How do I design "SCREEN GRID DRIVE" - diyAudio
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Old 6th May 2006, 02:41 PM   #1
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Default How do I design "SCREEN GRID DRIVE"

Hi,there
I seen Screen Grid Drive topology from Berning EA-230, Milbert BAM-235 Amplifier which used 6SN7 drive through 6JN6 with Screen Grid input and short Control Grid to Cathode (or Ground on some schematic).

My question: If I want to use EL34 instead of 6JN6, How do I design for its Grid (G1, G2)?

Thanks for advance.

ANALOG GUY
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Old 6th May 2006, 04:32 PM   #2
hacknet is offline hacknet  Singapore
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apparently EL34s are awfully screen drive candidates because of their 'unlinear' screen transconductances...
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Old 6th May 2006, 04:47 PM   #3
SY is offline SY  United States
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Worst possible output tube choice for screen drive. Or, conversely, worst possible drive choice for that output tube.

Berning chose his output tubes for specific reasons. Just to get your feet wet, try building an EA-230 clone and start playing around.
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Old 6th May 2006, 06:35 PM   #4
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Default Re: How do I design "SCREEN GRID DRIVE"

Hi Analog Guy,

Quote:
My question: If I want to use EL34 instead of 6JN6, How do I design for its Grid (G1, G2)?
Please ask yourself why in hell and for what reasons one should screen-drive an EL34? Really, this makes no sense at all.

If you have this amp and want to replace those 6JN6, just stick to them or as similar types as possible - EL34 is vastly different.

Tom
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Old 6th May 2006, 06:55 PM   #5
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According to David Berning's writings, "I found that I could get the same tube linearity at one tenth the idle current normally used if I drove the tube from the screen as opposed to the grid. I further found that this only works well on TV sweep-type tubes and not very well on audio tubes, and it requires a much more powerful driver stage." Link here.
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Old 10th May 2006, 07:05 PM   #6
radical is offline radical  United States
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Any suggestions for output transformers to use for
a Berning EA-230 clone and/or perhaps a much higher power 100W-200W version?
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Old 10th May 2006, 07:22 PM   #7
SY is offline SY  United States
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The 230 used some surplus trannies from HH Scott, IIRC. Those would be about 5000 ohms plate to plate.
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Old 11th May 2006, 02:30 AM   #8
AJT is offline AJT  Philippines
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Quote:
Sonically, the sound is much cleaner and more transparent at low listening levels when the transformer is not in saturation. The higher the idle current (more Class A) used, the harder it is to keep the absolute dc flux balance within the one to two milliamps range where saturation occurs in most push-pull output transformers.
this makes a lot of sense to me...
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Old 11th May 2006, 11:45 AM   #9
cerrem is offline cerrem  United States
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A good tube for screen driving is the 6L6GC or the beefed up version known as the 7027-A.....You need to calculate the screen current you will encounter at the max drive signal you plan on using...then calculate the appropriate driver impedance to handle the job...I see the schematic you posted has the cathode followers for driving... This is stuff right out of the 1940's and 1950's RF designing techniques...
The gm is low as heck for G2 drive.... Just like the McIntosh Unity coupled output stage, in that you will need to produce large voltage amplification in the driver stages then into a follower to feed into G2....
Keep in mind if you plan to drive the screen, then you need the proper curves to design this...The standard I-V curves won't help much...You need the plate characteristics for G2 modulation... Check out these curves....
http://www.triodeel.com/6l6gc_p7.gif

Look close and you will see they are for change in G2 while G1 is held to 0 volts...
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Old 11th May 2006, 11:49 AM   #10
SY is offline SY  United States
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Yes, and do note the drive level and consequent screen current (compared to max ratings). I'm a big fan and long-time user of 7027A, and these are not a good choice for this service.
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