EMRFD Message Archive 11999

Message	Date	From	Subject
11999	2015-12-14 06:09:14	farhanbox@gmail.c...	IRF510 : how high?
whats the maximum drain voltage one can use with the IRF510? i have pushed them to 48v. some blew up on the load test at 20 watts (single ended). i am wondering if i can get 100 watts from a pair at 14 Mhz. at 30v I am clocking 30 watts. - f
12000	2015-12-14 07:05:22	bobtbobbo	Re: IRF510 : how high?
You can get over 50 watts at 13.8 volts from a pair of IRFZ24Ns. Amp described in June 2010 QST. Have that amp running in my HB SSB xcvr with no problems. I used a regular, larger, heatsink and small low-noise fan instead of the heatsink/fan arrangement used in the article which was much too loud. Bob, K1AO
12001	2015-12-14 07:26:23	Jim Strohm	Re: IRF510 : how high?
Farhan, The datasheet claims 100V, but that's DC and not an SSB signal. Best to test experimentally and stringently document the device mfg. AND the source from which you purchased it, along with your test data.  We already know that "IRF510" is more of a guideline and not a specification; because of its popularity in hobbyist apps it is a frequently counterfeited device. When you get a fake device, all bets are off.  When you get a "sorta like, at DC maybe" device like most NTE "replacements," many or most bets are off. Because of those two issues, you may consider using a higher voltage device with similar input/output capacitance.  Please remember that when you began developing your BITX20, the IRF510 was cheap, plentiful, and almost never counterfeited.  And at that time it was an excellent, well-reasoned choice.  But nowadays it's one of those devices that if I were to specify it in a product, I'd require a top-tier vendor with full ISO traceability on each delivered device, AND subject 100% of incoming parts to a significantly rigorous in-circuit test -- and reject entire batches if one unit failed. Historically, I recall that more than one person has done some casual comparative testing on the various so-called IRF510s available, and all those good folks found a wide range of operating conditions among the casual sampling of devices they used. And of course, going a little farther back in history, remember that television sweep tubes were once favored in lower-end HF rigs and amplifiers because they were cheap, plentiful, and reliable.  Times change.  73 Jim N6OTQ
12002	2015-12-14 08:21:41	farhanbox@gmail.c...	Re: IRF510 : how high?
jim i had read that irft10 dies were used by other manufacturers. this device is getting a littl long in the tooth. i dont know of any other device that is as ubiquitous as this one. thus, i continue to experiment with it. some of mrf devices are so much more attractive. i would love to build mrf150 linear. - f ------ Original message------
12003	2015-12-14 08:44:29	Lasse Moell	Re: IRF510 : how high?
Farhan, You may know these... but to me it looks like a true tube replacement! NXP XR family: https://www.youtube.com/watch?v=8ziYqjMQGEQ /Lasse SM5GLC 14 december 2015 17:21:37 +01:00, skrev farhanbox@gmail.com farhanbox@gmail.com [emrfd] :   jim i had read that irft10 dies were used by other manufacturers. this device is getting a littl long in the tooth. i dont know of any other device that is as ubiquitous as this one. thus, i continue to experiment with it. some of mrf devices are so much more attractive. i would love to build mrf150 linear. - f ------ Original message------
12004	2015-12-14 09:17:29	iq_rx	Re: IRF510 : how high?
Hi Farhan, A major variable is class of service, or more specifically the drive to the gate and load on the drain.  A tremendous amount of work has been done over the last few decades on high efficiency modes, beyond the old familiar class A, class AB, and class C from our youth.  Class E, F and inverse F, and mode J all shape the waveform at the drain for efficiency and sometimes linearity by intentionally loading the drain with specific complex impedances at the fundamental lower harmonics.  Those loading networks tend to be low-pass, so sine wave power is delivered to the load, but the waveform at the drain is anything but a sine wave and may have significant voltage excursions. Device robustness is also related to gate drive.  Some high efficiency modes tend to imagine the device is either or off, but if the device switches fast enough, has a big drain inductor to the power supply, and a reflective load at the harmonics, then that's a recipe for drain voltage spikes, particularly if you also allow the load at the fundamental to make wild excursions from 50 ohms.  One reason the IRF510 was popular for those great 40m class E experiments at Cal Tech was that they were so cheap and easy to replace. It turns out that designing a single-band transistor amp for something like old class AB with a sine wave at the gate and a bigger sine wave at the drain is good for getting reasonable performance and long life out of a solid state device. Turn on the FET with a little quiescent dc bias, provide a drive network that will impress the input signal across the considerable gate capacitance, perhaps with some added resistance to make for a more benign gate load.  Calculate the approximate drain load resistance using the classic V^2/2R = power equation, and design a simple network to transform 50 ohms to the calculated R as a starting point.  Then adjust at the bench for output, efficiency, and linearity. There are a number of nice more modern FETs for linear SSB service, and those make good amplifiers using circuits in the application notes. Have fun with the experiments, Rick KK7B
12005	2015-12-14 09:51:27	Ashhar Farhan	Re: IRF510 : how high?
