Ameritron AL-1200 to gs35b Conversion PDF Print E-mail
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Sunday, 01 July 2007


Ameritron AL-1200 to GS35B Conversion 


Front View of AL-1200 with GS35b


The Ameritron AL-1200 Amplifier is in current production. In its factory configuratin it uses a single 3CX1200A7 (triode)  tube, grounded grid configuration. Some of the nice features of this amplifier include;

  • Peter Dahl transformer
  • PI-L output circuit
  • Full 160m-10m coverage
  • Large chassis with accessible components


Some of the 'Russian tubes' that have been available for the last 20 years are very reasonable in price/performance.  Re-tubing of an amplifier can be a good choice (and fun too), if a tube of similar plate dissipation and operating characteristics can be found.  Considerations need to be given to filament voltage, input signal feed, biasing, tube cooling and tube mounting.  The output network's impedance is really related to the operating point of the tube - the plate voltage and current  (the formula for Plate Load Impedance itself is an approximation) - R(load) = Vmax / Imin. If you're not changing the power supply, and the operating point is similar to the old tube, within the linear operating region (see the ARRL Handbook, or the old "Radio Handbook" by Orr, chapter 15) with a reasonable bias, it could be very easy to re-tube. 

It's also really helpful that others have done the a similar/identical re-tube before you. 

WB8WJU and others have re-tubed the AL-1200 with a GS-35b with success; in consultation wtih Tom, I found out that the gs-35b is nearly a drop-in. Besides the cooling and mechanical issues, the following needed to be done:

  • New 12.6v filament transformer or power supply (from DigiKey))
  • mounting of a  W4ZT 'bias board'
  • bias feed through an additional bias choke (per W4ZT's suggestion)
  • Re-use the existing  filament chokes

Last Updated ( Thursday, 19 July 2007 )
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