opinions on a issue with this loop

[12AX7]

My amp is a 6v6 powered amp thats very similar to the 2204 and the Soldono SLO from the stage b4 the cathode follower thru the PI. Pretty much exact cept for 22uf filters instead of 50uf. So i copied the SLO effects loop exactly and i wanted to see if anyone can look at the schematic in the link below and tell me if they see anything that would cause the issues i'm having before i give up and rip it out. The issues are two fold. First, it seems to compress the signal. I thought it was the effect i'm using at first, but today i did more extensive listening tests and noticed it's present when i keep the loop running but patch the send and return with a short cable. Then i bypass the loop altogether (i installed a DPDT bypass) at the compression is gone.
Second, theres a very strong low mid punch this amp has and that remains with the loop running with a cable patch, but with the DSP plugged in i lose that low mid punch completely. The DSP is a old Boss rack effect that can be used for both guitar and line levels via a -20/+4 switch and lots of level adjustment including a global output level that goes to 200%. So maybe it needs to be gain staged differently but i haven't been able to get that punch back by doing so. I;'m not sure if the input level somewhere in software must be low with the out high or visa versa,. But there are about 4 levels and only one tells yu whether it;s a in or out level so it's tricky. I can look up the boss unit's specs if you need any.

Can anyone suggest changes to the loop in this SLO schematic that might rectify those issues?   http://schematicheaven.net/newamps/soldano_slo100.pdf                         

[Ed_Chambley]

Since the voltages on the lop are not on the SLO schematic, how do you know if you have the same headroom.

Also, try a 12At7.  Plenty of things you can do to correct compresson.  Have you tried changing RL as the schematic states.  You may need a different value here.

If you have a 5k pot try subbing it for the 2.2k referenced to as RL and dial it down.  May even try reducing the plate load resistor on V4 to increase voltage.

[SnickSound]

The SLO loop is not exactly transparent, it uses a DC-coupled cathode follower as the driving stage, which adds some compression. It's part of the sound of the amp (it's not bypassable on the original design), and one of the key differences between the SLO and a Dual Rectifier (which uses an AC-coupled cathode follower AFTER the tone stack instead of before it).

If you prefer the sound of the amp without the loop in, you might want to move to a different implementation.

That said, it's also possible it needs some tweaking due to voltages, which I'm assume are lower due to this amp running 6V6s (the real SLO has a fairly high B+). This might affect the behavior of the cathode follower driving the loop.

[tubenit]

The D'Mars 6V6 has an OD channel that certainly adds some significant gain.  So, here are some things that you might want to try:

1)  Use a 12AT7   (I use 12AY7's or 12AV7 in all my FX loops)

2)  Add an FX send pot

3)  Add an FX return pot prior to return gain stage

4)  Try a paralleled FX loop

With respect, Tubenit

[Willabe]

Tubenit why did you tie the pots x end to the wiper on the FX return pot? 

[12AX7]

Since the voltages on the lop are not on the SLO schematic, how do you know if you have the same headroom.

Also, try a 12At7.  Plenty of things you can do to correct compresson.  Have you tried changing RL as the schematic states.  You may need a different value here.

If you have a 5k pot try subbing it for the 2.2k referenced to as RL and dial it down.  May even try reducing the plate load resistor on V4 to increase voltage.

I will indeed try a 12AT7 and try a 100k on the plate. I thought about doing the latter but it didn't hit me about the lower voltages possible causing it. (should have...embarrassing that i didn't !) I can also take the power from b4 the PI to get even more. And yes, i tried reducing the 2.2 cathode feed as the schematic suggests. No joy.

The SLO loop is not exactly transparent, it uses a DC-coupled cathode follower as the driving stage, which adds some compression. It's part of the sound of the amp (it's not bypassable on the original design), and one of the key differences between the SLO and a Dual Rectifier (which uses an AC-coupled cathode follower AFTER the tone stack instead of before it).

If you prefer the sound of the amp without the loop in, you might want to move to a different implementation.

That said, it's also possible it needs some tweaking due to voltages, which I'm assume are lower due to this amp running 6V6s (the real SLO has a fairly high B+). This might affect the behavior of the cathode follower driving the loop.

As above, i will try and get voltages up as much as i can. I can go before the PI to source it, tray a 100k plate, and use a smaller dropping R and see what happens. Even if i can't stop the compression, if i can just reduce it till it;s not notible that would be ok. What won't is the tonal change i mentioned which is a loss of that low mid punch the amp has. I just don't know if thats a product of the DSP's design or some sort of impedance or level mismatch due to gain staging or whatever.

4)  Try a paralleled FX loop

That was what i wanted to do in the first place but i couldn't be sure what might work. Not to mention i have no more room on the back panel for pots. the reason i chose the SLO's series loop is i figured since my amp was so similar my chance of success would be great. I have already tried one other series loop that someone helped me design, but it was a failure as far as transparency too and more so than this one. So i'm really not wanting to try another one. If i can't "fix" this one i'm done and will go back to using a second amp as effects only.

[12AX7]

Another thing thats odd and makes it harder to A/B the loop's sound with the DPDT bypass switch, and maybe someone can tell me what causes this. When i turn it from bypass to loop active, the volume at first is gone and it slowly comes up to normal after about 10 seconds just like when you turn the amp on and you hear the amp slowly come up as the tube start to work. The only thing i could thing was caps, but i replaced the .1uf at the cathode to the loop input and the 22uf on the loop's first stage cathode. Any ideas? Whatever it is maybe it could be related to the tonal issues?

