Message-ID: <4D19B3707137430CBE04857FBA5C80B1@gnat>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <162a4797468-1db9-bdf2@webjas-vae168.srv.aolmail.net> <1UQROqZht1.1c1VdzC0S69@optiplex980-pc> <f5cdf11c-819e-a83d-15cd-9590f7a07194@n1bug.com> <DM__180409110420_14031131174@smtp.eclipse.co.uk> <3a0c0669-018b-13a7-b56b-dd3f63fab464@n1bug.com> <197556217.20180409124931@gmail.com> <37574432-b72e-f2d1-0e85-af19dd970817@n1bug.com>
Date: Mon, 9 Apr 2018 15:21:48 +0100
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Subject: Re: LF: W1VD amp help - more waveforms
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Paul
I think you need to step back from this a little. One point, design/cable 
routing at 137kHz is normally not that critical and it is unlikely it is 
having much effect. Many us have ''haywired'' PAs at these frequencies 
without major troubles....even using croc-leads!!

You are unhappy about the drain waveforms.......why?? the ringing is on the 
''off'' device and so doesnt really matter in terms of efficiency. Consider 
what is happening. The FET drive goes off and the drain current starts to 
drop inductance in the drain feed causes the voltage to rise to try to keep 
the current flowing. Then on top of this the other transistor switches on 
and the transformer connection for the off side sees a rising induced 
voltage out of the transformer. There is bound to be overshoot but it does 
not absorb much power. It may be the generous ''dead zone'' (whilst a good 
idea) that is producing this picture.

The critical part is the the flat zero volt line when the FET is on. The 
current pulse rings a bit, but the current drops in the second half of the 
pulse.....why?? This, I think, is really the loss of power. Does Jay's show 
this? The point I am making is don't get drawn up blind alleys because a 
trace just does not look ''pretty''.

I think we have mentioned this before but the transient performance of the 
power supply may be a factor. There should not be great changes in the 
current draw because you are switching the PSU load from one transistor to 
the other but there can be higher speed transients that can affect power 
supply regulators in unexpected ways.

As Andy said efficiency at 12v is not very meaningful because of the high 
rds of these FETs. It is usefull to run up a PA at low volts say for safety 
when the antenna needs to be tuned.

Finally the scope probes are placing an. albeit small, capacitance across 
the drains (I am assuming they are correctly trimmed) The waveform can often 
be affected by where the ground is picked up from.

Stick with it because you are close to being ''there''

Alan
G3NYK


----- Original Message ----- 
From: "N1BUG" <paul@n1bug.com>
To: "Chris Wilson" <dead.fets@gmail.com>; "N1BUG" 
<rsgb_lf_group@blacksheep.org>
Sent: Monday, April 09, 2018 2:22 PM
Subject: Re: LF: W1VD amp help - more waveforms


> Hello Chris,
>
> There are plenty of possible causes here I think. When it comes
> right down to it, I'm not at all happy with the end result of my
> construction on this amp. If it ends up not working, it's because
> I'm not a good enough builder. Despite building for 35 years
> including many QRO tube amplifiers, I felt this pushed my skills to
> the limit... or beyond? It's not the fault of the design of course,
> it's me. This requires a very different skill set from what I've
> developed through the years.
>
> The driver board you see is my fourth attempt yet I'm still very
> unhappy with my layout and on-board routing of connections.
>
> I probably made a huge error in design and mounting of the driver
> board. Everyone seems to agree my gate leads are too long. This may
> take a while. If I'm starting over I'll have a few more goes at
> trying to come up with a more sensible layout and on-board routing.
> I'll probably have to design the new board so it can be soldered to
> the ground plane for mounting. The only other way I can think of
> would involve complete deconstruction of the amp to drill and tap
> new holes in the heat sink for relocated board mounting standoffs.
>
> I literally spent two weeks and likely 50 attempts at winding the
> output transformer, still not happy with it. I'm not sure the
> primary winding is acceptably tight.
>
> Today I'll try swapping out the cables connecting amp to LPF and LPF
> to dummy load. No reason to suspect bad cables, but I've seen
> strange stuff happen. The cable between amp and LPF is a bit long at
> 4 feet, maybe it doesn't like that. I'll have a look on the PSU side
> of that choke with the scope too. Good idea. Could be some RF there
> causing the PSU to go insane.
>
> I haven't added bypass caps on the driver board yet. I'm out of .1
> uf caps. I have some 1 uf ceramic but that may be a bit much? I'd
> probably get shorter leads by tacking them on the back side of the
> board. Bear in mind I'm testing at 13V which gives about 65W output
> so there's not massive amounts of RF around yet.
>
> Still puzzling over why my gate waveforms look OK (I think?) but
> drains don't. To my usually wrong way of thinking that suggests the
> ringing problem is caused by some defect in the drain circuit or
> outside the amp (cables, LPF). I think I'd like to try and eliminate
> those possibilities before I go back and completely redesign the
> driver board and mounting to shorten gate leads. No doubt it'll
> still come to that but I can hope not a bit longer. :)
>
> Paul
>
>
> On 04/09/2018 07:49 AM, Chris Wilson wrote:
>>
>>
>> Hello Paul,
>>
>> If my similar amp exhibits ringing on the drains (which I monitor in
>> real time), it means an antenna mismatch or a dicky connection (PL
>> type plugs were the cause, cheap junk off Ebay, a change to branded N
>> types fixed that issue). So if the output transformers are the right
>> material and wound right, the snubber resistors and caps are correct,
>> and the gate driver wires are really short, and you are looking into a
>> decent dummy load, well, it's a mystery! You have put decoupling caps
>> on all the Vcc pins? If you have a nice PCB and don't want to make
>> new, and the IC's are socketed you could solder the decoupling caps
>> direct to the IC pins for a test. Or even if they aren't socketed I
>> guess :) Without them my amp suffered with RF getting back into stuff
>> and  "funny  things"  happened. You should also have a `scope about on
>> the PSU side of the choke.
>>
>> Monday, April 9, 2018, 12:29:18 PM, you wrote:
>>
>>> Thanks Hugh and everyone else who commented and provided tips on my
>>> earlier post about this amplifier.
>>
>>> I will wait for Jay to have a chance to comment but I *think* I'm OK
>>> on timing of the phase transitions. However, something is causing
>>> major ringing (is that the correct term?) on the drains. I believe I
>>> need to find the cause and do something about that but I have no
>>> idea what to look at.
>>
>>> I've been building and even designing QRO amplifiers for 35 years
>>> but I'm a vacuum tube HF/VHF/UHF guy! This FET class D/E stuff is
>>> all new to me and I don't learn new tricks as easily as I once did. :)
>>
>>> Paul
>