ghost note sub tone half volume on up

[rickdog]

I've been fighting a ghost note sub tone on a 36W TMB. I've upped the 1st and 3rd filter caps about 40%, 56 uF total each. I installed 9H "universal" choke between 1st and 2nd filter caps, still no luck. The amp is roughly following Richie Hall design and Ceriatone 36W TMB. All cathodes are bypassed, smaller values help quelch it in earlier stages, but this seems to be a bandaid provided by the shelving, not a real fix. I even provided a 27K NFB resistor which did little to help. I guess I could up the 40 uF filter for the preamp stage 10 uF or so and see if that helps. Running a new JJ GZ34 fairly close to the PT. OT is a little close to one bank of EL84s, don't know if that could have this effect, it hasn't before, but you never know. The amp works fine otherwise with good power and very little hum. It's in both normal and TMB channels, so its coming probably PI and or power section? I have built many of these without encountering this problem to this extent, so I'm at a loss here. Any suggestions???

[doctornorbert]

You could try putting the preamp tubes each on it's own power supply node. I had the same problem, and doing this helped.

[rickdog]

Thanks Doc. That is a good idea. Matchless and others do this as a matter of course and I'm probably going to start making my boards to do this all the time. I never really understood stringing them along in the first place, it looks like asking for trouble. thanks again!

[doctornorbert]

At one point I had the phase inverter and one 12ax7 preamp tube on one node (normal enough for an 18 watt), then due to a wiring mistake, had an ef86 on the same node (though I thought it was on its own). Three tubes, one power supply cap, ghost note galore.

[rickdog]

A little research showed that the ghost note or following note problem is all too common in EL84 amps, makes me wonder if your idea about separate filter nodes isn't on to it. I'll try separating them probably tomorrow, although it does seem a bit better now, I replaced the cathode bypass on the EL84s as I suspected a bad cap there, and it seemed to help, but it's still slightly there. At lower volumes you don't hear it at all, but 12:00 on the master and vol on up brings it in. It is not there in the lower register, it starts noticeably at about D4 on the G string and goes on up from there, less noticeable at C5 on up.
thanks!

[doctornorbert]

That's exactly right - it only happens at high gain, when the tubes are drawing a lot of current and taxing the caps.

I put 8-16 uF on each tube. Using whatever intervening resistor values you would see in a Vox AC30.

The rest of the PS (PI, el84s, etc.) is straight out of the regular 18 watt schematics on this site.