The L4B amplifier is a great little amp. As with anything though, it can be made better. I got my L4B a few years ago. I've used it as my main amplifier for just about all of that time. It does an easy kilowatt and never complains. The L4B was manufactured from the early 70's so they can be almost 50 years old, if you have an early one.
I added vacuum relays for antenna switching. There is a small control board that is mounted in the bottom compartment, next to the soft start module. It incorporates bias diodes for the 3-500's, soft key circuitry, and timing for the RJ1's.
Recently I became dissatisfied with the amount of cooling air that came from the original blower. It's a nice design with low velocity air and a motor that is basicically trouble free, as long as you lubricate it according to the manual. The L4B was designed back in the days when legal limit was 1000 watts INPUT! Running it at 1 kilowatt output was really pushing the design a bit. The tubes did not have enough air to allow the higher plate dissipation.
Here is the installation that I did to my L4B and I am quite pleased with the results. Lots of air now but it is no longer as quiet. It eliminated the vibrating case parts but you can hear the increase in air flow .. although not bad nor objectionable in any way.
Click on any picture below to see a larger version with more detail!
First, I removed the old blower and took the Dayton blower apart to get an idea of how well it would fit. The Dayton blower is model 1TDN4 and I found the best price on Amazon. It fit the space well but I was going to need to move the plate blocking capacitors.
Overall it is a good price for a Dayton blower and readily available.
The blower flange was the biggest problem. I removed the entire back part which allowed me to place the blower all the way back to the rear L4B back panel. I also removed a small part of the front which intefered with the 3-500 chimney. I did need to redrill another mounting hole because the one that was provided was right on top of the tube compartment partition.
I found some neoprene gasket material and made a custom gasket to isolate the blower housing from direct contact with the chassis.
Here's the final mounting position. I used #12 hardware and flat metal and rubber washers to isolate any metal to metal contact. I also used small rubber grommets in the mounting holes. When those two mounting screws are tightened down, the blower is solid. I also put foam stripping on the back panel, between the blower and the panel.
Since I marked and drilled the mounting holes without consideration for that weatherproof strip, the housing was pressed firmly into the foam and no screws will be necessary to make that solid.
I mounted the blower motor and relocated those plate coupling caps. I used half inch brass strips that I purchased at a local hobby store.
There is plenty of clearance between the rear of the tuning cap and the motor. Click on the picture to get a bigger view.
It was necessary to slightly alter the blower intake hole. The new blower sits about a half an inch higher and about a quarter inch more to the side. In addition, I installed a blower speed control to allow the blower to be adjusted to suit the operation. On SSB the speed can be idles back a bit to move less but still adequate air. This was not necessary but a nice addition to allow you to tune the amplifier to the way it is being run.
It wasn't the finished look that I wanted so I installed a computer fan blade protector. It covers the air input hole perfectly and it has a washable foam filter that can be cleaned occasionally and will allow cleaner air to enter the amplifier.
Here is the vacuum relay controller board and the Soft Start board. The actual relays sit in the area occupied by the old frame relay .. the frame relay? It made the Thursday garbage pickup!
You always have to look for results. Originally, a piece of tissue paper (Kleenex) held above the tubes, would flutter slightly with the old blower. You can see that the airflow has greatly improved.
The other thing that this mod addressed was the vibration of the case and the mechanical hum that it made. Looking at the old motor mounting scheme, where four brackets held the motor to the rear panel, made the motor noise transfer to the whole rear panel. As the grommets became stiffer with age, they didn't isolate the humming the way they did when they were new! There's no such humming now.
The 3-500's are happy with the increased air and so am I!