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A.G.C. for Sideband and c.w.

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A"TECHNICAL Correspondence." letter(1) by Mr. Cranfield prompted a review of the Woods article(2) and that triggered off a letter to George Luid:, WOBFL. Reason for all the action was a "hang a.g.c." circuit for sideband(3) based on rectified i.f. that George improved(4) by using rectified audio instead of i.f. The letter inquired of WOBFL if he had run into any of the troubles recounted by Woods, since they hadn't shown up in the rectified-i.f. version. Here is the reply:

"I have received numerous letters from fellows who have tried out the audio-hang a.g.c. and have `improved' on it. One even went so far as to say he improved his by putting a 100K resistor from the a.g.c. his to ground. Of course, he missed the whole point of the circuit. Woods' circuit looks to me like it would operate exactly like the conventional a.g.c. in commercial receivers, except that it is audio driven."

"I have been using mine for over three years now, and I have not experienced a single tube in the controlled stages that exhibited any leakage to ground from the grid. I have found several 12AU7s and 12AT7s that have too much leakage to be used as discharge triodes. However, it is easy to find a good one, and the S meter shows absolutely no fade from peak reading. It is entirely possible that with no signal and the antenna disconnected there will be a slight positive potential on the a.g.c. line, from contact potential, but that voltage is to ground and can never be positive to cathode, since increased plate current will automatically increase the bias developed by the cathode bias resistor. Thus the effect cannot possibly be harmful."

"Just received my third Christmas card from VK6HC, who used our circuit and is very pleased with it. Shortly after the article, heard G2MA describing his homemade receiver with an audio hang-a.g.c. circuit that he described as 'fantastic;' called in and he said he had it installed within four days after receiving his QST."

"I built the a.g.c. circuit in a small Minibox, to fit into a 75A-4 right behind the PTO, with a cable that runs through a hole in the chassis to connect on the audio-output tube socket for voltages. Some models have a 330K resistor from a.g.c. bus to ground that must be disconnected. Used it on two A4s and it worked fine except that the attack time was too long, and a really strong signal would cause 'over-throw' and pop when the signal started. These were not my receivers so could not modify them, but I figured it was caused by the time constant being too long in the isolating filters in the grid returns of each controlled stage. In my own receiver I long ago substituted small r.f. chokes for the 10K isolating resistors, and I have absolutely no over-throw or pop. That is the only improvement I have been able to make in three years."

The diode suggested in Mr. Cranfield's letter was used for bias application by WOBFL, as pointed out in the original article.


  1. "Fixed bias with audio a.g.c.," QST, January, 1961.
  2. Woods, "An improved audio-driven a.g.c. circuit," QST, September, 1960.
  3. Goodman, "Better a.g.c. for s.s.b.- and code reception," QST, January, 1957.
  4. Luick, "Improved a.g.c. for sideband and c.w.," QST, October, 1957. See also, Single Sideband for the Radio Amateur, page 91.