Tightening low end/adjusting tone - Weber 6S100

[BobL]

Hi All -


I just built up a Weber 6S100 bass head (Sunn based, KT88s) to get my bass player off of his solid state rig.  At home I was feeling like it was sounding really good with my Jazz Bass, but ran into issues when I brought it to practice and he plugged into it tonight.  I've replaced dang near everything of consequence from the kit with higher quality stuff after being unimpressed with a lot of it - all pots are alphas now, tubes are EH KT88s and EF86, TAD 12AU7 in V3 and 5751 in V1, filter caps are JJs.


Low end was really farty and with a harsh lower midrange - couldn't get round, edge of breakup nice bass sounds like I was getting at home.


His cab is an Ampeg classic (maybe 6 speakers? it's similar to the 410 I have at home, but has the casters like an 810 fridge and seems taller - his is 4ohms, while mine is 8), so not dissimilar to what I'm running, but not exactly the same.


His bass has a great big (seymour duncan?) humbucker in it, and I think that might be part of the problem - that he's hitting the front end of the amp too hard.  He put a Hartke bass preamp pedal in front of it when we weren't able to get any good sounds, and that really seemed to tame things - added some compression and probably reduced signal to the front end of the amp?  It did much better with that added in... but...


I'm looking to maybe make some adjustments so it gets along with his bass in addition to mine.


First thought would be to replace V1 with a lower gain tube - I have a 5781 in there now.


Also wondering if adding additional filtering might help. 


Already has a solid state rectifier (this copper tube socket mounted weber dealie).


Would reducing the value of C9 to .047 or .022 help, maybe?

Wondering what ideas folks have... I'm too inexperienced to have a good idea of what all is actually doing what.


Schematic here: https://www.tedweber.com/media/kits/6s100_schem.jpg

[SnickSound]

Reducing C9 might help, and even 0.022uF would still leave most of the roll off outside of the important frequencies for bass.

I notice that the "Pull Boost" bypassed the NFB loop, that can't help, don't use that. In fact, you might want to try increasing the NFB by reducing R4. Before you warm up the soldering iron you can use alligator clips to parallel another 4.7k to see if you like it better at 2.35k. Then try lower and lower. More NFB tightens up the bass response of the power amp, a lot.

As for filtering, yeah... effectively a mere 50uF for a 100W amp used for bass is not a lot (unless that's the sound one is going for). I'd at least double that.

Perhaps reduce C17 and C18 too, but lower than 0.047uF would start eating into that low E string.

[sluckey]

The 6S100 has a guitar preamp, not a bass preamp. I would put a real Sunn bass preamp in it and see if it works better. Would not take much work to mod to the Sunn 200S preamp...

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_200s2.pdf

[MFowler]

I would change to 6550 tubes those KT88's are sensitive to screen voltage and the weber circuit doesn't lower it down much.  The Sunn was using ultra linear OT as well.


The KT88 grids are 220k/220k should be 150k/150k where the Sunn uses 100k/100K


Tone stack seems fine both have 56k slope resistor.


Mark

[BobL]

The 6S100 has a guitar preamp, not a bass preamp. I would put a real Sunn bass preamp in it and see if it works better. Would not take much work to mod to the Sunn 200S preamp...     https://el34world.com/charts/Schematics/files/Sunn/Sunn_200s2.pdf

Looking at the 200S schematic:


We're starting to stretch my comprehension there, but trying to follow along... this is good for me.


It looks like for starters, the connections on V1 are reversed in the Sunn vs. Weber schematics - e.g. 1 is 6, 2 is 7, 3 is 8. Assuming that is of no consequence, just opposite sides of the same thing.


I'm always hazy on what makes up the preamp section in these - where things start/end, but clearly dealing with the upper left quadrant - everything left of the KT88s is considered preamp, I guess?


I also start getting a bit lost since the controls on the two circuits are different - e.g. no master, mid or presence on the Sunn - and also because the Sunn is just using the two preamp tubes...


I have no Lo/Hi boosts, so if am I understanding correctly I would just be adding a resistor in both locations - (10M and .0075uF on the lo boost, and 22k resistor on the Hi?).


The other differences that are jumping out - my C9 is .1uF - they show a combination of .0075uf and .002uf, though the connection order is slightly different (does this actually matter? If not why? 


Mine, I show V1:1-> .1uf, splitting after to the pot and the 470pf off of V1:7.  The Sunn I show V1:6, split with .002uF to pin 7 and .007uF to 10M to the pot...


Either way, the change in values from .1uf to the .002 and .007 seems quite large...


Once past that, other than the grids from the KT88s mentioned by MFowler, I get confused about what all is happening with the single 6AN8 vs the EF86 + 12AU7.


What are the differences between the bass/guitar pres that I would be trying to change? Trying to understand, but reaching...

[Willabe]

Once past that, other than the grids from the KT88s mentioned by MFowler, I get confused about what all is happening with the single 6AN8 vs the EF86 + 12AU7.

