[AMPS] Linear with 4CX800A

[Phil Clements]

At 10:19 PM 9/6/97 -0400, Joe Subich wrote: 

snip......

>If you are going to continue holding yourself out as the final arbiter of 
>the right and wrong of amplifier design, and deride those with significantly 
>more experience than you as "recognized experts,"  at least get your facts 
>right.  Your comments are only true if one is using two tubes to generate 
>twice the output power at a constant plate voltage eg. 77SX vs 77DX).

So far, so good.....but throwing stones from the protection of a glass
house is not good operatinf practice, methinks.


 
>
>1) The design of an output network, be it a simple pi network or a pi-L 
>   is based simply on the tube optimum load impedence which, in turn, is 
>   determined by the anode voltage, anode current, and conduction angle 
>   (class of operation) of the tube(s).

Absolutely correct.....several numbers to "plug in here"....

>   for example, the output network for 2 X 3-500Z would be identical
>   to that of a single 3-1000Z as long as the two amplifiers are operating
>   at similar output powers.

You just contradicted your statement in paragraph 1)....output power has
nothing to do with determining an output network. (Except how big the parts
need to be)
For example, 2 3-500Z's:
operating at an Ep of 4000 volts, drawing 600ma of Ip (class AB2,GG) will
put out +/- 1500 watts on a good day. On the other hand, a 3-1000Z, operating
also at its maximum Ep of 3000 volts and 800 ma of Ip is also putting out
+/- 1500 watts. I would call that fairly similar output powers. However,
the output networks would have to be very different to maintain a target Q
of 10-15 because the plate load impedance of the pair of 3-500Z's is almost
twice that of the 3-1000Z....3,703 ohms vs 2,083 ohms. In a simple pi network,
the 3-1000Z would require 57% more C1 and 60% less L1 to arrive at the same
operating Q on 10 meters. As you stated in your opening paragraph, plate load
impedance is a complex formula, dealing with three variables. "Similar output
powers (or input powers, for that matter) are not one of the variables.

 
>
>2) It is more difficult to achieve the required low "C" for an output 
>   network tuning capacitor with two tubes than with a single tube simply 
>   because one must account for the output capacitance of TWO tubes instead 
>   of one.  In the case of 2 X 4CX800A the output capacitance is 22 pF 
>   (11 pF x 2) plus circuit strays where the single 4CX1600B is 12 pF.  
>   This capacitance appears in parallel with the plate tuning capacitor 
>   effectively dominating the minimum circuit capacitance and forcing 
>   HIGHER network Q on 10 meters (often well above 20) contributing to 
>   higher circulating currents, and increased losses in the plate tank.
>
>   In the case of this set of tubes, the 4CX1600B operates at a higher
>   anode voltage (3.3 KV vs 2.1 KV) and lower plate current ... thus a 
>   a higher optimum load impedence, a larger plate tuning capacitance, 
>   and potentially a more efficient output network.

Let's assume a Q of 10 on 10 meters. The tubes are at their maximum Ep...
3.3 kv and 2.1 kv.; the same input power to both...700 ma on the 4CX1600B,
and 1.1 amps on the pair of 4CX800A's. To resonate on 10 meters (pi network)
at our target Q of 10 we need 21 pf for the 4CX1600B, 12 of which is furnished
by the tube and 9 pf, furnished by strays and C1. L1 is 1.7uh in this case.
Finding a C1 cap with a minimum of 4-5 pf is not easy, but doable. (1000 pf
vacuum variables have a typical minimum of 4-5 pf) Looking at the pair of
4CX800A's, we find we need 47 pf for C1 (Q=10) 22 pf from the two tubes in
parallel, and 26 pf from strays and C1. (L1=.99uh) Now we can use almost
any vacuum
variable and even a few "bread slicers" will allow a Q of 10. So you see,
the 4CX1600B requires 34% LESS C1 for a Q of 10 on 10 meters at the same
power input. (not larger)
snip......
>
>In general, the single tube design is simpler and more forgiving (unless one
>is fixated on "tubes with handles" and extra-legal power levels).

Even single tubes with handles are more forgiving and last a LONG time when
operated at less than half their rated output.

(((73)))
Phil, K5PC


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