[Amps] Wanted, dual GS35b HF amps actual results

[TexasRF@aol.com]

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 at 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 at 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
>  
>