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  • 5e7 voltage problem

    Hey Guys here's my voltage readings
    V1, 1-211, 3-1.62, 6-207, 8-1.62. V2, 1-203, 2-1.7mv, 3-1.58, 6-309, 7-205, 8-204.
    V3, 1-232, 2-3.9mv, 3-1.87, 6-294, 7-26, 8-69. V4&5, 3-436, 4-438, 7-minus 45, 8-50.5mv.
    V6, 2&8-449, 4&6-369.
    Filter caps 1-362, 2-438, 3-449, 4-minus 46
    These seem alot different than the Fender schematic. Also my presence control does nothing. I have checked all parts placement against the layout & reflowed all solder joints. This is a Weber kit with Sylvania 5881's & a Sylvania 5u4g. Thanks for any help offered.
    Last edited by jbb52; 12-15-2008, 02:24 AM.

  • #2
    Voltages look about ball park, perhaps even on the low side. Wall voltages have risen since the Fender schems were drawn. I'm assuming that you are listing your filter caps from R to L (1 = preamp, 2 = PI, 3= B+ & Screen supply).

    Looks like you have a 12AX7 in v1, the schem shows a 12AY that will return less plate voltage in the same circuit.

    Are you sure the presence does "Nothing" as opposed to "not very much"?

    What plate current do you have? -46v for the bioas voltage sounds a little low?

    Comment


    • #3
      I do have a 12ax7 in v1. I,ve added a bias pot & now have it set 14.7 per tube. I have pin 3 - 394.7v & across the 1 ohm resister on pin 8 - 74.5. Does that look ok? If the presence control is doing anything I can't hear it. Also when I turn off the standby switch I hear a crackling sound. Thanks for the help.

      Comment


      • #4
        Do you just have one x 1ohm resistor from pin 8 of BOTH tubes to ground (e.g. 1 resistor for 2 tubes), or 1x 1ohm per tube?

        I'm assuming that's 14.7W per tube for plate dissipation?

        Please confirm (it's most likey what you mean but doesn't hurt to check).

        Comment


        • #5
          I do have a 1ohm on each power tube. Someone told me the bias was too high & need to be around 12 watts per tube. I couldn't get that low with the pot & resistors that I had si I changed the 6.8k to a 3.3k & put an 8.2k after the pot to ground. I now have pin 3- 464, pin 5- -53.5 & across the 1ohm resistors 26. If I,m thinking right I now has just over 12 watts per tube. Does that sound right? Thanks.

          Comment


          • #6
            "If I,m thinking right I now has just over 12 watts per tube. Does that sound right? Thanks." 26mv is not totally out of the ball park, but perhaps I'd be looking for 30-35mA per tube. So have a listen at 26, then 30, then 35 and see what you prefer?

            There is no "right" bias current per se, as long as you are not stressing the tubes & transformers, whatever sounds best to you is right. 12-15W is typical for a 2x 6L6 Fender. But it's best to judge the tone by plate current specifically than by Wattage dissipation, use the wattage figure more to ensure that you're not going to burn anything up.

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            • #7
              Originally posted by jbb52 View Post
              I do have a 1ohm on each power tube. Someone told me the bias was too high & need to be around 12 watts per tube. I couldn't get that low with the pot & resistors that I had si I changed the 6.8k to a 3.3k & put an 8.2k after the pot to ground. I now have pin 3- 464, pin 5- -53.5 & across the 1ohm resistors 26. If I,m thinking right I now has just over 12 watts per tube. Does that sound right? Thanks.
              No, I don't think so.
              Most of the real tweed Bandmaster, Pro and Super amps seen to have had their power tubes running much hotter in idle current and they had lower B+ too.
              The typical B+ is around 410-430vdc and the idle current I'd find, when the mains would be set for about 117vac, would be around 16-19 watts for each power tube.
              Older tweed amps ran their power tubes even hotter.
              Bruce

              Mission Amps
              Denver, CO. 80022
              www.missionamps.com
              303-955-2412

              Comment


              • #8
                "Older tweed amps ran their power tubes even hotter." Yes, but pre 5E# 6L6 Fenders were cathode biased (as was the 5E5) and didn't have to worry about current flying up when the amp was played through, unlike the 5E7.

                If Jbb52 can get his B+ down to 430v, with the PT supplied with his kit, without burning up his tubes (say <40mA, might get away with a bit more), then all well and good. If not, then I'd aim for a respectable plate current, that he likes the sound of and just live with the voltage that he gets...after all he doesn't have a genuine 5E7 PT and his ability to rebias will skew the final voltages somewhat.

                Comment


                • #9
                  Originally posted by MWJB View Post
                  "Older tweed amps ran their power tubes even hotter." Yes, but pre 5E# 6L6 Fenders were cathode biased (as was the 5E5) and didn't have to worry about current flying up when the amp was played through, unlike the 5E7.

                  If Jbb52 can get his B+ down to 430v, with the PT supplied with his kit, without burning up his tubes (say <40mA, might get away with a bit more), then all well and good. If not, then I'd aim for a respectable plate current, that he likes the sound of and just live with the voltage that he gets...after all he doesn't have a genuine 5E7 PT and his ability to rebias will skew the final voltages somewhat.
                  The PT in the Weber kit used to be a Chinese version of my custom wound, Heyboer 798 PT... as authorized by me a few years ago.
                  Assuming they are still using the same basic PT specs, it has two high voltage taps 660vct and 720vct, both at 200ma.
                  When used with the lower voltage tap and a NOS 5U4GBs, I run the power tubes at between 40ma and 50ma each depending on the OT chosen, my mood and my customer's "tone taste".
                  Most love the amps at around +45ma and about 18-20 watts per tube with the smaller 35 watt output tranny.
                  Bruce

                  Mission Amps
                  Denver, CO. 80022
                  www.missionamps.com
                  303-955-2412

                  Comment


                  • #10
                    Aha! Indeed, I clean forgot about the spare, lower B+ secondaries! Just as well you're here :-)

                    Comment


                    • #11
                      I just built my first amp, also a weber 5e7, and have gone through much of the same process in terms of looking at the old fender schematic voltages and comparing them. once I put a 12ay7 in the first stage I had pretty close to the original voltages except on V2 where I had much higher than expected. I'm still a bit confused by that because my B+ nodes are all within 5V of the old schematic, I'm more confused at how fender ever got close to 20% of those readings because from what everyone tells me and if you look at a 12ax7 load line, it seems like their voltages should be higher.
                      Also I was pretty sure my presence wasn't doing anything. JB, did you happen to ground the presence cap to the back of the pot itself? thats what I did, I think I overcooked the pot. I also put a feedback bypass in using the (unused) ground switch, I figured if the presence wasn't working I'd try the amp with no FB. I ordered a new pot (a much better quality one) and once I put it in the presence worked, though it's a very subtle effect, I think I may try to convert it to a mid control since the amp is already pretty 'present'.
                      If nothing else I hope this helps put your mind at ease.
                      cam

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                      • #12
                        Cam, I did ground my presence cap to the pot. My problem was the ot wires to the power tubes needed reversed. Works fine now. I,m still trying to get my head areound this bias thing.

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