Jim, You wrote,
<(which in the 8K is something like 4200 volts, I believe)
The 8K plate is 5400 VDC no load.
Rich wrote,
> In order to
> >produce this much anode current, c. 400w of drive is needed.
250-300 watts typically gives 5000 watts out.
Jim Wrote,
In the case of the Henry 8K, the inductance of the tank
> is tuned, while the capacitance is fixed by doorknob
> caps, I think.
YES, tank C is switched by band frequency range, L is roller inductor as
shown on my site.
So, I see no harm to the 8K in operating it at low drive,
> and constraining it's power output to legal limits, where
> it evidently provides very linear operation; but at the
> cost of probably very poor efficiency. After all, the
> AC mains are probably delivering 8 or 10 kW of power
> to the 8K power supply, and with only 1500 coming out,
> that is only about 15% efficiency, hi.
More like 50% at lower drive, depending on many factors..... Tuneup at
specific power level, frequency, load, etc.
I believe the result is very long life of the amp
> components, and a very nice on-the-air resulting
> signal, SSB, CW and AM (since the amp transfer
> characteristic resulting is apparently so very
> linear, a low power transceiver's AM signal
> can be amplified very nicely by the 8K, I think).
>
Yes, buyers of 8K ULTRAs do so for many reasons (there are many different
places to use them), but mostly because at or below design power out, it
provides a clean, reliable signal. Don't forget, the ULTRA buyer is never a
first time amplifier owner. Usually he had to prove to himself indeed that
other attractively priced amplifier is NOT the "same for less" he hoped for!
A lesson relearned every day.
73, Dan Magro W7RF, (member SCDXC, SCCC)
Rugged, Reliable Henry HF Amps are "Instant on" for RF Power now!
DXers/Contesters/Ragchewers get your HENRY from RADIODAN for LESS!
***Subscribe to the HENRY reflector at www.qth.net <http://www.qth.net>
***
www.radiodan.com <http://www.radiodan.com> ??? RFpower@radiodan.com
<mailto:RFpower@radiodan.com> A trip to our web site is worth the click!
> -----Original Message-----
> From: owner-henry@qth.net [mailto:owner-henry@qth.net]On Behalf Of Jim
> Reid
> Sent: Tuesday, December 08, 1998 1:01 PM
> To: Radiodan W7RF; Henry reflector; amps@contesting.com; Rich Measures
> Subject: [Henry] Henry 8K at 1500 watts Out -- Warning Long
>
>
> Jim, KH7M wrote, in part:
>
> >>In fact, with 100 watts into the 8K, and it's 13 dB gain, you will
> >>get out around 2000 watts, so better cut the drive in back until
> >>the Bird in the 8K reads no more than 1500 out with the key down,
> >>SSB peaks.
>
>
> Rich, AG6K responded:
>
> >However, the tank of the Henry 8k is designed for a much lower RL --
> >i.e., the tank is designed for 2.5a of anode current. In order to
> >produce this much anode current, c. 400w of drive is needed.
> >>......
>
> And, presumably under this plate current, the QL, the
> loaded Q of the tank will be something above 10, or so.
>
>
> However, Rich has caused me to revert to Terman, hi.
> I think the result of driving for only 1500 watts out, is a lower
> efficiency of use of the AC power operating the 8K. I have
> heard from 8K owners who do just this, operating both
> SSB and AM (!), who report that they receive glowing reports
> about their signals audio quality, so I would guess very
> linear operating by the 8K.
>
> Now, I also think, that lowering the drive to the grid, with the
> amp designed as Rich points out, results in a low Emax on the
> grid, and a smaller conduction angle of plate current, but I
> do not know how the Henry engineers have the bias set
> up in the 8K. My rather early copy of Terman discusses
> RF power amps only in terms of Class C operation, but
> some of the same ideas apply, I believe, when figuring
> what is going on in the circuit.
