Well that sound sounds just like your filter caps discharging and it is normal - nothing to worry about. A few of my amps make a similar sound on discharging
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Updated my schematic finally !
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but the thing is, it is the remnant of what WAS a problem at one point. At one point i tried a different value of the PI input cap and it motorboated constantly with the amp on. After i put the original back it only did it at shutdown. So whatever is causing it is on the verge of doing it and i suppose at shutdown it's just a but more vulnerable to it as the caps discharge. Doesn't matter i suppose.
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Re the presence pot, another option would be to use a 50k ohm inverse log pot intended for a fender tremolo intensity control - the bug opto coupler type. Should be near out of circuit on '0' but with good resolution as it's turned up - 50% should be 5k, 75% should be 500 ohms. And easy to source.
Re motorboating, have you tried different 12AX7 tubes? Sometimes an odd one can push normally stable amps close the the edge. Peter.My band:- http://www.youtube.com/user/RedwingBand
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Daz,
The way you have your presence wired is NOT how I drew it or described it. right now you have the pot AND the original shunt resistor acting in parallel as the NFB shunt. This substantially decreases the amount of NFB from where you had it. This could also be why you didn't get linear operation from the circuit originally. Look at the drawing I have attached. The idea is that the pot serves double duty as the shunt resistor AND the presence adjustment. This will return your feedback ratio to where you had it the last time you said it sounded good.
Chuck
P.S. I'll be out of town for a few days."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Ahhh....i see, and i did it. Works great, thanks Chuck. The MB stopped today but i'm sure it will be back. With all the changes i made lately i now have the amp at work where i can turn it up and i'm finding the truth about these tweaks. Some are definatly good, others i may be changing. One thing thats been bothering me for a week is i think i may have a bad OT ! remember i posted about the other amp like this one my friend now owns and the OT blew? Well, just a couple days after that i may have ruined mine by starting it w/o speaker. i played a chord and realized the speaker was unplugged and turned it off immediatly. Is it possible to damage the OT that quickly? I didn't think so because AFAIK it has to be on with signal for long enough for the windings to heat to the point the windings melt, right? yet my amp's volume seemed much lower after this happened. so when the new OT for his amp arrives i will tack it into mine to see. i may be ordering may second replacement OT within a couple weeks !
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Originally posted by daz View PostOne thing thats been bothering me for a week is i think i may have a bad OT ! remember i posted about the other amp like this one my friend now owns and the OT blew? Well, just a couple days after that i may have ruined mine by starting it w/o speaker. i played a chord and realized the speaker was unplugged and turned it off immediatly. Is it possible to damage the OT that quickly?Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
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Thats what i thought, but it really seemed a lot quieter after that. I have to clip the new one in to be sure because this thing was too loud to turn way up w/o killing my ears. Now it's easy to handle at full volume. And this happened b4 i began messing with the PA and NFB so it's not a gain change from tweaking those things.
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RUnning a tube amp with no speaker doesn;t ruin the transformer with heat. it can happen in an instant.
What happens is the voltage spikes that are reflected back from the unterminated secondary can arc your transformer windings. or potentialy the tubes. it all depends upon how large a spike is generated, and whether or not it punctured the insulating layers. It might happen the first strum, or you might get away with it for a long time. But nothing has to build up.
The longer you play that way, the more opportunities the wavefrom has to make peaks, and all it takes is one that crosses the magic threshold inside there.Education is what you're left with after you have forgotten what you have learned.
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Originally posted by Enzo View PostRUnning a tube amp with no speaker doesn;t ruin the transformer with heat. it can happen in an instant.
What happens is the voltage spikes that are reflected back from the unterminated secondary can arc your transformer windings. or potentialy the tubes. it all depends upon how large a spike is generated, and whether or not it punctured the insulating layers. It might happen the first strum, or you might get away with it for a long time. But nothing has to build up.
The longer you play that way, the more opportunities the wavefrom has to make peaks, and all it takes is one that crosses the magic threshold inside there.Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
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Very nice schem. May I ask what you use to draw 'em?
on a side note, that schematic no longer represents it's current circuit. It has changed a lot since then. at the moment it's probably better than it's ever been and i've been mostly just playing instead of tweaking lately with few exceptions. When it sounds this good it's hard to stop. (much thanks to Chuck who really sent me on the right path not long ago)
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right on, that seems like a great way to do it. only drawback is not being able to test the circuit and view voltage/current characteristics in the software before even building it. However, so far I find building a circuit to be the most reliable way of deciding whether or not it sounds good and works.
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