[ARC5] Receiver current drain

[Kenneth G. Gordon]

On 5 Jan 2011 at 19:56, Mike Hanz wrote:

> Like everything in life, "it all depends", Ken.  A clue is provided in
> the 15mA cushion for external devices.  If the B+ is 240v, you are
> pushing the audio output of the receiver to a full 400mW audio output
> shown in the bench test procedures, and the BFO is on, the total
> current drain can approach 45mA.  I seem to recall 35-40mA in normal
> operation, but it all depends on the conditions.  I'm curious as to
> why you might need it so precisely?

Well, I have a number of power supplies which were built and sold by Fair 
Radio Sales back in the 1970's time frame. They built several versions of 
these, around some power transformers which they had had especially built 
for them.

One of the end-bells has FAIR 818 stamped in it, in fact.

Their earlier versions were built using 5Y3 rectifier tubes, but their later 
versions used a pair of diodes in place of the 5Y3.

These were built on stripped dynamotor bottom plates, so all you had to do 
was to snap them into the dyno mounts and turn them on.

Output was some +HV and 24 -28 VAC for filaments.

The power supplies are properly designed....except for one thing: the B+ is 
around 350 VDC when unloaded, despite their having included a bleeder 
resistor across the output, and a resistor in series with the diodes. 

The filter cap is a dual-30 MFD at 450 VDC can-type with a 2.5K resistor 
between the two sections.

What I wanted to do was to load the output with a power resistor of the 
equivalent resistance to mimic the receiver load in order to discover how 
much of a voltage drop might occur.

I used a 4167 ohm power resistor (250 VDC at 60 mA), and the voltage 
drops to 165 VDC...which is a pretty big drop!

The transformers are fairly big, and don't get terribly hot at that load (about 
40 mA), but I am concerned about the damage that could occur to the old 
can-caps in the ARC-5 receiver caused by the too-high voltage when first 
turned on.

The older 5Y3 version probably wouldn't have had much of a problem that 
way since its filament has to warm up too, but the diodes DO cause a 
problem.

I think what I am going to have to do is to add a switch to turn on the B+ after 
the filaments have warmed up.

I dunno....

If I use your figure for current drain of about 40 mA, then I need to adjust my 
resistor to 6250 ohms.

But what am I going to do about the too-high voltage at turn-on?

Ken W7EKB