Ameritron AL-80A restoration project

W7TDC article on restoration of the AL-80A RF amplifier.

 

The Ameritron AL-80A uses a single 3-500Z triode to produce over 1KW of power with only 65 watts of drive.


I purchased it in the beggining of July 2003. It had several problems when I got it. The tube was bad, a VHF parastitic had occurred, causing several components to fail. As far as mechanically, there was only one problem. The plate tuning capacitor has a plastic coupler that connects it to the knob. This coupler was cracked were it slipped over the capacitor rod, allowing it to slip.

 

What is a VHF Parasitic? This is when a VHF voltage causes an self-resonant fly-wheel type occilation in the tube. When this happens, all hell beaks loose and many things make a day turn bad. There are two things that need to be done to solve this problem. First, the Q of the VHF self-resonant circuit needs to be reduced, and to reduce the VHF voltage-gain of the amplifier stage. To solve this problem so that a parasitic is less likely to occur, the anode and cothode self resonant frequencies need to be farther apart by reducing inductive reactance. This is done by shortening lead length and tuning it out by bypassing the grid to the chassies by using small capacitors. This increases the self-resonant frequency of the grid circuit to point where the tube will have less amplifing and osccilation ability.

 

Below is a list of all the things I did to the amplifier with an explanation.

 

1) Replace the VHF Parasitic Supressor
2) Add a step-start circuit
3) Add an additional capacitor to the anode choke
4) Add fast T/R switching
5) Adjust filament voltage to 4.8VAC
6) Add diodes across meter shunt resistors for glitch protection
7 ) Add rectifiers across HV filter caps.
8 ) Replaced the cathode bias zener dipode with several rectifiers.

 

1* I replaced the original VHF parasitic supressor which was a 100ohm resistor with a inductor around it, with Nichrome wire and two 100ohm resistors in parallel. The Nichome wire has an extremely small amount of resistance and handles large current and voltage being close to 22AGW.

 

2* The step-start circuit I added used a simple 10A 120VAC DPDT relay and 2 25ohm 10watt resistors.
Current went through the resistor and after about 1 second the voltage was high enough for the transformer to produce 120V. This closed the relay contacts allowing current to bypass the resistors. This simply protects the circuity and especially the filament from voltag skikes and current inrush.

 

3* The original capacitor was .001uF 7.5V. I added another capacitor to make it 10pf.

 

4* Even though I dont do QSK often I do use Vox during the heat of a contest. The amplier does switch to transmit pretty fast when I key down. Thats not really the issue. The issue, is that the T/R relay is hot switching. In other words, the radio is keying down and putting power out before the amp relay can even think about switching the relay. So the amp relay is switching while 100 watts is on it, which is very bad for the contacts. You can tell by seeing a SWR spike when you first key down. To fix this, I made an entire new circuit board that not only contained the fast T/R switching, but the electronic cathode bias switching also. There are two different relays for the fast T/R switching. One input and output. The output relay is a high speen Kilovac HC-1 vacuum relay, while the input relay is a RF reed-relay. This will insure that when the tranceiver Xmit is brought to ground, the amp will switch the relays before power is put through it.

 

5* One of the most important things is filament voltage. A 3-500Z has a minimum of 4.8V with a max of 5VAC. If there is a 3% increase above 5VAC, it will decrease the life of the tube by one half. I measured the voltage of mine and it was 5.2volts, which is way to high. I replaced the filament wire with 24AGW and added some resistive wire to one end. Using smaller wire is much easier then trying to find a .25ohm resistor. This brought it down to a perfect 4.8VAC. Even worse than 3% higher voltage is a under voltage. That will literally kill the tube in a very very short time.

 

6* If there is ever a glitch, where the HV arcs to ground, the meters will be blown out of the panel. I added some 200A PIV diodes across the meter leads to that they would be destroyed rather than the meters.

 

7* The next thing I did was add rectifiers across each HV filter cap. Electrolytic caps are detroyed very easily be a reverse voltage, especially that high. So if the HV were to ever arc to ground, the diodes would be destroyed while the capacitors are beeing nicly bled down by their resistors.

 

8* The next thing to do was replace the 7.5V 10watt cathode bias zener diode with a string of rectifiers in the foreward operating position. This not only allows adjustment to 75mA of plate current wich is optimum, but also helps retain 5v of cathode bias voltage. My fast T/R circuit contains a transistor used as a switch to auto switch the bias when transmitting.