In the Weber schemo, the 12AU7, that feeds the power tubes, is the PI (phase inverter). The 2 half's of the 12AU7 are wired in parallel for more current/drive to the power tubes. The EF86 is the PI driver. (And tone stack recovery.) The EF86 can sound different than a 12AX7, more grit. Everything before that EF86 is the preamp.

The Sunn bass amp schemo, the 2nd half (triode) of the 6AN8, that feeds the power tubes, is the PI, the 1st half is it's driver (small bottle pentode). Everything before that 6AN8 PI driver pentode is the preamp.

The PI in both amps is a split load PI.

[sluckey]

You only need to change the preamp circuit. No need to bother anything to the right of the wiper of the treble pot. Don't let the tube pin numbers bother you. I've played through both of these preamps and the Sunn 200S is definitely voiced for a bass. Here are the two preamps side by side so you can more easily see the similarities and the differences...

[Willabe]

I have no Lo/Hi boosts, so if am I understanding correctly I would just be adding a resistor in both locations - (10M and .0075uF on the lo boost, .....).
Mine, I show V1:1-> .1uf, splitting after to the pot and the 470pf off of V1:7.

The 10M on the lo boost switch is so the switch doesn't pop when closed.

On the Weber the 470pF on the volume pot is just a treble bypass cap.

[BobL]

You only need to change the preamp circuit. No need to bother anything to the right of the wiper of the treble pot. Don't let the tube pin numbers bother you. I've played through both of these preamps and the Sunn 200S is definitely voiced for a bass. Here are the two preamps side by side so you can more easily see the similarities and the differences...

Comedy - I was making literally an identical image and just came here to post it...


The differences I can identify seem very, very minor...





So, my understanding is:


1. The Sunn effectively greatly reduces the value of the C9 cap (down to like... 2% of the .1uF?) - I'm not sure what type of capacitor(s) I would want to use here?
2. C7 changes from the 250pF I have to a 470pF cap.  This seems like a small change?
3. Because the Sunn doesn't have a mid control, that 22K resistor is effectively setting the mid to a fixed value in place of the 25K pot, and the Hi boost would just bypass this?


Is that literally all we're talking about?


Of note are some changes from the Weber values that I made when trouble shooting some negative feedback issues I had, which I thought were close enough to probably not matter, but... maybe they are?


The 250 electrolytic, C8, I currently have a 220uf there.


The 250pf (C7) is currently a 220pf.


Surely I'm missing important things here?

[BobL]

Well, and the pot values - I'm not sure how that plays in other than giving greater granularity when adjusting, or how much the master has a negative impact on all of this...

[Willabe]

Don't forget the 47K and 0.0075uF cap to the right of the bass pot.

I don't understand how the changes in the Sunn preamp work. Their not common in guitar/bass amp models.

But, I know that Sluckey has built many amps and has helped dozens and dozens of guys here over many years, voice/revoice their amp.

More so, he has played through both of these preamps, I'd take it to the bank that the Sunn has more bottom end. 

[BobL]

Ah yeah, and a 22k resistor in there too... and the .022uF going to the wiper on the Mids vs the wiper on the Bass... Trying to picture how that all would fit together...

[Willabe]

The 22K R under the bass pot, you have it right, is setting the mids.

[BobL]

Would this be correct?  I wasn't quite sure on what would need to happen with the mid/bass controls, but this was my guess.





Would I want to replace pots with the values from the 200S schematic as part of this?

[sluckey]

If you want 3 tone controls look at the Sunn 2000S. It's a BIG bass amp.

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_2000s.pdf

[BobL]

Interesting... so I could just use that preamp in my existing one and things should work out...  that looks fairly straight forward.


If I'm eliminating the bass boost (though I guess I could use my current switch for boost here instead), would I just run both the .0075 and .002 together? Is a .0095uF a thing?


What type of cap do I look for for those tiny values?


Would I keep my inputs as is, with the 68K resistors to V1:2?

[sluckey]

I would build it exactly as the schematic shows, including any switches. Pots have to be the correct values too. I'm not picky about cap types or brands.

[BobL]

Ok... so I think it would look like this once laid out, but with the switching bit on the right in place of the double capacitor crud ('This can't be right')


What have I gotten wrong? Some thing(s), I'm sure. :)


I'm guessing I'd put the stuff off of pin 8 on the board - am I reading it right on the 2000s schematic - that cap from pin 8 is .22? Hard to tell, exactly.


I'm also confused about the differences on the inputs... mine only has two...

EDIT: Removed Image - see updated below.

[sluckey]

Bass pot is not quite right. Should only be using two lugs. Speaking of Bass pot, I would consider using a very common 1M pot with push/pull switch for Boost.

[BobL]

Bass pot is not quite right. Should only be using two lugs.

For the 200S?

200S uses all 3 lugs on the bass pot.   

2000S - I was actually not entirely sure on that one - I thought the break at the end of that pot in the schematic was possibly a photocopy error.


I have one of those push/pull pots, but I don't really care for them, which is why in the original build I used a little carling switch for the boost - I can use that for the bass boost as well, I think.  I guess a guy could wire a footswitch into that if he cared to, eh? Use a shorting switch so when the footswitch isn't plugged in the switch on the amp is active, but when plugged in, it bypasses the switch and uses the footswitch?


