Need advice

[daz]

I don't think you're going to be too happy with that arrangement. If you are taking the unattenuated output from the tone stack directly into the extra tube stage, you are just going to overdrive it pretty badly. A better arrangement would be to set a voltage divider in place of your current MV at the MV sweet spot and feed the additional tube with that. There is little point in using a 1M MV after the additional tube stage; the changing impedance will interact with the PI and change your tone anyway. As I mentioned in my earlier post, the best arrangement is probably going to be replacing the resistor to ground in your voltage divider with a lower value pot; It will act like your current MV from 0 to the "sweet spot" that you set at maximum. This arrangement will also give you additional headroom in the extra tube stage as you turn the volume down. A 12ax7 probably has WAY more gain than you need to just re-amplify the attenuated MV output unless you sweet spot is very low on the pot rotation. You might want to consider using a 12AU/T/Y7 instead. You probably can just determine by experimenting.

Thanks for the replay. I'm sorry about the voltage divider not being there, it was a mistake. As i have said several times, the intention here was to replace the pot after the tone stack with a voltage divider, which was the whole reason for doing this in the first place....to drop the level there to get the tone i like at low level. I simply forgot to add it to the drawing.

As to your idea regarding the MV, I was wondering if a lower value MV would be better, but then when you consider my goal with this is to take the tone that is finalized after the voltage divider coming from the tone stack, wouldn't a smaller pot's lesser load cause a tonal change? I put that treble bleed on the drawing there as a way to make the tone linear from low to high and would experiment both with cap value and with a trimmer in series with it to tray and get the low volume tone to match the higher volume as close as possible. I will try lower gain tubes, but i was also consider and split load to trim the level into the master/PI. I could use a pot to determine the best resistor values.

[R.G.]

Thing is RG, you're looking at it from the perspective of tweaking it so theres no treble loss. But my issue is that i WANT that treble loss from the master because thats where it sounds good. But since at that point the level coming out of the pot is so low the amp's volume is at home or bedroom level. .

I understand. I see that one of the things I meant to write didn't actually get written down. If you can set it to no treble loss by some means, then you can attificially introduce just the right amount of treble loss by putting in fixed loading caps.

I know that sounds silly - get rid of all the treble loss, just to reintroduce it - but the difference is control. You can reintroduce just the right amount, and have that not be a side effect of the pot setting. Or you can reintroduce it with the dual gang pot thing, but this time use the pot section for messing with the treble change just right instead of the master having to do the right amount of treble loss at the same time as changing volume.

It may not work for you. Just a thought.

[daz]

Thanks RG, but i just did the xtra tube mod and this is awesome. I think this is probably why those high end boutique amps have more than just 3 stages. This is perfect. I put a 1m trimmer as VD at the treble pot output to the grid of the new tube, which is just standing about the PI tube on long stiff legs that are the heater leads. The rest is all hanging out in space, but no noise or any issues. It's perfect ! I am definately cutting another tube hole and making this permanent. i'm still playing with values, but as it stands it's like the schematic i posted aside from the missing 1M trimmer i forgot to draw.

It works exactly as i'd hope. Heres the way it's configured now in the drawing below. I will likely change some things, and the trimmer at the treble pot may end up being a switch between two settings or even a fine tuning pot on the back or some such thing. Not sure yet. master seems to stay the same even w/o a treble bleed, but i may eventually add one and cut some highs at that plate with a snubber or something.....so many ideas to consider. but in the end if i leave it exactly as is the amp is worlds better. It's amazing to get that same sweet tone cranked up some ! The tone does change to some extent of course due to the obvious unchangable things like the munson curve and speaker, etc. But the goal was accomplished, and that was to retain the same sweet saggy feel and harmonic complexity without getting real harsh and hard as i turn up. I'm a happy camper !

[daz]

Chris, I took your advice and it worked great. that being the small pot in place of the voltage divider ground resistor. Left the old master at 10 (will replace with 1M R) and came to find a 5k pot was perfect. Retains the sound a lot better then before even. I'm now about to install a new socket and make this permanent. few other changes since the last drawing too, so values and one or 2 other things aren't exactly the same. Thanks for your help, that was really the trick i believe.

[g1]

Please post a schematic of the final permanent circuit (unless it's a trade secret ).

[daz]

Please post a schematic of the final permanent circuit (unless it's a trade secret ).

I was about to try and edit my schematic, tho it will be a bit of a chore so it may be a while. Or maybe not, but i will. It likely won't be final tho, as i'm constantly trying different ways to wire it and different values. I think theres much more potential here and i intend to find it. The added bonus is better tone overall at any volume. Still gets hard at high volume, but i think thats the small bright room and EV speaker. More work to do, but i'll work on a schematic and edit that as i make changes.

