SS Cathodyne PCB - LND150

[tristanc]

Was thinking of upgrading my FireFly to be a 'proper' push-pull instead of self-split. Rather than adding another tube or touching the preamp I'll simply add a basic solid state cathodyne splitter using an LND150. This looks good in LTSPICE and increases the clean power output over self-split a bit.

Anyone see any issues?

[tristanc]

I've updated the schematic a bit - added the ability to apply from NFB from OT to grid and added a cathode bypass cap - seemed to make the frequency response between the two outputs more symmetrical. These ideas borrowed from the Marshall 1 Watt JTM.

I'm getting a couple of boards made up and if they work I'll put the source files on my GitHub https://github.com/tristancollins/amps

[stephenl]

I played around with a source-o-dyne feeding a pair of KT66's a while back, it behaved well and was transparent. This is the implementation I used. Probably should have G>S protection diodes and a gate stopper...

source o dyne.jpg

[tristanc]

Thanks Stephen - good to hear a positive outcome. And also referring to it as a source-o-dyne - might have found more hits if I'd searched on that.

I think you have the output caps drawn the wrong side of the 22k resistors? But I get your point! Fixed bias too - had thought of that but copied from the 1W Marshall design. Will update once I have the board up and running.

[stephenl]

Hah, yep, caps in wrong place. I scribbled it up quickly to stick in the post

[tristanc]

In my haste to get a board made I forgot about the problems with using FETs and self bias... So I've modified my PCB to be fixed bias. I can easily modify the boards I have had made (cut a trace plus a couple of jumpers) so no real issue. Will report back when I have it working.

https://github.com/tristancollins/amps/ ... -Cathodyne

[stephenl]

Cool, looking forward to hearing how this works out

[stephenl]

Did you ever get a chance to play around with this?

[tristanc]

I've populated the board, made the change to fixed bias (cut a trace and needing a jumper from memory) and I'm now just waiting for a free afternoon to get it in to the build. Ideally I want to take some measurements of max clean output and max overall output before and after the installation. I should get a bit of a bump according to LTSpice. Will report back.

Tristan

[tristanc]

Did you ever get a chance to play around with this?

OK, so finally got this installed, and it passes sound. But I think there's an issue. The attached shot of the scope shows what's happening:

Light blue is amplifier input (200mV)
Pink is speaker output
Dark blue & Yellow (inverted) are the outputs of the cathodyne/souce-o-dyne.

Why would the outputs of the cathodyne sharp clip at ~1.5V? What I hear is, even with the MV on full and gain on very low, I'm getting slight break-up when I'd expect it to remain clean.

I'll go back in today with a scope and take readings at the input to the cathodyne and the two outputs.

[martin manning]

Schematic looks fine. What voltages do you have for supply, drain, and source, and the junction of the 1M5 and 470k? What's the input signal level?

[tristanc]

Thanks Martin - found the issue. Would help if I had applied a bias voltage to the LND150 gate... Forgot an all important jumper. All working as it should.

Some readings:

Clean out sine wave: 2.8Vpp (0.11W) with self-split, ~4Vpp (0.25W) with source-o-dyne into 8ohm load
Max out square-ish wave: 7Vpp (1.5W), 9.6Vpp (2.88W)

So in line with what I had expected from simulation. The amp itself sounds much like it did before, which is also a good result.

[stephenl]

What were your HT, G,S,D, bias voltages?

[tristanc]

What were your HT, G,S,D, bias voltages?

I didn't make a note I'm afraid.

I was in a bit of a rush, so wired it up, checked continuity etc, turned on and scoped it. Spotted the issue and verified with a quick play-through. Of course, in my haste I left off a jumper to apply the HT to the 'top' of the voltage divider for the bias voltage. Less haste more speed etc.

I'll go back in soon and take some readings - although it's pretty tight in there to get a probe in! HT should be ~265V.