[Boatanchors] TZ-20 Amplifier Questions

[Jim Wiley]

Unless I have been misinformed,  which is always a possibility around 
these parts,  the Taylor series of "TZ" tubes were an early attempt at a 
zero-bias tube.  The designations were something like:  TZ-40 = Taylor 
tube, Zero Bias, 40-watts plate dissipation, and so on.  So, using this 
idea, a TZ-20 would be a 20-watt zero bias triode.  Sounds logical, but 
don't have a tube manual for the  Taylor tubes to prove it.


- Jim, KL7CC



mac wrote:
> 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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