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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th Feb 2009, 9:49 pm   #1
JHGibson
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Default Better AGC

When the pentode used in a AGC controlled stage has its suppressor grid going to a dedicated pin, it is usually connected to the cathode or ground. If however the suppressor is connected to the AGC line at a point where it is decoupled to ground with a large capacitor, the AGC response of the radio will be enhanced.
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Old 16th Feb 2009, 9:55 pm   #2
Phil G4SPZ
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Default Re: Better AGC

Interesting tip, thanks. I've been trying to fathom out why and how it works, but apart from the AGC voltage making the suppressor grid slightly more negative, I'm not sure. When you say the AGC response will be enhanced, do you mean the speed of response to changes in signal strength, or the range of AGC action, or some other improvement? Did any manufacturers design their circuits this way?
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Old 17th Feb 2009, 3:16 pm   #3
Neil Breward
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Default Re: Better AGC

Interesting one this. For most RF/IF pentodes the grid base for the suppressor grid is fairly high - i.e. several tens of volts - as the grid is an 'open' spiral and not as close to the cathode as the control grid. Consequently I wouldn't expect much control to be present from the odd few volts on an AGC line. However, you do see suppressor grid control in the Limiter type of IF amp in some VHF/FM sets where the negative voltage from the FM discriminator is fed back. See the Bush VHF 61/62 circuit, for example, where the second EF89 IF amp is controlled in this way.

Feeding the G3 of a standard IF amp to the AGC line, at high impedance to DC although fully decoupled to RF, could have two effects. One, grid current could produce a negative DC voltage on the suppressor grid (cf. grid-current bias in AF amps with a very high grid-leak) which could raise the AGC voltage slightly. This might reduce the overall gain a bit by raising the bias on the controlled valves, but it might make the AGC a little more responsive.

The other possibility is that the G3 is acting as a clamp diode, preventing any positive voltages from slightly 'soft' valves getting onto the AGC line and damping the input circuits?

Any other ideas welcome!

Cheers,
Neil
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Old 17th Feb 2009, 5:09 pm   #4
JHGibson
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Default Re: Better AGC

By enhancement I mean that the audio output increases less with increasing RF input.
The effect of applying the same AGC voltage to the suppressor as well as the control grid is demonstrated in fig 2.14 on page 23 of the Radiotron Designers Handbook. The mutual conductance is shown falling rapidly with increasing AGC voltage.
I have seen this technique used only rarely in commercial radios. It is mentioned in the RCA Receiving Tube Manuals in application notes for the 6BA6.
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Old 17th Feb 2009, 6:37 pm   #5
igranic
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Default Re: Better AGC

Quote:
Originally Posted by JHGibson View Post
The effect of applying the same AGC voltage to the suppressor as well as the control grid is demonstrated in fig 2.14 on page 23 of the Radiotron Designers Handbook. The mutual conductance is shown falling rapidly with increasing AGC voltage.
The Radiotron Designers Handbook was the proprietary name used in Australia and USA. Elsewhere it is probably better know as The Radio Designers Handbook, by F Langford-Smith. The specific figure and page numbers apply to the Fourth Edition published in 1953.

IIRC Radiotron was the brand name employed by Amalgamated Wireless Valve Co of Australia

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