18Watt.com

Hey all, I've got some ghost notes happening in my mutt amp build, and I wondered if you folks could assist me with a fix. It seems to fit the profile - i.e. additional tones that are audible when playing on the G sting around the 12th fret. I've swapped PI tube, and still hear it. Just to be sure, I've used a few different cabs, removed anything between guitar and amp, etc, to isolate it to something in the circuit.

I've read some articles suggesting a snubber and some suggesting the power supply isn't filtered enough. I can certainly update the filter caps, but honestly am not fluent enough in power supply design to know what I would do and what the impact would be. Increase cap value at B+, maybe? If so, by how much? Increase all power supply cap values? No idea what impact that would have, aside from increasing the filtering...

The amp is a 10watt 5F11 power supply and power amp, except the PI has been changed to an LTP based on one of Josh's designs here. No NFB loop.

Everything else about this amp sounds outstanding - but those few spots producing the extra notes are driving me crazy.

Is that a SS or tube rectifier? SS is more tolerant to oversized B+ filtering. But you can always clip on an extra cap across the 1st filtercap to try if that changes things.
Ain't no harm in trying that for a test.
You can also try clipping a 10 to 220pf cap from plate to cathode on 1st and/or the 2nd amplification stage or PI tube... a 2204 has a 47pf on the 1st

Thanks for the input! It's a tube rectifier.
I'm assuming that any caps I clip in parallel will still need to be electrolytic, right?

cstenger5 wrote: ↑

Thu 12/23/21 1:55 pm

Thanks for the input! It's a tube rectifier.
I'm assuming that any caps I clip in parallel will still need to be electrolytic, right?

Yes Chris.. and preferably 450V too
Watch out for specific tube specs regarding max capacitance. The sum of your 1st filter cap assembly shouldn't exceed that

That's not necessarily true. The only thing that's for sure is the voltage has to be higher than your operating voltage.

In some cases adding a film capacitor a couple orders of magnitude smaller than the filter cap might be the correct choice, but it depends on the cause of the issue, and the specific cap you're talking about.

Here's some opinions:
https://www.mercurymagnetics.com/dealin ... occaforte/

https://www.thegearpage.net/board/index ... st-2382035

Thanks,
Josh

I can't argue with that..
But for test situations Chris shouldn’t worry too much about values and such. See if something helps or not first...
Ghost notes are terrible troubleshooting..