Following on from the discussion a few days ago regarding qrp vs qro and how it might be nice to have a little amplifier for those hard going days I had a little poke around ebay to see what kinds of things sell there (as you do). Maybe buy a pair of power fets and build something?
I came across rather an enigmatic entry from wsplcbargains for a 100W card for the flexradio 1000 saying “We have absolutely no circuit information” sitting at £10. This is rather odd as wsplc currently sell the same card for £462… I did a bit of research, found the circuit diagram, the board is well populated with all relays, switchable low pass filters, swr bridge, pair of irf510 drivers, pair of 2sc2879, looked worth a punt.
I got it for £55, £2 less than my maximum bid. It looks like it might be ok. A binary selector for the lpf will be very easy. A diode and transistor will do rf vox equally so. There’s no protection but a dc amp from the reflected power sensor to a pair of pin diodes would work as a mismatch attenuator. Only problem is its max 100w out from 1w in, but a front end attenuator would be easy enough, standing current on the devices is meant to be fairly low so it won’t be too bad.
Worth the punt I think:
The only problem will be trying to explain to Ian why I’ve just given him an extra 7AH SLAB to carry…
I was about to say the same. You may need 24v or more to get 100W output - but it may run OK on 13.8V and give you 30-40W out - which is probably more than enough in any case. Not sure if it came with a heat sink, if not one for 100W s going to be larger than one for 40w. Also running at a lower power means if the transistors are hit with a bad VSWR there’s a better chance that they will survive.
Ed.
EDIT: PS - here one RF Vox circuit from Australia:
It’s designed for 13.8V 0.5 watt in 100W out, as part of the Flex SDR-1000 but reduced output at 12v would be ideal for me, I don’t want a 100W PA! And as you say at 40W it will be far more robust to bad swr, not that I “plan” to give it that…
The heatsink has to be good for 100W ssb duty cycle, but also it comes with a fan kit to add to the flex 1000 enclosure. I’m hopeful that under “sota” conditions and reduced output the heatsink will be just fine as is, but I might paint it black!
Rather then fit a large Pi-pad Andy, I would fit a 0.5dB pad rewind the input/interstage transformer and take out the IRF510’s You do not really need them driving from an FT817, plenty of drive at 5W to take it to 40W to 60W. You will save some power here and thats good for obvious reasons !
Pretty easy to calculate the turns required. If you dont like maths, use an MFJ antenna analyser to find the near Broad Band approx 50+0j point.
I had considered that, there is way too much gain, but a) I don’t have an MFJ antenna analyser b) I’m not sure what maths to use, electronic design of this type is way after my 1970s electronics degree and I havent got the stomach for doing that much butchery! It would require way too much dismantling to get to the other side of the board to do it properly. Initially I’m going to put up with the inefficiency run the 817 at lowest output and see what happens.
I started with a love affair for Valves - and still do love em… Waiting for the day I get chance to use these 572b’s here. Homebrew is still where its at for me.
It’s a lovely piece of kit and a bargain at that price. But it looks huge and there’s a lot of faffing needed to get it ready for SOTA use never mind the bigger battery (7A SLAB: how 20th century, 22A LiFePO is de rigeur for SOTA QRO). All that for 9dbW over the 817. Or one and half S-points. Seems a lot of effort for not much QRO.
Great buying. Just wind the FT817 back to 1 watt and put in a 6 dB pad. You will have maybe 50 W then. KISS.
Conditions on HF have been poor lately for “local” ie VK work. Those running 40 W were worked but a number of the 5 W stations were never more than a mumble in the noise. Size does matter! 10 dB makes all the difference when conditions are marginal.
I grew up building with valves from about age 9, I doubt there were many other kids in my primary school who knew the resistor colour code! Thanks to my om (SK) it was a good upbringing. That lovely cherry red colour from a well exercised valve I’m a great fan of the 4cx250 series with homebrew for 6, 4 and 2 here, 2.5w in 250w out from a single no problem. Can be exciting at times. None of them will ever see SOTA!
But it looks huge and there’s a lot of faffing needed to get it ready for SOTA use never mind the bigger battery (7A SLAB: how 20th century, 22A LiFePO is de rigeur for SOTA QRO). All that for 9dbW over the 817. Or one and half S-points. Seems a lot of effort for not much QRO.
I’m a great fan of the 4cx250 series with homebrew for 6, 4 and 2 here, 2.5w in 250w out from a single no problem. Can be exciting at times. None of them will ever see SOTA!