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Author Topic: homebrew linear  (Read 223 times)
K3ZD
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Posts: 129




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« on: May 24, 2002, 02:39:56 PM »

I am considering building an linear amp based on an old ARRL plan for an amp with four 811 tubes in grounded grid.  I have two questions.

Would the ouput tank circuit for a junked sb-220 work in the output?

I can't find a decent band selector switch.  Is there any reason you could not use toggle switches instead?  Use a sepaate switch for each band with one terminal of each switch in common with the others.  The other terminal from each switch would go to its respective tap on the coil.  

tnx
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WB2WIK
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Posts: 20565




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« Reply #1 on: May 24, 2002, 04:08:33 PM »

The SB220 tank should suffice, bearing in mind that it doesn't cover 160m and does not have optimum Q for 17 or 12 meters, either.

A suitable rotary switch is available from many sources, including Harbach Electronics and Ameritron, who makes all their "spare parts" available for sale at component prices.  The switch from an AL-811H, for example, would be very suitable, and it's not expensive.  Fair Radio Sales and Surplus Sales of Nebraska also have suitable rotary switches, quite often.

Using toggle switches is not a good idea, since most are only rated 220Vac maximum and can break down at less than 1 kV.  The actual application here involves standing off substantially more voltage than this, as the peak RF tank voltage on the anode side of the tank can be as much as Q*Epk, which for an 811A amplifier might be in the range of 5kV or more.  Not worth the risk and likely problems, when good ceramic rotary switches are so readily available.

Have fun with the project!

WB2WIK/6

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W9GB
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Posts: 2616




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« Reply #2 on: May 26, 2002, 08:47:10 AM »

With a Heathkit SB-220 available (junked?), sounds like a restoration of that platform would work about as well as the 4 811's

Here are some web sites for building/design references

Richard Measures, AG6K amplifier site
http://www.vcnet.com/measures/

SB-220 notes
http://www.vcnet.com/measures/sb-220.html

Parts and assemblies
http://www.harbachelectronics.com/

http://wd7s.home.att.net/triode_control.htm

http://rmrlent.dtgnet.com/amp/

http://www.tomstubes.com/

73 de w9gb
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