I believe stability is the main problem with a triode. In non grounded grid
operation there is no shield between plate and the active grid. It most
always needs to be neutralized.
The fact that a tube draws grid current or doesn't isn't what causes
distortion. Drawing grid current or not determines class of operation.
A class B amplifier draws plenty of grid current weather it be grounded grid
or grid driven. There is no difference in amount of rotten signal produced.
However the GG circuit does provide some negative feedback.
There is nothing inherently wrong with drawing grid current if the amp is
properly designed to do so. A passive grid circuit will work fine for AB1
(no grid current), AB2 or even class B. The grid resistor would provide
plenty of swamping at the grid. However a well regulated bias supply would
be in order the same as it would with a tuned grid circuit.
Where problems arise with grid current is in an AB1 amplifier with an
unregulated bias supply and driving it into grid current. Now there is
"feculence".
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of TexasRF@aol.com
> Sent: Friday, February 17, 2006 2:10 PM
> To: dezrat1242@ispwest.com; amps@contesting.com
> Subject: Re: [Amps] Passive grid triodes?
>
>
> Bill, this is a wild guess on my part and assuming a linear class of
> operation:
>
> Bias voltage for triodes is much lower typically than a similar power
> tetrode. To achieve a decent power output level, there is going to be
> quite a bit
> of grid current. As Rich has pointed out many times, grid current is
> usually
> accompanied by rf "feculence".
>
> On the other hand, grounded grid operation automatically provides a large
> amount of negative feedback that will mitigate the "feculence" to a large
> degree.
>
> When a typical tetrode is operated in a passive circuit, there is usually
> zero grid current and the "feculence" is avoided.
>
> Tetrodes operated as grounded grid will benefit from the negative feedback
> and grid current can be drawn as a result.
>
> Grid current is related to rf drive power and plate current is increased
> in
> the process. Higher plate current will generate more power output. More
> output
> power is nice only if the signal stays clean. Grounded grid operation
> helps
> with this because of the negative feedback.
>
> 73,
> Gerald K5GW
>
>
>
>
>
> In a message dated 2/17/2006 12:35:52 P.M. Central Standard Time,
> dezrat1242@ispwest.com writes:
>
> I don't recall seeing any passive grid triode circuits and I'm
> wondering why. They always seem to be tetrodes. Is it a neutralization
> issue?
>
> 73, Bill W6WRT
>
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