Anybody running 4CX250B's on 6m?

[Captain Kilowatt]

If I were you I'd put the 250B's on 6 meters. Especially if you have plans for a two tube 3-500Z HF amp. They will make a very nice 6 meter amp if you like that band. Because you have 100 watts drive available I wouldn't even consider running the tubes as tetrodes. Yes the IMD will be better but then you have to deal with a screen regulator that can handle negative screen current. You need to apply negative feedback to neutralize the tube. If you over drive it once the grid will go bye bye without overload protection.

It would be easiest to ground the control grid and screen grid directly to RF ground. This should make neuturalization unnecessary. You could add some positive screen voltage to boost the gain however, don't forget about negative screen current if you do. You need a shunt regulator here because when the tube conducts heavily, secondary emissions are generated.

This causes the screen grid voltage to climb because the secondary current flow wants to drive the screen voltage towards where the plate voltage is through the secondary electron flow. Most builders ignore this issue and the result is unstable output power that wants to increase the longer you are keyed. This is similar to thermal runaway with transistors and can melt the screen grid into destruction if not controlled.

Good advice.I see we were posting at the same time. LOL.I have decided to go for 6m with this amp plan BTW. I do have 100 watts available but was planning to have a fixed ALC feedback from the amp so that whenever the amp is powered on the radio would automatically be throttled back. Using a fixed external ALC voltage I should not have any overshoot on key up either. I was simply going to derive the ALC feedback voltage from a spare filament winding.

 

[Moleculo]

Let's see some pics of this project!

 

[Captain Kilowatt]

I see patience is not one of your strong points.

I will post pix of the power supply components tomorrow. Don't laugh,the last time I used some of these components the thing weighed in at a tad over 100 pounds. This time there is a bit more iron involved.

 

[Shockwave]

Captain,

Might I suggest looking into purchasing "The Tetrode Board" kit available online for this project? This board contains all the essentials to make a properly working tetrode amp. It will provide you with adjustable active shunt screen regulation and fully adjustable bias. It also offers full overload protection for the screen grid and plate current with on board meter shunts to drive your panel meters. If you're into doing more home brew work, the designer of the board also publishes the complete schematic so you can make it on your own if you choose.

I would still exercise caution with making this a tetrode amp with a 100 watt exciter. With any reasonably efficient control grid input circuit, drive requirements should be only several watts unless your resistively padding the input. You might want to run some tests on your exciter to insure the ALC can effectively clamp the drive power where you want it and hold it stable before proceeding with building it grid driven.

The 4CX250 has relatively small interelement capacitances and therefore does not require a large neutralization capacitor to feed output back to the input 180 degrees out of phase. You can often mount a small flat strip of copper on a stand off insulator so the strip is facing the anodes of the tubes to form the cap. Trim the cap so that minimum RF is detected at the input when a small amount of drive is fed into the output on a cold tube.

 

[Captain Kilowatt]

I was looking at the tetrode boards online a few weeks ago. Still up in the air on it right now but leaning that way. As for the drive, yeah I know about the low drive requirements. I forgot to mention that I also have a 20 watt Bird 6 dB attenuator pad that I was concidering puting on the input to allow better isolation from the exciter as well as tame the drive levels. The exciter is an FT-857D and seems quite stable in regard to power spikes on key up when using a fixed external ALC. I can lower the drive to less than 5 watts this way and with the pad inline that means less than 1.25 watts of drive and maybe even less. I still have lots to consider for now but the power supply will be built with tetrode operation in mind. It can always be used for other projects in the future.

 

[paws264]

I see patience is not one of your strong points.

I will post pix of the power supply components tomorrow. Don't laugh,the last time I used some of these components the thing weighed in at a tad over 100 pounds. This time there is a bit more iron involved.

Bump

 

[Toll_Free]

The exciter is an FT-857D and seems quite stable in regard to power spikes on key up when using a fixed external ALC. I can lower the drive to less than 5 watts this way and with the pad inline that means less than 1.25 watts of drive and maybe even less.

Same radio here. Don't forget the brown wire. Use it. It activates 20 watt mode, and all of a sudden, you're good to go for ??cb?? linears

Also, you set the drive power needed with the grid resistor and the step up / down xformer. Tet's with passive grid input are the way to go... The grid samples the voltage off the resistor, and 0 watts drive needed. Most leenyar way to go, long as you get the operating voltages right.

--Toll_Free