[ARC5] Plate impedance...

[Kenneth G. Gordon]

On 14 Dec 2008 at 22:47, Roy Morgan wrote:

> On Dec 14, 2008, at 10:02 PM, Kenneth G. Gordon wrote:
> 
> > Anyone know what the plate impedance is of a pair of 1625
> > at, say, 7.0 Mhz, a plate voltage of 550 VDC, and a plate
> > current of about 150 mA?
> 
> Ken,
> 
> Nope, not readily.  There was an RCA Ham Tips about values of PI 
> networks for common tubes of the day that.  I interpolate the numbers 
> given for other plate I an V values and I get about 1500  ohms.  The 
> RCA HB-3 pages for the 1625 or 807 aren't much help with this.

I thought as much.

There is a rather simple formula used to calculate plate 
impedance of, I believe, a triode or triodes operating in 
grounded-grid, which is usually Class B.

Now, the ARC-5 final amps operate in some class other 
than Class B, somewhere between Class B and Class C, 
and are not triodes.

Nonetheless, we can get some sort of an idea about the 
plate impedance by comparing this formula and the 
information that Roy has supplied us.

That formula is the plate voltage divided by 2 times the 
peak plate current.

As I said, it isn't particularly accurate, but it suffices to 
enable a builder to at least choose components that will do 
the job.

In the case of the ARC-5 at 550 VDC and 150 mA, the 
value arrived at is about 1833 ohms, which compares pretty 
closely to Roy's 1500 ohms.

So, that means, to me, that if the plate impedance of a pair 
of 1625s is well above 1000 ohms, and the antenna to 
which those are coupled is between 5 and 12 ohms, there 
must be some sort of impedance transformer between the 
plates and the antenna.

Most impedance transformers of the period (1945) were 
frequency dependent. I.e. they were frequency sensitive, 
and exhibited significant selectivity.

Therefore, any impedance matching device was, 
automatically, capable of harmonic reduction, if it was 
properly used.

In order for it to match an antenna at, say, 7.0 Mhz, it 
automatically had to REDUCE output at any other 
frequency.

So, I repeat, if one maxes out the link, and reduces the 
loading to zero, harmonic suppresion would also be 
reduced, and effectively, eliminated.

This also means that harmonic suppression WAS built into 
the ARC-5 transmitters.

Ken Gordon W7EKB