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Rework of 3cx3000 input and output circuit

D

Duppy Conquerer

Member
Jun 25, 2012
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0
11
NEC
Hello fellas,
I am in the process of reworking a 3cx3000a7 multi-band to a mono band for the lower spectrum of 27mhz optimized. What I would like to know specifically is the values for the input and output pi tank circuit. The plate choke value, bypass cap value (at bottom of choke) doorknob cap value and the dimensions for the tank coil.
73
Sid
 

For the tank circuit on output, what plate volts at under load roughly, how hard do you plan on driving it? this is going to affect amp draw. Need this before i can give a value for tank coil. Are you doing a PI network or an LC for input?
 
Also what Q do u plan on using? Q is going to affect what size of coil and how much or how little cap you will need. Generally higher Q means more cap. Lower Q means bigger coil and less capacitance. Depending, you may not even need a Tune cap, A small value doorknob would suffice. Depending what is needed. Have to take into account capacitance from anode.
 
Unless I am terrible wrong I am pretty sure you do not need to alter the input tuning at all to narrow the out put tunning. You could leave the input brand band with selector to select what band you wanted but alter the output to be more specific. This way you only need to alter one half of the amp tuning circuits not both input and output. This way should you ever want to put it back to stock you do not have to do much.

Many amps had multiple configurations with 90% of it staying the same and just a few cap values and turns of wire here and their difference. In one form it would be 6m in another 10m and sometimes the same amp would have a broadband setup for legit Ham HF use.

Also why mono band? For the input tuning if you needed a ready made set-up their are ready made kit's that are super easy to use. No one makes kit output tuning circuits for obvious reasons. I just do not want you to think you are stuck doing both the front end and back end just because.

I am not designing my own input tuning circuit for my tube amp project. I using a kit. You just have to select the right value silver mica caps to finish it the kit has all the other parts including the printed circuit board.
 
Judging by his question, If it was a working amp, you shouldn't have to worry about plate choke, blocking cap or bypass cap. Simple retuning input and output. 10 meter would have been pretty close for lower 27mhz band. channel 6 or channel 11. So that is why I stated what I did.
 
Hello Electrontube,
thank you for you input. You raised a very valid point with regards to keeping the other freqs capability should I ever want to return it to its former glory days...but I'm gonna be realistic as can be..if I dont use it then I'm gonna lose it...furthermore, if I should need to return to stock it wont be a problem for me. Just wanna make some room in the deck because it is pretty cram inside and could pose an hazard should I try to optimize my freq of choice with the current layout. Anyway, I thank you for taking the time to respond to my post, you guys have been very helpful. Thank you.
 
Duppy Conquerer said:
Hello Electrontube,
thank you for you input. You raised a very valid point with regards to keeping the other freqs capability should I ever want to return it to its former glory days...but I'm gonna be realistic as can be..if I dont use it then I'm gonna lose it...furthermore, if I should need to return to stock it wont be a problem for me. Just wanna make some room in the deck because it is pretty cram inside and could pose an hazard should I try to optimize my freq of choice with the current layout. Anyway, I thank you for taking the time to respond to my post, you guys have been very helpful. Thank you.
Click to expand...

To really answer your question, Can you give us an idea or pic of what is there? Alot of those amplifiers like the Gates, collins and henrys, had enough tuning range to tune everything between 1.6 and 30 Mhz. I dont understand why people say they are "optimizing" for a specific band. If it tunes 11 already, then leave it be. Switching to a mono band design will net you next to nothing. And the only loss you would really be getting rid of would be the band switch loss. Thats even if one is used. Usually the tank circuits are so well built that a band switch was never needed or feasible at those power levels.

What I am trying to say is, If it tunes 11 now, then the values you would need to convert it are the values you just achieved using the components that are there already. So if you are dead set on mono banding it, then just tune to 11 meter and de-energize everything. Then just measure your capacitance and inductance and buy narrower range componets to replace them.

