Hi! First sorry that not in german but noone really would like to read my poor german

Recently I build a SLO50 with Tube town transformers and in general it is working quite good, zero hum, very low noise, very stable but I have some doubts regarding tone. I've never played orignal SLO so it is hard to tell is it good or not but here are my thoughts
1. Bass response is weird (to much not very tigh bass) with no possibility to tweak it without any eq at input.
2. Weird mid range - when I'm playing with attenuator it is not so clearly audible but when I'm playing loud something odd is happening with mid range (is hmmm... woody, don't know better description). and it is getting even worst warmed up (after 2-3h of loud playing)

I read on forum three advises for 50W build but without deeper explanation
1. Change NFB resistor from 39k (I guess it should be enough to connect it to 8ohm out rather than 4ohm) to get the same level or maybe You have other thoughts?
2. Rise voltage because it can get to "browny" whatever it means and what rise voltage really means any one tried it?
3. Change PI coupling caps to 22nF for better bass respond (is it true?)
4. Also I read somwhere that depth mod can create some issues.

Do You have any advices that I can check for such problems?

Below You may find sound sample
https://drive.google.com/file/d/1PrV6FD-98Y3ixY64_3GQvXf5jKYFdI46/view?usp=drive_link

I will appreciate You help!
And sorry that post in english but noone wants to read my german :d

Hello and welcome to the forum.

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1. Bass response is weird (to much not very tigh bass) with no possibility to tweak it without any eq at input.
...

The SLO design is aproaching close to 40 years in age, the overly tight guitar tones that exist now didn't exist back then. The stock preamp therefore isn't really tight enough for todays standards without modding it or adding an pedal in front of the amp.

I'd personally swap the 500kA Gain pot for a 250kA and increase the 470k that bypasses the 2200pf after the first gain stage to something like 680k or 1M. You can also try to lower the value of the 2200pF to 1000pF or an even lower value. This will cut bass in the preamp and make it sound tighter. I'd also install an depth mod to get more clean low end from the amp.

...
2. Weird mid range - when I'm playing with attenuator it is not so clearly audible but when I'm playing loud something odd is happening with mid range (is hmmm... woody, don't know better description). and it is getting even worst warmed up (after 2-3h of loud playing)
...

I would advise against using an attenuator with an master volume amp like the SLO. Most attenuators are purely resisitve and interfere with the ongoing interaction that is happening between the cab and the powersection. Presence and especially Depth controls rely heavily on this interaction to sound properly. The more an resisitve attenuator is used to pad down the signal the more (usless) midrange you will get.

...
I read on forum three advises for 50W build but without deeper explanation
1. Change NFB resistor from 39k (I guess it should be enough to connect it to 8ohm out rather than 4ohm) to get the same level or maybe You have other thoughts?
2. Rise voltage because it can get to "browny" whatever it means and what rise voltage really means any one tried it?
3. Change PI coupling caps to 22nF for better bass respond (is it true?)
4. Also I read somwhere that depth mod can create some issues.
...

1. To get the same amount of negative feedback the 100W version has you need to either decrease the feedback resistor or use a higher tap on the output transformer. Using the 8Ohm tap on your 50W head (while keeping the 39k feedback resistor) will give you the same amount of nfb as the 4Ohm tap on the 100W head.

2. Preamp voltages in an SLO are already on the higher side. I would not increase them (yet).

3. Smaller coupling caps will cut the bass response so it will actually do the exact opposite. They will also cause more unnecessary phaseshift at lower frequencies in the feedback loop. I would not go lower than 47nF at this position, maybe instead increase them to 100nF.

4. The depthmod is pretty simple and straight forward to implement not much that can/should go wrong here. The value used for the bypass cap is important as it determines the cutoff frequency. Start with an 4.7n and then try 6.8n or 10n here. The larger values will offer less maximum boost but will sound deeper and wont affect the lower mids as much.

Hi Marcin,

here is a link regarding voicing an amplifier for your reference. There is a lot of understandable explanation too.
https://robrobinette.com/Voicing_an_Amp.htm
I also attach an article by Alan Ratcliff which is not online anymore.

The SLO lead channel - and I assume, we're discussing the lead channel here - was never meant to be a tight modern metal sound. Mike Soldano wanted a thick, fluid higain solo sound, which works with a variety of pickup styles and pickup output levels.

The preamp design does not cut too much bass and low mids considering the amounts of distortion you can dial in, especially with the gain knob way high.

I found it helps quite a bit to use low output, PAF-style pickups. This prevents to force too much bass and low mid content through the first gain stages.

I also experimented with circuit details borrowed from the Peavey 5150 II or 6505+. Instead of the 2,2nF/470k stage before the gain pot, it uses just a 470pF cap there (IIRC). This tightens things up, but you may want to have a depth control in the poweramp. I use a 6,8nF cap there, with 4,7nF I found to get too much low mid mud.

Thank You guys! I will try all of it as soon I will get my amp back (it is borrowed now) and will let You know for sure. I'm quite familiar with electronics (graduating electronics designer with quite a few years of experience;)) but designing/calculating/simulating is a easy part but voicing amp is more like magic rather engineering so it is faster and easier to ask someone who already have experience

I think it's a matter of developing you own taste, and maybe a gut feeling, what does what and where in the circuit. I'm doing this for 20 years now, and still learning new things and aspects each time I stick my nose into some piece of audio electronics. 

Also the non-scientific approach is what keeps me fascinated

Yeah, getting amp work is an easy engineering task, You can calculate and measure everything (I'm and electronics engineer with 10+ years of expierience ) but making guitar amp sound good is a completely different, more magic than hard engineering. I mean everyting can be eventually calculated but there is a lots of experience behind what sounds good even tho it is not proper engineering solution. (fe stability of JCM800 )

Alright I did finally some modification.
1. As You stated I played a little with filter after first stage and ended with divider 1Meg/1n with 250k pot.

New vs old filter comparison


Additionally I changed screen resistors on power tubes (2xSovtek 6L6WGC) to 1kOhm and rebiased power tubes
Now I have
B+ = 507V
Ua = 505V
Ic (Iscreen + I plate) = 27mA
Rscreen = 1k
V Rscreen = 1,3V
Bias = 70% (assuming Pmax = 20,5W but I saw different datasheet with 20,5W, 24W and 30W for 6L6)

And here I have some questions about power section and biasing etc. I noticed that colder bias (60%) gives more fluffy and loose bass. What are Your recommendation regarding biasing power tubes and power section in general.

All Best
Marcin

Sorry, for the late reply. Obviously, I'm not here that often these days.

To be honest I don't play around with bias a lot, because I think its impact on tone is often overestimated, especially when we're discussing altering it in 10..20% margins. So I can't give you a sincere opinion or experiences on that 

The 30W PD limit is correct for the original 6L6GC, which is the strongest of the 6L6s/5881s.
This would mean around 36mA plate current for 60% PD.
For other 6L6 types check the manufacturer's datasheet.
PD = plate dissipation.