The AM Forum

THE AM BULLETIN BOARD => Technical Forum => Topic started by: KB5MD on May 10, 2019, 08:42:11 PM

Title: Plate on 4-1000
Post by: KB5MD on May 10, 2019, 08:42:11 PM
What is the minimum plate voltage that a 4-1000 will operate?  The minimum, not maximum.

Title: Re: Plate on 4-1000
Post by: W1ITT on May 10, 2019, 09:04:20 PM
I ran a 4-1000 for years and had the plate supply on a variac.  I recall running it at 2400 volts during early parasitic tests.  It was happier at 4 or 4.5 kv however.  But it made useful power, again unrecalled from this distance, at 2.4.  I was running 500 volts on the screen all the time.  Back then, I wasn't thinking about experimentation at lower plate voltages, so I never plumbed the depths below that.  The plate is rugged, but it would probably make sense to watch screen dissipation as you run plate voltage lower.  My recollection is that maximum screen dissipation is 75 watts, but check the data sheet to be sure.  It's a rugged and forgiving tube.

Title: Re: Plate on 4-1000
Post by: K1JJ on May 10, 2019, 09:26:18 PM
Yes, the 4-1000A is a workhorse.  It will generally run cleanest and most efficient at the higher voltages around 3500 to 5500V. But there are exceptions:

If the HV is reduced too far down, getting a proper idle may be difficult.  With the grid bias at zero, you may be able to ramp the screen voltage up high enuff to compensate.  How low do you want to go? For real low HV, try this...

Quick story: The little brother triode, the 3-500Z is a tube I once used as a driver. The GG 3-500Z is kinda like running the 4X1 in GG, but with much higher mu.  Eimac published a 3-500Z data sheet that showed tremendous cleanliness when run with only 1500VDC and positive overall tube bias or truly zero bias with no zeners or diodes in the cathode. (We want the grid to be the same or more positive than the cathode to pull up the idling current)  The 3rd IMD was -45dB or better.  It worked great in class A as a driver for a larger tube.

Now, lets apply this to a 4X1...  Is the 4X1 going to be in GG service?  If so, you can experiment to see how low you can go by putting a variable negative supply in the cathode (filament CT)   of say 1A    0 to -50 VDC. (Look up the curves first)   This will idle the tube high enough while you bring the HV way down.  With tone tests you will be able to find the sweet spot of best cleanliness and lowest HV it will run at.

If you are talking about running it in grid driven RF or audio service, put a positive bias on the grid (just as if it were negative)  or you can still put a variable negative supply in the cathode as before and go from there.  The whole question is as HV is decreased but idled at normal current, will the IMD remain reasonable or even improve like the 3-500Z did? I never tried it so you will have to find out.  And yes, use a variable screen supply, (and regulated) if used, to adjust for the proper screen current. (max 140 mA)  And keep an eye on grid current when making it more positive... there is a 40 mA limit.


Title: Re: Plate on 4-1000
Post by: KD6VXI on May 11, 2019, 09:11:07 AM
My 4x1 runs at 3k volts.  Grounded grid it gives appx 1200 pep out.

I've run them up to 8 kv idle. At that voltage they really sing!


Title: Re: Plate on 4-1000
Post by: Opcom on May 13, 2019, 12:47:16 PM
In class C with plate modulation, I have had good results at 2250V. The screen was run at 400-500V.

Careful loading to keep screen current down. I have no evidence about it but don't like it over 100mA. There has got to be a screen current limit but I have no idea what it is.

Output was about 350-400W carrier. More than the usual grid drive was needed to get legal limit peaks. About 20W.

I probably have a very good tube. It will make 100W carrier with zero screen voltage.

Screen is choke modulated. From the screen supply through a 2000 Ohm resistor, through an 8H choke, to the screen.

Title: Re: Plate on 4-1000
Post by: K1JJ on May 13, 2019, 01:18:25 PM
After running extensive tone and IMD tests I get the cleanest signal from a class C  4-1000A  plate modulated final by running about 50 mA on the grid and 140 mA on the screen.  The grid is driven about 10 mA above its 40 mA suggested value but makes a definite difference in higher audio peaks and brings the flat topping threshold up.   Running 140 mA screen current is the recommended value and is not as critical to performance as the grid.

I like about 4 KV on the plate in class C plate modulated service as a maximum. I have 2500 V and 3200V as alternative levels that I use often when condix are good.  But 4KV gives the best performance when using 4X1 modulators.  A variable (Variac controlled)  screen supply gives the option of running any plate voltage and running the plate loading anywhere we wish.

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands