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SVT 3 PRO - hum

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  • #1

    SVT 3 PRO - hum

    This came in with no sound. The 125ma tube voltage supply fuse was blown. I am pretty sure it's a fast blow, but should be a slo blo. However, all I have are 500ma slo blos right now, and the fuse isnt blowing with that value.

    There is a prevalent hum. The hum is ONLY there with both J11 and J12 driver tubes installed. Being that the heaters are in a series DC supply they must both be installed in order to work. I replaced both with new tubes and still hum. Grid resistors and grounds are good. DC voltages look normal. The +/-16v and +/-65v supplies are good as well. I disconnected the fan, still hum. Of note is that the turn on delay is about 20 seconds. Seems quite long. The fan blows immediately upon throwing the power on. Oddly, the hum almost completely goes away when the TUBE DRIVE pot is cranked. I get a fluctuating DC voltage of ~50mv on the speaker output with no load and about the same amount of AC voltage.

    Any ideas? Im curious if the split load tube stage could be the problem. Im not familiar with that setup.

    Preamp
    http://music-electronics-forum.com/a...55-534xxhb.pdf

    Power amp (not sure if this zip file link works)
    http://music-electronics-forum.com/a...g-svt3-pro.zip
    Last edited by lowell; 06-15-2015, 05:09 AM.
    Tags: None
  • #2
    Try going in the Power Amp In jack.
    If that stops the 'hum', then for sure it is a preamp problem.

    Pulling J11 kills it because you broke the chain to the power amp.

    Do any controls affect the hum?
    Pulling J9 & J10 has no affect?

    I would check the coupling cap at J11 (C19) for leaking Vdc.

    Comment

    • #3
      Forgot that detail. I had plugged a dummy plug into the PA-in jack and it didnt stop the hum. No controls affect hum. Not even the MUTE. The Tube Drive, as stated, largely reduces hum when turned up.

      I'll check C19

      Comment

      • #4
        Put your current meter in place of that fuse. If there is more than 125mA, it should help you locate the fault.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

        Comment

        • #5
          C19 is good. I'm stuck. I even DC grounded the grids of V1/J11 pin2, and then V2/J12 pin2, hoping that might narrow it down. Neither had any effect on the hum. Again, pulling those 2 tubes (V1/V2) on the PA board kills the hum. I'm guessing at this point, but it seems that either shutdown circuitry, fan circuitry, or DC filament supply may be the problem. The +65dcv on top of the DC filament supply is good. Q17 Vb is ~53vdc. I can poke around for voltages, I'm not confident I could accurately interpret the data. If this seems like a good approach please do tell. And please specify which points at which to measure. Thanks in advance.

          Comment

          • #6
            Originally posted by lowell View Post
            Of note is that the turn on delay is about 20 seconds. Seems quite long.
            It seems that you asked about it several times. And I responded (also several times) that this is normal. I would say that the turn on delay in this amp is between 25 and 30 seconds. So 20 seconds is not long at all.
            I'm not sure where exactly is C19 capacitor but it seems to me that you have problem with power supplies for the tubes. And you have there just a few components: diodes D14 to D17, C14 capacitor (page 3 of the power amp), C4 capacitor and Q1 transistor (page 1). Does the voltage change on emiter of Q1 when you crank up the TUBE DRIVE pot? Most probably yes because it kills the hum but this is the first thing you shoud check. Is Q1 shorted? As a test you may remove the transistor and short emiter with collector (I mean bypass the transistor). Does the amp play OK in this case? If yes, you have just 3 or 4 components to be checked. Maybe a ground connection is missing. Have you check the PC board from the bottom? Are capacitors soldered correctly (especially C14 and C5)?

            Mark

            Comment

            • #7
              Rock on Markus, I think you're right. I'm getting 0-10ohms across Q1's pins with power off.

              Why would this cause hum? I'd think if it's shorted then plate voltage would simply be higher than norm. Also, is Q1 a current source for V2/J12?

              Comment

              • #8
                How about desoldering Q1 and checking it outside the circuit?
                I wouldn't call it voltage source. This is kind of a regulator called capacitance multiplier. It is described here: Capacitance Multiplier Power Supply Filter
                The capacitance appears on the output as multiplied by the gain of the transistor. If the transistor is shorted, it has no gain hence the circuit does not work correctly. Check the transistor.

                Mark

                Comment

                • #9
                  Ok thanks for the link. I will definitely be testing it out of circuit.

                  Comment

                  • #10
                    Q1 was indeed shorted. Tested shorted out of circuit. Replaced it and the hum is gone and the amp is fully functional. Thanks for the help!

                    Comment

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