rick, your two part paper on linear amplifier design is my standard text. it delineates the metho so simply. i have been experimenting with push-pull narrowly tuned amplifiers of IRF510 class. My regular shack amp is built using those rules of thumb. Here is the beast :http://hfsignals.blogspot.in/p/25-watt-linear-for-40-and-20.html it is interesting to apply link coupled, narrow tuned, air core network to these devices. it works surprisingly well as long as the device side is above 10 ohms. i have pushed them to 60 watts at 28v (Meanwell SMPS cranked up from 24v). I was just wondering if I could tide this arrangment over 100 watts. - f
12007	2015-12-14 13:17:45	Graham / KE9H	Re: IRF510 : how high?
Dear -f: Just be sure to be wearing safety glasses when you go for 100 Watts. --- Graham ==
12008	2015-12-14 14:48:00	Tayloe, Dan (Noki...	Re: IRF510 : how high?
If you really want 100w using these devices, I suggest trying to use four of them.  2x vs. 2x in a push-pull configuration.  I have not had a problem getting 7 or 8 w at 12v, so a 24v supply seems high enough to allow 25w per device.  With 2x drain voltage peaks, that gives some margin to the device voltage limits. A push-pull configuration has the nice property that the input C looks to be 1/2 that of a single device.  Using 2x devices on each side brings the input C back up to a 1x of a single device. - Dan, N7VE
12009	2015-12-14 18:21:51	kb1gmx	Re: IRF510 : how high?
Farhan, Its a very capable device but I find getting the heat out of it the greatest challenge. I'm using the WA2EBY (HF_USE _FET in files under my call)  circuit and I get 55W using 28V.   At 34V (limit for that supply) its near 75W.    FYI at 28V on 20M the input current is about 3.9A  (110W).  THe device dissipation limit is 43W so 2 of them at 50% you can get to 80W but not  for long due to heating.   That power out is with about 2W of drive.  Too much drive and they  go bad fast as well.  Idle current per device is 100MA with thermal feedback and electronic  current limiting.  The power supply goes from constant voltage mode to current limit  mode at 4.5A (linear supply using LM723). I also used a design for 6M (OZ1PIF) using 4x4 push pull at 34V for 220W.  I use a well  matched antenna. The limit is that the device is a 100V device and IF the signal plus DC exceeds that its gone. Also at 28V ('EBY design) I've found it quite robust but at 34V the die in the package heats  faster than the heatsink (big!) can absorb it, the to220 case is has high thermal resistance. Also While testing I found that at 34V moderate swr was all that was tolerable (2.5:1) where at 28V I've never failed a device (open, short, wrong filter).  I spent a lot of time  abusing it (actually testing it to extremes) before calling it good and putting the screws in. Also by big heat sink I mean a 4x6x.25 inch thick  copper plate mounted to a 8x4x2 inch  (fins 1.75" tall) aluminum heatsink.  During testing to get the heatsink fins to 100F the  device cases (using IR thermometer) were at 190F (88)C.  The actual die by calculation  was over 200C.  That hot stability and life is limited.  At the time the devices were being  asked to deliver 92W at 34V on 40M, thats 165W DC input or 3.7 time rated power and at max rated drain current, they failed after 3 minutes.  Suggestion stay under 60W power  device DC input peak and insure the efficiency is not less than 45%. The key is if the power-out/power-in is less than about 50% the output circuit (transformer) is wrong and your converting DC to heat rather than RF.    I did use the BN-43-3312 balun  core as substitutes did not deliver good efficiency and power.  However for the secondary I found that The suggested #20 teflon wire was a good start but I found the secondary if  wound with 7 strands of #30 silver plated Kynar insulated wire (parallel) gave better power at 10M  by 4W (37W out vs 33).   Allison
12010	2015-12-14 19:49:51	iam74@rocketmail....	Re: IRF510 : how high?
