[Boatanchors] TZ-20 Amplifier Questions

[Niel - W0VLZ]

Solved it! Now I'm able to drive my TZ-20 amplifier to 75 watts input 
and 45-50 watts out. Once I fixed my wiring problem it all fell into 
place. See http://w0vlz.blogspot.com/ for the whole story.

tnx es 73,
Niel - W0VLZ


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
>
>
>
>