[Hammarlund] HX-500 trials

[Kenneth G. Gordon]

On 25 Apr 2016 at 20:08, Jeff wrote:

> Current finals are 6146As.  I swapped them out with another pair of 
> 6146As, and got the same results.

OK
 
> I checked the actual tube pin at pin 5: -50V on the tube itself.

OK

> Coax attached to the rig and into a watt meter/dummy load shows zero 
> output, so if they are going to a parasitic, it's beyond my wattmeter's 
> ability to see.

Shorted coax or short in output? Almost zero impedance. Open coax? Infinite impedance?

> Finals are pulling 230 mA as measured between the output of the HV 
> supply and the plate choke.  This is in standby.

Hmmmm....weird...

>  When I turn the mode 
> switch to CW and go to MOX, there's no change (nor output).

No. There wouldn't be.

> I don't have access to my spectrum analyzer where I am, so I can't see 
> if a parasitic is out there somewhere.

OK.

> Another test run: when the neutralizing capacitor is fully meshed, the 
> tubes draw over 400 mA, so there seems to be some effect on the 
> condition via that (if that makes anyone have a brainstorm.)

Sounds to me as though the finals are oscillating strongly. The question is, "Why?" Grid 
"seeing" the plate or vice versa?

> Results are the same whether or not the driver tube is in place.

OK

> OK!  Looks like we have a winner (maybe!)

No. See below.
 
>  From Alan - K6RFK:
> 
> Hi Jeff,
> I think that -50 on the control grid is not negative enough for the high
> plate voltage and 250 on the screen.

No. For a 6146 in  Class AB1 as linear amplifier, grid #1 voltage is -50 VDC for plate 
voltages varying between 500 and 750 VDC and screen voltages 200 VDC and below. 250 
VDC on the screens is a BIT too high, but not enough to cause this problem.

He could completely remove screen voltage or even ground the screens and this should 
drastically reduce his plate current.

>  Pin 14 on the accessory socket is
> connected to the cathodes of the 6146s.  Use a milliamp meter or volt meter
> on this pin and adjust the bias pot for 50 Ma or 0.5 volts from the pin to
> ground.  If the current is much more, that is why the tubes are getting hot.

He already said that plate current is 230 mA and up. Cathode current should be somewhat 
higher since it includes all grid voltages.

What you are doing with the above procedure is setting the idle current, but since the finals 
are already pulling over 250 mA, this will tell you nothing. You MAY be able to use the bias 
pot to apply enough bias voltage to the finals to bring the plate current down to a 
"reasonable" level, enabling you to do some more troubleshooting without destorying either 
your finals or some other circuit component, but the bias voltage will be WAAAAAAY 
"above" (below..more negative) what is necessary for Class AB1 operation.

The basic problem is that the finals are "running away", but so far, we have not been able to 
determine why.

At this point, and since he measures NOTHING on his wattmeter, I suspect either an open 
or shorted coax between the final and the load.

As I said, the fact that adjusting the neutralizing capacitor effects plate current suggests to 
me that the finals are oscillating very strongly.

This can be caused when there is no load on the finals. Look for an open coupling capacitor 
to the final amp tuning network for instance.

Ken W7EKB