This is probably the most sensible approach, one taken by the French BINGO developers (q.v.) among others. The IRFxxx series has always been a little suspect for RF. Lately it seems to have gotten worse. Check out the IRFZxxx and BUZxx series for good substitutes, especially the BUZ 24 and BUZ44. Again, running 4 or 6 in parallel push-pull makes a lot of sense. john Ad5YE ---In emrfd@yahoogroups.com, wrote : If you really want 100w using these devices, I suggest trying to use four of them.  2x vs. 2x in a push-pull configuration.  I have not had a problem getting 7 or 8 w at 12v, so a 24v supply seems high enough to allow 25w per device.  With 2x drain voltage peaks, that gives some margin to the device voltage limits. A push-pull configuration has the nice property that the input C looks to be 1/2 that of a single device.  Using 2x devices on each side brings the input C back up to a 1x of a single device. - Dan, N7VE
12021	2015-12-16 14:43:29	kg4ljb	Re: IRF510 : how high?
Dan, Perhaps I'm missing something, but I'd think Cin would DOUBLE - when you put FETs in parallel. Can you clarify? Thanks - Steve KG4LJB
12022	2015-12-16 15:10:12	Tayloe, Dan (Noki...	Re: IRF510 : how high?
When they are in push-pull (two devices total), the push pull drive makes it look like the Cin of the two devices are in series, thus the driver sees ½ Cin of a normal device.  When each side of the push-pull uses 2x devices in parallel (4x devices total), there is 2x Cin (2x devices in parallel on one side) in series with 2x Cin (2x devices in parallel on the other side) so the driver “sees” 1x Cin.   -          Dan, N7VE
12023	2015-12-16 18:02:02	Clutter	Re: IRF510 : how high?
I just recently changed over from 12V to 24V with my WA2EBY amp. I managed to blow up a pair of IRF510 finals, but it took a good hour of ragchewing on 40CW having forgotten to turn on the heat sink fan, to do that. My conclusion is that they are pretty rugged at 24V and 50W output. I do agree with others who suggested four IRF510 for 100W output level- but there are many better choices in RF power FET's today, pretty cheap, too. BTW remember, in EBY's original article, he points out that efficiency is 'peaky' over the lower end of HF- he got 75W output from two IRF510 on 30 meters, his best band (at 28V). That same amp produces about 20 watts less on 40 meters, its least efficient band. I switched from a 'medium' size heat sink, looks to be aluminum or 'pot metal', perhaps removed from an old car radio. I'm now running my EBY amp on an all-copper heat sink from an old Motorola server. It has the same ~4x6x0.25 inch slab of copper like yours, Allison- but no copper to aluminum interface, it's all copper with soldered or swaged thin copper fins, lots of them. I'm cooling it with two 40mm server fans. It's hard to detect any heating at all. Key down for a while produces slight temperature rise on the tops of the transistors. I've never noticed any perceptible rise in that heat sink. It has 3-4 heat pipes embedded in the copper slab, FWIW. Perhaps they help. They connect to an offset area with additional fins. Amazingly, I have been able to buy these on that auction site for around 10 dollars. It just takes some patience searching out the bargains. Allison- did the bundle of 7 wires only help on 10m while not hurting the lower bands? If so, I may try that change to the transformer myself eventually (so far, only 40m operation here). 73, David K3KY (Allison wrote:) Farhan, Its a very capable device but I find getting the heat out of it the greatest challenge. I'm using the WA2EBY (HF_USE _FET in files under my call) circuit and I get 55W using 28V. At 34V (limit for that supply) its near 75W. FYI at 28V on 20M the input current is about 3.9A (110W). THe device dissipation limit is 43W so 2 of them at 50% you can get to 80W but not for long due to heating. That power out is with about 2W of drive. Too much drive and they go bad fast as well. Idle current per device is 100MA with thermal feedback and electronic current limiting. The power supply goes from constant voltage mode to current limit mode at 4.5A (linear supply using LM723). I also used a design for 6M (OZ1PIF) using 4x4 push pull at 34V for 220W. I use a well matched antenna. The limit is that the device is a 100V device and IF the signal plus DC exceeds that its gone. Also at 28V ('EBY design) I've found it quite robust but at 34V the die in the package heats faster than the heatsink (big!) can absorb it, the to220 case is has high thermal resistance. Also While