Best efficiency is obtained when the plate tune and load caps are tuned for
maximum output with a given amount of drive. Tuning up at maximum power with
full drive as is normally done gives best efficiency at that power level.
Now if you reduce drive, power out drops and efficiency also drops.
An amp that has 60% efficiency tuned at full power will have 30% efficiency
when drive is reduced enough so the output power drops to 1/4 of full power.
This is true if the plate tune and load controls are not touched after
initial tune up. This is how you would operate an amp on AM.
If you want to operate with full efficiency at lower output power then
retuning plate and load caps for maximum output with reduced drive is the
way to go. (will not work on AM though)
If you want to maintain the same load transformation ratio then you need to
lower the plate voltage and plate current along with drive.
If plate voltage is cut in half and plate current is cut in half, from full
power, you will have the same plate impedance at 1/4 power as you have at
full power. This way you can maintain full efficiency without retuning when
reducing power to 1/4 power out.
This can be done at other power levels also (other than 1/4 power) by the
proper selection of plate voltage and plate current ratios that give the
proper plate impedance for the power wanted.
73
Gary K4FMX
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Steven Cook
Sent: Saturday, September 03, 2005 8:31 PM
To: Phil Clements; Amps@contesting.com
Subject: Re: [Amps] CW and High Voltage
Thanks to all for your collective wisdom.
I assume manufacturers added the CW position on older amps to meet FCC
limits on input power for that mode.
Did this serve to maintain circuit Q, or were there other considerations;
e.g., power supply limitation, et al.
Just seems weird to operate an output stage that is not fully "dipped and
peaked." Old habits are hard to break!
When I tune up my Kenwood TL-922 in the CW mode, and then decrease my
exciter output to around 50W, I notice a "HUGE" residual "dip and peak"
remaining in C1 and C2 respectively and a 200+ watt increase in output
power. What's up with that???
I guess I'll just have to get over the urge to "fine tune" things.
Grrrrrrrr <smile>
-Steve (WG7K)
----- Original Message -----
From: "Phil Clements" <philc@texascellnet.com>
To: "'Steven Cook'" <sccook1@cox.net>; <Amps@contesting.com>
Sent: Saturday, September 03, 2005 12:01 PM
Subject: RE: [Amps] CW and High Voltage
>
>> Team,
>>
>> I've acquired an older Henry 2K-4, which only has one High Voltage
>> setting
>> ~ 3500V no load.
>>
>> It runs a pair of 3-500z triodes and will be driven with ~100W
>>
>> My question concerns running the amp at reduced output on CW and if it is
>> better to:
>>
>> 1. Apply full drive, and adjust C1 and C2 until the desired level of
>> output is achieved...
>
> This is never done under any circumstances on any mode!
>
>
>> 2. Apply full drive, adjust C1 and C2 until maximum power is achieved,
>> then reduce the exciter drive without readjusting C1 or C2.
>
> This is correct for all modes.
>>
>> What is the ramification of starting out with reduced drive, (like 25 or
>> 50 Watts) and adjusting C1 and C2 until maximum output is achieved?
>
> Because the tank circuit is designed to operate at only one impedance
> transformation and Q, and it is usually designed at the maximum key-down
> anode voltage available and maximum rated anode current for the tube(s).
>
> For best harmonic attenuation, cleanliness, and efficiency, always use
> method 2 above.
>
>
> (((73)))
> Phil, K5PC
>
>
>
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