SLOclone schematic omission?

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SpiderWars

SpiderWars

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I built a SLOclone several years ago using Onetics trannies and always thought it sounded too gainy, rough, harsh. There was something about the schematic (whether it be the old typical Soldano labeled schematic or the SLOclone schematic) that bothered me but I forgot about it until recently.

The input of most guitar amps has a 1M resistor to ground and a 68k resistor in series (or two 68k resistors in parallel). I don't know the theory but allegedly that 68k resistance (or some resistance in that range) is necessary to for the input to sound good and not clip excessively. The FX Return jack on most guitar amps has a similar setup, 1M to ground and ~68k series. But the typical internet schematic for the SLO doesn't have that 68k resistor on the FX Return. It has the 1M to ground but NO series resistor at all. The jack connects straight to the grid which just seems weird.

So I threw a 68k in there (right at the socket) and it really seemed to clean up the rough, harsh stuff without reducing the saturation. And the tone controls now seem to work better. The voice of the amp is unchanged, just the quality of the distortion is better.

I don't know if this resistor is in a real SLO, seems unlikely it would have been overlooked by whoever made the schematics. But if you have a SLOclone and it doesn't have this resistor it's easy to try it. And FWIW, I don't think this 'fixes' the loop or even addresses that in any way.
 
That would be a grid stop resistor and is there to prevent blocking distortion in high gain setups, hence the 470k grid stopper in the preamp. At the input it also serves, in conjunction with the miller capacitance, to block radio frequencies. The fx return is not high gain and thus the grid stopper is omitted. Really it sounds like there may have been some other issue going on as most of the preamp distortion occurs far before the fx return.
 
But the return does clip, that's part of the SLO sound. I'm not sure what you mean by 'not high gain' there unless you're referring to the fact that most of the gain is before it and not after it. The stage itself is high gain. It suppose it could be another issue that is somehow fixed by the grid stopper. The circuit seems very sensitive to small changes.
 
The return should not be clipping at all from a line level input.
 
When I say Return stage I'm referring to both the gain stage and cathode follower pair. The clipping isn't excessive and just seems to round off the sharp edges.
 
The original SLO doesn't have that 68k grid stopper on the return so I didn't include it in the schematic. If it tames noise and/or makes the amp sound better then I'd leave it in. I think everyone agrees that the SLO loop is not the best design.
 
Ya, it really seemed unlikely that it would have been an omission but you never know until you ask. It doesn't seem to affect noise but it just makes the distortion seem way better even though it's a fairly subtle change. I'm turning the Treble/Presence way higher than ever before...not because the voice/tonality changed but because the treble is now 'good' treble. I never REALLY liked this amp until now. Thanks for your input!
 
No problem. I actually need to get new power tubes for mine it's limping along at the moment. I have a chassis a boards to build another and some original PVC series caps just have to find time. I also want to mod it a bit tighten up the low end a bit further. I might even mess with 39k cathode resistor. Also will install the loop bypass as I never use the thing anyway.
 
What power tubes do you prefer? I built mine with some 6L6GC I got from Tubetramp called Shuguang Red Box. I've recently tried Tung Sols and some unknown GT 6L6GC that were in my 5150 and the Red Boxes were the clear winner there.

I've tried a few things to tighten up the low end but usually I change it right back because it loses something or gets too mid-focused. I've messed with the 39k and it always goes back for me. In my case, the only thing that really stuck without affecting other things too much was a film/foil .68uF (looks similar to a Sozo) instead of 1uF on that first cathode.

Here's a couple of full frontals from when I first built it (~8-10 years ago). I did the layout myself (borrowing heavily) and hand cut/drilled/staked the circuit boards. I've added a 3-way bright switch for the OD channel since but that's about it.
 
JJKT66s. They aren't cheap. I have a bunch of 6L6GC of various manufacturers but always go back to the KT66s. The chinese ones can be good but I have to sort through a bunch of them.

That's a pretty clean build. I don't really know if I can tell a big difference between the mallory pvc caps or any other polyester caps other than the fact the pvc's are huge.
 
Lots of amps don't have that resistor or have an insignificante value
I think the rectifier and the framus cobra, both soldano-based don't have that
The 68k is the standard value just because most builders started from marshalls that had that value
Having lower values or no resistor at all actually helps with dynamics and thickness, but some amps might get a bit flubby if you don't cut some bass after the first gain stage
My lee Jackson jmp didn't have any grid resistor in the input, but it sounded flubby and bassy because it didn't cut enough low end after that first stage
it had a 10n coupling cap and no bright cap in the gain pot
Using a lower value capacitor and adding the bright cap solved the issue
 
Modern tight sounding amps usually have 1n coupling caps for the lead channel
2.2n tops
 
I have a quad of Chinese KT66 I should try then. Looks to be plenty of space for proper cooling.

I decided with this particular amp that I wasn't going to mod it much right after I finished. Simply because there have been previous builds where I would mod away and think it sounded better but when compared to other amps it had become way too mid-focused (but tight). Then I'd mod it more and eventually it wouldn't be as pretty inside and I was basically back where I started.

Your comment about lower value grid stoppers (or none) at the input being more dynamic/thicker agrees with what I'm hearing by adding it at the Return stage. It sounds like it's not getting hit as hard on peaks and the bass isn't flubbing out (which can have high-endy artifacts). The amp does sound a little less bassy with the Return stopper but turning up the Bass just a little gets it right back...but without the flub.
 
SpiderWars":3kj6b5nq said:
What power tubes do you prefer? I built mine with some 6L6GC I got from Tubetramp called Shuguang Red Box. I've recently tried Tung Sols and some unknown GT 6L6GC that were in my 5150 and the Red Boxes were the clear winner there.

I've tried a few things to tighten up the low end but usually I change it right back because it loses something or gets too mid-focused. I've messed with the 39k and it always goes back for me. In my case, the only thing that really stuck without affecting other things too much was a film/foil .68uF (looks similar to a Sozo) instead of 1uF on that first cathode.

Here's a couple of full frontals from when I first built it (~8-10 years ago). I did the layout myself (borrowing heavily) and hand cut/drilled/staked the circuit boards. I've added a 3-way bright switch for the OD channel since but that's about it.
Click to expand...
Nice and clean work there,thats impressive!
 
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