4cx250 grounded grid amplifier grid
My first encounter with this tube was generously helped by G3MY, who supplied details and photos of his single tube six-metre amplifier. I completed my own very quickly and it worked just like Mike told me it would. Having a decent number of tubes at my disposal I decided to try a pair of them in grounded-grid configuration. It paid off just too well.
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3CX800 amp(s)
I was thinking of using a or balum and increasing the grid resistor value to or ohms to reduce the amount of driving power required. With the 4cx a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a ohm termination That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1.
Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required?
I've only read about them. G2DAF wrote extensively on pasive grid linears. I think the ultimate value of grid resistor will depend on the grid capacitance and layout parasitics.
The articles I've read use Ohms as the grid swamping resistor but with a 4CX Ohms might be possible. Here is an article you may find useful. Please Let us know how yours shapes up if you decide to build one.
I have a pair of used 's 4cx's with glass seals but I have no idea how much life if any are left in them. My idea was to build an amplifier chassis that could be used to experiment with several tube configurations.
I've found a source for a VA toroidal power transformer with two v and two 6. In a bridge rectifier configuration with a capacitor input filter using large capacitors it should yield about volts under load.
With the bridge rectifier configuration the center tap of the HV secondary will provide volts that can be regulated down to the volts required for the screen of a 4CX The target amp would be between to watts output a nice afterburner for a watt QRP rig I'm thinking about.
The defining feature of a G2DAF amplifier is not the untuned passive grid input, but the method of grid biasing. It has no control grid bias and the screen voltage is developed by rectifying the RF drive. Most passive grid tetrode amplifiers use conventional fixed bias voltages on g1 and g2. G2DAF's published articles on amplifiers are on my web site, and the RSGB historical collection still has his original receiver, transmitter and amplifier, along with many hand-written notes.
The 2 x A amplifier that featured in his published articles still exists, but at some later time he had removed the screen rectifier tubes and converted the amplifier into a very basic grounded-grid configuration.
This should jump out at you with the proper circuit analysis. Just analyze the circuit with a parallel tank load on the plate, and look for negative resistance showing up at the grid. Your grid swamping resistor would need to take care of the worst-case negative resistance.
Then spend a month tearing your hair out over parasitics that increase the plate-grid coupling over and above what the data sheet says they are. Hey OM: I don't see plate grid coupling with a grounded screen and the cathode at negative screen voltage? You only needed 20W of drive power and a single HT supply. I have one using a pair of s, 10W from my TS drives it to about W output. It looks to be exactly the same layout as the one on Ian's site but has had the valve rectifiers replaced with solid state at some time.
Steve H. Ive seen one amp that I know worked using a 4cxB using a transformer. He used tha amp on 6M. I think he still added a little capacitance to tune out some of the inductance in the transformers. The best I remember he really didnt have to do this. He just did it so he would see a perfect SWR beteween the exciter and amp. I modified an old 2X amp with tunned grid by ripping out the tuned circuit and replacing it with a ohm resistor.
Yes, that's what I meant. And if you're looking to figure out the largest parallel grid resistance that will still effectively swamp out the instability, I think that's the best approach to take. In practice its probably vanishingly small. But for many tubes you'll have to spend a long time optimizing the physical layout of the circuit before you have more coupling through the tube than you have around it.
I was recently playing around with a 6DQ6 PA and it did not require netralization with 2K ohms on the gridI didnt try anything higher than that. Using broadband transformers will introduce some reactance that may have to be tuned out. I suspect this may not become a problem until you get up to 10M or so.
Passive grid linear. Reply to author. Report message as abuse. Show original message. Either email addresses are anonymous for this group or you need the view member email addresses permission to view the original message. Anybody here ever build a passive grid linear amp? That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly.
Svetlana recommended that configuration with their 4cx, 4xc and 4cx tubes, and it should work well with more commone surplus 4cx's. Grumpy The Mule. Howdy, I've only read about them. Tim Wescott. Stev eH.
Classes of Operation
Glowbug Index. I have never been very active on 70cm and have in fact now taken down my old 14 ele beam. But back in the s there was a very active 70cm ATV group in Bristol who encouraged me to put some signals on the air in order to join the fun. They have now moved on to 23cms, 10GHz, etc and left me far behind. The anode resonator fits over the anode fins of the valve, and cooling air is blasted through from the grid box into the tubular resonator, and then out of the anode box at the far end, where there is a PTFE insulating sheet to act as a decoupling capacitor. It is a very neat design, and its mechanical features appealed to me. I have always found the 4CXB to be an extremely willing and well-behaved device, and have never found it necessary to use exotic techniques such as neutralisation.
4CX250B/7203
My MHz PA. The design: The amplifier is build from two identical aluminum boxes measuring xx76 mm with the top box being the plate compartment and the lower box having a small room for the input cathode circuit and space for filament transformer, power for the blower and metering, keying and bias circuits. The DC connection is made through a low ohmic resistor, in this manner I can measure the grid current directly across the resistor. Plate compartment. Plate circuit is a stripline x mm made from 1,5 mm copper sheat. The tuning flapper is grounded at the end wall and moved by a teflon string going to the lower box. The output flapper is placed at the rear side and adjusted by a teflon screw through the top cover.
Oh no, there's been an error
Post by Don Haworth » Tue Nov 04, pm. Post by philwashere » Tue Nov 04, pm. Post by Rune » Tue Nov 04, pm. Post by Curt Reed » Tue Nov 04, pm.
Metering of a Grounded Grid Amplifier
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Red Wave Radio
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2C39A as the Output Tube
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Problem 4 4CX250B's amplifier blowing screen resistors
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Russian hf amplifier
As writing this we are still stuck with amplifier design. MSRP: Amplifiers are a favourite homebrew project. A switch-mode amplifier would work for those, would be simpler and more efficient, and could use much cheaper transistors. Protected against antenna SWR over 2 to 1 and. Device test jig.
Antique Radio Forums
The tube needs also V on screen and —84 V V or more on rx on grid G1. The primary design objectives were that it be inexpensive yet work well. List posted soon. EXE Tuned Networks.
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