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Help with voltages (no load) deluxe AB763 type build

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  • #1

    Help with voltages (no load) deluxe AB763 type build

    Hi All,

    Hammond tech support came through, the replacement power trans arrived this week. Got it all wired up. No load voltages look too high. With my trusty 250 watt light bulb current limiter, measured some voltages.

    Note: I do have the Immortal Mod's ss rectifiers installed on pin 7 to 6 and pin 5 to 4 on the rectifier socket. I have the hv wires going to pin 7 and pin 5. I checked with the "diode check" setting on my cheapo meter and they read OK, not shorted, not open.

    Line voltage: reading 122vac.


    With power off, and circuit all wired up (filter caps drained)

    Read 174 ohms across the choke, but this is with it wired in to the circuit. Can't tell whether this is useful info.


    Powered up, power switch on, standby off (doesn't matter since no rectifier)

    Without the rectifier tube installed:
    At the rectifier socket:

    pin 2 to 8 5vac
    pin 7 to pin 5 717vac

    Across green wires 6.3v

    Look OK.


    Powered down, plugged in the rectifier tube, powered up OK, no smoke no sparks.

    At the rectifier socket:
    pin 6 to ground 356vac
    pin 4 to ground 356vac (this is the circuit side of the SS rectifiers)

    standby OFF:
    pin 8 to ground 489vdc Eeek!

    Standby ON:
    pin 8 to ground 488vdc

    With standby on, getting about same voltage downside of choke.

    OUCH my filter caps are only rated 475v!!! (They are F&T 16uf 475v)

    Powered down, drained all filter caps.


    The Fender schematic reads 420 to 410 vdc on sides of the choke.

    Questions:

    Did I toast the filter caps?
    Do I need to swap out for 500v filter caps?

    Will the B+ voltages drop to values closer to what the schematic reads after powering up with all of the other tubes installed and the speaker hooked up?

    Thanks.
    Last edited by mikepukmel; 11-18-2017, 06:00 PM.
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.
    Tags: None
  • #2
    Please install the tubes.
    The Vdc voltages will drop accordingly.

    The caps are fine.

    Comment

    • #3
      The voltages are OK. It will come down a lot when you have a load. The caps do have a surge rating that allows them to typically take about 5% over rated voltage for a minute or so. If you allow to warm up before operating the standby you'll likely be well under 450V.

      Personally I would fit 500V caps in due course, but don't sweat it . Many amps have run like this for decades.
      Experience is something you get, just after you really needed it.

      Comment

      • #4
        Thanks Jazz P, and Nick! OK next step, all tubes in, speaker in and hopefully ... amplification!

        When I was looking for parts, the 16ufs I could find were 475's. The next up 500v were 22uf or maybe 20uf. Not knowing I was worried about straying too far from the values written on the schematic.

        It was less than a minute or two total, I think. A few 10's of seconds, just to get the measurements after turning the power on.

        How did Fender get by with 450v rated caps back then?
        The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

        Comment

        • #5
          That is how the standby switch came about. It was cheaper than higher voltage caps.
          Experience is something you get, just after you really needed it.

          Comment

          • #6
            Got one more question if you don't mind: with everything wired up, power off, no speaker connected, (no tubes in, only the rectifier tube is in) Im measuring 1 ohm between the 8 ohm lead of the output transformer and the black common wire from the output transformer. I checked a few resistors, and readings all look OK. Is there a simple way to test that the OT is ok before powering up? The choke reads about 174 ohms. The input side of the output transformer is reading about 404 ohms
            The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

            Comment

            • #7
              Originally posted by mikepukmel View Post
              Got one more question if you don't mind: with everything wired up, power off, no speaker connected, (no tubes in, only the rectifier tube is in) Im measuring 1 ohm between the 8 ohm lead of the output transformer and the black common wire from the output transformer. I checked a few resistors, and readings all look OK. Is there a simple way to test that the OT is ok before powering up? The choke reads about 174 ohms. The input side of the output transformer is reading about 404 ohms
              It's OK. The OT secondary has very low DC resistance otherwise all the power would be dissipated in the OT rather than the speaker.
              Experience is something you get, just after you really needed it.

