Last edited · 15 revisions  

 


Why is this important:

The uBitx has plenty of power at 80m and a decreasing amount for every band you go up with barely 2W at 10M.

 

Why does it happen?

The devices used are incapable of the needed gain at higher frequencies and have poor performance at higher power that being the 500 milliwatt to 1Watt level.  The IRF510 has been widely blamed for this but measurements show the driver putting out 400-500 milliwatts at 80m and barely making 60-70 milliwatts at 10M. 

The irf510 is making the best of nothing.  A better device is still going to get near nothing.

 

Optimal form of the amp:

Ignoring the IRF510s and paying attention to the weak driver and stages before.  Because they do not work well at all.

Since we have three stages and need 47db of gain that means the average gain is 16db (round number is 48).  For that you need a device with a FT of not less 
than 1200mhz (certainly not 3904s at 300FT).  The driver stage needs to do 1W for comfort.  A pair of decent parts like 2n3866 or 2n5109(and others)
can do that easily.  That's not 4 of them but two.  But gain at high power is harder so at best we can get 13db from them.  So the prior two stages need to produce about 35db.  Best again is a single 5109 for the pre-driver and we get more gain at lower power say 15db.  That leaves the Q90 stage needed to produce 
a few milliwatts and 20db of gain.  A 2n5179, BFR106, MAR6, and others easily can do that.  Of course all the transformers need to be redone and 
the output feed and transformer is pretty poor.  The board layout complicates matters being much too tight.

 

 

Thanks for the annotated schematic!

 

 

 

Possible solutions:

First there is no 1 pill solution as the IMD of th base amp is also very poor.

See thread #49729 May 19th.  Also many other yabuts and how about this postings.

 

Prefix....

Do so at your own risk!  In the 20 to 30 mhz randge there are spurs in the SSB TX and none of these amp fixes improves that, be aware.  No fixes published for that as  Sept/15/2018.  If the instructions are too vague for you likely you will be in over your head wait for a someday maybe a kit.

 

This is a list of things that worked better.  Getting this amp to better still is hard due to space and other constraints with layout.  If you think the full bore is radical i have built one that does 3-30mhz with the same power out, less than 1W variation at 10W.  It cannot be done without a separate board due to space and isolation keeping the input far from the output in a straight line.

Also do not email me with "I don't have 2n2222A(to18) will xyzzz work?" as the answer should you get one is "tried them, they were poor" or try it and let us know.  Same for ferrite, transformers and so on.

 

==================== Easy ======================== 

Easy change list.

Before going into this the biggest bang for the buck is swapping out all the 3904s in the Pre-driver and Driver with 2n2222A.  This improves the 80-17M performace over the 3904s.  Gets a little 10M improvement.

Change Q90 to BFR106 and C81 to 470pf (tames the high power on 80) Make sure the current for this part is under 20Ma preferd to be 12-15ma.  [Shoot for 10 mA.  source]

 

==================== end easy ======================== 

 

 

Full bore change list:

====================Not easy==========================

IF all goes well 6w on 10M when RV1 is set for 14W on 80m. 

IF it doesn't, I'm not accepting complaints.

 

==================== first amp q90 ======================== 

Replace Q90 with BFR106.  (Mouser about 18cents each) . Reset bias of Q90 with BFR106 , Set bias for less than 10ma (r86 2.7K for mine).

Make C81 470pf, and R83 8.2ohms. (lower power at 80m improves 10m)

 

To set bias and measure simple measure across the devices emitter resistor voltage/resistance=current  so 100ohms with 1Volt is .01A or 10ma.

Change C82 to 470pf (peaking for 10M and less power at 80m, optional if your not playing at more than 20mhz.)

 

Remove T8, remove wire and wind with 8 turns #28 Bifilar, pay attention to wiring.  Ignore pins 1 and 6 as they are not used.

 

==================== Pre-driver ======================== 

All the 2n3904s in the predriver replaced with 2n2222A or better. Hint I mounted them from the underside with very short leads as it makes 

the top less crowded.  If 2n5109 is used add a ferrite bead on the collector lead. the part if FB43-unknown as I had a few handy they are 4mm long and the hole is large enough to pass a transistor lead.

 

R87, R88 emitter resistors for pre-driver reduced to 11ohms (add parallel 22ohm). 

R86 replaced with 1.5K ohm in series with 2.7uh SMT choke.  (resets bias and adds peaking.)

 

Remove T9 and take one turn trifilar off of the 10.  Reinstal pay attention of lead placement.  IF available 6turns #28 trifilar on BN43-302 

 

==================== Driver  ========================

All the 2n3904s in the driver replaced with 2n2222A or better. 

Hint I mounted them from the underside with very short leads as it makes the top less crowded.  IF 2n5109 is used add a ferrite bead on the collector lead.

 

R911,r96, R941, R942 emitter resistors for driver reduced to 11ohms (add parallel 22ohm).

R92, R95 replaced with 1.5K ohm.  (resets bias)

 

Remove t10 and take two turns trifilar off of the 10.  Reinstal pay attention of lead placement.

 

==================== final amp  ========================

 

Replace [T11] with BN43-202 (or BN61-202)  wind turns ratio such as 2 turn primary and 3 turn secondary.

NOTE: this only use two of the three windings so ignore pins 4 and 3 (unused).

 

Ferrite heating here is likely due to two possible causes:-

inefficient transformer due to how its wound,

not enough ferrite  (more than 12W) or wrong material.

 

Despite claims the RD16HHF is NOT a panacea and does not materially help 10M result.  At 10$ its your fun.  I tried them and have used them elsewhere with good result but not that much better.

====================   end   ========================