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