Tomm, there might be interest in this subject by others so I will send to
the list also.Your Rp numbers look fine to me. I haven't studied the GS35
closely but the data I have seen impliess a gain of about 13 dB. This would
suggest that 100w drive would produce about 2000w output. You may well need
more
drive to exploit the potential of a pair of tubes. As with other triodes
commonly used, the higher the plate voltage, the higher the gain.
One of your options for high power to low power switching is to use the
higher plate voltage for high power operation setting the Rp as needed for that
and then switch to the lower voltage for low power operation. If you reduce the
drive power, the plate current can be lowered to the needed value to fit the
Rp set for high power. If the plate voltage is dropped to 70% and plate
current is also dropped to 70% the resulting Rp is unchanged. In this case,
output power will be about half of the high power setting.
This is by no means a new idea as many old amplifiers had a 1000w and 2000w
pep switch that worked in this manner. In fact, that may well be the reason
your PWD transformer has the voltages you mentioned.
Sounds like you have a good handle on all this. Have fun with it all!
73/k5gw
In a message dated 2/21/2005 2:59:59 P.M. Central Standard Time,
KD7QAE@ARRL.NET writes:
Thanks Gerald, I was planning on a dual GS35b design to have a very
robust <2kW amp as opposed to a high power >3kW amp. The transformer I
have is an Alpha 77sx unit from PWD that delivers 1800 or 3000 vac @
1.2A CCS. That would be approx 2500 and 4200VDC from the filter caps.
Using the accepted formula Rp=Ep/1.8*Ip I get Rp=4200/1.8*1.2=1944 Ohms
at the high end and using that I get Ip = 2500/1.8*1944=0.71A at the low
voltage end of operation. At 60% efficiency, roughly 1000W and 3000W
output. Iterating on the Rp value by lowering Ip to .8A gets me an Rp of
2317 Ohms, a 2500V Ip of 0.47A and low and hi powers of 714W and 2016W
respectively. Will my 100W transceiver drive this? Have I figured this
correctly?
Tomm
TexasRF@aol.com wrote:
> Hi Tomm, measuring plate load impedance is a tough job for a typical ham
> project. For that reason we tend to calculate that and hope it is close
> (it will be).
>
> In practice, the tuning process optimizes the plate load for maximum
> power transfer so there is no real need to measure it directly.
>
> As far as the other data, you will be very close by taking the single
> tube data and scaling it by two. ie: plate current X2, drive power X2,
> with the plate voltage X 1 of coarse. The input impedance would be
> divided by two as would the plate load impedance.
>
> You can also operate a pair of tubes with the single tube data and have
> a super conservative, long tube life design. In this case the plate idle
> current can be set at 2 X single tube for improved linearity and/or
> reduced drive requirements.
>
> 73,
> Gerald/K5GW
>
>
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