Updated my schematic finally !

[tubeswell]

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

[daz]

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.

[pdf64]

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.

[daz]

I'll keep that in mind, tho i think what i'll likely do is a 10k pot with the trace cut at one side so it's fully open turned all the way CCW. As for tubes, i don't think thats it but i'll give it a shot. thanks

[Chuck H]

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.

[daz]

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 !

[tubeswell]

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?

Nah 1 chord won't do it. I've switched on amps that I've modded in the past and started to play before I realised the speakers weren't hooked up, and the OTs didn't get wrecked. The energy/heat has to build up for a little while before the current can start to interact with the insulation

[daz]

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.

[lowell]

Very nice schem. May I ask what you use to draw 'em?

[Enzo]

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.

[tubeswell]

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.

Oh well - seeming as how I am inviting trouble, can I change my story then your honour? I can see how if you have a big enough voltage spike it will puncture the insulation, but I was generally thinking of spikes that aren't that big, but are made more destructive with the addition of heat. Does that sound any better? (Not that I'm expecting to be let off the hook .)

[Enzo]

I'll have to think about it. I'm closing up for the night and going home.

[daz]

Very nice schem. May I ask what you use to draw 'em?

Photoshop. What i do is make the symbols like resistor, cap, etc, then after i've made one the rest i can copy and paste. You can select anything then copy and put it wherever you want, rotate them, etc. With certain functions like that you'd be surprised, but it can actually be easier than many dedicated schematic apps, at least the few i tried. once you figure out all the things you need to move things around any way you want it's quick and easy.

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)

[lowell]

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.