# 2xGi7b final Pi coil question

Discussion in 'Amateur Radio Amplifiers' started by SV2SBE, Oct 7, 2018.

1. ### SV2SBEHam MemberQRZ Page

Hello again,
I am at the stage finalising the RF compartment of my amplifier.I have made a coil that is from a 3/16" copper tube and the internal diameter of 2".And the total of 18 turns.I know that we use the copper tube for the higher bands.But i want to ask if i can use it with 30 and 40 meters.From your experience with the size of the coil how long i can go?2 tubes,2200v anode and air capacitors from SB-220
i dont want to use 80 or 160 meters and 10.
Thanks
George

2. ### W1QJHam MemberQRZ Page

Hi George, firstly I would like to thank you for using our scale of inches. hi hi I see no reason why you can't use 3/16" tubing for a 30/40 meter coil. The only problem I see is that the coil will have to be pretty large, perhaps even larger that 2" diameter. Have you determined that 18 turns will give you enough inductance for 40 meters? If you have the room to fit such a coil I think it is good. Most of the time space is of main concern. Lou

3. ### SV2SBEHam MemberQRZ Page

Thank you Lou,
will make some calculations and come back again.

4. ### N8CBXHam MemberQRZ Page

Have you done any tank value calculations? With two tubes that will affect the C1 value, but might be okay using the C1 & C2 from a SB-220.
For the lower bands I use the Airdux coils from MFJ and then cut to the right amount of turns.
Jan N8CBX

5. ### SV2SBEHam MemberQRZ Page

i think that the capacitors will be enough (i think!! maybe not ) can anyone give me a simple program to make the calculations that works?

6. ### W1QJHam MemberQRZ Page

I have constructed numerous tank circuits and calculations are ONLY ball park at best. I use calculations as a starting point and add extra inductance at first attempt to test the circuit. Then simple trial and error quickly homes in on your specific value of inductance that fits your particular stray values. As far as C1 and C2, in those cases I once again use a value that should be more than adequate. By measuring the actual capacitance of C1 and C2 at the proper settings I can determine if I can lessen the capacitance to give broader tuning. NEVER be surprised if a calculation is quite a bit different than the actual required value. As a side note, the key to a highly effective amplifier with maximum obtainable efficiency, your input circuit(s) must also be right on the money. Input circuits are just as important, if not more so than the tank circuit.

KK4RSV likes this.
7. ### N8CBXHam MemberQRZ Page

Why not consider using the two L1 coils directly from the SB-220? The plate impedances might be close enough. What is your calculated plate impedance for your amp?
Jan N8CBX

SV2SBE likes this.
8. ### SV2SBEHam MemberQRZ Page

Hello Jan
I dont have the final coil from the sb220.I have found the capacitors in ebay and build the amplifier from scratch.It is my first amplifier and thats the reason im asking soooo much questions.