Trying to understand the tonestack

[Bob-I]

I’ve been fairly inactive here for awhile, mostly because the amps I’ve build are great. I use a 20 watt 1x12 combo high plate non HRM with reverb for most gigs, some larger venues I use a 50 watt 2x12 combo HRM also with reverb.

Recently I’ve been debating the tone stack in these amps. I use a fairly well used tonestack in all of my dumble style amps.

Treble 390pF/.002uF mid boost, 250k
Mid .01uF, 250k
Bass .1uF, 250k or 500k, .001uF bypass and 10k tail
No jazz/rock switch
PAB with 2-22M

I find it’s voiced ok, but fairly ineffective. You have like the voicing because the controls are fairly subtle. I think this is due to the de-coupled mid and bass. I coupled one of my amps just to see if this was the case and the controls are much more effective. However now the mid control, with that big 250k pot, almost acts like a variable boost. From 0-5 it works as expected, from 5-10 there’s a significant boost in gain. I like the increased effectiveness, especially on the treble, and I can reduce the size of the mid pot, but I’d like a better understanding of this tonestack.

I’m curious if anyone here understands the de-coupled mid/bass better than me and explain why it’s so common in dumble’s amps.

Inquiring minds want to know.

[Bob-I]

So I guess this means no one truly understands this tone stack.

I’m also trying to voice the OD channel. I find this amp a bit cold and one dimensional. My other builds are warm and 3D but this one just leaves me a bit flat. Experimenting tends to confuse my ears.

[martin manning]

Well, I agree with you that the D stack is not as wide ranging a the typical Fender TMB, and modeling backs that up, but I always thought that was a good thing since the extreme bass or treble cut settings on the Fender stack aren't very useful IMO. Cold and sterile sounds like something isn't biased correctly. Have you looked at all of the preamp tube cathode resistors and caps carefully?

[Bob-I]

Well, I agree with you that the D stack is not as wide ranging a the typical Fender TMB, and modeling backs that up, but I always thought that was a good thing since the extreme bass or treble cut settings on the Fender stack aren't very useful IMO. Cold and sterile sounds like something isn't biased correctly. Have you looked at all of the preamp tube cathode resistors and caps carefully?

I agree about the extreme settings on the FMV stack, but the skyliner falls a tad short for me.

I’m thinking you may be right about bias on V2. I measured all the resistors but that doesn’t always mean the bias is correct if the voltages are off. That’ll be my next step for sure. I already reduced the size of the bypass caps, 10uF was all I had at the time and it’s just too much. I went to 1uF but I don’t have any 5uF so I’ll pick some up and experiment. But like I said, my ears play tricks on me after awhile.

[dorrisant]

Just a thought... Put a smaller value, like 33k, in series with a 100-250k pot wired as a variable resistor. Install that in place of the slope resistor. You might want to block the DC from the plate... use a 0.1µf for that. You can dial in your slope value by ear or drill a hole for it on a panel somewhere. Lots of flexibility with that.

[Bob-I]

Just a thought... Put a smaller value, like 33k, in series with a 100-250k pot wired as a variable resistor. Install that in place of the slope resistor. You might want to block the DC from the plate... use a 0.1µf for that. You can dial in your slope value by ear or drill a hole for it on a panel somewhere. Lots of flexibility with that.

Great idea. I may just try that without drilling holes first. The slope resistor can make a big difference,

[dorrisant]

Might get away with using a trimpot on your board for a trial. Maybe even leave it there if you find "that sweet spot" setting.

[talbany]

Hey Bob
Good to see you here again. I don't know if this helps you?.hell you might already know this
I had a similar thing going back and forth from a Fender style stack to the Skyliner and took me a while to get used to it but obviously, you have played these amps for quite a while now. The only thing I can tell you is much of the range of the stack is constricted due to the higher value 250K mid pot and since the ground of the mid pot is the ground reference for the stack. The larger the pot value the further from ground you go the less range you get from the stack.(Lift it and you have a Megaboost) Because of this you have to make sure and measure out the values and tapers on both middle and bass pots. Some linear taper pots (especially Alphas) are not so linear, sometimes you have to test the range of the pot on the bench before I install it just to make sure its evenly tapered. The same with the Bass pot. The higher the value the further from ground you go. (You can, of course, compensate this by changing the 10k tail resistor). If you think the later Skyline has a crappy range! try the earlier 2nd generations stack with the 1M bass and 1M treble and 250K pot with the funky 2 cap James configuration across the bass lugs. That thing gave you hardly any range but if you set everything 12 Oclock it sounded great with both singles and buckers no need to tweak it weird