testing I found that at 34V moderate swr was all that was tolerable (2.5:1) where at 28V I've never failed a device (open, short, wrong filter). I spent a lot of time abusing it (actually testing it to extremes) before calling it good and putting the screws in. Also by big heat sink I mean a 4x6x.25 inch thick copper plate mounted to a 8x4x2 inch (fins 1.75" tall) aluminum heatsink. During testing to get the heatsink fins to 100F the device cases (using IR thermometer) were at 190F (88)C. The actual die by calculation was over 200C. That hot stability and life is limited. At the time the devices were being asked to deliver 92W at 34V on 40M, thats 165W DC input or 3.7 time rated power and at max rated drain current, they failed after 3 minutes. Suggestion stay under 60W power device DC input peak and insure the efficiency is not less than 45%. The key is if the power-out/power-in is less than about 50% the output circuit (transformer) is wrong and your converting DC to heat rather than RF. I did use the BN-43-3312 balun core as substitutes did not deliver good efficiency and power. However for the secondary I found that The suggested #20 teflon wire was a good start but I found the secondary if wound with 7 strands of #30 silver plated Kynar insulated wire (parallel) gave better power at 10M by 4W (37W out vs 33). Allison
12024	2015-12-16 19:30:27	Clint	Re: IRF510 : how high?
You might want to look at the HF Packer amp design.  It uses a pair of IRF510s running at 24 vdc and puts out close to 35 watts.   http://www.hfprojectsyahoo.com/yahoo_site_admin/assets/docs/ManHFPAV4.20454748.pdf   A common problem with the HF Packer amp is blowing out the finals due to high SWR.   73 Clint
12025	2015-12-16 22:48:43	Ashhar Farhan	Re: IRF510 : how high?
I get 35 watts with just one IRF510 on 7 MHz at 28v. The EMRFD's IRF530 gets over 50 watts on a single IRF530 at 28v. I guess, the consensus is to use multiple devices to spread the heat sources and keep the drain voltages down but ... "Here be Dragons" 1. Higher drain voltages provide better IMD. In our simple quest for power, this is the first causality. Probably paralleling in push pull will provide better performance from these devices. 2. Instability. Given that the input and the output are reactive with these devices, The lore that emerged from south indian operators was that the IRF510 linears were inherently instable with 'dipoles'. In turned out that this was symptomatic, unless the linears were well matched, the instabilities set in. 'It works well on the dummy. but with the antenna it goes haywire' was a common refrain. In all, the ubiquitous IRF510s and IRF530s provide interesting challenge. They cost two for a dollar in Hyderabad but I take each blow out very personally (hi!). The other device quite common with the local crowd is the IRF830 that they do push to 100 watts. They just keep a jar of IRF830s by the linear I guess. Most of the interesting work on these devices is coming in from Indonesia. There's an Indonesian facebook builders called Kommunitas Radio BITX. They use these devices well above 100 watts with most ingenious arrangements. Those who are on facebook can take a peek at their projects on https://www.facebook.com/groups/105478459588498/ . I don't follow all of it as most of the discussions are in Indonesian. - f
12026	2015-12-17 01:41:08	bubnikv	Re: IRF510 : how high?
I am getting 110W on 40m from a pair of IRF520's at 24V when driven close to saturation with 3W input CW signal, but I fear the FETs have too high thermal resistance to survive that mode for long. But 4 IRF510's, two and two in parallel, shall be electrically equivalent and if cooled sufficiently, thay shall be reliable. http://ok1iak.blogspot.cz/2013/12/irf520-amp-prototype-testing.html    73, Vojtech OK1IAK
12027	2015-12-18 06:02:18	Ancel	Re: IRF510 : how high?
Can't you run a 6-32 machine screw direct to the heat sink thru the Transistor TO-220 TAB from the top of the board? The FR4 isn't a good conductor.
12028	2015-12-18 07:27:31	Ronan McAllister	Re: IRF510 : how high?
Just an observation -- those gray colored soft  power device insulators (bergquist and pthers) have poor thermal conductivity (from experience) -- even if heat transfer compound is used. Recommend either a direct connection to aluminum or copper heat sink (this may be undesirable at RF and may require an insulated heat sink) , or, use mica insulator or better, beryllium oxide insulator... Just my 0.02 Ronan/KB6NHQ
12029	2015-12-18 09:05:22	kg4ljb	Re: IRF510 : how high?
