Posted by Goneill (A) on July 15, 2006 at 21:55:13

In Reply to: Re: Ella mods: diodes, resistors and bias pots posted by RHone on July 3, 2006 at 18:00:41:

OK, I’ve tried the screen diodes change and I have to agree with Al (aka tubegarden). It does seem to improve the sound, making it smoother, more musical and having more presence. Of course I might be imagining it as I’ve done quite a lot of changes recently, but I feel it’s worthwhile. Besides, it’s easy to do and if you kept your old power supply diodes when you changed them to Schottky’s then it’s free.

I found this information about what the change achieves and how it works:

Dennis Grimwood

There’s a lot to read in there, but my interpretation of it is that the screen (grid 2) is positively charged to help draw the electrons off the cathode, past the negatively charged controlling Grid (grid 1) towards the plate. Without this not so many electrons move, which is why in triode mode you lose nearly half the power of the amp. The problem is that the screen not only helps the electrons on their way to the plate, but being positive it actually attracts some of them itself. This sets up AC currents on the screen circuit as well as reducing plate current. By adding a diode to the screen the circuit is broken for AC which means the electrons continue to the plate, resulting in a cleaner sound (no AC messing up the OPT) and about 10-20% more power. As Dennis says, it makes the pentode behave more like a triode but without the resulting loss of power.

Apologies to Dennis if I’ve got this all wrong, and I recommend you read the original pages as there is a lot of interesting stuff there. This change is described in item "4. Silicon diode feed" as well as the introductory pages on Screen Grids and Ultra-Linear.

Dennis did give me one warning though. By putting the extra current through the plate you can drive the tube beyond its normal operating range. The best way to avoid problems is to make sure the grid has enough negative charge to control it. In the Ella this is easy as each tube has its own bias – just make sure you haven’t set the bias reading close to the maximum mV reading recommended for the tubes. On my KT88’s I use about 480mV which is well below the maximum of about 600mV and I have had no problem.

I’ve updated the photos and diagrams on my web page:

• My web page   (Open in New Window)

• tried it today - bcherry 00:37:41 07/21/06 (0)
  In Reply to: Re: Ella mods: screen diodes posted by Goneill on July 15, 2006 at 21:55:13:

  It's a significant improvement. More quiet between notes, less busy, smoother. Pretty good result on the value scale. I used 4 x in4007 (1000v/1a).
  Thanks for that one Al, and Graham for posting the drawing.

  Brian
  
  Take control! DIY.
  
  

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• Screen diodes - better explanation - Goneill 07:55:57 07/19/06 (0)
  In Reply to: Re: Ella mods: screen diodes posted by Goneill on July 15, 2006 at 21:55:13:

  Dennis has sent me the following description of the mod which is rather more accurate than my summary. Thanks Dennis!

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  In a normal Triode, Tetrode or Pentode amplifying valve it is well established that variation in DC plate voltage does not cause a change in through current.

  The Control Grid (Grid #1) is most usually used to control electron/current flow through the device.

  In the case of a Tetrode or Pentode the function of the Screen Grid is to regulate DC current flow through the device to a predetermined ratio between change in Control Grid voltage to change in plate Current for all input signal levels. This is effected by installing the Screen Grid between the Control Grid and Plate but considerably closer to the Cathode than the Plate. Triodes do not have a Screen Grid.

  This physical relationship between the elements in the valve enables the Screen Grid to increase the attraction of electrons to it, thereby increasing electron flow, and to shield the Control Grid from electrons reflected back from the Plate.

  Thus the Cathode and Control Grid, not being able to discern the difference between Screen Grid and Plate, always see a "Triode" - but once electrons reach the Screen Grid, because it is made of a wire coil with gaps between the small diameter individual wires (similar to a picket fence) most of the electrons just pass through to continue on to the Plate for collection.

  Managing the interaction between the Control Grid (Grid #1) and the Screen Grid (Grid #2) results in improved efficiency compared to a Triode - eg more gain, more output voltage and more power output for any given DC supply voltage.

  But in so doing the Screen Grid consumes power, which is lost as heat but also, and most importantly, diverts some of the available AC power output away from the load.

  By adding a correctly polarised diode into the line feed to the Screen Grid in the manner described, the DC supply relationship to the power valve circuit remains unchanged but the AC current in the valve is prevented from accessing the Screen Grid as an output terminal, hereby offering a shorter current path. Consequently all the available current is directed to the Plate, as in a Triode.

  Since the AC current through the device is the source of output power, the result is improved performance.