>
> In any case, the result from low drive is that there is a lower
> voltage developed across the plate tank circuit, and thus
> Emin, the difference between the plate supply voltage
> (which in the 8K is something like 4200 volts, I believe)
> and the EL, the maximum voltage across the tank is
> much greater than the applied grid voltage, Emax.
>
> So there will be an impact on the loaded Q of the plate
> tank since this Q depends upon the square of EL
> divided by a product term which includes the plate
> current squared and the term RL, as Rich has said.
>
> Now with lower than the 8K "design" drive of 400 watts,
> per Rich, it is not immediately clear what will really
> happen to the loaded Q of the plate tank, as both
> EL in the numerator is lower, and the current in
> the denominator is lower, and both are squared terms.
> The inductance of the tank in the denominator is constant,
> but the R, plate resistance is going to differ from that which
> would exist with 2.5 amps of plate circuit current.
>
> So, I am not in a position to complete the analysis,
> as I do not know all the particulars, nor do I have
> a copy of the Eimac data sheet on the 3CX3000A7
> used by Henry in the 8K.
>
> However, if the loaded Q drops, the result will be,
> perhaps, an interesting tuning problem for the
> amp user, not sure about this either, hi. In the
> text by Terman I have out here on Kauai, he
> writes that it is not uncommon for "very high
> power Class C amplifiers" for values of loaded
> Q to be as low as 2 or 3! Again, I do not know
> how far toward AB or B operation the 8K is
> from a Class C triode transfer characteristic.
>
> Terman goes on, in a text footnote, to offer that with
> such low loaded Q values, it will be necessary to use
> a tuning procedure that causes the tank circuit to
> still offer a resistance impedance to the tube,
> even though this impedance may not be the
> maximum impedance obtainable from the load.
>
> In another section of his text, Terman discussed the
> behavior of parallel circuits with low Q. He writes
> that when circuit Q is low, the curve of circuit impedance
> as a function of frequency still has a shape that resembles
> a resonance curve, unless the Q approaches, or is
> less than unity. At low circuit Q, the maximum impedance
> 'no longer necessarily occurs at the frequency of series
> resonance, and the condition of unity power factor does
> not necessarily occur either at the frequency of resonance
> or when the impedance is a maximum. the actual behavior
> for a given Q will depends upon the division of circuit
> resistance between the inductive and capacitive branches
> of the tank. Terman goes to say that what occurs in
> the low Q tank of a power amplifier is going to depend
> upon the distribution of resistances between the capacitive
> and inductive branches, and which branch is tuned to
> effect resonance.
>
> In the case of the Henry 8K, the inductance of the tank
> is tuned, while the capacitance is fixed by doorknob
> caps, I think. In this case, Terman says that maximum
> impedance and unity -power- factor conditions will
> only coincide if all the circuit losses are concentrated
> within the capacitive branch. I presume this would
> be the case in the Henry 8K via the use of doorknobs,
> and the rather large and healthy appearing conductor
> of the plate tank inductor as shown on the Henry
> web page.
>
> So, I see no harm to the 8K in operating it at low drive,
> and constraining it's power output to legal limits, where
> it evidently provides very linear operation; but at the
> cost of probably very poor efficiency. After all, the
> AC mains are probably delivering 8 or 10 kW of power
> to the 8K power supply, and with only 1500 coming out,
> that is only about 15% efficiency, hi. But not sure the
> fellow who buys and 8K is that concerned about his
> power bill. Who is to say??
>
> Will really appreciate learning what else I ought to know
> about the situation of applying low drive to these big
> amps. I believe the result is very long life of the amp
> components, and a very nice on-the-air resulting
> signal, SSB, CW and AM (since the amp transfer
> characteristic resulting is apparently so very
> linear, a low power transceiver's AM signal
> can be amplified very nicely by the 8K, I think).
>
> 73, Jim, KH7M
> On the Garden Island of Kauai
>
>
>
>
> ---
> Submissions henry@qth.net
>
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