Designing Ham transmitter |
I thought, since I am designing a transmitter for myself, I would walk through all the steps here. First, I decided to eventually run a pair of 6146 tubes. I want to measure the difference between 6146 tube types at HF, and document if they really are as different as claimed. These tubes are also a good choice for CW because they have high gain and are compact for the power level. The 6146 data looks like:
Looking at tube characteristics we can pick a zero drive bias point based on limiting to 20 watts dissipation at 700 volts (20/700= .029 amps or 29 mA safe no-drive current). If we look below at the -50 volt curve (just below G) we would have safe plate current with just over -50 volts bias. This will eliminate any need for a screen clamp tube!
The rated output of a class-C 6146 at 700 volts is around 65 watts. This would be with about 600 volts of peak anode voltage swing, or about 400 volts RMS voltage (because the waveform has high harmonic content, RMS voltage swing is less than .707*peak we might expect). At 130 watts for two tubes, the plate load impedance would be about 400^2 / 130 = 1230 ohms. If we compare this estimate to the approximation Ep/2Ip=R we have 700/.26*2 = 1346 ohms. I will design around my estimate of 1230 ohms optimum plate load resistance. It is probably closer, although this value is not critical. Many articles suggest a network Q of 10-12, but in reality there is very little difference in harmonic performance between a Q of 5-6, and a Q of 10-12. The calculated difference in 2nd harmonic voltage going from a Q of ~7.5 to a Q of 12 is only 1.4 dB. If harmonics are an issue, it is more effective to find other means to suppress them than to increase tank Q. Good harmonic suppression is more a matter of construction and layout than it is tank Q, so long as a somewhat reasonable Q is selected. Minimum Q for a Pi-network is generally accepted to be 1+(sqrt of Rp/Rl). This means if we plan for the highest impedance transformation we should be OK at all other settings. If we don't do that, the network might act "spongy" or tune backwards, with less loading capacitance decreasing plate current at dip. If we plan on a 1.5:1 load SWR, we should use sqrt of (1230/33) +1= 7.1. Q would be 7.1. This also covers us at reduced power, because tank Q increases at a faster rate than the required increase in Q as the ratio becomes higher. We will never have a condition, at reduced power, where tank Q is too low. We will plan for a Q of 7.1 on all bands into 33 ohms (not 50 ohms). This gives us headroom for SWR errors!!
Because of the large values required, 160-meter tank values determine the physical area required for the tank circuit. For proper headroom, we need just over 500 pF at 1kV for the plate tuning capacitance. I decided to use a 208 pF capacitor with a 360 pF padding capacitor for the plate. This will provide a tuning range of 375-568 pF on 160 meters. The tuning capacitor will need padded on 160 and 80 meters. For loading, I have an 1100 pF air variable. The loading capacitor will need padded only on 160.
Power SupplyAlso see this link I've settled on a power supply. The schematic below is a p-spice model of the supply. R5 and R6 actually represent internal resistance of the transformer. R1, R2, and R3 represent the load by external circuits, including voltage dividers and vacuum tubes:
This supply moves the choke into the negative rectifier lead so I can extract bias from the AC voltage appearing across the choke. I've done this in a Globe Scout and it works OK. This is a full wave bridge that uses the transformer center tap to obtain half voltage for the low level stages and screen grids of the PA tube. R1 is the HV load, R2 the low voltage load, and R3 the bias system. Diodes can be conventional 1N4007's.
Here is how the real-world supply actually tested in a load-pull as R1 was varied. It looks like I have a good 100 watt supply, and a marginal 150 watt supply. This should be suitable for a single 6146B tube, or perhaps a pair:
Supply ESR looks to be around 350 ohms or so. I tried several chokes and couldn't find anything better, and no matter how reasonable bleeder current or how much I increase inductance I can't seem to get less than 700 volts no load from my 700VCT transformer. I'm working on stabilizing the supply more if possible. |