My guess here would be that Dumble liked the 500Hz bump the 250K gave him but in doing so had to sacrifice the stacks overall range. Also plate driven stacks are not as efficient as say one that is cathode follower driven ala Marshall.

BTW. A picture of the amp with what component types/brands you used might contribute to the sterile sound. Do you have a gut shot handy?
Tony

[norburybrook]

I've found that the amps are so well balanced you don't actually need to mess with the tone stack that much. Only thing I ever seem to do is remove some bass depending on the situation, or flip on the bright/mid or Jazz switch to taste.


If you look at Robben Ford or Larry carlton's amps they almost always have the tone stack set mid way with the bass rolled off a little.

Here's Larry's settings on my amp.

IMG_20181004_235639.jpg

M

[Bob-I]

BTW. A picture of the amp with what component types/brands you used might contribute to the sterile sound. Do you have a gut shot handy?
Tony

I agree with that 100%, however I'm using components from the same stash as several other builds. I know the MPP caps are not highly thought of, but I've had good success with them.

I measured the preamp voltages...

V1
P1=178 C1=1.75 ...Rp=220K Rk=3.3K/10uF
P2=181 C2=1.84 ...Rp=150K Rk=2.2k/10uF


V2
P1=196 C1=1.51 ...Rp=220K Rk=3.3K/1uF
P2=201 C2=1.6 ...Rp=150K Rk=2.2/1uF

[Bob-I]

I've found that the amps are so well balanced you don't actually need to mess with the tone stack that much. Only thing I ever seem to do is remove some bass depending on the situation, or flip on the bright/mid or Jazz switch to taste.


If you look at Robben Ford or Larry carlton's amps they almost always have the tone stack set mid way with the bass rolled off a little.

Here's Larry's settings on my amp.

IMG_20181004_235639.jpg


M

I agree. Like I said, I'm trying to understand the tone stack, pretty much just trying to learn. In actual practice I pretty much just set it at nickles and rock.

[talbany]

BTW. A picture of the amp with what component types/brands you used might contribute to the sterile sound. Do you have a gut shot handy?
Tony

I agree with that 100%, however I'm using components from the same stash as several other builds. I know the MPP caps are not highly thought of, but I've had good success with them.

I measured the preamp voltages...

V1
P1=178 C1=1.75 ...Rp=220K Rk=3.3K/10uF
P2=181 C2=1.84 ...Rp=150K Rk=2.2k/10uF


V2
P1=196 C1=1.51 ...Rp=220K Rk=3.3K/1uF
P2=201 C2=1.6 ...Rp=150K Rk=2.2/1uF

Bob
So here are a few things that come to mind looking at the shots.
1st.. I don't think I would necessarily correlate a thin sounding/harsh or sterile amp directly to the tone stack or a problem with the stack unless something was either not wired correctly or I have a poor or fractured solder connection (which I've found generally happens on the little toggle switch connections)
2.. An oscillation of some kind can cause the amp to do it. It's hard to gauge the depth in photos but I see a plate wire run directly under the 180k grid resistor?. The other thing I see is you have the Presence wires (looks to be) tied to the AC wires powering your relays. The presence wires are connected to your PI and carry signal directly off your OPT. I would separate these wires from one another any other wires since they are out of phase with the rest of the amp. (Of course, an oscillation can come from anywhere and permeate throughout the entire amp)
3rd. Are the famous Brown Dale RN65 pate resistors. I know these were the resistors everyone used here back in the day, however these sound absolutely "nothing" like the diamond cut 1975 RN Dales Dumble used in his early ODS's. The new Dales can contribute to making the amp sound harsh on the top end in either plates or signal path.
4. The Xicon carbon films you used sound great in a clean amp but I've found in an amp with OD can color the sound with some 3rd order harmonics. They work well in an aggressive sounding amp if that's your thang (Like a Trainwreck) but I would not consider them a smooth sounding resistor as per Dumble used.
5. The MPP's you have in there are fine and I don't think they are causing your problem. They have a nice tight pleasing low mids about them that work great in the PI and in the OD section.
6. I would replace the 180K OD entrance resistor with a stock 220K CF
7. Sometimes a poor or corroded switching loop jack can cause that problem. jump them out and see if it helps.
8. Your voltages on V1 and V2 look good although it looks like the V1 tube is drawing a bit more current but that shouldn't be a problem
9. Did you check your bias on the output tubes sound like they might be running cold or badly mismatched?
10. It could also be a phase or a loading issue with your reverb section. I have not built this design so this is purely speculation!!