  Thus it might be said that the addition of the diode causes the valve to behave similar (but not identical) to a Triode but with the increased efficiency of a Tetrode or Pentode.

  The sound produced is more similar to the bright, clean sound of Tetrode/Pentode than Triode.

  In the case of connecting a Tetrode or Pentode as a Triode, by strapping the Screen Grid to the Plate, the valve behaves as a Triode. Inserting the Diode as described has a similar effect on the electron/current flow but the sound will still remain the "triode" sound - albeit it cleaner and brighter.
  

  • Full description   (Open in New Window)

  
  

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• Calling Al - bcherry 05:09:02 07/17/06 (7)
  In Reply to: Re: Ella mods: screen diodes posted by Goneill on July 15, 2006 at 21:55:13:

  

  Al, just want to confirm if you have connected your diodes the same way?
  Brian
  Take control! DIY.
  
  

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  • Re: Calling Al - Goneill 06:38:28 07/17/06 (6)
    In Reply to: Calling Al posted by bcherry on July 17, 2006 at 05:09:02:

    Hi Brian,

    I think Al is using his Ella in triode mode, so it will look like:

    
    
    

    Note that in both UL and triode cases the diode faces towards the screen, pin 4. Al, can you confirm this?

    Thanks,
    Graham
    
    

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    • Re: Calling Al - tubegarden 15:33:12 07/17/06 (5)
      In Reply to: Re: Calling Al posted by Goneill on July 17, 2006 at 06:38:28:

      Yes, that is the correct orientation, cathode toward the screen :)

      On mine, I just put the diode from pin 6, where the 220R was already tied, to the plate, pin 3.

      Previously, I converted to triode mode by snipping the UL tap from pin 6 and using some tasty silver plated copper wire to jumper to pin 3.

      As the diodes I was able to find were a bit overspec, I used the silver wires to tie to the monster leads :)

      The amp sounds fine.

      I have ordered some VH caps for the output coupling. A friend thinks they are 'really something'.

      Will report after I put them in and get the required 400 hours break in on them ...

      
      
      

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      • V-caps - Goneill 16:41:58 07/17/06 (4)
        In Reply to: Re: Calling Al posted by tubegarden on July 17, 2006 at 15:33:12:

        Thanks for the note. That sounds like a good way of mounting the diodes. I believe the resistor should always be as close to pin 4 as possible to stop any oscillations so using pin 6 as a tag point is a good way to go.

        Regarding the caps, anything that says "cost no object" worries me! Which ones have you ordered (0.1, 0.15, 0.22)? And what have you got at the moment that you'll be comparing them to?

        Please do let us know how it goes so we can start saving the pennies!

        
        
        

        • Teflon caps   (Open in New Window)

        
        

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        • Re: V-caps - tubegarden 09:24:36 07/21/06 (3)
          In Reply to: V-caps posted by Goneill on July 17, 2006 at 16:41:58:

          Teflon Film Tin Foil 0.1uF V-caps arrived yesterday.

          I replaced the 0.22uf Obbligato Copper in the Ella output coupling.

          Still breaking in. Seem a good choice. YMMV ;)

          Happy Ears!
          Al

          
          
          

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          • Re: V-caps - tubegarden 04:13:51 09/14/06 (2)
            In Reply to: Re: V-caps posted by tubegarden on July 21, 2006 at 09:24:36:

            Months have sung by. I now have a pair of V-H TFTN on one channel and a pair of Obbligato Copper on the other. Both are 0.1 uF. They seem to have settled in well. Next energy burst, I will try and find some way to describe the differences. Don't wait up ;)

            
            
            

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            • Re: V-caps - Goneill 18:43:40 10/12/06 (1)
              In Reply to: Re: V-caps posted by tubegarden on September 14, 2006 at 04:13:51:

              Hi Al,

              Another month has gone by and I've only just noticed your update! Please do try and explain how they sound.

              Thanks,
              Graham
              
              
              

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              • Re: V-caps - tubegarden 12:23:18 11/29/06 (0)
                In Reply to: Re: V-caps posted by Goneill on October 12, 2006 at 18:43:40:

                V-Caps sound like the speakers and microphones are several meters closer together, at any volume level :)

                Take that! Stereophile!

                Happy Ears!
                Al

                
                
                

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• Re: Ella mods: screen diodes - RHone 21:20:58 07/16/06 (0)
  In Reply to: Re: Ella mods: screen diodes posted by Goneill on July 15, 2006 at 21:55:13:

  Thanks. Excellent reference and Dennis is a fellow Aussie. More than I could have asked for!
  Rod
  
  

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