>Rich Measures wrote:
>>>
>> My strategy, Ian, is to choose a value of grid termination resistance
>>that roughly affords the needed peak RF drive V to the grid with exciters
>>that develop 100v-p (100w rms) to 141v-p (200w rms) across 50 ohms.
>
>OK, that makes sense, but until now I don't recall you mentioning the
>need for a range of input transformer ratios to bring everything into
>roughly the right ball-park.
It's discussed briefly in Figure 5 in *Amplifiers*. However, only two
examples are given: the bifilar-stepup (200 ohm grid termination), and
the trifilar-stepup (450 ohm grid termination). In my opinion, more
people are interested in building with tubes that use 200v to 350v of
grid bias. Trying to build an amplifier with multiple Svetlana 4CX800As
(12.5 ohm termination, 50v bias) makes less sense to me than using a
single, larger Svetlana tetrode---and the dollars/kilowatt ratio of
larger tetrodes is much better. For example, at full tilt, the Svetlana
8171 costs under $69/kilowatt. There is no gold plating on the grid to
worry about if it decides to oscillate at 65MHz.
If you think it would be appropriate to add the long [AMPS]
version--which includes the 12.5 ohm/50v bias case--to *Amplifiers*, I'll
do so.
...
>>... In your opinion, Ian, is a
>>regulated supply, or an unregulated supply, most likely to have an output
>>R of 100k?
>>
>I do not interpret that as a recommendation to deliberately choose a
>poorly regulated grid supply.
IMO, regulated supplies are characterized by low output Z. For instance,
the 2kV max. 723/TMOS-FET screen V regulator in *Amplifiers* has an
output Z under 1 ohm.
If the grid draws Zero Current. The only thing that will change the DC
grid V is a drop in the electric-mains V. I have built AB1 amplifiers
both ways, regulated bias and unregulated bias. It matters not a jot if
a regulated supply is used. Regulating the zero current bias supply makes
zero difference---regulating the screen supply does.
...
>>You've got to be putting me on, Ian. AB1 means zero grid current. .
>>
>Yes, but there's another way to achieve that, namely to adjust the
>drive, either manually or by ALC.
>
>>>and the adjust the screen current for the
>>>recommended ZSAC.
---Oops. The above contains a typo. It should read "adjust the screen V
for the recommended ZSAC." Sorry.
>>I thought I read it in *Care and Feeding....*. After spending about 15
>>minutes therein, I couldn't find it.
>
>Nor could I.
---Maybe it was in Eimac's *Single Sideband*?
>
>> However, the practice makes sense
>>to me. To explain: There are two means of setting ZSAC: by adjusting
>>grid V, and by adjusting screen V. Grid V adjustment is constrained by
>>the requirement that zero grid current be maintained---which leaves
>>choosing to adjust the screen V. Does this make sense to you?
>
>...... I appreciate what
>you're saying, but don't agree that it's the only "right" way to do it.
>
Trying another door.........OK, I don't touch the screen V adjust.
However, what can can go wrong if one uses the grid bias V adjust to
adjust ZSAC in Class AB1?
The benefits of using the screen V adjust to set ZSAC are:
1. You wind up with the highest screen V--i.e., more RF output. (the
Esc^3/2 power law)
2. You don't run the nasty IMD risk of drawing grid current.
Rich---
R. L. Measures, 805-386-3734, AG6K
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