Kegger kt88 SE-UL amplifier as a winter build

Discussion in 'Tube Audio' started by robnec, Oct 12, 2017.

  1. robnec

    robnec Active Member

    Messages:
    376
    Location:
    NY NY
    Hello. I was thinking for a lon KT88-SE_UL_fbk_LED_6DJ8_ECC85_425V.gif g time to finally make myself a SE amplifier. I decided to build a Kegger designed one. I have tubes, power transformer, choke and the rest. The missing element are output transformers which I have to order. Before I do it I wanted to ask if the schematic I have is a final one and it should be followed. Robert.
     
    Northwinds likes this.
  2. primosounds

    primosounds AK Subscriber Subscriber

    Messages:
    1,363
    That looks like a good circuit. The most critical part in Single ended is power supply and layout. Since there is no noise cancellation as in a push pull circuit you need to be extra careful about layout of wiring and components. The filament wiring should be tightly wound pairs. Power trans and choke along with the rectifiers is best put as far away on the chassis from the input tube. You do realize that this amp will be very heavy unless you make monoblocks. I have been making my high power single ended amps with a separate power supply chassis. this has the added benefit that the AC nasties can be isolated by simply moving the power supply chassis. Do you have a schematic for the power supply?
    I have used Edcor and Electraprint output transformers. They both sound good with the Electraprint sounding a little more refined. Hammond also makes a good OPT for this use and supposedly it has the best bass.
     
    Last edited: Oct 12, 2017
  3. robnec

    robnec Active Member

    Messages:
    376
    Location:
    NY NY
    Hello. Thank you fir taking time to answer. I have built PP kt120 amplifier so I am aware of weight and scale of the project. I do it for myself so time and expense is not a main consideration, to certain level. I want to use dc for input and output tubes heaters. My main concern is the schematic because I have seen several versions of it. Most of the builders had used Edcor output transformers so I think I will do the same unless there are reasons for a change. Robert.
     
  4. Tinkerbelle

    Tinkerbelle Everything in Moderation ------- Hudson Valley, NY Super Mod Subscriber

    Messages:
    6,324
    Location:
    Hudson Valley, NY
  5. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,220
    Location:
    Hoboken, NJ USA
    Or using a SE KT120 is probably a good option also.
     
  6. robnec

    robnec Active Member

    Messages:
    376
    Location:
    NY NY
    Hello. Yes. That is a thread I was trying to find. Thank you.
    My plan is to use kt120 tubes that I already have installed in PP amplifier. The only missing elements are output transformers. Again I think I will buy Edcors as Kegger himself suggested. Robert.
     
  7. primosounds

    primosounds AK Subscriber Subscriber

    Messages:
    1,363
    Edcor makes good trannys. The only drawback is the wait while they make them.
    Using DC for the heaters is not necessary and just makes things more complicated. My SE KT120 amp has just a faint hum with your ear in front the speaker but that is on Altec 19 speakers which are very high efficiency and with other speakers you don't hear anything. Some SE builders claim that using DC for the heaters has a detrimental sonic imprint. My suggestion is to consider running the power tubes under fixed bias, this will give you more power and also allow you to change power tubes with just adjusting the bias to accommodate the different tubes. Well, you also need to be able to adjust the B+ by using different rectifier tubes. This can make for an extremely flexible power amplifier.
     
  8. prelius

    prelius AK Subscriber Subscriber

    Messages:
    580
    Location:
    NorthEast USA
    I would not do DC heaters for this amp. Kegger's KT88 SEUL is great, I've built a few of them. Never needed DC for the heaters. I also converted this schematics from auto to fixed bias, as promosounds suggested.The amp will definitely benefit from this mod. In addirtion, I made an "integrated" amp based on the SEUL schematics (low gain single stage preamp in front), and I just elevated the heaters 30-40v above ground... No hum heard with my Cornwalls

    If anything, I would suggest following Kegger's PSU design. This is where most of the noises can be filtered out. Good luck!

    Cheers, Paul.
     
