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Tough to track down tube amp problem

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  • Tough to track down tube amp problem

    I picked up an old westinghouse console at the local recycling center and pulled out the tube amplifier. It's an HF-101x. The only schematics online are paywalled; I bought one but sharing it feels illegal (?). No idea what the law is on that.
    It's ~ 2x5W stereo amp with a 12AX7 preamp tube, a 12AX7 driver, 2 EL84 power tubes and a 6CA4 rectifier. I did a quick test before I took anything apart (perhaps a mistake) and everything seemed/sounded alright; volume and sound quality were fine allowing for an ancient turntable and speakers.
    I changed all the electrolytic capacitors, replaced some cracked bakelite tube sockets and of course replaced any components or wires that crumbled when I had to bend them out of the way. So, basically a complete rewire except the transformers.
    This was strictly a phonograph amplifier with no line level inputs or a radio in the console and no preamp on the thurntable, but when I plug a turntable in it's so quiet that you'd swear I plugged the turntable into a line-in. This is both with a turntable I know is working correctly and with the original. Further, if I add a preamp ahead of the amp it sounds fine, with no distortion and an appropriate output for it's wattage. It's the same regardless of speakers. It behaves exactly like the preamp in the circuit was just replaced with a pair of wires. I didn't think it was really possible to have a tube amp circuit not working and still have the signal pass unmolested.
    I've changed every tube with either one known to work (12ax7s and el84s) or a new one (i have no other amp that takes a 6CA4 to test it on.) The voltages on the tube sockets read correctly and all the controls work smoothly.
    I didn't find anything tracing the circuit against the schematic, although presumably I wired something together wrong or subbed in a component of the wrong value(?). If you're getting the correct voltages from the transformers they must be ok, right? All the solder joints are fresh, I even re-flowed the ones I hadn't had to touch because I was running out of ideas. I'm feeling the problem must be before the power amp section but beyond that I'm thoroughly confused.
    Any thoughts on where you would start looking for the cause of this?

  • #2
    Sorry, I am not clear on what the problem is you are trying to chase down. Can you try to restate it more clearly?

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    • #3
      Simply put, the preamplifier stage of the amplifier is not providing any gain. It's still letting the signal through and not providing any noticeable colour to the sound. The passive tone controls between the pre-amp tube and the driver tube still works. It's gone from being a complete phonograph amp to just a power amp with an equalizer on the front end.

      Before I started "fixing" it, it was plenty loud.

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      • #4
        [QUOTE=. Further, if I add a preamp ahead of the amp it sounds fine, with no distortion and an appropriate output for it's wattage. It's the same regardless of speakers. It behaves exactly like the preamp in the circuit was just replaced with a pair of wires.[/QUOTE]

        Based on your findings above, I would check plate & cathode voltages of the preamp tube/s. It's passing signal, but voltage could be low enough to impact gain. It's something common to both channels, so make sure your grounds are good. If this amp uses the chassis as a ground path back to the power supply caps, check for any resistance to the node of that preamp tube. Without seeing the schematic, it's tough to diagnose.

        ** As always - keep one hand in your back pocket when probing high voltages.
        Last edited by tom_s; 09-19-2018, 04:06 PM. Reason: Safety tip added
        Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.

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        • #5
          Since it has el84 and 6ca4 in it I'm going to assume it's going to use 6 volts on the heater of the 12ax7. 12AX7 has a center tapped heater, it will work with 6 or 12 volts. A problem will show across both halves of the tube identically if there's a pin9 problem. Should see around 300 milliamps through pin9.

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          • #6
            Originally posted by dj-sandwich-ham View Post
            I picked up an old westinghouse console at the local recycling center and pulled out the tube amplifier. It's an HF-101x. ...
            Any thoughts on where you would start looking for the cause of this?
            A thought:
            Due to the age of this console - the Phono circuit was likely designed for older Ceramic cartridges that have higher Output Voltage of a Magnetic Cartridge ( I presume you are using )
            Before I started "fixing" it, it was plenty loud.
            Same phonograph/turntable?
            "Not a Speaker Designer - Not even on the Internet"
            “Pride is your greatest enemy, humility is your greatest friend.”
            "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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            • #7
              tom_s: For plate voltage I'm getting 121V and the schematic calls for 135V. At the cathode I'm measuring 1.04V with a target of 1.22V. I was assuming that was all pretty close? I guess I don't really know how close things have to be.

              devnull: yeah all of the tubes are getting 6.3V.

              Sydney: yeah, the original turntable is still much quieter than before I began tinkering.

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              • #8
                Those voltages are a touch on the low side, but not nearly enough to cause such a loss of gain. It has to be something in common to both channels - power supply, a grounding issue, or even a bad shared cathode resistor or bypass cap on the output tubes. Here's a thread I found on AudioKarma about ceramic cartridges -
                I resurrected an ancient Voice of Music Stere-o-matic four-speed changer today, and it seems to run pretty well now that I've gotten it going again....
                Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.

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                • #9
                  Are you able to link or post the schematic? My guess is your signal wiring (shielded cable?) has a fault, most likely up front.

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                  • #10
                    Schematic is a no-go until I figure out my scanner or find the original file somewhere in my backups, but I am going to try replacing the input wiring tomorrow on your suggestion. Most of the wiring was going to go anyway when I put it into a separate cabinet, so it's worth a shot.

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                    • #11
                      You can always take a pic (of the schematic).

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