Kegger/blueglow kt88 project

Discussion in 'Tube Audio' started by Tubeglowpio, Jan 6, 2019.

  1. Tubeglowpio

    Tubeglowpio Active Member

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    Happy new year everyone! I've been really busy but have wanted to build the Kegger se kt88 for awhile now and after seeing blueglows youtube videos I'm sold. I enjoy my 5 watt se builds but have a party coming up this spring and need more power so am ready to get started but have a few questions. I'm going to follow the schematic blueglow posted pretty closely but wonder how he got around 450 volts on the kt88 plates using a 360 volt power transformer. Using psud2 I'm getting around 450 when using a 400 volt transformer with 5AR4. I'm thinking either psud2 isn't as accurate as I thought or the impedance of the power transformer must be pretty small? I'm also am trying to think if I should run it closer to 500 volts and run kt120's for a tad more power.
     

     

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  2. Tubeglowpio

    Tubeglowpio Active Member

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  3. UncleBingo

    UncleBingo AK Subscriber Subscriber

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    I may be missing something but we always learned it as 360 X 1.414 = 509 (rectified) then subtract your resistive losses (choke etc)
     
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  4. Tubeglowpio

    Tubeglowpio Active Member

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    That is good for an estimate but you need to take in account to the transformer secondary resistance, so if I physically measured the ac output of the transformer that math should work but I just was curious if that Edcor power transformer really has that low of secondary resistance compared to Hammonds. The hammonds 400 volt is about 95 ohms on the secondary, Edcor doesn't list any of there resistance specs for power transformers for some reason. Screenshot (8756).png
     
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  5. Tubeglowpio

    Tubeglowpio Active Member

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    Actually I now see the transformer used was a 380 and 360 was just another one listed on the schematic, so that sounds more possible. I notice a note saying the 360 volt transformer is if you want to run kt88 and el34, shouldn't a 380 or 400 volt transformer be fine for a el34 as well especially if using a 5u4?
     

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  6. GordonW

    GordonW Speakerfixer Subscriber

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    The Dynaco Mk II used a 430-0-430 PT with 5U4 and EL34s. That worked fine, as long as the caps were taken care of appropriately. So, a 400-0-400 should also work fine.


    Be aware- the one thing that is necessary with 380-0-380 and 400-0-400 transformers, that is not necessary with a 360-0-360 transformers, are supply caps with voltage ratings IN EXCESS of 500v. With a 5U4 (fast start) rectifier, a 360-0-360v PT will cause the power supply voltage to just touch 500v briefly, on startup (before the output tubes start conducting)- so, 500v caps would be OK. With a 380-0-380 PT, the voltage goes up to 525v, and with a 400-0-400 transformer, the voltage is closer to 560v briefly. That would require caps with at least that high of voltage rating (525v caps with the 380-0-380, and 600v caps with the 400-0-400 or the 430-0-430 of the original Mk II).

    .
    Regards,
    Gordon.
     
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  7. Kidmoe

    Kidmoe AK Subscriber Subscriber

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    He's using an Edcor power transformer. Most Edcors that I've used have a pretty low secondary resistance. That probably explains the difference. Are you using the full secondary resistance for your calculations or just from center tap to one end?
     
  8. Dave451

    Dave451 AK Subscriber Subscriber

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    I also think this will be my next build. Have been wanting to try SE and was impressed by Mark's videos. I was also convinced about the quality of the circuit based on his noting of the long series of optimizing in Kegger's thread. Watching this thread with interest! The ability to use different output tubes is also very intriguing and I'm also interested in Mark's planned preamp build as well.
     
  9. Tubeglowpio

    Tubeglowpio Active Member

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    Thanks for the input, I ended downloading a handy bias app on my phone so I could quickly check different bias points for all the possible tubes. The 380-0-380 seems pretty good actually, I will just have to add a switch with two different cathode resistors to hit the bias points and then also switching between 5u4 and 5ar4 I should be able to use any el34,6l6,kt66,kt77,kt88 and kt120. Yeah I already decided I would use 630 volt caps at least for the first 2 couple of caps when I first saw this circuit, I feel like 500v caps are going to worry me in the long run.
     
