effects loop idea

[Chuck H]

That .1uf bypass cap should have increased frequencies above 1k gradually to a 3.5dB increase around 5k!!! Are you saying you didn't hear that?!? If not then you MUST have something cattywhompus.

There's shouldn't be that much noticeable difference in tone with a loop. Maybe a tad. Seriously, a tad. Did you try the amp WITHOUT the DSP in the loop? If you don't have a switching jack bypass set up you can just plug a cable into the loop. The thing I worry about more with loops is added noise. Be sure you're wiring is all correct. and that the DSP itself isn't responsible.

Also, even if none of this turns out to be the case you still have an ACTIVE circuit happening. That means you can recover losses and tune the circuit if you have the patience and relative peace of mind to NOT just go "Nope! Now it shall always be bad. So sayeth my closed mind."

And hey! You already have the tube in there. You could always try rewiring as a parallel loop. You should be able to set up your DSP effects to 100%. Then the only fidelity loss would be to the effects and the dry signal remains just as it was before the mod.

You gotta try to be a little less jumpy

[daz]

Do you have a grid stopper on the input to the PI? My experiments indicate there is a subtle effect on attack at the PI input that is controlled by grid current and coupling caps there.

No, but i really can't imagine that helping because it's not just one thing. The biggest problem isn't even that hardness it's a big loss of harmonic complexity. The tone is just too different for a single component to make that kind of difference. But if you still think so after this i'll try it.

[daz]

I'm not saying it didn't, but it's nothing that remotely changes the tone in a way that helps so i wasn't even thinking about that, too busy thinking about the dull tone. It should have, but then it's not a OD gain stage so it's not nearly as noticeable as it would be there. I just think it's a matter of too much going on or maybe the circuit either being in a location thats not good for this. The latter i mention because the tone stack knobs seem to work quite a bit differently. Maybe i should try it between the treble wiper and master with some component changes to compensate? That would take out the possibility it could be the tone stack's response being affected by the circuit. I do feel the middle is much harsher than before so that might be where i'm getting that hardness from.

That .1uf bypass cap should have increased frequencies above 1k gradually to a 3.5dB increase around 5k!!! Are you saying you didn't hear that?!? If not then you MUST have something cattywhompus.

There's shouldn't be that much noticeable difference in tone with a loop. Maybe a tad. Seriously, a tad. Did you try the amp WITHOUT the DSP in the loop? If you don't have a switching jack bypass set up you can just plug a cable into the loop. The thing I worry about more with loops is added noise. Be sure you're wiring is all correct. and that the DSP itself isn't responsible.

Also, even if none of this turns out to be the case you still have an ACTIVE circuit happening. That means you can recover losses and tune the circuit if you have the patience and relative peace of mind to NOT just go "Nope! Now it shall always be bad. So sayeth my closed mind."

And hey! You already have the tube in there. You could always try rewiring as a parallel loop. You should be able to set up your DSP effects to 100%. Then the only fidelity loss would be to the effects and the dry signal remains just as it was before the mod.

You gotta try to be a little less jumpy

[Chuck H]

Hey daz... You know what the loop did? It inverted the phase 180*. At least some of the affect your hearing on your tone is likely because you're still sitting in the same position playing the amp the acoustic responses are a polar negative to what you had before (slaps forehead).

Please report on my post above. Did you hear a difference with the .1 bypass cap on the recovery stage? Did you try the amp without the DSP in? And now, additionally...

Bypass the circuit in place. Just lift the output of the recovery circuit and tack the top of the original cathode follower in there. Still sound bad? Because that's the way it was before so it should sound great. You (or the amp?) seem prone to doing one thing and causing a different problem. I just want to be sure that's not what we're dealing with.

[Chuck H]

This may sound crazy... Try swapping your speaker +/- leads.

[daz]

I answered the .1 question in my last post. But i will try both the speaker phase swap and return to before, tho the latter will be tough due to the way it's physically laid out due to no room. And i DID find a mistake....no 1M load on the return. But oddly it made no difference when i added it. I guess because the DSP was the load and with the jack patched together w/o the DSP it wouldn't work? In any case didn't matter.

[daz]

reversed it. seems to sound better but still not there. But it may have been the level more than the speaker because when i did that it also hit me to check the level which i hadn't done since i first fired it up and it was hitting the ceiling probably due to changes since i 1st tried it. But i can't recall the timeline. In any case it's not sounding bad, but it still is missing that richness and theres a thinner sounding top end. Maybe I'll go back and try the 22uf on the cathode again.

EDIT: reversed the speaker BACK to original and it DID make a lot of difference because now it sounds thin and hard again. It took going back to original speaker phase to really hear the full difference. I think that was most of the difference, tho it still isn't as good as b4. But much better. How would i go about reversing the phase of the circuit? Maybe that would bring it all the way back. But once it's there i will revert back to before the SLO loop and see how much is missing or not. At this point it's hard to tell with my ears starting to play tricks after several different sounding incarnations.

[Chuck H]

Well first it was dull and now it's thin. Is it dull and thin? I know this stuff is all subjective but I need a launching pad.

Just don't jump the gun and tear it out yet. There's still work to do. And remember to reverse the speaker leads for the circuit bypass test. As few connections as it takes to bypass, it might be an idea to set it up on a switch so that AB in circuit tuning is easier to check in real time. That sort of thing is a big help in fine tuning a circuit.

[daz]

With the speaker reversed from normal it's neither, it's close to normal....I THINK. Like i said, hard to be sure till i revert back to original b4 SLO. But i first want to know....can the phase of the circuit itself be reversed? I don't want to have the speaker reversed permanently in case i decide to run it with a second amp at times. What exactly causes the phase difference, the second cathode follower?