I made some revisions, below - am I on the right track?

[BobL]

Tapers would be Audio for the Vol and Bass pots, and Linear for the contour/treble, yeah?


And I could keep the Presence control as is, I'd imagine...


I'm also not sure if I've got left/right lugs on the pots correct?

[Willabe]

2000S - .....

I thought you were still doing the 200S, I then figured out you went to the 2000S.

So, I took my post down.

 

[BobL]

Seems that these are some harder capacitor values to find.


I see old stock .0075uF caps on ebay... again, not sure what composition would be appropriate?


Looks like .005uF, .001uF, .002uF and .22uF can be had in Orange Drops.  They have .0068uF as well, which seems close to .0075uF - maybe not close enough?


I also am not finding 10M resistors in my usual places - they go up to 8.2M - is that close enough for that switch?


Speaking of .22: is that a .22uF coming off of pin 8, or a 22uF? It is hard to read on the schematic - I think it says .22?

[SnickSound]

Seems that these are some harder capacitor values to find.


I see old stock .0075uF caps on ebay... again, not sure what composition would be appropriate?


Looks like .005uF, .001uF, .002uF and .22uF can be had in Orange Drops.  They have .0068uF as well, which seems close to .0075uF - maybe not close enough?


I also am not finding 10M resistors in my usual places - they go up to 8.2M - is that close enough for that switch?


Speaking of .22: is that a .22uF coming off of pin 8, or a 22uF? It is hard to read on the schematic - I think it says .22?

Don't worry too much about getting the exact value, given the high tolerance of these parts. 0.0068uF or 0.0075uF is a 10% difference ... if you can hear the difference I have a dog whistle to sell you :D

As for those 10M resistors, they're just there so the caps can discharge even when not engaged without bringing down the impedance of the circuit. 8.2M will do.

And yeah, I read that as a .22
A 22uF there would ensure full gain at all frequencies, a .22uF (or 220nF if you will)  will limit voltage gain under 485Hz. It's a nice way of rolling off some low-end to tighten things up without actually killing it.

[BobL]

Don't worry too much about getting the exact value, given the high tolerance of these parts. 0.0068uF or 0.0075uF is a 10% difference ... if you can hear the difference I have a dog whistle to sell you :D

As for those 10M resistors, they're just there so the caps can discharge even when not engaged without bringing down the impedance of the circuit. 8.2M will do.

And yeah, I read that as a .22
A 22uF there would ensure full gain at all frequencies, a .22uF (or 220nF if you will)  will limit voltage gain under 485Hz. It's a nice way of rolling off some low-end to tighten things up without actually killing it.

Thanks!


Oh, and 10M just wasn't in the list by size like the rest... so I actually found that one.

[BobL]

Oh, and about those resistors on the inputs - I'm totally in the woods there - other amps I've made w/ 2 inputs have a 68K resistor there, as does the 200s, but the 2000s has 4 inputs and 47K, plus the 270K and .001uF on the other two... If I'm only going to have the two inputs, do I need to still switch the values to 47K, and have that 150K to ground instead of the 1M to ground that I currently have?

[sluckey]

Use your 1M and two 68Ks.

[BobL]

Does that layout look like I've got it right?  The one thing I notice is that I could probably move that 56K resistor all the way on the right over horizontally to the left of the .022 it connects to, and run a single under-board wire from the .001uF cap to it to clean it up a bit.


Mostly worried about making sure I have the circuit right.


How do you tell left/right lugs on a pot in the schematic? Orient yourself 'facing' the wiper and then left/right? That was how I was viewing it...

[sluckey]

I 'think' they are right.

[BobL]

Appreciated!

[BobL]

Finally got parts (minus the 500K pot for the Volume, which USPS was kind enough to destroy by walking all over the package, so I've got the 1M pot in there still until a replacement arrives), and had some time today to rip out the old preamp and build the 2000S preamp into this.


It works, which is good, and my initial impression is that it sounds really good with a lot of low end, and seems possibly louder, which was a bit surprising - maybe it's just less choked?


Will try it w/ the band on Tuesday or Wednesday, but initial impressions are good - thanks for all the input!

[BobL]

Ok... bass player played with the band through this last night - huge improvement over where we were, but thinking some adjustments still need to be made.


Whereas before the lows were farting out and loose, they are sounding really nice, however there are a LOT of mids and high end, which means we had the middle control turned all the way down and were still getting a lot of high mids coming through, and he needed to pull the tone on the bass itself off quite a bit to smooth things out.  This is workable (this was w/ a bass with combo J and P pickups - the humbucker bass was even heavier on the mids - very Rickenbacker sounding, which is cool, but not really what we're after), but it'd be nice to have a more useable mid control.


I have put that 500K pot in the volume, which I think pulled some highs...


I'm wondering what might be some good things to try to adjust those controls - it seems like I'd want to adjust both the treble range, and pull my starting point for the mids down - I'm not really sure what frequencies would be affected by each section of the tone controls.