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From: <jrusgrove@comcast.net>
To: <rsgb_lf_group@blacksheep.org>
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Date: Mon, 9 Apr 2018 10:07:18 -0400
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Subject: Re: LF: W1VD amp help - more waveforms
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Paul

Sorry for slow reply. I agree with all of Chris's comments.

I seem to recall you posted a link to a pix of your amplifier last week ... but it must already have 
been deleted here since I can't find it. Please send the link again so I can have a critical look.

Moving the driver board close to the FETs should be a priority. Rather than drilling and tapping you 
could mount the board (at least temporarily) with short thick wire lead 'standoffs' soldered from 
the driver board ground to the amplifier ground plane.

The ringing on the drain looks 'clean' so it's probably not related to instabilty or rf getting into 
the driver. I run about 1' of cable between the amplifier and LPF ... never tried it with a longer 
cable. In addition to the what Chris mentioned, excessive ringing can be caused by cross conduction 
at the phase transitions. It was hard to tell from your scope display ... I'd suggest moving the 
traces so they share the same baseline ... essentially on top of each other ... to better see what's 
going on at the trasnition.

What are you using for the bypass cap at the center tap of the transformer?

My unregulated HV supply (xfmr, rectifier and big filter caps) is on a powerstat which is convenient 
for testing ... key the transmitter and dial up the powerstat while watching the drain waveform. Any 
abnormality in the waveform (including excessive ringing) will likely be spotted before it becomes a 
problem.

After seeing the pix I may have more comments. BTW, I might have a spare driver PC board if you're 
interested. Can't remember if I have any left. Was going to order another batch, but like too many 
other things, I never got around to it.

Jay W1VD


----- 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 9:22 AM
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
>