Lafayette Kt600 pre amp question

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

  1. robsrockin

    robsrockin Member

    Messages:
    65
    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

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    7,551
    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:
    6,525
    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 AK Subscriber Subscriber

    Messages:
    521
    Location:
    Louisville Ky
    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:
    172
    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:
    3,548
    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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  7. robsrockin

    robsrockin Member

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    Hi folks,sorry about just going away for a bit,had to go N.C. for a bit to help out my brother.I do appreciate all of this info that you have all sent to me.I'm guessing that a complete recap is in order here.I have been looking for a kit and or the caps and have found a guy on the bay that has a complete kit for 150 or best offer,My concerns now are should I try to get new cans or just put in the new caps that are with this kit? Also is there any chance that there may be a person out there that may have some experience with this pre amp that may have some lit.on what they did.I do know some about this work but tubes are pretty new to me still and I don't have a clue about the coupling caps or where they are,and there are some power diodes that come with this kit that I'm not sure about either.OK guys one more question and I promise I'll shut up and go away.do any of you know of any resistors that are jumping out at you that I should replace?Thanks again for any and all of you input and advice I am sure this will be very helpful to me.
     
  8. shelly_d

    shelly_d Not An Audiophool

    Messages:
    6,525
    Location:
    Alameda CA
    OK. Decide what you want with this. On the can caps there are basically 3 ways to go. A)Search for replacement cans that have the same or similar values, B) use modern caps wired into the circuit and cabled tied in place somewhere there is room under the chassis, or C) open the existing cans, remove the innards, and slide in modern caps (restuffing) with the leads leading out the bottom to hook up to the existing terminals from the old cap.

    Solution A is the easiest, it is drop in. The down side is that recent manufacture cans may not be available in the sizes you need and NOS can be very risky because electronics very definitely have a shelf life and you don't want one tat is close to end of life. Unless you can read the date codes on the caps and know what they actually translate to, you have no idea how old a NOS cap is. Personally, I don't trust ANY NOS caps.

    Solution B is probably the next easiest to implement. I will probably lower in cost then A but you must have the room under the chassis and use great care in insulating the leads and even cases of the caps (sometimes) to prevent shorts.

    Solution C costs as much as B and is a lot more work. You need to carefully select the caps for size to fit under the cleaned out can as well as for size and voltage. You need to do the work of removing the can, then carefully removing the metal crimp around the bottom of the cap, separating can from fiber board bottom, then cleaning out the insides of the cap. Then the new ones must be inserted, leads fed through holes existing or drilled through the fiberboard bottom and hooked to the terminals, avoiding any short circuits, then gluing the can back to the fiberboard base. It is far and away the hardest to do but the result is modern, safe caps with the look of the original.

    Once that is done your cap replacement is the same as for solid state units. Any cap that looks like an electrolytic or paper needs to be replaced by a new one with the same or greater voltage rating and same capacitance.

    Coupling caps are caps that have different DC voltages on each side of them. For example, a cap that has one side attached to the B+ or Plate of a tube and the other side to the Grid of another tube would be a coupling cap. If it could have DC on one side and there is only AC on the other side, that too is a coupling cap.

    Hope this helps a bit.

    Shelly_D
     
  9. rickl

    rickl Member

    Messages:
    88
    Location:
    Twin Cities
    I did A on mine. Rob, I'd suggest you keep it simple and swap out power supply caps and coupling caps. I had "black" coupling caps in my 1959 version. Orange drops made a difference.

    I didn't find any resistors I needed to replace. And I don't recall updating the PS diodes. Its been a while since I did this. Swing by and I can give you a hand.
     
  10. robsrockin

    robsrockin Member

    Messages:
    65
    Hi guys,Thanks for the info.I will be looking into how much room I have under the chassis,I have already ran into the problem of finding replacement cans for it can't get them all so I may have to do a little A and B and now it's time for me to go and find the coupling caps so I can change them as well.I'll let you know how it goes as I get moving on this repair.Have a good eve.
     

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