Dumb science project: 5150 II -> SLO

[jamesmafyew]

Just wanted to get some comments on this. I've acquired a 6505-Plus for basically no $$. I've fixed dozens, if not hundreds, of these things over the years, and know the circuit really well. I've modded a few of them when players requested it -- adjustable bias, lowpass filters, adding chokes, whatever.

Anyway, since the Plus/II model has independent clean/dirty channels, it's sort of a closer cousin to the SLO than the original 5150. For this project, I'm ignoring the clean channel and may return to it later. I can work on the drive channel without altering the clean sound in any way, unlike the original 5150. I'm just curious about what's possible, and more about the relationship between the 5150 and its obvious progenitor the SLO... they do sound different (though not much), and I mainly just want to nail down which differences are mostly responsible for that. Is it just component values, or do component materials/selection matter as well? People who want to justify their $3.5k invesment in an SLO often say buzzwords like "mil-spec construction" but I have no idea whether the components themselves matter anywhere near as much as their actual values or the layout.

If you convert the 6505+ component values in the first two gain stages to 5150-I values, those are in fact also the SLO values for those stages. I'm going to do that (not shown below). The +/II model already has the correct tone stack values for the SLO, where the 'I' model had an extra treble cap. I also know that the SLO's power stage is 'nicer' than this amp, given that it has a choke and more normal screen resistor values. Might do that too, but I suspect it's not that big a deal.

Beyond these things, it seems like the differences are just minor lowpass filtering differences (probably changes made by Peavey to avoid getting sued?), except for the cathode followers. The SLO has two of them, DC coupled -- one to drive the FX loop, then another for the tone stack. The Peaveys just use an anode follower for the tone stack.

Below is what I drew up on the 6505+ schem, based on comparing schematics with the SLO. Looks like I wouldn't have to cut any traces even!



SLO schem I'm using: Prowess Amplifiers - Misc - Schematics - Soldano SLO100
6505+ schem: http://schems.com/manu/peavey/peavey_5150-II.pdf

Any thoughts on this? I'm wondering whether the perceived greater clarity and thickness in the SLO has to do with the DC coupled CF stages and all the 2nd-harmonic content they add, along with the lower output Z driving the tone stack. Plus I ASSUME that the Peavey anode follower is being overdriven normally, and I have no idea how AF overdrive sounds compared to CF overdrive. Plus there's the fact that CF-driven tone stacks have slew rate limiting in cutoff, which adds its own character.

(I don't care about the amp's resale value and already have all needed parts on hand.)

[tedmich]

Interesting project! Don't forget about the Sloclone forums,those guys know the SLO circuit backwards and forwards and could be a real help. I believe the SLO OT is much more of a high fidelity unit than your typical PV... but I could be wrong good luck!

Maybe you could reamp the exact same riff under controlled recording conditions both before and after modding?

[jamesmafyew]

Maybe you could reamp the exact same riff under controlled recording conditions both before and after modding?

I will try to do this, though I'm a little short on recording equipment around here. :-/

[Gregg]

Make sure you get those two cathode followers with their respective component values in the chain as per SLO schematic even if you're not using them as an FX loop. For much more stuff go to the slo forum.

[Enzo]

(probably changes made by Peavey to avoid getting sued?)

Nah. Unless they copy the board art and the schematic drawing, there is nothing to sue over.

[jamesmafyew]

Make sure you get those two cathode followers with their respective component values in the chain as per SLO schematic even if you're not using them as an FX loop. For much more stuff go to the slo forum.

Well it'll get one CF, but I'd have to add a tube or do some dramatic rework to get both. I'm not trying to build an SLO clone (which would be a lot easier to build from scratch than converting a PCB-based amp like this), but more to see how much more like an SLO this amp will get with these changes.

I'm not sure to what extent the FX send CF in the SLO gets overdriven. Obviously the gainstage it's DC-coupled to gets slammed pretty hard. But since there's almost no load on the thing, with that tapped output, I sort of doubt it's AS important as the CF that's driving the tone stack. I started a thread a while back (unfortunately my scope images aren't there anymore) about the specific asymmetric wave shape that CFs produce when driving a heavy capacitive load like a tone stack:
http://music-electronics-forum.com/t31760/
In the amp that I was building at the time (Marshall 2203 inspired), that turned out to be one of the biggest contributors to the preamp distortion character. In a situation where you're mostly creating a square wave, it's a source of asymmetry. The DC-coupled cathode follower always compresses the positive peaks (as per Merlin's Valve Wizard article), but the slew-rate limiting from the capacitive load also makes the negative peaks triangular/trapezoidal. So while I may miss out on some of the positive-peak compression from the FX-send CF, it wouldn't have the same character/importance as the CF driving the tone stack. I think I may get dramatically more mileage out of the latter.

