[ARC5] Audio power output and BFO fixed - 10 meter BC-454

[WA5CAB at cs.com]

Brian,

Actually, the turns ratio of the audio output transformer in the R-26 is 
well known as it's given in the parts list as 4000 turns primary and 500 turns 
secondary, or 8:1.  The primary is center tapped (you have to go to the ARA 
parts list for the turns count) and the neon lamp is across half the 
primary.

However, apply the rest of your calculations and the situation is even 
worse.  So something is rotten in Denmark.  Ken, you need to recheck your 
secondary voltage measurement.  Try using an audio voltmeter with 250 ohm load 
(speaker transformer disconnected).

Robert D.

In a message dated 02/18/2013 20:48:13 PM Central Standard Time, 
brianclarke01 at optusnet.com.au writes: 
>     If the output impedance of the 12A6 is 7000 Ohm (RCA design value)
> and the secondary of the OPT is exactly matched to 500 Ohm, the turns
> ratio of the OPT is sq rt (7000 / 500) = 3.74. (This, of course, is
> researchable hypothesis, as I am unaware of the design specs of the
> OPT. All I have seen are wire gauge and resistance specs - probably
> for trouble-shooting.) 
> 
>     3.74 x 72 Vp-p = 269 V. This is more than the rated output of the
> dynamotor, and doesn't allow for the loss of swing because of cathode
> bias. It is also much higher than the ignition Voltage of the neon
> across the OPT primary. I reckon your output must be highly distorted.
> So, it probably sounds loud because of that distortion. I suspect
> there is something odd here. The likelihood of running the 12A6 plate
> from rail to 0V is negligible, especially if the screen is still at
> rail. 
> 
>     In some of my Command receivers, the wiring in the chassis is glued
> down with glyptal. The use of single-core wire throughout was another
> piece of Drake's genius - to remove possible sources of
> vibration-induced capacitance change. 
> 
>     73 de Brian, VK2GCE.
> 
> ----- Original Message -----
> From:"Ken Gordon" 
> To:"Arc5 mail list" 
> Cc:
> Sent:Tue, 19 Feb 2013 01:33:12 -0000
> Subject:[ARC5] Audio power output and BFO fixed - 10 meter BC-454
> 
> I rechecked the audio power output here and tried to be a bit more
> accurate this time.
> 
> Peak-to-peak voltage at 400 Hz into 500 ohms on my HP o'scilloscope is
> 72 V.
> 
> Peak voltage is thus 36 V
> 
> Erms=36/sqrt2=~25.5 V
> 
> P=E^2/R=1.296 watts.
> 
> Even on the cheap Radio Shack CB speaker with the 8:500 ohm
> transformer I installed in it, 
> it will literally rattle the windows.
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480