[Amps] Passive grid stability calculation for a tetrode?

[Markku Oksanen]

Hi Peter
Looks like I should find a circuit to compensate for the high input C .I did build a GU84b amp and that uses a pi-circuit where the grid cap is on one side.Kind of sorts things out for 10, makes input SWR a bit worse on 15 but still acceptable, below 2.This gets a bit tricky with 120 pf.
If I understand this correctly, compensating the input cap "away" brings the stability criteria back to just swamping resistor "regulated" and thus perhaps the resistor value should be then that 150 or so, not more than that.
Yes, these things take a lot of power from every corner.  Fun to build though.  And as long as the kilowatts remain on EXCEL, this is safe too.
Markku
> Date: Fri, 15 Nov 2013 14:16:09 +0100
> From: g8on at fsmail.net
> To: ww1c at outlook.com
> Subject: RE: [Amps] Passive grid stability calculation for a tetrode?
> 
> Hi Markku,
> 
> Looks like a pretty fearsome electricity bill with over 500 watts for the filament alone! You would only need 600 volts p-p for 10kW PEP (14.25 kW DC in) At 1500 watts out, it would be around 250 volts p-p (allowing for losses) so 78 watts in the 100 ohm swamp resistor About 160 watts at 3kW out....
> 
> Above even 1500 watts, things like antennas and cables get a bit heavy duty...
> 
> The 120pF input capacitance will be a bit of a problem, too, looking like -j47 at 28 MHz.
> 
> But it would be stable with 100 ohms and even with 150. 200 might be pushing it a bit, though.
> 
> 73
> 
> Peter G3RZP
> 
> 
> ========================================
>  Message Received: Nov 15 2013, 12:44 PM
>  From: "Markku Oksanen" <ww1c at outlook.com>
>  To: "g8on at fsmail.net" <g8on at fsmail.net>
>  Cc: 
>  Subject: RE: [Amps] Passive grid stability calculation for a tetrode?
>  
>  HiA more careful calculation of 4CX5000 gm gives about 0.03 so:
>  
>   
>   
>   
>   
>   
>    Frequency MHz
>    
>    
>    30
>   
>   
>    Tube mutual
>    conductance A/V
>    
>    0.028571429
>   
>   
>    2 pi f
>    
>    
>    188400000
>   
>   
>    Plat grid cap pf
>    1
>    
>    1E-12
>   
>   
>    Load
>    resistance of tube
>    
>    1700
>   
>   
>    Grid resistance
>    
>    
>    100
>   
>   
>    
>    
>    
>    
>   
>   
>    H, stability criteria
>    
>    
>    0.915085714
>   
>  It looks like a relatively low grid resistance is needed for this and with 300V perhaps voltage this is a lot of watts, 600 w by my calculation.
>  Markku
>  > Date: Fri, 15 Nov 2013 11:44:07 +0100
>  > From: g8on at fsmail.net
>  > To: ww1c at outlook.com; amps at contesting.com
>  > 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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