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K&K Audio / Lundahl Transformers

Tubes more powerful than the 6550

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Hi, Kevin,

I remember you mentioned som time ago you are trying out some tubes more powerful than the 6550. Is it the KT120 or somthing else ? Can we have some update on this ? Many Thanks.

Alan

Follow Ups:

RE: Tubes more powerful than the 6550 - KevinC 13:05:11 12/19/12 (9)
We have done many experiments with the KT-120 and have built four PP amps around the KT-120. It is the tube to use if you have lots of current and voltage to supply it with. We have used it with voltages up to 600V and currents up to 95mA and it has held up well. The sound is excellent especially at the higher dissipation operating points in triode, where it has no equal for ease of use and watts per tube.

In fact I have modified the ST-70 upgrade kit to suit the higher currents that are preferred with the KT-120 and will soon have a power transformer available that is compatible with the original or replica ST-70 chassis and will provide higher voltage, as well, to get the most out of a triode-connected ST-70. My prototype is running at a B+ of 535VDC and 75mA per tube, and sounding great.

Kevin Carter
K&K Audio
www.kandkaudio.com
- RE: Tubes more powerful than the 6550 - akltam 22:48:09 12/19/12 (8)
  Thanks for the update. I am currently using the LL1669A for a pair of the 6550 in triode mode using LL1663.
  Do you think I will benefit by swapping the KT120 into it ? Will the LL1669A supply voltage to be too low for this ? The LL1669a can only supply around 430V (when drawing 60ma per tube in triode connection) If so, will there be a Lundahl transformer able to supply the higher (suitable) voltage.
  Thanks.
  Alan
  
  Edits: 12/20/12 12/20/12
  - RE: Tubes more powerful than the 6550 - KevinC 04:22:28 12/20/12 (7)
    One of the amplifier designs I referred to uses an LL1669A for the B+ power supply for 4 output tubes. I am using a separate transformer for the filaments and connected the 4 filament windings in series with the high voltage windings to get more voltage. However, at 70mA per tube (and 30mA for the input stage) the LL1669A runs hot, so there is no more current to be had there, assuming I want to have the LL1669A still working a year from now ;+)). The sound of this amplifier with LL1682AM/PP as outputs is really excellent.
    
    So I suggest turning up the bias current for your amp as it is now to perhaps 65mA and listening with the new operating point. If that motivates you for more, a higher voltage power supply with more current and KT-120s will likely be well worth it for you.
    
    It would be possible to use the LL1651 as the power transformer and get about 680-700VDC for the B+. I have used the KT-120s no higher than 600V so far, but they are rated for more voltage than this. With a 700VDC supply and the tubes operated with a common cathode current sink (Class A1) for bias the KT-120s would have about 620VDC from plate to cathode and should operate fine. Unfortunately, the LL1651 is not big enough to supply 4 output tubes at these voltages and currents of more than 60mA, so monoblocks it is for really heavy-duty Class A operation.
    
    Kevin Carter
    K&K Audio
    www.kandkaudio.com
    - RE: Tubes more powerful than the 6550 - akltam 05:18:46 12/20/12 (6)
      For this pair of amp I got, the LL1669A is running monoblock and powering only 1 pair of 6550. As I only need 2 of the filament supply, I can squeeze another dozen of volts using remaining filament supply. Thank you for the tips. Do you think I have advantage by simply swapping in a pair of KT120 here ?
      Is the new mains transformer you are getting for the ST70 capable of powering 2 pairs of KT120. If so, can you tell us the spec of this new transformer.
      I know that the Lundahl has a bigger core than the LL1669a. It will be interesting if they can produce a more powerful mains transformer that can power 2 pairs of KT120.
      Alan
      
      Edits: 12/20/12
      - RE: Tubes more powerful than the 6550 - KevinC 08:54:57 12/20/12 (5)
        
        In Reply to: RE: Tubes more powerful than the 6550 posted by akltam on December 20, 2012 at 05:18:46
        
        If you want to operate your output tubes at or near the 100mA bias current point, which your power transformer and output transformer will support without a problem, you will need to change to KT-120s. I recommend operating at the higher bias point, because it sounds better.
        
        The new ST-70 compatible power transformer will have dual 120VAC primaries, and 840VCT @ 475mA (with tap), 2 x 6.3V @ 4A, and 5.0V @ 2A secondaries.
        
        A larger core for higher voltage/current Lundahl power transformers does exist, but the transformer would likely cost over $350 each. That will limit the sales severely, so I'm not sure it's worth it.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Tubes more powerful than the 6550 - Brinkman 19:19:00 12/20/12 (4)
        
        In Reply to: RE: Tubes more powerful than the 6550 posted by KevinC on December 20, 2012 at 08:54:57
        
        "The new ST-70 compatible power transformer will have dual 120VAC primaries, and 840VCT @ 475mA (with tap), 2 x 6.3V @ 4A, and 5.0V @ 2A secondaries."
        
        Kevin,
        
        I've kept my mouth shut since you mentioned this power transformer to me in person back in October... but is it too soon to ask when you expect this to be for sale and how much you expect to be selling it for?
        
        I want one.
        
        Ben
        
        RE: Tubes more powerful than the 6550 - KevinC 06:56:23 12/21/12 (3)
        
        In Reply to: RE: Tubes more powerful than the 6550 posted by Brinkman on December 20, 2012 at 19:19:00
        
        Such admirable restraint! I should have them by the end of next week. I haven't really thought carefully about a price, but there will be two-tiered pricing, one for ST-70 upgrade kit owners and a higher one for everyone else. You. of course, are in the former category.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Tubes more powerful than the 6550 - Richard P 09:06:07 12/21/12 (2)
        
        In Reply to: RE: Tubes more powerful than the 6550 posted by KevinC on December 21, 2012 at 06:56:23
        
        Kevin,
        
        This is a most excellent bit of news. In addition, have you made further headway in developing a drop-in replacement timer for the 5AR4? Just curious. Have a most pleasant holiday!
        
        -Richard
        
        RE: Tubes more powerful than the 6550 - KevinC 11:25:06 12/21/12 (1)
        
        In Reply to: RE: Tubes more powerful than the 6550 posted by Richard P on December 21, 2012 at 09:06:07
        
        I have a time delay relay available right now for the ST-70 upgrade. Check it out on the Power Amplifier page.
        
        Even better, though, is that I have the first batch of the small/medium Lundahl transformer enclosures on the way. These will house LL1682/PP or LL1663/PP (and LL1660 sized transformers for other purposes) and fit in place of the Dynaco outputs. When I get mine, I will retrofit my ST-70 with LL1682/PPs and take pictures to show how to do it.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Tubes more powerful than the 6550 - Richard P 09:13:01 12/22/12 (0)
        
        In Reply to: RE: Tubes more powerful than the 6550 posted by KevinC on December 21, 2012 at 11:25:06
        
        I need to check the website more often! Thanks!
        
        -Richard

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