Amazed and Disappointed

[Arthur B.]

The harshness may have to do with the cold clipper stage. Replacing the cathode in the third stage with one that biases it into the linear region. Removing the cathode bypass capacitors maybe necessary to bring the gain down since the resistor swap will increase the gain.

[Tim Hall]

Rob, Bruce's battery trick sounds like a great idea. DC heaters had occurred to me, but that's a lot of extra components to stuff in that little box. If it turns out to be the heaters, hopefully shielded wire will nix it.

Arthur, I'll keep that in mind as I head back to the drawing board...I have a feeling this design is going to be overhauled. I've seen a schematic and heard clips of an amp that uses this topology, only it has a TMB tonestack and uses a 12AU7 instead of an ECC99, and it sounded pretty good.

I think I may have skipped an important step in this process: rough testing. I think I'll try this all out in a much bigger chassis with cheaper components, where I can work easier. I'll try to stuff it in a stomp box when it sounds good .

[Steve Conner]

How have you connected the PT/rectifier to the first filter cap? The connections from the rectifier to the filter cap carry nasty pulsating currents that will induce a loud spiky buzz sound, either by induction, or more usually by a conductive path shared with a signal ground.

These two wires should lead straight from the rectifier to the capacitor terminals, and shouldn't be used for any other purpose. IOW, all other connections to these two nodes should come off the capacitor terminals too.

[Tim Hall]

Ok, I've decided I'm not a layout genius.

Steve, your point is noted. I tried as best I could to keep things logical, but the filter caps being so big made it very difficult. Unfortunately I ran a wire from the rectifier to the first cap. And I ran a wire from the the second filter cap to the third. Basically it was either have long ground wires or long supply wires, and I guess I chose the wrong one.

Your description of a spiky buzz could very well be the secondary "hum" I'm hearing. There are two different sounds it seems like to me, and grounding the heaters kills one of them.

I'm guessing there's no good work-around for this one...just bad layout.

[Steve Conner]

Hi Tim,

The wire from the rectifier to the first filter cap is not a problem. As long as you then only connect your other wires to the filter cap end, not the rectifier end.

Tech geek explanation: The high pulsating current in the wires between the rectifier and first filter cap causes a voltage to appear across them. If the negative wire is part of your grounding system (ie the ground for other parts of the amp passes through it) then it will inject noise into your ground.

I always find this really hard to explain, though I can get it right every time in my own designs now. Maybe this page will explain better, though it's targeted at solid-state, the idea is the same:

http://www.tcaas.btinternet.co.uk/jlhearthing.htm