The AM Forum

How about the manual PDF file? 

Email me at kb3rdt@windstream.net

The two 5R4 tubes are used as parallel full wave rectifiers so with a solid state replacement you only need to use one socket, available rectifier diodes will easily handle the current.  A total of 6 1N5408 rectifiers should be used, 3 to replace each section of a single 5R4.  You will also need a plug to fit the 5R4 socket (or a salvaged tube base) and a 150 ohm 20 to 25 watt resistor.

Make two strings of three 1N5408 rectifier diodes putting the diodes in series (anode to cathode) in each string.  Each string will start with an anode connection on one end and will end with a cathode connection on the other end.  Connect the anode of one string to pin 4 of the plug/tube base and the anode of the other string to pin 6.  Keep the two strings well separated  but bring the cathode ends to a common point and connect this to one end of your 180 ohm resistor.  Connect the other end of the resistor to pin 2 of the plug.

When replacing high vacuum rectifiers with solid state I also add an inrush current limiter to the associated power transformer primary because even with the series resistance the initial inrush surge to the capacitors will be greater with silicon rectifiers.

Sorry thought you need TX1 schematic!

I meant 150 but actually a 100 ohm should be sufficient and will generate considerably less heat.  This will put the voltage reasonably in line with what the tube rectifiers would have produced, it will be a bit higher particularly under light load.

Roger, 

Since the 5R4 will drop about 67V at 0.25 A,  wouldn't R be as close to 268 ohms as you can get with standard resistors ?

Hi Mark,

It is difficult to truly simulate the dynamic resistance of a high vacuum rectifier because the voltage drop versus current isn't linear and as you approach the full normal current draw the drop (and power dissipated in the resistor) become very high.  The current draw is probably in the neighborhood of 500 mils in AM on voice peaks which would result in 135 volts drop and around 70 watts dissipation in a 270 ohm dropping resistor.  

The Apache uses a choke input HV supply with a fairly stiff bleeder which does help regulation and the voltage rating for the caps gives you a little leeway for increased voltage.

In general I usually leave high vacuum rectifiers in place, it is the MV units I don't trust :)  Exceptions for me are my Hallicrafters HT-30, HT-32, HT-32B, and HT-37 transmitters which have the known weak insulation for the rectifier filament winding.  For those I use plenty of series resistance and inrush limiting.

On edit:  MV tubes are very easy to properly substitute using silicon rectifiers.  Voltage drop across a MV rectifier is fairly constant ranging between 5 to 15 volts for a healthy tube within rated current flow.  I just add a small series resistor (in addition to inrush limiting) to reduce the initial heavy capacitor charging current.  You could almost perfectly simulate the voltage drop of MV by using a large number of lower PIV rectifiers (each with a ~.7 drop) instead of a few higher PIV units.  A combination of this plus a series resistor could more closely simulate the operation of a high vacuum rectifier but the number of junctions required would be impractical and is rarely necessary.