Modify AL-1200 HV power supply

WARNING – LETHAL VOLTAGES!!! DO NOT ATTEMPT WITHOUT PROPER SAFETY PRECAUTIONS!!! I’m not responsible for what you do with this information!

The Ameritron AL-1200 has a 3,600v power supply. Its filter capacitor bank of eight, 270mF 450vDC runs right at the limit of its design (8 x 450 = 3600). A common modification is to add two additional capacitors, along with the needed bleeder resistors, to increase the filter bank capacity to 4,500v. Keep in mind that the transformer still puts out 3,600v, so this modification essentially allows the components in the filter bank to have some overhead capacity.

W8JI and W7RY both offer a replacement PCB designed for 10 capacitors. W1QJ described the way he modified an AL-1200 for W2RE. I choose to adopt the W1QJ method, and will show how I did it here.

The modification involves adding two capacitors in series with the existing capacitor chain, with a bleeder resistor across the terminals of each capacitor. I’ve done this by constructing a small PCB that I’ve added to the amplifier. Then, move two HV wires, cut a HV trace and add a jumper for the metering, and add a jumper to complete the HV series circuit.

Conceptually, this is the modification. I’ve omitted the additional bleeder resistors on the drawing, but they are required! The existing bleeders are desoldered on one end to allow me to test them. The one on the far left had failed completely open, thus causing the capacitor to “tick”. That’s what has led to this modification. I had replaced all the caps and bleeders only five years ago.
Here’s my first test fit of the additional PCB. Note the markings on the board indicating the bleeder resistors, capacitor polarity, jumper to complete the series circuit, and destination for the 2 HV wires to be moved, and the new HV metering jumper.
The completed modification and reassembled amplifier.

10 Replies to “Modify AL-1200 HV power supply”

  1. Hello Scott from Finland! I have had the Ametritron AL-1200 linear amplifier for a few weeks and it works well. I’m mostly at 160m and I get out of it at about 1300W. I would be interested in adding two electrolytic capacitors to the power supply section. I understand that it raises the voltage to 4000 V, but does that voltage decrease when the linear load is applied? Is there well room for two additional capacitors? This would be an interesting experiment and I have a Harris RF302 tuner that can withstand higher power. Would you recommend me to make this change? I think I could take the maximum power with a lower anode current. Maybe the tube Eimac 3CX1200A7 will get longer life then? I would be grateful for an answer. 73’s Esko / OH3EM

    1. Hello Esko!
      I made the modification to add the two additional capacitors to give more headroom, as the original design with eight 450 vDC capacitors was right at the 3,600 v output of the Peter Dahl HV transformer. I had struggled with weak or failed filter capacitors bleeder resistors; the first symptom of this was usually sagging HV when going key-down. In those instances, the key-down HV would drop to ~3,150 volts on the panel meter. After the +2 capacitor mod, key-down voltage is rock steady at ~3,600 v.

      All the best,
      Scott W5WZ

  2. Thanks Scott for the reply. I also have a key down voltage drop of about 300 volts. Does it also affect the output power somewhat, I think so. I don’t dare make the change myself, but maybe sometimes I get it done. Best regards de Esko OH3EM.

  3. Thanks Scott. I managed to get 6 pieces 500 VDC and 2 pieces 450 VDC electrolytic capacitors. The common voltage at these is 3900 VDC. They are screw-mounted like the originals. I haven’t installed them yet. What do you think is this ok when the two pieces are at different voltages. They are connected in series. Best regards Esko OH3EM

    1. Esko,

      If it were me, I’d want all the capacitors to be the same value, as well as all the bleeder resistors to be the same value.

      The guts of this amplifier have very high voltages, and margin for error and safety is important!


        1. Hi Bob. The modification doesn’t really add any output power, as the transformer’s voltage has not changed. What it does do, however, is prevent output power from sagging when key down, and provides more overhead for the capacitors and bleed resistors. The original design of eight 450vDC capacitors is good for 3,600 volts, which is right at the output of the factory transformer. Adding the extra two capacitors takes this limit to 4,500 volts.

          –Scott, W5WZ

          1. Nope. Just stabilizes and keeps components from operating threshold of their design limits.

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