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Re: boosting the power on 28 MHz #ubitx

Timothy Fidler
 

Kees Talen builds and sells an amp using Mitsi mosfets? that goes 80 to 6 m.? He uses RD16HHF1 . not the VHF version . he is very conservative in design.

http://www.qsl.net/k5bcq/Kits/Kits.html

claims 16W out SSB with no artifacts?on the Spectro box? (20W nominal ) it maintains this Power all the way up to and inclu 52 Mhz. This is because he is using a Cu tubular core type TFR in type 61 material . The input Trafo does not matter as there is power to burn. He mentioned this to? me in some emails.? He also tried the old CT Primary and gave up - it seems there was too much variation from one coax ferrite to the next.? So he want over to the phasing choke method for power in.

So at 28Mhz what will be your limit after this change out is the Ferrites, specifically the OPT . Type? 43? is Very? bad at this frequency possibly 3/4 of the impedance is resistive loss .? There is data on this versus type 61 premium material? on the Antennas part of the site and I hve a bit more .?

?The thing about type 43 is it is cheap and there are several equivalents to it made in Europe.? Type 61 is the ONLY material to use above about 8 Mhz.? Because the Ue of 61 is? 125 versus the? 800 nominal for type 43 , if you do? a change out, you will need to change the transformer out to say two x? size 80 stacked, if they will fit so you do not lose too much in shunting on the lower bands? (NEEDS to be calculated not guessed) Ie you would need to? keep the primary inductance up to say min of 0.8 x what it is at moment or you compromise the design on 3 Mhz. If you don't change the ferrites and windings then you could have very capable set of Mosfets but still not much improvement.??

Sorry to be a bringer of bad news.? Obviously the Trafo of the driver set is also eating power so if you are up for surgery look there too.?

A recent poster made a comment re change out of drivers to 2n3866 . which is? hard beast to find.? 2n3053 would be as good at this frequency.? I don't know if he changed out all four or just two and threw the other sissy drivers away.? If you have big drivers with press on heatsinks on them and not too much drive to their bases, you may be able to go for gold and short out the emitter ballasting resistors which are costing you drive power.?

I have not mentioned Balun transformers on purpose.?


If you did all those changes I would guess you would get easily 12 W out even at 28 Mhz. Also the core temperature of the Mosfets will be lower.

By the sound of things.. power is severely compromised on high bands.? Flowers said as much when he did the 20 m coversion. He was getting only 2 W out he said before the RD15HH... then 6W after.? Given the kludging of the connections you are going to have to do I would not risk the higher gain Vhf Version. Flowers used it only as he found it easier to buy.?
?

Timothy E. Fidler : Engineer BE Mech(1) Auckland , NDT specialist AINDT UT /RT3 , MT2 CB #2885,?
Telephone Whangarei?? 022? 691 8405
e: Engstr@...



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Sent:
Wed, 22 Aug 2018 03:52:08 -0700
Subject:
Re: [BITX20] boosting the power on 28 MHz #ubitx


Would there be a clear advantage to replace them?


Il 21/ago/2018 23:09, "Timothy Fidler" <engstr@...> ha scritto:
Simple answere is NO .? But there are advantages as per below.
1.The Pinning on RD devices is GSD with device flat on its back. ( that means inter alia the final transformer has be direct wired to the? Drain of the RD - if the wire will stretch.)
2. RD16HH needs slightly? more? measured bias current? on D-S? leg to stay linear.
3. RD16HH is near idiot proof wrt SWR up to 15V (see datasheet)? but above that you are near one third of its? max operating condition and you take the risk of losing them if the output is unterminated. THe qualification voltage for these devices is 12.5V and many homebuilt amps with a pair have provided 16W out,? linear at 14V supply.
4. RD16HH has Source ie zero volts thermal/elec bonded to the tab, so if you provide approp support,? you can rejig existing sink arrangement and have both devices direct mounted on a common massive finned heatsink with NO mica washers under the devices.? Obviously to do that,? you would need a carrier plate that the PCB stands on via hex nut stand off legs, and the heat sink is mounted off that carrier plate.?

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