# [Amps] Passive grid stability calculation for a tetrode?

Fuqua, Bill L wlfuqu00 at uky.edu
Fri Nov 15 12:51:07 EST 2013

```    All the feedback theory that has been offered applies to RC coupled amplifiers.
In theory, even a triode should never oscillate since the feedback is out of phase with the input, right?
However, the real problem is when reactive components are present in the input ckt.
The classic tuned grid tuned plate oscillator. The feedback capacitance and the input
ckt which is resonance causes two problems. The resonant input, due to its Q actually
steps up the voltage. And if it is tuned somewhat below resonance, if I recall this correctly,
it along with feedback capacitance shifts the phase shift from 180 degrees.  Now it does not
have to be exactly in phase if you have enough step up voltage, there is something called
phase and gain margin for assured stability.
In fact there is a tuned plate untuned grid oscillator, commonly used for power oscillators,
where the plate is tuned and there is simply an inductor in the grid ckt.
The problem is that the input of the tube will not normally be simply seeing a resistive
load looking into the driver, but a complex impedance that must be swamped out.

73
Bill wa4lav

________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Carl [km1h at jeremy.mv.com]
Sent: Friday, November 15, 2013 11:22 AM
To: donroden at hiwaay.net; Amps
Subject: Re: [Amps] Passive grid stability calculation for a tetrode?

Carl
KM1H

----- Original Message -----
From: <donroden at hiwaay.net>
To: "Amps" <amps at contesting.com>
Sent: Friday, November 15, 2013 9:44 AM
Subject: Re: [Amps] Passive grid stability calculation for a tetrode?

> How much grid voltage swing for AB1 ?
> ( how many watts into a 50 ohm load )
>
> Don W4DNR
>
>
> Quoting Carl <km1h at jeremy.mv.com>:
>
>> Even CBers can build stable grid driven 4X5's!  It is a very hard
>> tube to make oscillate.
>>
>> The SK300A socket requires good screen bypassing for RF.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message ----- From: "peter chadwick" <g8on at fsmail.net>
>> To: "Markku Oksanen" <ww1c at outlook.com>; "Amps" <amps at contesting.com>
>> Sent: Friday, November 15, 2013 5:44 AM
>> Subject: Re: [Amps] Passive grid stability calculation for a tetrode?
>>
>>
>>> Markku,
>>>
>>> It may be shown that (i.e. as the books say, it is left as an
>>> exercise for the reader!) if you have H<2 as a result of solving
>>> the equation H=gwCpgRpRg  where g is the mutual conductance, w = 2
>>> times pi times frequency, Cpg is the pate - grid capacitance, Rp is
>>> the plate load resistance and Rg is the grid resistance, the stage
>>> is stable. Arguably Rp should be the plate load resistance in
>>> parallel with AC plate resistance of the tube, but sticking to the
>>> load resistance gives a bit more stability.
>>>
>>> vy 73
>>>
>>>
>>> Peter G3RZP
>>>
>>>
>>> ========================================
>>> Message Received: Nov 15 2013, 07:32 AM
>>> From: "Markku Oksanen" <ww1c at outlook.com>
>>> To: "Amps" <amps at contesting.com>
>>> Cc:
>>> Subject: [Amps] Passive grid stability calculation for a tetrode?
>>>
>>> All
>>> I am back on my 4CX5000 project and as it not done yet, I am still
>>> contemplating the circuit to be used.Earlier this year I got solid
>>> info from this reflector for GG operation and that look like a
>>> pretty good solution.However, I was wondering about passive grid,
>>> grid driven also.
>>> The question is:  How do a calculate the stability?  If I simply do
>>> math for tube voltage gain I get something like 25.Then if I
>>> calculate the feedback capacitive voltage divider, Grid to Cathode
>>> 120 pf, Plate to Grid 0.7 pf, I get less than 0.01 for the ratio.
>>> If the beast needs the voltage gain times feedback gain to be above
>>> 1 to oscillate, to me it seems this tube is stable no matter what.
>>> Where did I err if I did?
>>> Thanks!
>>> MarkkuWW1C
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>>
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>
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