L4B 160m modification

For the PACC contest 2005, we set ourselves to further improve our 160m station. For the CQWW CW contest in 2004, we already build a vertical and 2 K9AY RX loops for this band, all we needed now was a little more power. We didn't have any serious amplifier for this band, and the 200W barefoot power just didn't make it trough the high QRM levels often experienced on the other end of the line. So we decided to build a 160m extension to one of the available amplifiers. We choose the good old L4B by Drake. This amplifier gives a nice 1 kW output from 80m up to 10m. The best solution for us was to sacrifice the 80m position, and modify it for 160m. The build was done by Jelmer PA5R.

The Build
The idea was simple: just add enough capacitance in parallel to the existing anode and antenna loading variable capacitors, and increase the inductance in between. As a starting point, all values should be roughly doubled, and then fine tuned for best performance. Of coarse, we are dealing with a high power amplifier, so only suitable materials and components for these power levels can be used. The RF currents flowing in an amplifier output tank circuit can be massive!

After a lot of experimentation, the end result was an additional 160pF at the anode capacitor, an extra inductor made of 24 wdg heavy gauge Teflon coated wire on a big Amidon T-300 core, and an additional 1200pF in parallel with the antenna loading capacitor.

l4b_cap102 l4b_coil202

The green capacitor on the right is the 160pF/4kV shunted over the excisting anode tune capacitor. In the left picture, you can see the added inductor made of 24 wdg on a Amidon T-200/3 iron powder core (2kW HF type). The blue part just visible on the left is a 1200pF/3kV shunt capacitor added to the antenna loading capacitor. The original mica cap was removed. Make sure that the capacitors used here can handle the large RF currents flowing trough them. Ceramic disk capacitors for instance are not useable!

With the tank circuit tuned for 160m, the remaining  modifications could be realized. First of all, the anode RF choke coil had to be increased in value, because the impedance of the existing choke is too low for the 1.8 MHz RF signal, causing excessive RF current flow into the power supply, possibly even a complete burn out of the choke! So a second choke was added in series after the existing choke seen from the anode. This was done to minimize the risk of unwanted resonances causing a choke melt-down. Think about it! Of coarse the new choke was first measured using a network analyzer to check for these resonances and checking if the impedance was high enough for 1.8 MHz. Again, the value of the existing one should be at least doubled. In addition, an extra 4700pF disc capacitor to ground was added to further reduce any RF current flowing into the power supply.


The additional RF choke


  Extra 4.7nF capacitor to ground

The last step was to modify the 80m input matching circuit to obtain a decent VSWR to the driver transmitter. This circuit is a simple PI network in a low-pass configuration. Some extra shunt capacitors and series inductance using a small iron-powder core did the job. Not much effort made at the input circuit, because we use transceivers with build-in tuners. A VSWR of below 2:1 is more than sufficient. Be sure to use silver-mica capacitors as RF drive power can be as high as 100W or so!

There were no modifications to the RF choke in the fillament supply circuit because the existing choke plus the feed trough capacitors are able to deal with the 1.8 MHz signal by themselves.

The result

The result was a pretty 900W output power with 80W drive power in the SSB setting of the amplifier. This setting we also use for CW, as the anode voltage applied to the 3/500Zs is very conservative at only 2.4 kV. These tubes will last for ever with this low anode voltage.

I'd say:
160m modification for Drake L4B DONE!

The amplifier performed outstanding during the nightly hours of the contest, so we think it survived it's first test, and hopefully will stay alive for many 160m QSOs to come.

If you're interested in performing this modification to your amp, schematics can be requested via e-mail.