  9. robnec

    robnec Active Member

    Messages:
    376
    Location:
    NY NY
    Hello. Gentlemen. I am able to build anything you want but not without the schematic. Sorry. I know my limits.. If fixed bias is preferred let it be but give me schematic. To be honest the whole purpose of my posting was to find out the best version of this amplifier. Robert.
     
  10. prelius

    prelius AK Subscriber Subscriber

    Messages:
    580
    Location:
    NorthEast USA
    Then I would build it per Kegger's schematics first. You have the amp. Do you have the PSU schematics? You can use the same schematics as in Kegger's original KT88, but you have to adjust power tranny capacity, chokes, resistors in in that schematics to handle up to 350mA of current. Please let me know if you can't find it...
    Cheers, Paul.
     
  11. primosounds

    primosounds AK Subscriber Subscriber

    Messages:
    1,363
    If you want to go step wise do the cathode bias amp first but leave room for implementing fixed bias but leaving the option for fixed bias.In my case i wanted to use the many different types of power tubes that i have collected so fixed bias allows me to use 6l6, 6l6gc, 6ca7, kt66, kt88, 6550, kt90, kt99, kt120, even kt150. If i was just building to a specific output tube then cathode bias would be preferred.
    Fixed bias requires another source of DC volts which is made negative. I prefer to use a transformer and then full wave bridge rectifier with schottky diodes, and then filter caps sometimes with a small choke. Then bias pots. So, there is a lot more circuitry involved. It is essentially building another power supply. One of the benefits of the fixed bias supply is to remove the cathode resistor and electrolytic cap from the sound of the power tube. It also increases power output by about 10% or more depending on where you set the bias current. This is not insignificant where output can be around 10 watts. It also enables much greater flexibility with your amplifier with the adjustment of the power tube bias for tube rolling. Essentially, i felt that if i am spending a lot time and money building an amp i wanted to make it more flexible to get more out of it.
    Schematics are easily perused through a google search. The important thing is to note the primary AC voltage from the circuit shown and they you try to find a bias transformer to match. A lot of builders use a 6.3v or 12.6v filament transformer and wire it backwards to get 63vac or 126vac as the output. The bias circuit uses very little current so a small .3 or .5 amp filament trans is more than adequate.
     
    Last edited: Oct 14, 2017
  12. robnec

    robnec Active Member

    Messages:
    376
    Location:
    NY NY
    Hello. Thank you again for your interest. I will be using kt120 tubes so I understand cathode bias is preferred. I am planning to simply move power tubes from one amplifier to another. I will try to order transormers from Edcor early next week. Robert.
     
  13. srb02

    srb02 "Infinity Speaker" Man Cave

    Messages:
    254
    Location:
    Taunton, Mass
    Hi.
    http://audiokarma.org/forums/index.php?threads/kt120-150-se-ul-build.734130/

    This is the amp I built, Kegger setup the layout, and finalized the circuit and power supply for this amp, using KT120, and KT150.
    It was the SE parallel schematic, he adjusted for a single power tube.
    It is a wonderful sounding amp, gets used almost daily.
    Beware: there are a substantial amount of 120 and 150's that have problems running more that 65% (red-plating on one side) or so of rated specs. This is due to sloppy alignment when they were built.
    Buy from a trusted source and make sure you can send back any defective ones.
    Again just a spectacular sounding amp.
    my 2 cents Stu
     
  14. robnec

    robnec Active Member

    Messages:
    376
    Location:
    NY NY
    Thank you for posting schematic for kt120 tube with power supply. I need more power so I have to use two power tubes.
    Gentlemen. I have to order output transformers. What primary? 3.75k or 3.5?
    Robert
     
  15. primosounds

    primosounds AK Subscriber Subscriber

    Messages:
    1,363
    There is not enough of a difference in those 2 to make a difference in the circuit. If you look at a graph of distortion and load resistance (operation characteristics) of a GE 6l6gc, for example, it is relatively flat for those numbers. For the 6550A , the kt120 is an oversized version of this tube, the plate to plate load resistance is about 5K for lowest distortion and shows a fairly flat curve from 4500 to 5200 ohms. Again, choosing between the 2 values you want will not be an appreciable difference.
     
    Last edited: Oct 16, 2017

Share This Page