Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ronquest on April 11, 2023, 07:28:59 pm
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I've heard this too many times in other amps, maybe someone can help me get to the bottom of this. Some call it swirl but I'm not so sure about this.
Here's a sound file and schematic of the preamp. It's a pretty simple amp and just one 12ax7 before the phase inverter.
http://work2eat.com/Ghostprs.mp3
I've tried most of the usual ghosting/feedback/oscillating fixes. This sound is in the preamp for 99% sure.
I've removed the PI and power tubes and it's still at the output of the 12ax7.
Patched in signal from another pre to the power section and the power section is fine.
Even with the gain turned down a good bit / clean-ish, it's till noticeable.
Tried larger supply caps, choke in/out, coupling/bypass caps, other 12ax7s various grid leak & stopper resistor values.
I'll work on something a long time to figure it out, but I'm getting tired of this one. Hoping you guys can help!
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could it be lead dress issues? i've had all sorts of weird little problems cleared up just by moving or rerouting wires
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I can’t hear an obvious problem.
Ghosting is usually used in the context of a intermodulation between the signal and a low frequency, eg HT ripple or pickup magnets creating a node in string vibration.
I can’t hear that effect going on here.
Almost any clipping stage heard dry and unfiltered will tend to behave like that over the ADSR of the input signal.
It becomes more obvious if the depth of overdrive is lowish / there’s only one stage likely to clip.
It tends to be better to develop the overdrive over 2 or more stages, not overdriving any of the stages too hard.
And to boost the treble before any clipping stage, then the harmonics generated don’t totally change the signal’s tonal balance.
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Almost any clipping stage heard dry and unfiltered will tend to behave like that over the ADSR of the input signal.
It becomes more obvious if the depth of overdrive is lowish / there’s only one stage likely to clip.
A.D.S.R. – Attack, Decay, Sustain, Release -ya, I had to look it up
I've been thinking and reading a bit more about the guitar string physics.
some good info here - https://www.fuelrocks.com/the-physics-of-sound-guitar-strings/
I know that a single plucked string has more than a single frequency, but I expect (maybe want) to hear a smoother DSR transition of the ADSR
The sound sample was a SM57 on an ET65 at a medium volume, so with filtering, but yes, I agree with your statement.
I plugged into some Amplitube models to see if I could get the same result I was hearing and it's there. I could here it in Fender Tweeds to 800s to SLOs. It may be just the nature of the guitar and now that I've heard it I can't unhear it. I still think this circuit is accentuating the unwanted sound I'm hearing and it might be the too few gain stages as you pointed out. I can still hear this sound in most of my amps. It may not be ghosting, but it is haunting me :-/
*Lead dress has been addressed.
I run fixed tones and sweeps through the amp and don't hear anything out of the ordinary. I'll do a few more tests today.
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Ceiling fan?
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Ceiling fan?
I'm always laughing at your posts, so I'm not 100% sure if you're joking or not. No ceiling fans, multiple rooms, circuits, guitars, speakers, cables, relocated the cell phone, faced true north on one leg.
Are you planning any future get togethers? And have you started your book yet?
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One summer, some years back, every week a different person would ask about a funny indescribable sound. And several times it WAS a ceiling fan, the silent blades making a wup-wup reflection in the room.