[JN] Full Pentode operation with a CFB OPTx

pentode at netscape.com pentode at netscape.com
Wed Apr 12 19:17:25 CDT 2006

As far as ugly and uglier go, taking the TX apart and adding a strategically placed tap is leading the pack. The neat thing about adding a tap is that either pentode or higher percentage U-L are options...just swap the g2 connections.

Sooner or later there may be another option presented, and when it happens, thanks...:)

--- dcigna at localnet.com wrote:

From: Dave Cigna <dcigna at localnet.com>
To: undisclosed-recipients: ;
CC: joenet <sound at mailman.soundlist.org>
Subject: Re: [JN] Full Pentode operation with a CFB OPTx
Date: Wed, 12 Apr 2006 19:18:30 -0400


I think I have a pretty solid understanding of UL and it's variants. For 
the purposes of this discussion, let's not distract ourselves by the 
goals or end results; let's just look at the circuits. For simplicity, 
we'll consider SE.

Pentode mode: the screen is held at some constant voltage *WRT to 
cathode.* This is just how pentodes were intended to work in the first 
place and that's what we call pentode mode. It's easy to accomplish in 
the typical case where the cathode is grounded directly or at some DC 
voltage that is effectively ground as far as AC signals are concerned. 
In other words, the cathode is at AC ground. That means that the screen 
needs to be at AC ground too. B+ is AC ground, so we can just connect 
the screens there. We can connect them to any DC voltage provided 
there's a big enough cap to ground to keep the screens at a steady 
voltage WRT to cathodes.

Ultra linear mode: the screens are not held at a constant voltage WRT 
the cathodes, but are swinging some fraction of the signal at the plate. 
The easiest way (but not the only way) to accomplish this is to put a 
tap on the primary of the output transformer and connect the screen 
there. Remember: one end of the primary is at AC ground (B+) while the 
other is connected to the plate. You can put the tap anywhere you want 
to put any fraction of the plate signal at the screens.


OK, let's consider what happens if we use cathode feed back. Instead of 
grounding the cathode, let's take part of the primary of the output 
transformer away from the plate circuit and put it in the cathode 
circuit. If you draw it on paper (you should) it's helpful to put the 
ground end of the cathode winding next to the B+ end of the plate 
winding. After all, they're both AC ground. In fact, as far as the AC 
circuit is concerned, you can connect B+ and ground together and call 
the connection AC ground.

Now consider how this thing will work if you connect the screens to AC 
ground as with typical pentode operation. Effectively, the screen will 
be connected to a tap in the primary of the output transformer. It's not 
running pentode mode anymore; it's running ultralinear! To restore true 
pentode operation you need to keep them at a constant voltage *WRT the 
cathodes.* AC ground no longer works when a winding on the output 
transformer is used to apply cathode feedback to a pentode. Ok, it 
works, but it's not true pentode operation anymore, it's UL.
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