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8877 amplifier design questions

Jul 26th 2011, 09:33

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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An ARRL member asked this question:

I'm building an 8877 linear similar to the one described in several issues of the handbook mid eighties. The cathode is isolated by a rf choke, 100uh @ 1000ma but no cat. #. I've not found a source either and wondered if it might be just as well to wind one on a small ferrite toriod?

During standby the cathode is allowed to float positive by a 10K resistor. If the filament is at ground potential is there a danger of exceeding the breakdown of the filament to cathode. Why can't I float the filament at the cathode voltage since there is no good reason to ground the filament ct?

The goal of my project is to build a small desk top amp capable of about 800 watts output using a seperate 3000v supply. The 8877 should do well at that power and is smaller than all of the other popular tubes. Other space saving ideas I've incorporated into the design is toriod wound filament transformer, toriod inductors for the input circuit (like the ones used in the K2) and toriod cores in some of the plate tank elements. When all done, I think it will fit in a new cabinet the size of Heathkit HW12. If you are interested, I'll keep you posted.
Jul 27th 2011, 11:24

AE5ZZ

Joined: Apr 27th 2011, 13:15
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Total Posts: 0
you might look at Allied Electronics 895-3792
http://www.alliedelec.com/search/productdetail.aspx?SKU=8953792
for your choke.I don't have any answers for the other questions, but good luck on your project and feel free to keep me posted on it. Hope that helps. If you click on their catalog page you will see some other options.
Jul 27th 2011, 13:26

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
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Total Posts: 0
I'll post a link to a scan of the Handbook article, probably sometime today. That may allow more complete discussion. I'll delete this post when I post the link.

Ed, W1RFI

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