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VHT Lead 40 Unfunctional

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  • VHT Lead 40 Unfunctional

    I tried powering on my VHT Lead 40 a few weeks ago and after about 2 minutes of power, it went dead. No indicator lights are on, and the preamp tubes aren't glowing, but the power tubes are glowing. I have replaced all fuses inside the chassis as well as the power supply caps, and a few other caps that looked bad. Checked all solder joints and all seemed fine. Still no luck. Not sure where else to check for problems. I would appreciate any advice you all might have here.

    Thanks

  • #2
    A schematic would clarify but I suspect the silly DC heaters syndrome.
    Like a lot of amplifiers that use Chinesium pre amp valves that are inherently heater cathode noisy, applying a rectified AC supply to the heaters helps to reduce noise from heater cathode leaks due to poor manufacturing techniques.
    Look for a bridge of 1Amp diodes feeding the 1.2Amp heater supply, usually 1N400*, I fit 1N4392 1.5A 100v. Under rated diodes cause failure. Obviously this is only experience and not exact to your amplifier as I have no schematics.
    The special 20 has many fuses and a common heater supply, so as I say, just a guess.
    Last edited by Jon Snell; 07-25-2020, 08:48 AM.
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #3
      Here is the Lead 20/40 schematic.
      I have seen references to the 40 being a 'prototype' as some had (2) 5881 output tubes while the schematic shows (4) EL84s.
      Attached Files

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      • #4
        The LV supply in the schematic (lower left area) runs the preamp tube heaters and probably the indicator lights as well. Check the outputs of REG1 and REG2, should be 15 and 30V. (TP18 and TP19)
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #5
          If there is a 30volt rail, from the seriously stressed regulators, check R12 & R13. They reduce the 30volts down to about 24 ish for the series wired heaters.
          Last edited by Jon Snell; 07-26-2020, 10:33 AM.
          Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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          • #6
            Thanks for the replies everyone, and the schematic Jazz P. Tried googling that thing for hours and couldn't find it. Looks like there's a bit of a consensus on checking the regulator circuit. I'll have to pull out the voltmeter this week when I've got time.

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