[sluckey]

Maybe your bypass mod has left the tube's grid floating in one switch position?

[12AX7]

Maybe your bypass mod has left the tube's grid floating in one switch position?

Smart man ! You nailed it. In any case, that was a bother A/B'ing it but no matter now. I'm done with this loop and tearing it out. I tried everything mentioned above and it just sounded worse. So rather than screw with it endlessly, i think i will consider the likelihood that it's never going to get me close enough to be happy. Back to the 2nd amp for effects scenario. Maybe i'll get a wild hair and try a parallel loop one day if i find one that somehow seems like it will work. Thanks all for the ideas.

[SnickSound]

Maybe your bypass mod has left the tube's grid floating in one switch position?

Smart man ! You nailed it. In any case, that was a bother A/B'ing it but no matter now. I'm done with this loop and tearing it out. I tried everything mentioned above and it just sounded worse. So rather than screw with it endlessly, i think i will consider the likelihood that it's never going to get me close enough to be happy. Back to the 2nd amp for effects scenario. Maybe i'll get a wild hair and try a parallel loop one day if i find one that somehow seems like it will work. Thanks all for the ideas.

No need to throw away the loop completely, but the SLO implementation is anything but a transparent one.

You could do it the Mesa way: AC-coupled cathode follower followed by recovery stage, after the tone stack:
http://i911.photobucket.com/albums/ac317/cscotto_photo/2-mesaboogietremoverb3-1.jpg

I'm using a variation of this in my own build (buffer stage is not a cathode follower design, but still feeding the loop via the cathode, it's from Merlin Blencowe's book).
And since I'm filming the whole thing, you'll be able to witness first hand whether it works well or not (it also is on a bypass switch): http://el34world.com/Forum/index.php?topic=21387.0

[Ed_Chambley]

No, don't rip it out yet.  Check out the attached file and see if you can convert yours to this.  A member gave me this and I tried it and IMO is the best tube loop I have ever used, however I do prefer a Mosfet one I lifted from a Freidman amp.


Also, Tubenit has made a ton of loops and has always helped me to get what I needed if only a simple loop in a Princeton for a Delay.  Let the guys help because I know you will prefer having it if it works well.  If you are frustrated, just switch it off and leave it for a few days.


I do know the feeling of wanting to button up and play the amp.

[12AX7]

Thanks, but thats pretty involved and unless i have good reason to think it'll work in my amp i'm just going back to no loop. Too much time spent over a few months trying to get a loop to work transparently.

[12AX7]

Just had one of those "ah haaa" moments. I realized it was the DSP's output level that is crucial. I thought anything within reason was going to sound the same and that the DSP input was the crucial thing. But i now realize it goes from unity to huge compression gains if it's global out level is way up. So it overdrives the loop's tube. I thought it wouldn't do that as long as the input level was low and the amps volume was the same with or W/O the DSP/loop,  but it does. It's a bit more complicated really, but the short version is with a 12AT7 and a 100k plate resistor i;m getting very close. the trick seems to be adjusting the DSP's output level till the tone is the same as no loop. I don't know if i can get it exact because it's a pain with all the DSP settings i have to change and re-patching etc to compare the 2. But i think that output level is the key. The DSP's input seems not to matter as long as the meter doesn't show clipping.

Anyways, thats where that compression was coming from and by lowering the global out level i can get it exactly the same as non loop tone. The question now is how close i can get it as far as that low mid punch and volume plus get the high end the same. (something i only just started noticing is the highs may be extended) I'm burned out after all this and my ears need a rest so that'll have to wait. But theres a good chance i can nail it. maybe not 100% transparent, but at this point i think it may be a pretty subtle difference.

[2deaf]

The SLO loop is not exactly transparent, it uses a DC-coupled cathode follower as the driving stage, which adds some compression. It's part of the sound of the amp . . .

That's the crux of the bisquit, there.  Unless you want the characteristics of this loop, it is not suitable for transplant to another circuit.

As far as parallel loops go, don't most effect devices have some form of mixing which constitutes a parallel strategy?

I have attached a suggested loop that I believe is no more involved than what you are currently working with.   

[2deaf]

I have encountered a problem with the above attached loop.  The 3.3uf capacitor takes a really long time to charge and puts a high voltage on the Send jack.  I lowered the 1M resistor to 100K, but was still concerned with the initial voltage.  So I put butt-to-butt zeners in and that cured the high voltage, but it retains a slight DC offset.  So I put a .01uf capacitor in front of the return grid to prevent a bias problem. 

I've seen plenty of large blocking capacitors on effect loops, but I don't recall ever seeing zener diodes on one, so I suspect I'm missing something here.

[PRR]

Why 3uFd? That CF can't drive the implied 3K load, and your load is likely 10K 22K or 100K.

[2deaf]

Why 3uFd? That CF can't drive the implied 3K load, and your load is likely 10K 22K or 100K.

The 3.3uf capacitor allows a low impedance path for 60Hz hum that is induced in the patch cable.  3.3uf seemed to be a breaking point for this setup with larger capacitors yielding no additional benefit.

I drew some more lines last night and altered the design to eliminate the problems.