>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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