Dan, Thank you for your reply. I'm still having difficulty with your assessment. I would think that in order for the input capacitance to halve, the 2 devices in a push pull amp would - somehow - have to be in series. Again, maybe I'm missing something but it seems clear and obvious to me that if anything, they are in parallel, therefore I'd expect C in to double. In a classic Push Pull amp circuit, The Source connections are usually tied to ground - clearly they are in parallel. The Gate connections have a low resistance connection between them, afforded by the input transformers secondary. For all practical purposes, they are in parallel. The drains have a similar situation with respect to the output transformer. How can the 2 devices not be seen to be in parallel - with resulting additive capacitance's? For that matter, how can they possibly be seen to be in series? Sorry to belabor the point. Cheers and 73 - Steve KG4LJB.
12030	2015-12-18 12:02:12	kb1gmx	Re: IRF510 : how high?
>>>>Allison- did the bundle of 7 wires only help on 10m while not hurting the lower bands? If so, I may try that change to the transformer myself eventually (so far, only 40m operation here).<<<< No cost below 20M and improved above 17M. You found the biggest flaw in the IRF510, that die on a package that can transfer  heat well would be interesting. FYI the temperature testing was brick on the key at max power (not at -1db compression) It took a good while with that cooling arrangement. Also no fan. I've built many amps for higher and lower power and cooling is everything. Allison
12031	2015-12-18 12:06:22	kb1gmx	Re: IRF510 : how high?
The Packer amp is a packaged design based on the WA2EBY amp, Look at the schematic. The differnce between the two, none save for the packer add an boost power supply to get  24V in a 12V system. The thing that will kill the FETs the fastest is over power on the input.  5W will do it if the  SWR is over 3:1.  At 1-2W I found it hard to cook mine, and I tried.    Allison
12032	2015-12-18 12:53:44	kb1gmx	Re: IRF510 : how high?
"Here be Dragons" Indeed!~ 1. Higher drain voltages provide better IMD. In our simple quest for power, this is the first causality. Probably paralleling in push pull will provide better performance from these devices. HIgher voltages are good to a limit.  The limit is a 100V device with 25V power in a high SWR may see 100V peaks on the drain.     The other place damage occurs (all power fets) is there is capacitance from drain to gate and if the coupled power and the drive power are high enough the gate punches through.   Its a balancing act between over voltage failure and enough voltage to stay in the linear region.  I've found  it less a problem at higher drain currents but then cooling has to be good. Last area is marginal bias.  A IRF510 without enough heat sink gets hot fast and will thermally take off. Thermally controlled bias and a heat sink to take the heat away is a must.  So to help that I use attenuator at the input.  For the EBY amp on all bands 2W hit max power or exceeded it. So a 4W radio (some of my HB radios) needs a pad to reduce that to under 2W.  I built a pad into the amp so that the drive gets me to 75% power.  The result is it also inures the input impedance is good and the gates are well loaded which helps stability. 2. Instability. Given that the input and the output are reactive with these devices, The lore that emerged from south indian operators was that the IRF510 linears were inherently instable with 'dipoles'. In turned out that this was symptomatic, unless the linears were well matched, the instabilities set in. 'It works well on the dummy. but with the antenna it goes haywire' was a common refrain. I've seen this and every case I investigated the device was not well matched to the load.  Especially on the gate side.  Often the gate is lightly loaded to keep the drive down and the price is poor stability every time.   that and poor output transformer design or construction helps the matter.  Its still a triode device so feedback from internal coupling or layout is bonus problems.  That leads to a common fault I see, ungrounded heat sinks.  The drain and heatsink if not isolated and the sink grounded its a big antenna  near a high impedance circuit (the gate side), often with the device standing up.  I try to lap the device  down flat, I cut off the drain lead at the case, use a insulator to the heat sink and a solder lug to attach  to the tab going to the output circuit.  Makes for a longer layout but the goal is to keep the output far as  possible from the input.   Last item on current peaks ground must be ground.  If that sounds silly  there's a point there.  Most of the board layouts at high current have the FETs mounted with a long  Source lead (bad) and the connection to ground is often long (electrically) too, instability, low gain and  low power are often the result. I will repeat it.  