#ubitx Power output on 40 / 80 m with high current levels #ubitx

Alex - PA1FOX

Hi all,

I'm working on my uBitx which is completely homybuilt, so not the currently sold PCB version. By building it from scratch you encounter a lot of interesting things and learn a lot about it. Receiving of the rig is very good now but I am struggling with the TX. When aligned for output power of 8 watts, using a sine wave generator and full modulation level, the rig draws about 3,5 amps of current on 40M. At 80M, power output increases to about 12 watts, but current jumps up to around 4 amps. Sometimes my IRF's get killed at that level. There's now way so far I've got it set to say 8 watts with less than 2 amps of current. Things I've done so far are:

Play around with the HF power gain pot RV1 in combination with more audio drive level
Play around with the IRF's bias adjustment. Setting it too high will give a current thermal runaway and a fried IRF within a second. Careful with that...
Output toroids of the IRF balance, changed the windings bifilair, trifilair en more in both directions, so in fact play around with the output impedance with the IRF's see. The trifilair original option seems to be the best bet, but with these high current values. Toroid is a 37-10 with 7 windings trifilair.
Changed the emitter resistances on the 2N2219 to decrease the input power.

I've measured the LPF and it has a cutoff at 7300 KHz so that should be fine.
Testing done at 12volts, when at 13,8 volts, the output power and IRF amp values increase dramatically (got over 16 watts of RF) but this often kills one of the IRF's. No way I would keep the IRF's alive on 24 volts.
I don't have the LPF's for the higher bands in place yet, but it seems that at 20m and higher the overall gain of the TX chain decreases. When measured WITHOUT LPF (yes on dummy load, for sure) output power decreases to less than a single watt... This could also be an effect of the bidirectional part of the TX chain where the gain decreases with higher frequencies. For now I'm concentrating on the difference between the behaviour at 40 and 80 meters.

Could it be I am just using a wrong brand of IRF's? I have a load coming in from a second supplier to check.
Any other ideas? I have got rechargeable batteries built into my rig and they would last much longer if I could keep the total peak current below 2 amps.

Oh by the way, I've got a very large cooling heat sink and never got it really hot, just 'warm' 

Lovely experimental rig!

73, Alex

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