AL811 and AL811H Fan Kit (revised Sept 1, 2024)
We are offering a complete fan kit. A lot of this kit is to make airflow correct and as easy as possible. This kit restores airflow to the original design values.
Fan Kit Instructions (Rev 1) PDF file
If you are not familiar with tack soldering connections watch this: Video
Initial Design
Temperature rise in any closed space is a function of air mass exchange rate and the total power dissipated as heat. I designed the AL811 and AL811H amplifiers as the cheapest possible amplifier for medium power amateur SSB and CW use only. The design was long before FT8, and never intended for AM use. Correct minimum airflow is about 18-20 cfm on normal SSB and CW with three 811A tubes (25% less power), and 25-28 cfm on higher duty cycle modes for four 811A tubes. This is for a 40oF internal temperature increase at maximum average tube dissipation. The tubes safely dissipate about 60-watts per tube average anode heat, while filament and other power losses amount to about 120 watts in the AL811H and 95 watts in the AL811. Use of 572 tubes does not change the power ratings, it just makes tuning much more forgiving and allows the three tube 811 to run the same power as the 811H.
No matter what tubes are used in the 811 series when dissipated power is the same total thermal rise will be the same for the same airflow rate. A larger tube will not make the amplifier run cooler, it simply makes the tubes last longer and be less often damaged by long tuning periods.
Some Ameritron 811 amplifiers have far too little airflow for normal SSB operation. This Cooltron fan is especially bad. It flows roughly 1/3 of proper airflow. This fan has excessive temperature rise even on casual SSB use with the three tube models. If your amplifier has this factory installed fan, it must be changed. Your amplifier will last longer with proper airflow:
If you live in a country with 50Hz mains, this fan will only flow 5 cfm. Flow with 50Hz mains is too low even with the amp on standby.
13.3 CFM measured in cabinet on 60Hz mains
Airflow is difficult to measure, this is why we did this work for you. While not an absolute test, a good breeze should be felt some distance from the exhaust holes. A Zippo lighter will have problems staying lit over a foot from the amplifier side. In general, the more airflow, the longer components will last.
Our Kits KF811 and KF811H
Our new revised AL811 kit contains a small circuit board with Zener bias diodes and other parts.
The Zener bias has three selectable voltages that are set with a single movable jumper. The board bias voltages cover a wide range of tubes by selecting 0, 3.9, and 7.8 volts. These voltages add to any existing bias.
There is a protective GDT installed on the board, plus a voltage limiting resistor.
The fan kit also has two manually selected speeds via a toggle switch.
Our fan selection is a standard 45 CFM, 3.2 x 3.2 x 1.0 inch, 12Vdc 0.2 ampere fan. It is sleeve bearings, which are actually quieter than ball bearings. If you decide to buy your own fan, we recommend a Keyfanclub 45cfm fan. This fan draws 0.2 amperes and flows over 25cfm in the actual amplifier. (If you use your own fan, be aware many fans do not meet advertised rated free air. About half of fans tested failed to meet advertised specs in open air. All fans will flow considerably less than free air flow through an amplifier.)
By adding a dropping 5W 3.9V Zener, our fan selection flows near 18 cfm on low voltage (~10Vdc) and near 25 cfm at ~15Vdc, the typical AL811 full bus voltage. This flow rate is with the amplifier cover on.
The fan rating is about 45cfm free air, but be careful! A 45cfm rating does not mean a different fan will work the same in the amplifier.
This kit bolts in without any chassis or wiring modifications.
Either prewired kit is $48 plus $5 shipping to the USA via USPS.
Parts ORDERS or parts questions go to orders@ctrengineeringinc.com
