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Chinese Linear R.I.P.
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All,
So I set up a test station in the garage, and was trying out the Chinese linear again. The uBITX was feeding the linear in CW mode, and I was giving it toots with the mike switch. The linear was plugged into the LPF, and the LPF into a 30dB 20W Narda attenuator. The Narda was plugged into a cheap step attenuator, and the step attenuator into a tinySA. Yup, the 3rd harmonic still really high. Let's try another band... All of a sudden, the power supply started buzzing louder, and the uBITX screen went blank. Oh, no did I burn out the uBITX? Nope, it was the linear. Almost a dead short across the supply pins. It sucked all 35A out of the Astron power supply and wanted more. I'm guessing that one of the final transistors let out the magic smoke. So people - what's the class act in 100W ( or so ) final amps to boost a QRP rig into usefulness? I see the same Chinese vendors have kits that use an actual RF FET - instead of a switching FET. - Jerry KF6VB |
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Jerry I used to recommend that you set the bias pots for minimum current and then slowly increase one pot to increase the idling current by 100 ma, then increase bias on the other MOSFET until it increases by another 100 ma.? Now I am not so sure.? The device datasheet is probably your friend.? Calculate the load-line and find a current that is within dissipation limits and in a linear part of the load-line.? Gate capacitance (Ciss) should be the lowest capacitance device, usually IRF510, not IRF520 or IRF530. Arv _._ All, |
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On Tue, Jun 8, 2021 at 04:11 PM, jerry@... wrote:
to boost a QRP rig into usefulnessI am assuming your intention was not to insult the 7,101 other members of this list that find QRP very useful.? However, if you don't like your QRP gear you could probably sell it and added to the money a 100w amp would cost buy a decent used 100w rig. ? -- |
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On 2021-06-08 16:22, Terry VK5TM wrote:
*** Doubt it. That package is used only for dual RF devices. But theOn Wed, Jun 9, 2021 at 06:41 AM, jerry@... wrote:.....I see theI wouldn't bet on that, a large percentage of 'RF Transistors/FETs' device could be used/surplus, or failed-QC. And the amp I'm looking at uses a device that's characterized at 850MHz. The fact is, one proper HF LDMOS device costs more than an entire Chinese kit. - Jerry -- |
¿ªÔÆÌåÓýDiscussion of amplifiers of modest size (50-100w) is pretty common across several large QRP mail reflectors I subscribe to, it came across here as little more than a playful jab.As I recall, Ashar specifically designed the uBitx to allow the finals to operate off a higher voltage to increase RF output (25W+?). Ken, N2VIP On Jun 8, 2021, at 19:25, Doug W <dougwilner@...> wrote:
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Hi Doug - no offense intended. QRP is fun. That's what ham radio is for - for me, anyway. Fun. But it's no fun when people don't hear you. And at my QTH, with my antenna, people mostly just don't hear me with 2 watts.
Also, I'm a ragchewer, not a QSO collector. 599-599-tnx QSO 73, not for me. And the instant somebody exits a QSO because they couldn't hear me, my hand goes for the power button. However, if you don'tDoug, that's just unkind. Shame on you. Actually, I have a perfectly functional TS-590S on my desk, and an Alpha 78. And an IC706MKII in the closet. And a 30L-1 in the shed. And a bank balance that would let me drop $12K on an ultimate whiz-bang ham station without even sneezing. But what would be the fun in that? - Jerry KF6VB -- |
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John Cunliffe W7ZQ
Jerry,
Did you measure the input impedance to the changed low pass filter before you applied power through it? It might not present an impedance that makes the transistors happy. Also, given the vastly different output power on the different bands, are you sure you did not drive the FETs to hard on the band it died? John |
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On Tue, Jun 8, 2021 at 06:52 PM, jerry@... wrote:
Hi Doug - no offense intended. QRP is fun. That's what ham radio is for - for me, anyway. Fun. But it's no fun when people don't hear you. And at my QTH, with my antenna, people mostly just don't hear me with 2 watts.All very fair points.? I clearly misinterpreted an intention that wasn't there. Doug, that's just unkind.?You're right, it was.? I latched on to one word and ran with it.? As long as you're having fun with your radio the rest is noise. ? -- |
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Hi John,
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Well, it was coincidentally on 80M when it died. And that is surely the band on which the uBITX gives the most output. But if that's enough to fry the finals, the amp and the uBITX are just not well matched. I imagine one could design some sort of network that would gradually reduce the gain at low frequencies and up it at higher frequences AND maintain good SWR looking at the uBITX, but that's surely beyond me. I saw somewhere that these IRF5xx transistors are sensitive to excessive gate voltage, and the allowable voltage is quite low - on the order of 4 volts. Hindsight being 2020, I probably *should* have checked S11/return loss on the LPF with the nanoVNA before powering it up. I *did* check the S21 logmag, and it looked good. If the amp does give a lot of spurious, most of THAT energy is surely reflected back by the LPF... - Jerry KF6VB On 2021-06-08 16:53, John Cunliffe W7ZQ wrote:
Jerry, |
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