2024 Monkeymatic Howler

[xtian]

"Howler" is a custom build for a client, based on the THD Univalve (single-ended, KT88 power tube). My build is finished, but I'm trying to solve a 4KHz oscillation when gain is turned up all the way (it's living up to its name!).

I have 340v plate, 26v cathode (bias = 80%), putting out 13 watts into an 8-ohm load. I had no luck adding gNFB--could only elicit positive feedback, not negative! Chopsticking the OT primaries (brown and blue) changes the pitch of the oscillation, so I can try to improve my lead dress here--but what's sensitive to proximity? The power tube's cathode is the only thing that's close to the wires. Open to suggestions.

IMG_3428.jpg

THD_UniValve_1_41_062104.pdf

[Stevem]

You have grounds landing / deriving all over the place and that’s making for resistance which then acts like an antenna.

So I would try way more of a start grounding layout going back to where the recto bridge ground is, which should also be right where the first filter stack gets its ground.

I would maybe first try reversing end fir end one of those two coupling caps nearest and above that tube socket.

[didit]

Hello Aaron --

Open to suggestions.

The wires for the output primary, particularly the brown ground return, seem unnecessarily tightly coupled with the filament feed. Try unbundling it, and use the large space above your socket to float it a few centimetres off the chassis and away from everything else.

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[Colossal]

Aside from the annoying oscillation, which I'm sure you will get sorted quickly, great looking build, dude!

[xtian]

You have grounds landing / deriving all over the place and that’s making for resistance which then acts like an antenna.

It may look that way, but it's not. There is only a single contact point to the chassis, at the reservoir cap. The input and output jacks are all isolated from the chassis.

[cdemike]

Really cool build! Very satisfying to see PtP look so clean. Does it oscillate with nothing plugged in? The relationship to volume control settings makes me think there might be something happening around V1 -- maybe segment of the grid stopper's lead running between the terminal strip and resistor body is picking something up from the OT primaries?

[Phil_S]

Try routing the brown wire around the other side of the power tube socket. Occam's razor. Simplest is often the one. I offer this in response to what you reported for chopsticking.

[xtian]

The wires for the output primary, particularly the brown ground return, seem unnecessarily tightly coupled with the filament feed. Try unbundling it, and use the large space above your socket to float it a few centimetres off the chassis and away from everything else.

Good call. Raising the brown wire helped, but didn't solve completely. I noticed that placing my hand near the wires quieted the oscillation, so I used copper tape to shield the wires, and grounded the tape to the chassis. Much better.

I hate it when function wins over form.

IMG_3430.jpeg

[pdf64]

It can often be the case that putting the chassis into its cab, in which there's foil etc screening over the open top of the chassis, can magically resolve these kind of issues

But yeah, the OT primary wires are rather close to the input stage circuitry, a screening plate between them might be a good idea.

[didit]

Good call. Raising the brown wire helped, but didn't solve completely. I noticed that placing my hand near the wires quieted the oscillation, so I used copper tape to shield the wires, and grounded the tape to the chassis. Much better.

Excellent.

I hate it when function wins over form.

You are an adapt master at clean builds. However, using available space for point-to-point on tags and socket is good form. Think sculpturally. Visualize potential fields, starting right from layout sketches. Maybe take a few hours going through mid-century ARRL magazines and manuals for spatial design insights.

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[Stevem]

I forgot to ask, does the issue take place weather or not you have a guitar plugged in, but only when the volume is open above a certain amount ?

Also why 800 volts of filtering on the input section of V1, do you just have a ton of 400 volt filters on hand I guess?

[xtian]

I forgot to ask, does the issue take place weather or not you have a guitar plugged in, but only when the volume is open above a certain amount ?

Also why 800 volts of filtering on the input section of V1, do you just have a ton of 400 volt filters on hand I guess?

Issue is same with nothing plugged in, or guitar plugged in with volume at zero.

V1 has 47u/450v filter cap, V2 has 120u/400v, reservoir is 110u/700v effective (stacked caps). These values are similar to the Univalve schematic, but my voltages are much lower, so probably overkill.


Here is my finished layout diagram.

Screenshot 2024-06-29 at 2.32.22 PM.png

[xtian]

BTW, I used onshape.com CAD to draw the chassis, and I used the sheet metal function to flatten my 3D model for aid in fabrication. It was excellent! I'm happy to ditch fusion360. Onshape is free if you agree that all your work is public.

Here's my onshape project: https://cad.onshape.com/documents/78a48 ... 4c37bf8072

Screenshot 2024-06-29 at 2.34.08 PM.png

Screenshot 2024-06-29 at 2.34.22 PM.png

[cdemike]

I'm still thinking the problem is at V1 given how shielding the OT primaries helped and that it only oscillates with the volume turned up even when the input of the high-gain input is grounded; signal seems to be getting in somewhere between the high gain input jack and the volume control there if the volume control impacts the oscillation. Would you be open to swapping the positions of V1A's cathode cap/resistor with the grid stopper on the terminal strip? It might be hard at this point, but it could also be worth checking if you could route the OT primaries in a U-shaped pattern like in this drawing. Pardon the ugly artwork.

[Guy77]

BTW, I used onshape.com CAD to draw the chassis, and I used the sheet metal function to flatten my 3D model for aid in fabrication. It was excellent! I'm happy to ditch fusion360. Onshape is free if you agree that all your work is public.

Here's my onshape project: https://cad.onshape.com/documents/78a48 ... 4c37bf8072


Screenshot 2024-06-29 at 2.34.08 PM.png


Screenshot 2024-06-29 at 2.34.22 PM.png

Ohhh I like this software you used for the chassis. If I start making my own chassis again I will use this same concept/software you used. Using copper tape was a cool idea. I don't usually see that tape for sale.

Thanks for sharing. Can't wait to see the finished build!

Cheers
Guy