              Comment

              • #8
                Originally posted by nickb View Post
                It's OK. The OT secondary has very low DC resistance otherwise all the power would be dissipated in the OT rather than the speaker.
                Awesome, thanks Nick. OK here goes ...
                The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                Comment

                • #9
                  Originally posted by mikepukmel View Post
                  Awesome, thanks Nick. OK here goes ...
                  It.

                  Sounds.

                  Great!


                  Thanks to everyone.
                  The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                  Comment

                  • #10
                    Originally posted by mikepukmel View Post
                    It.

                    Sounds.

                    Great!


                    Thanks to everyone.
                    THAT, my friend, is why we do this
                    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                    Comment

                    • #11
                      Originally posted by SoulFetish View Post
                      THAT, my friend, is why we do this
                      Oh man, I wish I had a list of all of the people to thank, who helped with suggestions and answered my often annoying 'newbie' questions.
                      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                      Comment

                      • #12
                        It's crazy how wall voltage can effect the plate voltage , wall voltage at about 124 vac & the plates were around 440vdc , used the variac at about 119vac & the plates dropped down to around 415vdc .
                        "UP here in the Canada we shoot things we don't understand"

                        Comment

                        • #13
                          First time biasing the amp (Deluxe AB763 schematic) do these numbers look OK?

                          Hi All!

                          As you said, after I put all the tubes in, checked everything out, B+ voltages dropped quite a lot.
                          I ran through the first attempt at setting the bias, using the output transformer DC resistance and v drop method. The reason for that was my toy of a voltmeter was out of digits when I tried to measure the drop across the 1 ohm resistor. (Will remedy that soon, this weekend hopefully)

                          These are Electro Harmonix 6v6 tubes.

                          V Measurements taken with amp on, volume at 0, warmed up for about a minute.
                          Data after a few adjustments:

                          Inner 6v6 tube:
                          =========
                          Plate (pin3) to ground 433vdc
                          Voltage drop pin 3 to center tap 3.75v
                          DC resistance pin 3 to center tap 209ohms
                          3.75/209 = 0.0179A
                          433v*0.0179A = 7.769watts

                          Outer 6v6 tube:
                          =========
                          Plate (pin3) to ground 432vdc
                          Voltage drop pin 3 to center tap 3.70v
                          DC resistance pin 3 to center tap 198ohms

                          3.70/198=0.0187A
                          432v*0.0187A = 8.07watts

                          I was surprised at how sensitive the power changes are to a relatively small turn on the bias pot.

                          I don't have any frame of reference for these numbers. Is around 18ma or 19ma a reasonable number for these tubes and amp? Also, as these tubes are rated 14W max. Is roughly 8/14 = roughly 57% an OK bias point, for tube safety etc?

                          As an aside, I had the bias voltage set to a larger negative voltage for the first startup, so the very first measurements I got was around 3.3W to 3.5W and at that level the amp sounded pretty good. I haven't played it yet, it was after midnight when I did the bias.

                          Will redo everything when I get a better meter.
                          The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                          Comment

                          • #14
                            Biasing on the cold side will hurt absolutely nothing in the amp. Play it that way to enjoy the amp as long as you like. Once you get a more sensitive meter - to measure mV across the 1-Ohm resistor - then you can dial in the bias to whatever is recommended for long life and and smooth breakup. Nice work!


                            edit: BTW, what is your screen voltage?
                            If it still won't get loud enough, it's probably broken. - Steve Conner
                            If the thing works, stop fixing it. - Enzo
                            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                            Comment

                            • #15
                              Originally posted by eschertron View Post
                              Biasing on the cold side will hurt absolutely nothing in the amp. Play it that way to enjoy the amp as long as you like. Once you get a more sensitive meter - to measure mV across the 1-Ohm resistor - then you can dial in the bias to whatever is recommended for long life and and smooth breakup. Nice work!


                              edit: BTW, what is your screen voltage?

                              Thanks, will do! Oops, don't know the screen V, will measure it and update the post.
                              The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                              Comment

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