[AMPS] tetrode output

[measures]

>
>Hi Lane and reflectees !
>
>Re:   Homebrew 4CX1500B amp. spec.
>
>>1. The screen voltage has taps for 150, 250, or 350 vdc. I am using it on
>>the 250vdc tap. When I go to the 350vdc tap the ZSAC rises to about 1000mA.
>>>Why?
>
>
>This is what you would expect - you will need to find how the control grid is
>biased and increase the negative voltage on G1 - should be about -55V dc
>- make it more negative to reduce ZSAC back to 250mA or 300mA.
>However +350V dc maybe a bit high - should ideally be +325V 

The rated screen potential is 400v.  With this level of screen potential, 
maximal peak anode current can easily be realized. 

>but a 1500B
>will work well with +350V - the problem is the dielectric in the tube base
>screen decoupling cap., which may be rated at 400V dc, may eventually 
arc-through
>with the combination of +350V dc + RF.

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.  .  
>
>>2. I can not reduce the output out of my TS-850 any lower than 10 watts,
>>even with alc. What kind of variable attenuator network can I build on the
>>input side of the amp so I can vary the drive that the tube receives from
>>the exciter? There are times when 400 watts is all that is needed.
>
>The TS-850s has an internal power-level control (RF board R7)
>which you can set to any value you like from 1W - 100W, however
>it's not such a good idea to run such low drive levels
>(sigs. not clean from 850) - your new amp. maybe has tuned input
>rather than passive grid,

The tuned grid of an AB1 amplifier is a virtual open circuit.  850s need 
a 50-ohm termination plus a means of canceling grid reactance on the 
higher bands.  

>from the sound of the drive levels involved
> - a passive-grid (50ohm) 4CX1500B needs about 40W min. drive
>for 1400W output.

The 8660/4CX1500B needs very roughly 40v-pk drive.  With a 50-ohm 
termination, this represents c.16w of drive.  If you have a ts850, one 
way to drive an 8660 is with a 12.5 ohm grid termination resistor, driven 
through a 4 to 1 bifilar transformer using mu=40 core material.  .With 
the drive set to about 65w, the 8660 should be on the verge of drawing 
grid current.   
>.....
cheers, Chris

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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