Most of the power mosfets have gain that can extend to in excess of 100Mhz.  Do not  let the high input and output capacitance  obscure that as it has limited impact on power generating ability, gain, or power out.  There are challenges matching the low input and output impedances but its done (even the RF devices like MRF150 has 400pf input capacitance and works fine at 144mhz). >>>In all, the ubiquitous IRF510s and IRF530s provide interesting challenge. They cost two for a dollar in Hyderabad but I take each blow out very personally (hi!). The other device quite common with the local crowd is the IRF830 that they do push to 100 watts. They just keep a jar of IRF830s by the linear I guess.  Funny I've used the device and save for intentional destructive testing I've not fried any.  Then again I treat it as I would any device that has gain well into VHF (they do!). I'm running an amp using eight (8) in a 4x4 push-pull.  Layout is everything, input matching is also important and a transformer suitable for 6M (transmission line type no ferrite) and its stable and does 225W at the  28V level with good ole IRF510s.   The hunk of aluminum under them is impressive and each one is on  a ceramic heat transfer wafer as an insulator.   Gain for that amp is 12.5DB at 6M!  It also idles at 1A  (lots of current) so there is 28W of heat just turning it on.  >>Most of the interesting work on these devices is coming in from Indonesia. There's an Indonesian facebook builders called Kommunitas Radio BITX. They use these devices well above 100 watts with most ingenious arrangements. Those who are on facebook can take a peek at their projects onhttps://www.facebook.com/groups/105478459588498/ . I don't follow all of it as most of the discussions are in Indonesian.<<< If you run them class E (switch mode) the IRF510 pair will do a lot more power.  Very suitable for AM or CW. The trick with that mode (class E) is that the device is being drive and loaded to the limits of the Rdson and there are better FETs for that. For real power I just go to the MRF,  MRFE,  BLF series parts as they can do it and they are cleaner. If $270US is not too scary for one device the MRFE6VP61K part is a kilowatt plus and good to vhf. My preference are in the realm of BLF278 or BLF177 and those do 300-400W with a good 50V supply (and good for 15A!).  FYI: most of the RF devices used even push pull tend to want drain current is the  300-1000ma even for the moderate 150W devices like MRF150.    I mention this as its common to see IRF510s at 50ma.  If you run them at 150-200ma the gain is better and more stable but at 13V that  needs more than a puny few inch square heat sink. Things I've noted working with power fets and high power transistor amps in general. Allison
12033	2015-12-18 12:58:54	kb1gmx	Re: IRF510 : how high?
"Can't you run a 6-32 machine screw direct to the heat sink thru the  Transistor TO-220 TAB from the top of the board? The FR4 isn't a good  conductor." Cut a window in the board.  Use an insulator and thermal transfer grease plus a shoulder washer with a 4-40 (2.5mm) screw to hould it firm to as large a heatsink as practical. The back of the fet is metal for a reason. G10/FR4 has terrible heat transfer properties. Allison
12034	2015-12-18 13:09:57	kb1gmx	Re: IRF510 : how high?
For RF and good heat transfer there are two things available. Mica insulators, thin and good transfer with heat transfer grease. My favorite is a SIO2 ceramic slab about 1mm thick that both  provides heat transfer and good insulation again with heat  transfer grease. Both are inexpensive and stocked by most suppliers.  I paid  73 cents each (for 50 pcs) from mouser  just recently (532-4170 Aavid thermalloy). Don't forget the shoulder washer to insulate the screw (4-40 or 2.5mm). Allison
12035	2015-12-18 13:23:13	kb1gmx	Re: IRF510 : how high?
>>>The Gate connections have a low resistance connection between them, afforded by the input transformers secondary. For all practical purposes, they are in parallel.<<< True save for the signal applied to the gates are 180 degrees out of phase.  Parallel at DC but at RF  anything but.  That effectively puts the generator (effective RF source) in series from gate to gate. If you parallel the devices yes that NODE has twice the capacitance.  So going from a 1x1 push-pull to a 2x2 push-pull there is a doubling of each nodes capacitance,they are still in series for RF. The whole input capacitance thing is a red herring. Its big to start with and bigger is no worse. In the end you matching an impedance not a simple capacitance. If we take the IRF510 out  and use a really nice RF power FET say a MRF150, thats 400PF input capacitance. My 6M IRF510 experimental amp (successful) uses EIGHT in a 4x4 push-pull arrangement.  works fine with the right input transformer (9:1 step down) to match the roughly 10ohm gate to gate impedance. Allison