[Boatanchors] TZ-20 Amplifier Questions

[mac]

Niel,

According to the specs in my 1948 ARRL handbook you should be able to  
get something around 80 watts out of a pair of TZ-20s at 750 volts.   
Grid bias should be
-40 volts at 56ma grid drive for two tubes.  For self bias this would  
call for a 700 ohm grid resistor.  A stage capable of providing  
something like 10 watts of drive would be advisable here given circuit  
losses, impedance mismatches, etc.  The same circuit constants  
probably OK for 500 volts too but the power output would be  
proportionately lower.

Concerning the 4K ohm grid resistor recommended by Taylor for a single  
TZ-20, this value would be about right for a T-20 .(-85 volts, 18ma)  
according to the ARRL specs.  The 1948 Radio Handbook by Editors and  
Engineers gives the same figures as the ARRL book, BTW.  The TZ-20 was  
developed for class B audio use and not the best choice for RF.

Note that Taylor doesn't specify specific bias voltages for either  
tube, curious...

Swinging links were typically used to control PA loading back in the  
days these tubes and circuits were popular.  Note that you have  
wrapped a fixed link around the center of your PA tank.  Coupling in  
this case can be controlled by a series variable capacitor.

Plate resistance values are not relevant here.  The design of a class  
C amplifiers like this has been covered in depth in every ARRL  
Handbook 1935 - 1970 (and later).  Would recommend some study....

Dennis D.  W7QHO
Glendale, CA

******************

On Jan 8, 2012, at 9:18 AM, Niel - W0VLZ wrote:

> I've been working on a RF amp using a couple of TZ-20s in push-pull.
> Please see http://w0vlz.blogspot.com/search/label/TZ-20%20RF%20Amp Now
> that I have it up and running its not quite performing like I  
> expected.
>
> 1) The Taylor tube manual indicates I should get over 100 watts  
> input if
> B+ is 750V. I'm running B+ at 500V and getting only 25 watts input  
> max.
> An over 75% hit in power for dropping B+ 33% feels like a big hit.
>
> 2) The manual indicates 75% efficiency. At 25 watts input I'm seeing
> about 10-12 watts out. That's only around 50%, not 75%.
>
> Any combination of my three TZ-20s seems to produce the same results  
> so
> I don't think it is the tubes.
>
> Three things I can play with are bias, B+, and output coupling.
>
> I'm using a 2K bias resistor as recommended in the manual for two  
> TZ-20s
> in a RF amp (I assume Class C). Does this need to change when  
> running at
> 500V? Is there some way I can use the tube curves from the manual that
> are linked into my blog pointed to above to calculate a new/better  
> bias
> resistor value?
>
> Is the 75% drop in power to be expected given the 33% drop in B+? If  
> so
> I'll need to come up with a 750V supply.
>
> What is the best way to couple this amp to a 50 ohm antenna system?
> Again see the tube manual page I pointed to above. The plate  
> resistance
> of a zero bias TZ20 is 26.7K while the plate resistance of a T20 is  
> 8K.
> Typical triodes seem to be in the 8K and lower range. I believe the
> optimal size of the link is tied to the plate resistance and the  
> size to
> the tank coil, correct? On 80 I ended up with a 40 turn tank coil. How
> many turns should be in the link? or is there some other way to couple
> to the antenna that I should consider?
>
> Thanks for any help.
>
> 73,
> Niel - W0VLZ
>
>
>
>
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