Lets discuss amp tone and how it changes w/volume and why

[daz]

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It seems that too many variables are involved, making the amp hard to mange. So perhaps some compromises can be made, can you do without some of the controls such as the variable NFB and/or the presence control? e.g., can you get the tone/feel by setting the MV and the tone control on the guitar alone?

I already canned the presence and NFB controls since they do nothing and in one's place i have a cut control with a 2000pf cap. Works fine but i just feel NFB would work better. To be clear, i AM getting the tone and feel i want. It's just harder to dial it in. It also sounds much more marshally than it ever has due to what sounds like a shift in the mid frequency, tho i'm undecided whether i'm good with that. I like it a lot but i prefer a more neutral sound for versatility. So i'm thinking of trying a plate fed tonestack as i have done in the past. That actually made it sound less marshally so it may be just the perfect compliment to the BS master's sound. but i'm getting ahead of myself. It just has a sound now that is so much more tweakable than before. I can see a lot more potential to mold it to whatever i desire.

[jazbo8]

Interesting situation, sounds like you are really close. Since you don't like the post-PI MV and the pre-PI MV is hard to dial in, perhaps you can try re-locating the "master" volume one stage back (V2a on the Marshall 2203 schematic) and restore the NFB connection back to the PI, so they do not interact at all - hopefully it doesn't rob you of the Marshall-ness...

[Chuck H]

Actually... Considering that the PI tube probably won't be, and in fact isn't intended to be overdriven and clipped, why not use a 100k pot as the cathode resistor for the cathode follower!?! Feed the TS off the wiper! This would surely be better than placing the master AFTER the TS. The end result is the same as far as the PI is concerned and any impedance issues should be significantly improved. As would the cancellation of the NFB loop that the BS circuit causes.

[jazbo8]

There is DC on the cathode so you get "scratches" if you use a pot there to change the volume. With the post-PI MV, you can overdrive the PI, some (in fact many) actually prefer the pre-amp+PI distortion but not daz, that's why I am proposing moving the "master" further up the chain. Here is an old post by teemuk that is relevant to the discussion - the player has to decide for him/herself on what's acceptable sound/feel-wise - there is really no right or wrong, the MV is just a tool not a fix-all...

[Dave H]

Dave, "scope" the signal at PI cathodes in BSMV vs. generic MV. Now think what happens to the degenerative feedback at that node when you close the feedback loop. You'll see it.

OK, I finally got around to trying that. The signal level at that node increases when feedback is applied but what does that tell me about the BS master being inside the loop?

We could be looking at the wrong circuit here. The one Daz posted (see BSMV #1 schematic below) is incorrect. All it does is load down the tonestack which isn’t what he wants. I think the correct circuit is BSMV #2. This one clearly affects the feedback loop.

[daz]

OK, I finally got around to trying that. The signal level at that node increases when feedback is applied but what does that tell me about the BS master being inside the loop?

We could be looking at the wrong circuit here. The one Daz posted (see BSMV #1 schematic below) is incorrect. All it does is load down the tonestack which isn’t what he wants. I think the correct circuit is BSMV #2. This one clearly affects the feedback loop.

[ATTACH=CONFIG]26660[/ATTACH]

I'm using #2. I can't imagine #1 working right because i have tried lowering the 1M's to 44k (put 47k's in parallel with the 1M's) and the volume hardly changes. I'm not experiencing any of the problems whats his face suggested including not turning the volume off at zero. Mine absolutely does. When i was experimenting with it i think 250k for some reasons didn't, or maybe it was the single side one or single @250k. Whatever it was, this one turns off at zero.

And at the moment it sounds frigging great. Better than it ever has actually with loads of harmonic content. As to V2a, that would kill the drive as i turned it down. As for Chucks idea, sounds interesting and it one idea i never would have thought of or even considered to work right, but maybe i'll try that and see if i can live with the scratchy pot. At this point i'd be totally happy to leave it as is, but i still wish i had the NFB available because i'm not a fan of cut controls

[Dave H]

I'm using #2. I can't imagine #1 working right because i have tried lowering the 1M's to 44k (put 47k's in parallel with the 1M's) and the volume hardly changes...

..but i still wish i had the NFB available because i'm not a fan of cut controls

Right, you posted a schematic of #1 but actually used #2. It's no wonder I'm I can't imagine #1 working either. It has to be a mistake.

Try the circuit below. I think it's the original BSMV. It shouldn't affect the NFB so the Presence control will work as normal.

Cut controls work better if you use a log pot and reverse the ends so that it works like a normal tone control (max treble when fully CW). It makes the pot taper smoother.

[daz]

Right, you posted a schematic of #1 but actually used #2. It's no wonder I'm I can't imagine #1 working either. It has to be a mistake.

Try the circuit below. I think it's the original BSMV. It shouldn't affect the NFB so the Presence control will work as normal.

Cut controls work better if you use a log pot and reverse the ends so that it works like a normal tone control (max treble when fully CW). It makes the pot taper smoother.

