Lafayette Kt600 pre amp question

Discussion in 'Tube Audio' started by robsrockin, Jul 5, 2018.

  1. robsrockin

    robsrockin Member

    Messages:
    63
    Hi Everyone,I am working on this pre amp and have come across an issue that I have not found before.After installing 7 of my nos mullards I was able to power it up slowly and listen to it for a bit 20 or 30 minutes and the thing sounds great,then I started feeling around for excess heat and found that one of the multi cap cans was getting pretty hot the others not hot at all.Can anyone please tell me if this is normal or should I replace this can also can this thing murder my tubes?Any thought or suggestions about this would be much appreciated.I really do have concerns about these tubes,I do not want to kill them.Thank you to any and all who may respond to this and have a great day folks.
     

     

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  2. knockbill

    knockbill Addicted Member

    Messages:
    7,378
    Location:
    SE PA
    Old PS caps should be replaced in a vintage amp that you want to use,,, along with wax/paper caps, and checking the value of all resistors...
     
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  3. shelly_d

    shelly_d Not An Audiophool

    Messages:
    6,375
    Location:
    Alameda CA
    Caps should not be hot. Shut down until you get them changed out. I would use new caps in place of the old can. If you want to keep the look of the old can, there are many how to's that teach you how to open the can up, remove the innards and "re stuff" them with modern electrolytics. Since this is the power supply, it will have little to no impact on sound quality, but a very large impact on amp and tube life.

    Shelly_D
     
  4. wlgrant6

    wlgrant6 Active Member

    Messages:
    485
    The KT-600 is a great preamp. Well worth a thorough going thru. I have one that I completely rebuilt the power supply, and changed out the coupling caps for orange drops and I love it. Have it feeding a Bob Latino VTA-70. Bill
     
  5. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    77
    Location:
    NJ
    Oh Yeah ! Fantastic Preamp. Perhaps, the KT-600 is the finest early stereo vintage preamp. The engineer team who created these, including legendary designer Stu Hegeman, did their math and implementation very well. They sent closely matched parts in every kit or assembled unit, providing precise phono equalization. Early Mono discs sound delicious with this pre. The versatility of the KT-600 is almost unmatched, the sound delightful...

    In a working unit, when a power supply cap gets hot, it has got to get replaced. The power tranny in the KT-600 is not robust and they chose a half-wave rectified power supply; certainly not the best method. But, when working right, these preamps sound simply fine, very fine. If you run the preamp with a hot cap, the likelihood is that the diode and power tranny can be damaged. Hum will be imminent as well, as the cap derates with heat. By all means, upgrade the power supply caps, renew the diode, and replace any signal coupling caps which leak any DC voltage to the following stage. If your unit has the white cased coupling caps, quickly replace them all. Those white coupling caps some units came with are some of the worst caps ever found in vintage amps. If you have the black Pyramid or General Instrument "Imp" caps, some will still be okay for a while, but could be bettered....First, please get that power supply renewed....ENJOY the KT-600....
     
  6. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    3,295
    Location:
    New York City, NY
    All true. The ESR of older capacitors can climb to be several Ω and the constant charging cycles generates a great deal of ohmic heating.

    Moving to a full-wave rectifier will greatly offload the transformer and improve regulation. A half-wave stresses the transformer and filter because all of the current charging occurs in only half the cycle. The charging therefore becomes a square-wave of high current spikes with a low duty cycle.

    This increases noise because the transformer's inductance combines with its parasitic capacitance to form an LC tank circuit, and the square wave stimulates that tank to ring. This produces noise bursts which sap amplifier power.

    This is true, but large electrolytics poorly filter mains harmonics and high-frequency noise. That requires smaller decoupling capacitors.

    Sound advice but I'd go a bit further: all coupling capacitors must be replaced with new ones, and film is best. When (not if) a fifty-year old coupling capacitor fails it puts DC onto the grid which turns the tube fully on, chernobyling it. Very expensive when it blows an output stage.

    Those Japanese coupling capacitors were never good when new, and the point about the white ones should be well taken. The gray ones aren't any better.
     

     

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