Topband: W7IUV preamp info

[Larry Molitor]

Recent posts regarding the preamp circuit I have on my web page and which has appeared in ON4UN's book seem to indicate a lot more problems building the circuit recently. I hope to update the preamp info on my web page to minimize problems folks have duplicating the circuit but in the meantime maybe this post will help.

The schematic presented in ON4UN's 4th edition on page 7-98 is incorrect. The 47 ohm resistor shown in the emitter should be a 4.7 ohm resistor.

The schematic shown on my web page under the "Rotatable Flag" topic is correct but is optimized for my application. If the circuit is built **EXACTLY** like the schematic shows, it will work as specified in the text. Parts substitutions are possible but unless you have a good background in RF design, don't try it. I hope to update the web site soon with more parts options.

The power supply is specified as 12 volts. That means 12.0 volts +/- 0.1 volt. It does not mean 14 volts or 13.6 volts or anything else. The part values specified on the web page are chosen to maximize IMD performance with 12.0 volts applied. If you run more than 12.0 volts, you run the risk of damaging the transistor which is running near it's maximum ratings. USE A GOOD HEAT SINK!

Some of the parts are critical. Use disk ceramic caps or other caps that are known to have a low ESR and no HF parasitic resonances. If the cap has a plastic case, it is probably not the right part to use! Use 1% metal film or carbon film resistors. Most metal film resistors have low enough parasitic inductance to work fine in this application. Be aware that this circuit has a usable bandwidth of over 100 MHz so build it accordingly. The 4.7 ohm resistor in the emitter lead is critical, make sure it is not less than 4.7 ohms! 5 ohms is OK, 4.5 ohms is not.

The current drain should be 89 ma. +/- 5 ma. If it is not, you screwed something up. The transistor should be hot to the touch, but if the heat sink is adequate, you will be able to just barely keep your finger on it. If it is hotter than that, you either need a bigger heat sink or you are drawing too much current.

The 0.01 uF ceramic coupling caps that are specified may be increased in value to 0.1 uF if, and only if, the preamp is preceded by a very good high pass filter. Using a good 7th order or better high pass in front of the preamp will allow the use of the 0.1 uF caps. The higher value caps should be used if VLF operation is contemplated. The 0.01 uF caps have been specified to provide a small amount of BC band filtering for those applications which need a bit more BC rejection. I suggest using the 0.01 uF caps unless you really know what you are doing. (Design note: if this preamp follows a complex filter such as an elliptic function filter, you will probably need to use the 0.1 uF caps in order to present the filter with a proper termination impedance)

The 2N5109 and 2N3866 transistors have been tested in this circuit and will provide performance specified. Other transistors might be used but I have not tested any nor can I suggest any substitutes. Both of the above transistors are readily available from several mail order sources. Substitute devices need to be compatible with the Ft, hfe, NF, and power dissipation of the specified devices.

Have fun!

73, Larry

Larry - W7IUV
DN07dg - central WA
http://w7iuv.com