[AMPS] Re: Tetrode Output

[measures]

>Hi Rich and List
>
>Here is one I can sink my teeth into. Rich some of your comments regard the
>Grid Driven Tetrode design lead to some confusion on my part. Let me
>reference your specific comments
>
>> Indeed.  If a screen bypass C shorts, ground the screen, connect the
>> screen supply pos.. to chassis gnd and the neg. to the cathode.  For
>> tetrodes with handles, this is SOP. There's a grounded screen config.
>> schematic on my Web Site using the requisite screen shunt regulator. .
>
>First off I assume the schematic you are referring to is the one labelled
>Figure 5 (Class AB1 grid input circuit) on your web page.
>
Bob -
Figure 5 is more for the 8169 or 8171.  For an 8660, a larger shunt 
current - or a shunt screen regulator might be better.  

>2) Now if the screen is shorted by the bypass cap it is at RF Ground but
>still is floating at DC....RIGHT?
  
When the screen is directly grounded, it is not floating.  
>
>3) Your schematic does not show the pos. screen supply with a ground symbol
>Earth or Chassis. Is this an omission or am I missing something?
''
Fig 5  shows all supplies.   The pos is ''earthed''. The neg connects to 
the cathode.  
>
>4) Take these 2 points together if the screen is grounded and the pos.
>screen supply voltage is at chassis ground how does the screen current
>meter work?

it works ok, as shown on fig. 5. .  
>
>5) In a slightly different vain what you are suggesting is rather than
>elevate the screen above ground potential you recommend lowering the
>cathode below ground. Am I right?

correct
>
>6) Does one have to take an special precautions with the filament
>transformer as it's secondary will be hundreds of volts below ground?
>
yes.  Ground nothing.  Connect the heater to the cathode to avoid 
heater/cathode breakdown.  Use a bifilar heater choke.  

>7) Will this affect the bias voltage of the control grid? 

Not in the least.  The bias voltage is always the DC potential from the 
cathode to the grid.  

> Should the
>voltage potential of the all the tube elements remain the same wrt each
>other 

Yes 

>and ground potential?

the ground moves a bit.  
>
>8) You state that this practice is SOP for tetrodes with handles. How so,
>one would think that 5 to 10 KV bypass caps would be alot less trouble.
>What's the rationale?

A low inductance screen-ground is essential.  Caps have more inductance 
than copper foil.   Grounding the screen of an 8171 or 8281  requires 
eight copper foil straps - roughly the width of a finger -  between the 
SK-300a's screen collet and the grounded shell.  Screen-grounding is 
serious business because  - if one is to avoid oscillation - the screen 
must shield the grid from the anode.  .  
>
>9) Implicit in this design is the need to allow the Plate supply return
>line to float. Am I right?
>
Yes.  The cathode - and all that connects to it - floats.  The cathode 
connects to the negative lead. of the anode supply.  .   The floater is 
the screen supply.  The grounded-screen configuration looks a bit strange 
at first, but eventually it makes delightsome sense.  note -- in order to 
protect the floating cathode circuitry from the danger of positive hv to 
ground arcs, some unusual glitch protection diodes are needed.  See Fig 5 
for details.  //  There is no free lunch.  
>
cheers, Bob
>
>
>


-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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