EDIT:been going back and fourth between speaker phase and trying many things. When i just did go back to the original non SLO loop circuit tho it was obvious theres just a lot of richness missing. I can't really describe it because every time i try i realize i'm not really sure what it is thats wrong, or at least i can't figure out how to put it into wordsnand that just confuses you and others. I just know and feel it and it's not right. But the one thing i'm sure of is there is less harmonic complexity, and i thing that's where the other issue details stem from Rich harmonics give a dynamic feel and response with some squish in the attack that is very important. What i'm wondering is if now i have to redesign some details in the gain stages to compensate for what the loop changes.

[Chuck H]

It's the recovery triode that feeds the recovery cathode follower. Cathode followers don't invert, but regular gain stages do. That recovery amplifier is what boosts the signal back up and the cathode follower converts it to a lower impedance to feed the tone stack. So, +1 gain stage = inverted phase. You shouldn't sweat the phase of the speaker. The amp is now playing in the same phase it was before the new circuit. So if it was in phase with other amps then, it will be in phase now. Amps don't care about +/- symbols The fact is that many modern amps with switching options swap phase during operation when gain stages and effects loops are switched in and out. It's also a fact that some amps play forward (as it were) and some amps play inverse. Did you know that you can't effectively jumper the channels on BF Fender amps because the two channels ("normal" and "reverb") are out of phase with each other and it causes phase cancellation problems? It's not at all uncommon for players that run multiple amps to tune things in by inverting speaker phase on one or more amps. I can understand if it offends any sensibilities. In a perfect world the + goes to the + and the - goes to the - and if anything different is afoot then something must be WRONG! Not so. Did you know that Jensen speakers from a certain era (and Fender was using them) had the +/- markings inverse from what most other speaker makers were doing? And inverse from how they make them now!!! Really, don't sweat it.

But DO invert phase again for the loop bypass test to get an honest evaluation of what's up

If you absolutely can't stand it you COULD invert the OT secondary. That means the black lead is on the hot output jack lug and your impedance switching references - (probably ground. It is on most guitar amps without isolated jacks). Depending on how you've clipped your leads this could be a real PITA. But suffering the aesthetics is your decision.

[daz]

But DO invert phase again for the loop bypass test to get an honest evaluation of what's up

I did. Thats what i was saying above. (yes, put speaker phase back when i did) It sounds a lot better with the old passive loop than the SLO loop. If only the passive one allowed volume.

As i said before, the tone controls seem different with the SLO loop. Makes me wonder...maybe theres a balance there between 4v feeding the stack vs the SLO's CF. Would it be worth trying a different location, IE: between the stack and master? You picked the cf>tone stack because we were doing SS, but now that it's tube??? Maybe use just one triode or 2 that are both regular gain stages if needed (and would set the phase back) so the master would not run from a cathode follower? I could use a voltage divider at the treble wiper till the DSP's in level meter is in the center then use one gain stage into the master or 2 if needed. It might just change the whole dynamic in a way that brings the tone back. Just an idea, but what do u think?

[daz]

Well, i tried it and tho this is preliminary since it's getting late and i couldn't turn it up loud for more than a few seconds, but i think it did the trick and then some ! The tone seems even better than ever and when i turn it up it stays more smooth and rich than usual. I was loving it. But i will have to ask for your opinion on how what values to use. Basically it's treble wiper goes thru a cap to the send jack. Return jack to the grid and a .022 off the plate to the master. I left the same unbypassed 2.2k cathode. theres a bit too much signal out, but that can be remedied with bigger cathode R i suppose or even a voltage divider at the plate load i suppose. It's already a split load because i added a 120k in series with the 100k to make 220k like the schematic when i was messing with it before, but if i go back to 100k with a pair of 47k's and take it from the middle to lower it more that might do it. So i can adjust that. I'll wait for your thoughts on it, but i think if i simply lower the output a bit it will not only be perfect but better tone than it was before this whole fiasco. I'm so friggin relieved ! This is the location the metro device installs, and so many people swear itl;s totally transparent so i had to try it.

[Chuck H]

Well it's getting a little late for digesting circuits from a written description Be prepared, I may request a schematic.

But I'm happy you found a circuit that you like. So far. Knowing the actual circuit will help because there may be a new element happing now WRT clipping that wasn't there before. There's no rule that says you can't do that with the loop but it would be better to know just what's happening.

[daz]

It IS late, so this may be missing things but i just scribbled it quick. Note the values are mainly not chosen for this but just what was there. I would have changed some but i just wanted to see if it would even work.

[Chuck H]

Well that shouldn't work very well at all!

That doesn't mean it doesn't sound fine to you. The problems with that design are all technical. Nothing dangerous to the circuit, just performance issues. You would surely have trouble with some effects in that loop. That your DSP can be dialed down in sensitivity to accept something like 25V of signal without clipping isn't typical.

You could pad the signal from the treble pot with a large series resistor and then load it down to like 10k. That would get your impedance down some and the series resistor would isolate the tone stack from the load so character and adjustment wouldn't be affected. I had a loop like I just described in an amp for a while. It worked "ok" but could suffer from HF loss with cables longer than little shortys. Your loop will suffer the same problem and also not be able to run many effects because of the high drive voltage.

The module that you linked works differently from what you're doing there in that it has a buffer stage to pad the signal down and lower the impedance (I assume). Then it reamplifies the return. So, one more stage than what you have there.

Also... You are still in inverse phase from the stock circuit because you still have one more gain stage in series. Remember that the cathode follower doesn't invert phase.

I have an idea. I'll draw it up and re post unless you just want to roll with the path your on.