[ARC5] AM modulating an "ARC-5" transmitter...

[Fuqua, Bill L]

   Cathode modulation had been used for some time. It's efficiency is somewhere between plate modulation and grid modulation.
My electronics instructor back  in high school, over 50 years ago, used to run a 813 cathode modulated. 
    There is not much need for voltage gain since you want to modulate the cathode current and you have effectively Beta raised to the 4th power
of current gain, and the input impedance also goes up nearly equally well. However, to make sure I had enough drive I'd opt for a carbon microphone.
But as I just said, some negative feedback still is needed to stabilize the cathode current and also reduce distortion.
73
Bill wa4lav
 
________________________________________
From: ARC5 [arc5-bounces at mailman.qth.net] on behalf of Leslie Smith [vk2bcu at operamail.com]
Sent: Monday, October 12, 2015 7:33 PM
To: ARC-5 List
Subject: Re: [ARC5] AM modulating an "ARC-5" transmitter...

   Robert (and others)
   I distrust this circuit.  I suspect the circuit was never built and
   tested.
  Does any-one here know of this circuit being used - either on a
  test-bench or actually on-air?

  The circuit lacks stability for the DC bias (operating) point.  If
  used with a battery the operating point will drift as the battery
  voltage drops.
  The result?  Either a low degree of modulation or splatter.

  The claim that the circuit will operate with a crystal microphone
  seems to be unfounded - only Q4 supplies voltage gain;  Q1 to 3 supply
  only current gain.
  The operating bias for the 1625 will be somewhere between -35 and -65
  volts.  If this modulator is to work it will need to supply a peak to
  peak signal AT LEAST 50% of that range.
   Without the benefit of "spice", - just looking at the circuit I would
   say, "No way."

  That's my assessment

  73 de Les Smith
  vk2bcu at operamail.com


On Mon, Oct 12, 2015, at 08:13, Robert  Eleazer wrote:
> As for the transistors for the Simple Modulator, I would use 2N3394 for
> Q1-2-3 (I have a bag of about 100 of them) and a 2N3055 or the TO-220
> equivalent for Q4 (overkill).
>
> How does that sound?
>
> Wayne
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