[AMRadio] Can you have Class D and Class E tube amps?


Larry Szendrei ne1s at securespeed.us
Tue Dec 27 09:12:20 EST 2016


On 12/27/16 6:36 AM, Rob Atkinson wrote:
> Unless I am mistaken the Continental 314 was a class D tube rig.
>
I think the original person making the inquiry (Chris) was asking about 
using tubes in Class D or E service in the *RFPA* at 137 KHz. The "class 
D" (PDM, PWM) tube-type broadcast transmitters employ a class D 
*modulator* operating at, I believe, the 60 - 100 KHz range. The RFPA in 
these transmitters is conventional class C. My homebrew "big rig" also 
uses a class D modulator (an 833A) at about 80 KHz, but the RFPA is just 
a tube-type class C final with the negative lead floating, because the 
modulator is in series with the final from a DC and AF perspective. 
Because its a series modulation scheme, the DC supply needs to provide 
more than double the quiescent voltage required for the RFPA, but there 
is almost no extra energy wasted in the modulator. The HV supply needs 
to provide more voltage, but much less current, that a power supply that 
supplies both the RFPA and a "conventional" analog modulator (for 
example, class AB1, AB2, or Class B).

If a tube-type class D modulator can operate at 80 KHz, which it can, it 
is not a big stretch to expect it to be able to operate at 137 KHz in an 
RFPA. But this is the pulse repetition frequency, so there is 
significant odd harmonic content, and therefore significant energy that 
must be amplified, at much higher frequencies.

I have much less experience with/knowledge about class E amplifiers, so 
I cannot say much about that part of the question.

73,
-Larry/NE1S


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