For the input, just buy a broad band tuned input and dont mess around with a tune and load....alot easier
 
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DXHound,
Thanks for your input as well, and for the most part you make a lot of sense with your point of view. However, what brought me to the decision to at least gut the tank coil, is that, I don't believe it would be able to withstand full steam from the 3cx3000, and although I am able to load up on the lower side of 27mhz, its not centered to enable peak resonance at the desired freq., and as a direct result, lowers the efficiency at desired frequency. The following test results will help you to understand a little better:

With 4.2kvdc with load
50w pep carrier yields 200w and swing 1500w pep with 1.2A plate current
100w pep carrier yields 500w and swing 1750w pep with 1.5A plate current
200w pep carrier yields 750 and swing 2200w pep with 1.75A plate current

note: I did get a little more output - got more with less plate volts and drive, eg., 4kv plate and 45w pep yield 250w and swing 2700w pep with plate current at 1.5A - when I change dooknob from 500pf to 200pf but it almost moved me outside the tuning range of the Pi network...whereas I was only able to load up at the end of C1 and C2.

Now let me give a little background about the rig...its a HomeBrew capable of tuning 10 - 160 mtrs. It has ATI-6 input tuner board on input, on the output C1=0/500pf@15KV, C2=0/800pf@10kv, L1=(10mtrs coil - 0.8uh) B&W 160/850), L2=B&W 802 Choke - 110uH@1A, and 500pf doorknob@15kv

Power supply is a hefty pig that can deliver 5200@2.5A

Now let me make it clear that I don't intend to be running my high performance car like I'm at the Indy, but I would certainly like to be able, if and only, when the occasion warrants, eg. like when you are on the freeway trying to get around a drunken or half asleep 18 wheeler ..to able to step on the accelerator out of dangers way, lol.

Yes, I believe the input is fine...I am able to tune flat for zero reflect, so I 'm good there. The output on the other hand, the plate choke inductance seems bit too high at desired freq, also the max current capability of plate choke is at 1 amp, and will undoubtedly be a problem if the throttle is ever opened. There you have it. What are your thoughts?? I will try to upload some pics later. Again, thanks to all for your assistance.
 
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If your seeing an increase in current with a decrease in Po at 27Mhz, then the tank circuit should be redone as it is not loading for that freq....as you already stated.

C1 looks fine, but C2 should have at least double the capacitance. You could try shorting out a few turns of L1.

Do you have an antenna analyzer? or service monitor? If so you could use either to help tune the tank circuit and try diff configurations.

As for the math, Just remember that a tank circuit has a given frequency at wich it is resonant. And you also need to decide if you want Hi Q or Low Q. High Q will have a very tight tuning range, and net higher gain. Low Q will be easier to tune and will allow you to travel around the band without tuning as much, but will have less gain at the fundamental tuned freq.

Im not going to write all the math as there is alot.......you need to determine the resonant freq of each componet. Use LC resonant theroy listed here......Simple parallel (tank circuit) resonance : RESONANCE
 
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Dxhound,
I do have mfj259B handy and dual scope the runs up to 50mhz as well, and will attempt to get a close approximation of tuning for resonance with the carbon resistor substitution from plate to chassis after I get around to wrapping new coil, but truthfully, I have never done the procedure before..but suspect it will be a lot of fun. The relevant section of L1 already have two turns shorted, so it's about 3 turns that is active. Anyhow, thanks for the assistance, it's very much appreciated. Going to turn in now, I will check back in the morning. Goodnight.
 
Also should be noted, the higher the Q, the higher the plate circulating current in tank coil. Cicculating current is roughly Q x plate current. So if you do a Q of 15 and pulling 2A, that means you have roughly 30A going thru coil. Will have to use a bigger conductor instead of 1/4" tubing. 3/8" or eveb 1/2" tubing which can be a bear to work with.
 
You can purchase the accessory for the analyzer and make it a grid dip meter, I remember using the old heath kit grid dip meters years ago, really helps for making the tuned tank circuits.
 
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