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Author Topic: Input Balun on a 4 X 572B Amplifier  (Read 4026 times)
VE3XAT
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Posts: 3




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« on: November 24, 2004, 10:46:06 AM »

I have been doing some upgrades to an old amp I've been working on. This amp has four 572B tubes in it with no tuned input networks. There is no room for them anyway. Four 572B's have an input Z of ~ 200 ohms and are fed directly through .01uf capacitor. Would it be worth making a 4:1 balun with say a T106-2 core (not sure if current or voltage balun type at the moment). Would it help sine things up alittle. Thanks for any reply.
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WB2WIK
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Posts: 21837




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« Reply #1 on: November 24, 2004, 11:08:58 AM »

I'm trying to envision the amplifier that wouldn't have room for a 3"x3" square printed circuit board containing the tuned networks and a wafer switch section (that could be mechanically coupled to the end of the existing bandswitch shaft)...

Isn't it the flywheel effect of an input tank circuit that benefits the system?  A balun alone won't provide that.

I added a tuned-input board to an old Gonset GSB-201 (4 x 811As) and wow, what a difference that made!  Amplifier gain improved from <10 dB to about 13 dB without changing anything else.  It also allowed the use of SS exciters which were previously unable to drive the amp at all.  There was tons of space in the Gonset.  What kind of amp are you working on?

WB2WIK/6



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VE7ALQ
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Posts: 349




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« Reply #2 on: November 24, 2004, 05:36:31 PM »

I am thinking of a Clipperton-L amplifier.  Supposedly it does not have an input tuned circuit to make the exciter see 50 ohms, but of course it has a 'PI'-network on the business end of the 572B tubes.  I guess that the input impedance of four 572B tubes must be close to 50 ohms when used grounded-grid.  Actually, I find the lack of a tuned circuit on the grid side of the 572Bs an asset because I would like to be able to use the amplifier on the WARC bands (which is legal in Canada) and fixed tuned input circuits (such as on the Heathkit and Yaesu amps) would render that kind of difficult.
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VE3XAT
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« Reply #3 on: November 25, 2004, 04:41:15 AM »

Wow first post here and I mess up. I was looking at the wrong data sheet. The cathode Z for one 572B/T160L is ~ 215 ohms, so it would be a 1:1 balun. Yes I understand how a tank circuit works and if you got +3dB out of your amp with the tuned input thats great! It's an old Dentron 160-10L with lots of changes. There is room for an input tank network on the power supply side of the amp. I don't have a warm feeling about using the same swith to switch the input circuits. First not enough room behind the switch to add another section because of the tank coil. Also I don't really like to put the input and output lines so close together in case I make an oscillator. A seperate switch would be ok. But all I really wanted was one broad network to do the WARC bands too. I had seen some homebrew amps with a balun and thought it would be an easy way to do it. This type of input circuit at these links seemed interesting to. I'm working on a 813 amp also and have all the stuff to try it.
http://www.qsl.net/wb8erj/813input.htm
http://www.angelfire.com/tx5/wieser/813amp.htm
Thanks guys!
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