  10. Tubeglowpio

    Tubeglowpio Active Member

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    Yeah at first I saw the 360-0-360 transformer on the schematic and thought the secondary resistance would have to have been pretty low but then I saw he was actually using the 380-0-380 which makes sense. I have always used the full secondary resistance in psud2 because I thought they are asking for the full secondary resistance, honestly never thought about it too much, so I should always cut it in half then?
     
  11. Tubeglowpio

    Tubeglowpio Active Member

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    Yeah I've been watching his channel for years and I've been looking at the original schematic and threads on the kt88 build before his series and was already ready to build it but then wanted to wait to see what he thought about it and I had never seen him so excited about an amp, he compared it to the joy of holding your newborn in the hospital lol. I also just got the analog discovery 2 for free from a friend that is going for mechanical engineering so he said he wouldn't ever really use it, so it will be interesting to see how close my results are to Marks.
     

     

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  12. Kidmoe

    Kidmoe AK Subscriber Subscriber

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    Yeah, Measure from center tap to one end and use that as the secondary resistance or just use half of the full winding measurement. To do it right though open up the settings box and insert the values that you get for resistance of primary and secondary windings as well as the unloaded voltage readings off the secondary and at what wall input voltage to the primary. Your total impedance number should be higher than the resistances measured.
     
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  13. Tubeglowpio

    Tubeglowpio Active Member

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    I'm really considering building monoblocks because using two of those same 380-0-380 transformers will give me enough voltage and will be able to handle enough current to get a good use out of kt-120 tubes not to mention should improve snr a bit and amazing channel separation. Also this will be easier on the rectifier tubes.
     
  14. chuckworkb

    chuckworkb AK Subscriber Subscriber

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    Monobloc route would also make the wiring less congested. The only issue is the additional cost of 2 power transformers and two enclosures. I'd be curious what you end up using for enclosures when you get around to getting that far along in your planning if you take that route because I was thinking of possibly doing the same thing.
     
  15. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    Rather than making 2 mono amps it is better to build a separate power supply chassis. Containing the AC production on a chassis that is away from the DC components is a big plus in terms of noise in the output.
     
  16. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    BTW, this schematic is almost an exact copy of the "RH Universal" amp that was developed about 5 years ago. The only difference is the front end tube and the LED used on the cathode. You should check it out if you want to have more background on its development.
     

     

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  17. Squidward

    Squidward Scrappy Mod Staff Member Moderator

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    Kegger's amp is switchable from UL to triode, RH is pentode. Looks like the RH Universal schematic has a solid state device in the power tube cathode, Keg eschewed that design. Keg's input tube cathode arrangement is a specific and novel arrangement selected both by ear and use of a distortion analyzer.

    They're both small part count single ended amps, so there is some similarity in the schematic. That being said, I find that each amp is about as different as one can get with the parts selection for certain positions (tube cathodes), and output tube toplogy (UL & Triode or Pentode). About the only similarity that they have is the use of Shade short loop feedback, which at one point Alex erroneously claimed to have invented.
     
  18. Tubeglowpio

    Tubeglowpio Active Member

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    Well and I'm planning on using motor run caps for the output bypass caps and other film caps for the power supply. I'm just going use two of the Hammond steel chassis from mouser, I think its a 14*10*3 chassis that I will be using. The cost will be about 200 bucks more to do the mono blocks.
     
  19. Squidward

    Squidward Scrappy Mod Staff Member Moderator

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    Whenever I build an amp, years later I will remember the compromises I made more than the extra cost incurred by upgrades implemented. Monoblocks sound like a fun build, and you've got me thinking about my own Kegger KT-88 project list.
     
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  20. Tubeglowpio

    Tubeglowpio Active Member

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    I did consider that as well but don't like the idea of running some power wires between two chassis plus it would be nice to be able to have the mono blocks by each of the speakers. I'm still thinking about it but will be ordering most of the parts today.
     

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