[AMRadio] Pi-Net vs Link Couple

Brett gazdzinski Brett.gazdzinski at mci.com
Thu Sep 22 08:13:51 EDT 2005

I don't think there is any way to run a triode single ended with link
output, not in class C.

I run a pair of 812A's (or 811A's, or V70D's) push pull link coupled output
modulated by a pair of 811A's.
It works very well.

You can run them up to 2000 volts on the plates, or 1750, 0r 1500.

I can get 400 watts carrier out pushing them, 350 watts out has the tubes
showing no color.

That is very close to the legal limit with plenty of audio.

The 812/811 tubes are cheap, work from 1000 to 2000 volts, and
don't take up a lot of space.

I used small vacuum variable caps for neutralization, kilowatt
coils, and a 6000 volt plate tuning cap.
I got the small (2 to 30 pf @30,000 volts) neut caps out of old paging 
transmitters someone gave me, they ran a 4-400 at 3000 volts on 70? MHZ.


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Geoff
Sent: Wednesday, September 21, 2005 10:46 PM
To: Discussion of AM Radio
Subject: [AMRadio] Pi-Net vs Link Couple

SO, here we are, well past 1991 and the 'law' that went into effect 
saying that 1,500w PEP output is the maximum RF Power output that we 
hams can run, regarldless of mode.  That doesn't deter the homebrewing 
spirit, but it does suggest that acheiving 1,500w PEP output is much 
easier than producing 1kW DC input to the final.  With the mindset of 
still wanting to use the classic high-level plate modulation scheme, 
engineering a rig to use only one tube in the final (a 4-250, 250TH, 
304TH/TL, 4-400, etc), modulated by a pair seems to make more common 
sense.  That, and it's a bit more economic in filament requirements.

I've heard recently that matching the output of the Class C 
pate-modulated final to the antenna is better, and more efficiently 
achieved by link coupling, vs Pi-Net.  On the other hand, it's argued 
that Pi-Net coupling produces less RFI than link coupling does.

So, which is better?


What are the effects of nuetralizing a single tube in a balanced tank 
circuit?  If Pi-Net is to be used, does the final tube still need to be 

I know of a guy who wants to build a rig using a single 450TL in the 
final, modulated by a pair.  He wants to pi-net the output, but I've 
heard that's a bad idea.

I want to build a rig using a medium powered tride, perhaps a 250TH, 
modulated by a pair of 811's.  Pi-Net, or Link Couple?

I like seeing this kind of technical discussion on the list.  I'm 
looking forward to all inputs.

73 = Best Regards,

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