[daz]

Ok, here ya go. The resistance with the ? mark is there because i have a trimmer in that position and i'm not sure yet where i want that to be. Probably close to zero resistance tho. Anyone with advice please go ahead. That and pointing out mistakes. I looked over my schematic after editing it, but i was in a fig after all that and i may have missed something. Sounds really good tho. I expected to have my tone stay intact at higher volumes, and while i got that if not to the degree i'd hoped, i got better tone everywhere which is something i wasn't expecting. Hopefully with more tweaking i'll get the high volume tone even better. A very very good mod overall. Really glad i did this. Oh, and the tube is permanently in place now.

[yunger]

just observing here...and i don't quite think i understand what is going on really but:

so the resistance(whatever the top half of the trim pot is) in series with the grid of the 4th tube cuts high frequencies due to miller capacitance?
wouldn't a cap to ground do the same thing if the end goal is to dump some treble?

[daz]

just observing here...and i don't quite think i understand what is going on really but:

so the resistance(whatever the top half of the trim pot is) in series with the grid of the 4th tube cuts high frequencies due to miller capacitance?
wouldn't a cap to ground do the same thing if the end goal is to dump some treble?

oops !!!! That 1M pot should be a 5k ! I'll fix the schematic and have a corrected on up in a few minutes.

That was on the advice of chris above. he said replace the ground resistor in the voltage divider i was going to put there with a small pot, and It works great. I first put a 1M pot there just like there was before as the pre pi master to see what values i wanted in the VD. But i ended up putting a 5k pot as the ground R as per chris, and a trimmer there as the series resistor instead of a VD to see what i'd get, and the 5k works perfect as a master and the trimmer sets the overall volume range of the master and affects tone, which i have yet to decide on.

[yunger]

so if the ground leg of the VD is only 5k, the series resistance would have to be pretty large or you'll end up dumping bass right? now you will have grounded resistance parallel with the bass pot. i think the .022 caps in the tone stack would need to increase to get you back where you were. or maybe the change is for the better...if you like the sound now, disregard.

[daz]

so if the ground leg of the VD is only 5k, the series resistance would have to be pretty large or you'll end up dumping bass right? now you will have grounded resistance parallel with the bass pot. i think the .022 caps in the tone stack would need to increase to get you back where you were. or maybe the change is for the better...if you like the sound now, disregard.

You may have something there, as i am noticing some weirdness with the tone controls. Especially the treble. I'll try paralleling them and see if doubling them is helpful. Thanks.

EDIT: nope, no joy. Looks like i've found the flaw in this circuit. So i tried it as it was before the mod but with the same master as in the mod. Being right after the tone stack causes the tone controls to not work right. Seems like it should mimic a 1M pot turned low when the trimmer is at max resistance or close, but oddly i doesn't and i don't undertand what the heck could be causing that.

[g1]

If you are trying to mimic the original master off the treble, you need a resistor near 1M in series after the treble pot (marked as ? on your schem.).
Think of when the master was there and what values resistance the pot would have read, I don't think you are duplicating those values.

[daz]

If you are trying to mimic the original master off the treble, you need a resistor near 1M in series after the treble pot (marked as ? on your schem.).
Think of when the master was there and what values resistance the pot would have read, I don't think you are duplicating those values.

I know that, and the trimmer is a 1M trimmer that i generally keep at the 900k it maxes out at. I measured the original master at my fav tone position B4 i did the mod and it measures about 8k ground resistance and almost the full 900k series. anyways, i'm going back to square one and putting 1M master back at that location and using this other master after the 4th stage and see how it compare to putting another 1M in that spot. Lots of work ahead.

[cbarrow7625]

Great question about the MV. I always assume that a lower pot value will result in less tone change, especially considering miller capacitance and all the interactions R.G. spelled out, but honestly I cannot tell you for sure. Either version should be easy to try but no matter what you should be better off by limiting the signal going into the "reamplify" tube. Don't overdrive it. Probably not what you want.

[daz]

Great question about the MV. I always assume that a lower pot value will result in less tone change, especially considering miller capacitance and all the interactions R.G. spelled out, but honestly I cannot tell you for sure. Either version should be easy to try but no matter what you should be better off by limiting the signal going into the "reamplify" tube. Don't overdrive it. Probably not what you want.

I know, i was able to use the trimmer to hear where it started adding OD and that wasn't a good thing.

The thing that amazes me is with all the tricks people have come up with over the years, It seems no one ever figured out a way to implement a volume control that is tonally linear. Adding this tube allowed me to get it much better, but it's still a ways from where i hoped it would be. I had it very close a few times, but when i did some other thing wasn't right.