[ATTACH=CONFIG]26661[/ATTACH]

Ahh, i must have grabbed the wrong one because there were 2 as i recall, one being the one i'm using/. Anyways, i HAVE tried that one you just posted, the single sided one. The tone didn't seem as good and i think at that point it was not allowing the control to be fully off when down completely. But that may have been due to pot value because at some point i noticed lower values can cause that volume even when turned all the way down. NFB was active with one pot thos as you are suggesting, but it was VERY low. Maybe 10-20% of what it normally is, or at least it seemed so. Certainly much much less NFB. But i'll try it again with a 1M pot. Thanks.

[Dave H]

I'm reinventing the wheel. It came up in post #75. It's hard to keep track in a thread this long.

The MV probably doesn't go fully off because the pot CCW end isn't grounded, it's connected to the cathode so it can inject the signal in there through the pot's resistance. Lower resistance pots will inject more signal.

[daz]

Ok Dave, i tried a 1M and it worked just how i said it did when i tried it before with smaller pots. It doesn't fully turn off and the NFB is in effect tho just barely. I would say not the 10-20% i suggested about, but 10% max. Barely noticeable. But i will have to wait and see how it does louder because i can't make any noise at the moment. As for tone quality, that too will have to wait till i can turn it up. This whole experiment now has turned from wanting a master who's tone/feel doesn't change radically as i turn up to which method of BS master sounds best. Because i have found the overall tone quality is much better than with the pre PI master. I want to see if this single sided one sounds as good or better and i will use it if it does even with the lack of zero volume and full CCW. Thats not important anyways because it's lower than i'd ever use it. I will have to try a different pot tho because the 1M i tried was on/off at about 8:00. Went from the tiny bit of volume at full CCW to plating volume like a switch. I have some 1M's with better taper tho and i'll try one of those.

[daz]

I'm reinventing the wheel. It came up in post #75. It's hard to keep track in a thread this long.

The MV probably doesn't go fully off because the pot CCW end isn't grounded, it's connected to the cathode so it can inject the signal in there through the pot's resistance. Lower resistance pots will inject more signal.

The dual pot method does tho.

[Chuck H]

As for Chucks idea, sounds interesting and it one idea i never would have thought of or even considered to work right, but maybe i'll try that and see if i can live with the scratchy pot.

I never worry about inherently scratchy pots because I'm not turning them while I'm playing. I can understand how it's just aesthetically unpleasant to some though. Also unpleasant is the voltage rating of standard pots. Typical would be 500VDC for linear taper and 250VDC for audio taper. Since that circuit has about 200VDC on it you'll want to use the linear taper, which will feel clumsy in the low range. Or you'll need to buy an extra special audio pot.

[daz]

I never worry about inherently scratchy pots because I'm not turning them while I'm playing. I can understand how it's just aesthetically unpleasant to some though. Also unpleasant is the voltage rating of standard pots. Typical would be 500VDC for linear taper and 250VDC for audio taper. Since that circuit has about 200VDC on it you'll want to use the linear taper, which will feel clumsy in the low range. Or you'll need to buy an extra special audio pot.

Actually Chuck, your idea is great. But i'm not talking so much about as a master, but simply as gain reduction with a set divider. I've always felt this thing has too much gain before the PI and that it probably has negative affects on the tone. So i put a 47/50k divider (just added a 47k at the cathode pin and paralleled a 100k with the cathode R) there and i'm going to see how it sounds once the people are gone today. So far at very low volume tho i can see it really has a nice effect on the master's sensitivity. I can now adjust it much easier, and i always thought the amount of gain was the reason the master has always been sensitive. So if nothing more your idea may be very helpful in another way. Just gotta wait till i can turn it up to see how all this stuff works. Thanks 4 that idea !

[Dave H]

The dual pot method does tho.

When the dual pot is CCW the two triodes of the LTP are connected at the grids and at the cathodes. Any voltage between the grids and cathodes (across the 470R) is now common mode and is rejected.

[teemuk]

I can't imagine #1 working either.

Me neither. Obviously all it does is that it varies the overall resistance of the grid resistors. It doesn't introduce a voltage divider, which would attenuate the signal and by that means control it's amplitude. ...And I don't evem want to know what happens when you short grids and cathodes together.

The reason why scheme #1 even introduces very little difference in overall tone in the first place is this: in the usual LTP circuit the grid resistors are.... drumroll ...bootstrapped. (Ever wondered why that master volume scheme was named like it was). Bootstrapping in this case means that the resistor is in between a node (grid) having certain voltage and signal phase, and another node having almost equal voltage and signal phase, yet low source impedance (the cathode). Effectively this reflects the impedance of the resistor much, much higher to the circuit.

The 1M grid resistors, due to bootstrapping, may circuit-wise easily appear as 5M resistors. Decreasing the resistance naturally decreases also the impedance increased by bootstrapping, but still there's is very little difference in between using a, say, 220K grid resistor (that appears maybe as about 1M) and 1M grid resistor (that appears maybe as about 5M). The source impedance of the feeding stage, at least, is low enough that such wide tolerance imposes no concernable difference.