(The 5150 does have a CF-driven FX loop, but it comes after the master volume. I don't think I'm going to try to move it, since I will want to do something else with the clean channel later and am not interested in redesigning the switching system.)

[Pedro Vecino]

I'm just curious about what's possible, and more about the relationship between the 5150 and its obvious progenitor the SLO... they do sound different (though not much), and I mainly just want to nail down which differences are mostly responsible for that. Is it just component values, or do component materials/selection matter as well?

Have you played a SLO?. The 6505+ (5150 II) is an amp with a very soft touch compared to SLO. The latter is a stone until it sounds really with high volume. This is something that is frustrating for many guitarists that try it with low volume (at home, for example) or directly handle low playing pulse.
The 6505 + compressed much more and does not handle the strings separation and the dynamic behavior of of the Slo. Not even close.
The important thing is that I had several clones to supervise; some of them built in a religious form (voltages, supertransformers, passive components and tubes) and none of them have achieved these effects that I comment before.
In a 6505+, with another circuit and distribution, full of different capacitors (metallized polypropylene Illinois style, multilayer ceramic, electrolytics, 1/4 watt resistors, without choke, etc.) I see more than complicated, if not impossible.
Sorry to tell this but if it were mine I would modify preserving his original architecture. A better clean with with a more uniform transition at different gain settings, an approach to the 5150 original lead in the lows and a well balanced crunch with the latter trying to reduce the compression at maximum.

[Gregg]

I'm not trying to build an SLO clone (which would be a lot easier to build from scratch than converting a PCB-based amp like this), but more to see how much more like an SLO this amp will get with these changes.

In that case the changes you did on the schematic will do the job.

[jamesmafyew]

Have you played a SLO?.

Yes, several, and several other Soldano models as well, at very high volume. At that volume, I always felt like the SLO's dynamic response was more like a Marshall 2203 than a 5150, but with more of a "big iron" sort of punch (a very good thing!). Thus the thinking about the importance of a CF-driven EQ, which most people think sounds "harder" than plate-driven EQs. The experience begged the question... is the difference 80% the transformers & supply, and only 20% preamp topology and component values? Or is it maybe the other way around? Or maybe it's 80% lead dress. I want to find out.

[Gregg]

My experience with the "SLO type preamp" is it sounds good in almost all situations regardless of the iron size, part types, power supply etc. It doesn't necessarily sound like a real SLO but it's still very good and people like it.

[jamesmafyew]

Wanted to follow up on this.

No before/after clips, because, well, it sounded AWFUL. The master volume instantly became unusable, driving the power stage to full power at "2" on the dial. The DCCF that I added before the EQ was so far into overdrive that it swamped all voicing earlier in the preamp, producing a sound akin to a Boss DS-1 directly into a mixing board. However, with gain way down, pick attack was MUCH clearer, sounded more like what I expected from the changes.

Peavey has a 10:1 voltage divider right before the EQ, so I tried putting that back in. Made the MV more useful, but the tone did not improve. There's also a problem with capacitive coupling of high frequencies ("swarm of bees") between PCB traces on the long journey between the driver stage and the EQ. I noticed this because whereas before the amp was totally quiet with the MV all the way down, after the mods I could hear a fair amount of mids and highs through the speakers with the MV at zero.

So there's definitely a layout problem, but aside from that, it seems like it's impossible to produce the same gainstaging as the SLO without moving the FX loop (not doing that). The funny thing is that I feel I now have a really good handle on the process the Peavey engineers went through trying to get their amp to sound more like the SLO. The stock circuit values are pretty much exactly what it needs to make it sound better. I do think that a good bit of the difference in pick attack response between SLO/5150 is due to the AF vs CF drive of the EQ.

Anyway, awesome learning experience! This amp is going to get 5150 I values in the drive channel and a lowpass filter to kill some of the hair.

[nsubulysses]

That's too bad. I was very curious to hear the results of this undertaking. Thanks for sharing.