4CX1500B help needed ...what are normal operating conditions in AB2 grid driven?

[VE7 Kilohertz]

Hi Guys,

I had a screen current failure indication in my Collins HF-8020 PA yesterday, when I first turned it on and made the initial tune up. I am driving it with the 8640 generator and it's been working perfectly for a week. I have all the Collins manuals, 2 of them 3" thick, and so much bloody detail, I can't find a simple chart of normal operating conditions. They walk you through a very thorough detailed troubleshooting procedures which involves replacing circuit boards rather than looking for simple meter readings.

So, my Eimac data sheet says 2900VDC plate, 225VDC screen with -34 bias and 15mA screen I, and .75A anode I.  I am seeing about 3200VDC anode V, 225VDC screen V, 35mA screen, .75A plate I and -35 bias. Will a 300VDC increase in plate voltage cause this much discrepancy in screen current? My line voltage happens to be 240VAC now, and the amp xfmr tap is set for 230VAC. I can pull it all apart and change the tap to 240VAC on the plate xfmr and try that..which I will eventually do anyway, but it's a pain to remove everything to do it.

The problem arises during the tuning cycle, when the autotune functions are moving thru the various combos of coil/caps and it is in low power mode. It seems that when the power is low, ie 400W, the screen current is higher. As the grid drive is increased, the screen current drops and the plate current goes up. There is a screen current meter for both +/- so it looks like the screen current can normally be positive or negative, depending on conditions. I went through the alignment procedure and I actually have it working now, but the screen current is still 35mA at 1KW out, with all other parameters as noted above, but at least the fault light doesn't come on now.  

Anyway, some guidance from experienced tetrode users would be greatly appreciated.

Cheers

Paul

[W2VW]

Dunno about the autotune but changing loading should be able to move screen current lower.

I use screen current as an indicator of proper tuning with my 4CX1500B.

[VE7 Kilohertz]

Okay thanks! Just reading some more about this loading stuff....

So what are your normal operating parameters, to give me an idea of what might be happening here?

thanks

[W2VW]

2900 volts B+ under load.
Static plate I .3A Max occasional very brief tuneup just over 1A plate current carrier conditions.
0 grid current
0 - + 15 ma screen I unless loading is not correct. I'm afraid to run it much past that.
I do not know the grid and screen voltages.

[VE7 Kilohertz]

And the story continues..I tore the whole system apart and adjusted the line voltage taps for 240VAC and put it all back together and everything is exactly the same. Everything must be regulated.    And I found the normal operating specs for all meter switch positions.....it was in the power supply manual. Of course.  Anyway, with 2900 Anode V and 225 screen V, and -34V bias and .5A anode I, it should have less than 25mA screen current and this is 35-45 mA. I am running it into a dummy load and have tried all over the HF spectrum with the same results. This is supposed to be a new (NOS) tube...but who knows. It's date stamped 1983. It's entirely possible that when I first got this going 7 years ago, that I ran it with the tube it had and I didn't pay much attention to the details of the meter readings, which means there may be a new tube still in my shop.    It does however, put out full power easily, and then some. One thing I am not sure of, the tube data sheets states single tone and 2 tone measurements but they don't list just RF carrier. Is there a large difference between no tone and with tone? I am driving the RF in with my 8640 but with no audio tone, just RF.

I also found a publication from Eimac called "Care and feeding of Power grid tubes". Excellent dry techy reading, 170 pages, but I haven't had time to completely read through it.

So, it looks like loading has a large effect on the screen I.

Any other thoughts or experiences with this or similar power tetrodes that may help me?

Cheers

Paul

[Steve - K4HX]

Single tone is the same as carrier.