solder joints thoroughly.

  • #2
    Originally posted by tyler8611 View Post
    Does this seem like a power cap or maybe transformer problem?
    Neither. There's a choke between the main B+ and the second power supply node that feeds the 6V6's screen grids. There would have to be something awful wrong with that choke for the screen grid voltage to drop all the way to 153V. Let's measure the resistance between those choke leads. Power OFF of course.

    Also possible the hi voltage nodes are wired in an out-of-order way.
    This isn't the future I signed up for.

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    • #3
      I'm confused. You say, "B+ is 440". Then you say, "My screen and plate voltages on the 6v6s are both only 153v" The output tube plate node (B+) is before the choke and goes through the OT directly to the plates. You almost have to have something wired wrong.

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      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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      • #4
        Originally posted by Leo_Gnardo View Post
        Neither. There's a choke between the main B+ and the second power supply node that feeds the 6V6's screen grids. There would have to be something awful wrong with that choke for the screen grid voltage to drop all the way to 153V. Let's measure the resistance between those choke leads. Power OFF of course.

        Also possible the hi voltage nodes are wired in an out-of-order way.
        Ok I'll do that and report back. Also will check the doghouse again.

        Comment


        • #5
          Originally posted by The Dude View Post
          I'm confused. You say, "B+ is 440". Then you say, "My screen and plate voltages on the 6v6s are both only 153v" The output tube plate node (B+) is before the choke and goes through the OT directly to the plates. You almost have to have something wired wrong.

          [ATTACH=CONFIG]48992[/ATTACH]
          I meant the voltage from pin 8 of the rectifier to the first power cap node. That is 440 and the second node feeding the screens is 153. The OT plate voltage is also 153.

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          • #6
            Originally posted by Leo_Gnardo View Post
            Neither. There's a choke between the main B+ and the second power supply node that feeds the 6V6's screen grids. There would have to be something awful wrong with that choke for the screen grid voltage to drop all the way to 153V. Let's measure the resistance between those choke leads. Power OFF of course.

            Also possible the hi voltage nodes are wired in an out-of-order way.
            Between the leads is 154 ohms.

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            • #7
              Yes. I understand. If you look at "pin 8 of the rectifier to the first power cap node", follow it up through the OT. It connects directly to the tube plates. If you measure 440VDC there, you should also measure it on the plates, unless you have a horribly bad, hot, melted standby switch. Measure DC on both sides of the standby switch. Is it the same on both with the switch closed?
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

              Comment


              • #8
                Originally posted by tyler8611 View Post
                I meant the voltage from pin 8 of the rectifier to the first power cap node. That is 440 and the second node feeding the screens is 153. The OT plate voltage is also 153.
                Voltages to ground of course.

                Comment


                • #9
                  Originally posted by The Dude View Post
                  Yes. I understand. If you look at "pin 8 of the rectifier to the first power cap node", follow it up through the OT. It connects directly to the tube plates. If you measure 440VDC there, you should also measure it on the plates, unless you have a horribly bad, hot, melted standby switch. Measure DC on both sides of the standby switch. Is it the same on both with the switch closed?
                  True and everything is wired correctly. Weird problem.

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                  • #10
                    Originally posted by tyler8611 View Post
                    True and everything is wired correctly. Weird problem.
                    True that. It's very weird to have 440V at one end of that choke and 153V at the other. If that's really the case it should be shooting flames. I'm still holding out for a mis-wiring. If you have an electronically knowledgeable friend have them give it a look-see. Sometimes a different set of eyes can pick out what's amiss.
                    This isn't the future I signed up for.

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                    • #11
                      IMO, the problem is before the choke. According to post #1 the plate voltage is also low. The plate voltage is derived "pre-choke". I'd like to know if the same DCV exists on both sides of the standby switch. Then, if we're good there, we see what the voltage is on the OT primary CT. It should be easy to follow the voltage and see where it's not. The B+ caps should be the same electrical point as the plates as far as DC is concerned, yet we have 2 different voltage readings, so I agree- something is not connected properly.
                      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                      • #12
                        Don't know if this is your problem, but I replaced the filter caps in the dog house on my 78 SFDR.

                        When I powered up, something was not right, I never measured any voltages and nothing smoked... but I knew something was "off".

                        Powered down and checked my work... one of the cap leads I soldered into the eyelet was a bit long, and it missed the insulating card and shorted to the chassis.

                        Maybe have a look in that area.
                        If it ain't broke I'll fix it until it is...
                        I have just enough knowledge to be dangerous...

                        Comment


                        • #13
                          Originally posted by The Dude View Post
                          IMO, the problem is before the choke. According to post #1 the plate voltage is also low. The plate voltage is derived "pre-choke". I'd like to know if the same DCV exists on both sides of the standby switch. Then, if we're good there, we see what the voltage is on the OT primary CT. It should be easy to follow the voltage and see where it's not. The B+ caps should be the same electrical point as the plates as far as DC is concerned, yet we have 2 different voltage readings, so I agree- something is not connected properly.
                          I forgot to mention that I discovered earlier that on standby, the voltage readings were actually normal. Drop happens when I take it off standby. The switch is wired correctly with the leads going exactly where they should. Very strange.

                          I also measured the resistance of the transformer and choke leads and they were fine. I drained the filter caps and measured those and the readings began around 90k and kept creeping upwards past 150k, so I'm not sure whether that's normal.

                          I ran a flashlight under the filter cap board- no shorts. I'll have to keep searching.

                          Comment


                          • #14
                            Originally posted by galaxiex View Post
                            Don't know if this is your problem, but I replaced the filter caps in the dog house on my 78 SFDR.

                            When I powered up, something was not right, I never measured any voltages and nothing smoked... but I knew something was "off".

                            Powered down and checked my work... one of the cap leads I soldered into the eyelet was a bit long, and it missed the insulating card and shorted to the chassis.

                            Maybe have a look in that area.
                            I thought that could be a possibility also so I looked under with a flashlight and saw nothing grounding out. It's a garolite board on standoffs so I was able to see.

                            Comment


                            • #15
                              Originally posted by tyler8611 View Post
                              I forgot to mention that I discovered earlier that on standby, the voltage readings were actually normal. Drop happens when I take it off standby. The switch is wired correctly with the leads going exactly where they should. Very strange.
                              Are you saying that, when you have the standby switch closed, your 440V B+ drops at the filter caps?
                              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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