[Boatanchors] TZ-20 Amplifier Questions

Sun Jan 8 18:42:24 EST 2012

The Z stands for Class B audio only Jim

Since those tube are more efficient at the higher voltages Id figure on 
15-20W of drive for a pair considering input circuit losses at 500V. Once 
the design is peaked then the driver can likely be reduced.

Id also start with an external variable bias and once youve found the sweet 
spot then go fixed if you want.

If you cant get the grid current needed you will never get the plate current 
either.

You can also go zero bias Class B and see what you get for no signal plate 
current. Thats a fairly good way to test emission. Taylor claims 25-30ma for 
a pair at 800V, derate accordingly for 500V.

Carl
KM1H

----- Original Message ----- 
From: "Jim Wiley" <jwiley at alaska.net>
To: "mac" <w7qho at aol.com>
Cc: <Boatanchors at mailman.qth.net>; "Niel - W0VLZ" <w0vlz at aggienetwork.com>
Sent: Sunday, January 08, 2012 3:03 PM
Subject: Re: [Boatanchors] TZ-20 Amplifier Questions

> 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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