GS-15B 23cm Amplifiers
by
KD5FZX
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1 x GS-15B PA

Single GS-15B 300W High Performance Amplifier.

 
Water Cooled GS-15b

Operational Instructions & Data for 300W PA

W6PQL 1xGS-15b PA

Complete 1 x GS-15B "tabletop" PA by W6PQL.

Water Cooled GS-15b

Preparing the GS-15b to be water cooled.

Water Cooled GS-23b

Description & pictures of a water cooling system.

Single GS-15B PA for 23cm

1xgs15b PA bottom 1xgs15b PA side1 1xgs15b PA top 1xgs15b PA side2
Click on pictures to enlarge

This project is based on the excellent GS-15B PA built by N8OU. He sent his prototype for examination and testing; with this proof in hand that the GS-15B would perform well on 23cm, a second PA was designed using the N8OU prototype as a reference. The biggest change in this second PA is that it employs a 1/4 wave length input cavity instead of the original 3/4 wavelength design. This reduces the physical dimensions and increases PA bandwidth without any compromises. It results in an excellent match; no detectable input SWR with an input circuit return loss exceeding 23dB!! Anode tuning is smooth and easy.

Comments about tubes:

Variations in tube performance were observed. With 10 Tubes tested, three tubes worked stable up to 400W+, three worked stable up to 350W+, three worked stable up to 300W+, and one worked stable up to 275W+. The lower power tube showed a rapid de-tuning jump when drive was increased, indicating grid or screen distortion. In a more general test, 20 tubes were tested for operation at 300+ Watts output, 18 passed, the distorted grid tube already noted and an additional leaky tube were the two failures. See N8OUs article about tube behavior and power supply recommendations. This is a significantly better "yield" than found with the GS-23B (one of 5 to 7 is good for 23cm), and the GS-15B can be found for $15 to $30, delivered (NOS, old stock), which is significantly less expensive than the $150+ for each GS-23B. NOTE: There is a pulse version of GS-15B with the designation GI-44B. This tube is NOT suitable for use in this amp. The screen to plate distance is longer and this tube is designed for higher plate voltage. Also the screen to plate capacitance is lower so the cavity resonance frequency will be about 30MHz too high.

Representative Performance Data
Cooling: Water Preliminary, Air
   ParameterClass AB2Class B-55V Vg1Class ABClass B
   Plate voltage2300V2300V2300V2200V2200V
   Plate Current300mA260mA250mA  
   Plate Input690W598W575W  
   Screen Voltage320V320V320V320V320V
   Screen Current0 to -1mA0 to -1mA0 to -1mA  
   Grid Voltage-32V-40V-55V  
   Grid Current8 - 10mA3 - 5mA4 - 8mA  
   Idle Current25mA0020mA0
   Drive Power8W10W15W10W10W
   Output Power345W330W345W320W275W
   Efficiency50%55%60%>45%>55%

NOTE ON GS-15B FILAMENT PERFORMANCE:

The tube filament was tested to determine the voltage at which the heater reaches full emission. It was a pleasant surprise to find that 5.2 volts is the lower limit without any reduction in emission. It is therefore recommended that no more than 5.5 VAC be applied to GS-15B filaments. This will extend the tube life and reduce the effects of secondary emission (cathode back heating).

Measurements:

test set-up
Click to zoom
 

Left: Looking down on the desk with the PA and test set-up. Right: Drive (left) is 50W scale, output (right) is 500W scale. This power measurement was done with 10W drive in class B and shows 350W output. The screen voltage was raised to 350V for more gain and less grid-current. Tests to 500W output were successful.

  test readings
Click to zoom

Materials:

The cavity ring is cut from 4X3 aluminum tubing and is machined to final size. Aluminum is OK for this PA since the anode impedance is not too low. The screen - plate capacitance on GS23B is 11.5pF. This equates to 10.68 ohms at 1296MHz. The coaxial section of the cavity is 11 ohms to match this, and use of aluminum results in significant material losses. This makes silver plating inportant for the GS-23B 23cm PA. Not so with the GS-15B; The GS15B screen to plate capacitance is 1.8pF. This equates to 68 ohms at 1296MHz. The screen ring diameter is 42 and the anode is 12mm. If we use the value in between of 25mm, the impedance is 67.2 ohms. This is not very critical in this application; the cavity is not getting hot so the losses are very low. Silver-plating would have very little impact here. All other metal parts are brass and copper.

Sheet insulators are made from 0.125mm - (0.005") Kapton film.
Shoulder insulators are made from Teflon.
Plate ring finger-stock is Instrument Specialties model 97-135.
Screen and Grid finger-stock is Instrument Specialties model 97-221
(See http://www.lairdtech.com for more finger-stock info.)

Click on pictures to enlarge
1xgs15b PA dwg
Single GS-15B PA

Download a 1:1 scale (original prints out at 2/3 scale). Print using your browser's print function.

23cM-GS35-compl1.JPG
All machined parts for 1 x GS15B


1xGS-15b details

Single GS-15B PA cavity construction details.


four PAs

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Last Updated: 2 February 2005
Feedback: Paul S. Goble, III, ND2X
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