Specifically the L4B was made back when the legal limit for power was specified as 1000 watts "input", which is about 650 watts output. That's a lot easier to produce than the 1000 or 1100 watts output or even 1500 watts output, which is what we want from our amps today. The two factors that come into play is the plate voltage and then the cooling requirements.
- PLATE VOLTAGE The L4B has a separate power supply that can sit elsewhere. Normally it produces 2650 DC volts when resting and under full load about 2450 DC volts. That's being pretty easy on the tubes and probably why they have been known to last so long. Today's 3-500 amps, by comparison, use about 3650 DC volts on the plates and will do legal limit! The tubes are rated at 4000 max DC volts but running them at that level requires a much better cooling system.
- COOLING When the L4B is run the way it was designed, the amplifier just didn't challenge the dissipation capabilities of the tubes. Heathkit cooled the SB-220 and 221 with a muffin fan for the same reason. The fact that Drake used glass chimneys and a wimpy blower wasn't a whole lot better but it didn't need to be.
There are all kinds of mods available and some of them directly address these problems.
Here are a few of the things that I have done to mine:
First, I removed the old blower and took the new 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 $47.74. Overall it's a great price for a Dayton blower and it's readily available. It fit the space well.
I should mention the the blower has a rating of 49 cfm. Each of the 3-500ZG's need 13 cfm of air for each tube to realize the maximum rating of 500 watts of plate dissipation or 26 cfm for the pair.
The Dayton blower provides plenty of reserve capacity. In fact, we both installed a blower speed control that allows us to throttle the blower speed back when such cooling is not required. It cuts down on the air flow noise to match the operating environment better, like when you want to set the amp on the desk next to the microphone!
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 redrilled another mounting hole because the one that was provided was right on top of the tube compartment partition.
I wanted to isolate as much of the blower from direct metal to metal contact with the amplifier. I found some neoprene gasket material at the local auto shop and made a custom gasket to isolate the blower flange from direct contact with the chassis. I also found some foam gasket material with super stick on one side that I placed between the rear panel and the blower. Since I didn't allow much space for the foam gasket, it was pressed firmly between the two, providing a pressed fit between the two pieces and isolating direct contact in that area.
This is what the rear of the L4B RF unit looks like after the blower modification. The blower intake requires that you make the rear panel hole slightly larger. The intake is about 3/8 of an inch higher and about 1/4 of an inch toward the power plug from the old opening. I used a nibbler and a file but no matter how much I nibbled and filed it lacked the look.
Jim K8IH suggested using a computer fan protector to cover the enlarged hole and the protector has a washable foam filter that makes the air going into the amp a bit cleaner. The filter also slips out for cleaning! Instead of spending time every few months oiling the blower bearings as before, you can use the time to wash the filter off. Just make sure that it's dry before turning the amp back on!
The old frame antenna switching relay was removed and found the trash can very quickly. I started having trouble with it not returning to receive well. A pair of Jenning RJ-1's were installed to replace the old relay.
There are several ways to do this and all will probably work well. Others have used silicone sealant and simply set the relays in the sealant before it set up. I made an aluminum bracket to hold the new relays and used rubber grommets and pushed them into the grommets. The fit was tight so they won't be falling out soon and the rubber provided sound isolation. You really can't hear them switch, except for a very slight "tick" when they pull up.
The vacuum relay require very specific timing to make sure that they are not "hot switched" Jack WB8BFS designed a board that does that for his Kenwood TL-922 but it works quite well in an L4B. It creates the timing necessary for the relays and also provides an amp switching circuit that will protect your transceiver amp switching circuit.
It also provides diode bias voltage to the 3-500's, switched appropriately, and eliminates that crazy bias scheme from the HV supply. This board fixes several design problems with the L4B.
Jack's board is shown installed on the left and the small board next to it is the soft start board from Harbach Electronics. It provides protection for the front panel switches which have become impossible to find replacements for. It also makes life easier on the HV power supply components.
These are just a few of the modification that help the old gal into the modern age. If you decide to do any of these updates, or can suggest a few more, I would like to hear about your experiences. You can email me at my QRZ.COM listing email address.
-- Larry W8ER