BTW. I don't think the resistors your using is necessarily alone causing the amp to sound sterile or harsh. Just contributing to it
This is why I only mentioned them.

Hope this helps and good luck!

Tony

[10thTx]

Are the famous Brown Dale RN65 pate resistors. I know these were the resistors everyone used here back in the day, however these sound absolutely "nothing" like the diamond cut 1975 RN Dales Dumble used in his early ODS's. The new Dales can contribute to making the amp sound harsh on the top end in either plates or signal path.

Talbany,

What would be two alternatives to the RN65 resistors that you like better?

With respect, 10thtx

[martin manning]

Bob, if you want to increase the range on the Skyliner stack, decrease the tail resistor on the bass pot to 3k9 (more attenuation of bass and mid), and increase the 10n feeding the mid pot to 22n (more mid scoop). That will give you something closer to Fender BF range. Don't worry a bit about the resistor types ;^)

[Bob-I]

Bob
So here are a few things that come to mind looking at the shots.
1st.. I don't think I would necessarily correlate a thin sounding/harsh or sterile amp directly to the tone stack or a problem with the stack unless something was either not wired correctly or I have a poor or fractured solder connection (which I've found generally happens on the little toggle switch connections)

Yes I agree. I started there but the more I look and listen, this isn't a TS problem at all.

2.. An oscillation of some kind can cause the amp to do it. It's hard to gauge the depth in photos but I see a plate wire run directly under the 180k grid resistor?. The other thing I see is you have the Presence wires (looks to be) tied to the AC wires powering your relays. The presence wires are connected to your PI and carry signal directly off your OPT. I would separate these wires from one another any other wires since they are out of phase with the rest of the amp. (Of course, an oscillation can come from anywhere and permeate throughout the entire amp)

That's an illusion, the relay AC lines are laying on the chassis, the presence and reverb lines are flying above.

Good point on the PO though. I'll pull out the scope and look for it. I did have some PO when I first fired up this amp but solved that with lead dress on V1

3rd. Are the famous Brown Dale RN65 pate resistors. I know these were the resistors everyone used here back in the day, however these sound absolutely "nothing" like the diamond cut 1975 RN Dales Dumble used in his early ODS's. The new Dales can contribute to making the amp sound harsh on the top end in either plates or signal path.

Yea, I drank the koolaide and bought a bunch from mouser years ago. I haven't found them overly harsh and I kinda like the brightness. What would you suggest?

4. The Xicon carbon films you used sound great in a clean amp but I've found in an amp with OD can color the sound with some 3rd order harmonics. They work well in an aggressive sounding amp if that's your thang (Like a Trainwreck) but I would not consider them a smooth sounding resistor as per Dumble used.

Again, what would you suggest?

5. The MPP's you have in there are fine and I don't think they are causing your problem. They have a nice tight pleasing low mids about them that work great in the PI and in the OD section.

I've been debating swapping with 6PS, I may have some around.

6. I would replace the 180K OD entrance resistor with a stock 220K CF

Will do

7. Sometimes a poor or corroded switching loop jack can cause that problem. jump them out and see if it helps.

Check

8. Your voltages on V1 and V2 look good although it looks like the V1 tube is drawing a bit more current but that shouldn't be a problem
9. Did you check your bias on the output tubes sound like they might be running cold or badly mismatched?

Yes, this is a 6V6 amp and I've got it biased at 75%, matched within about 2ma, and I've tried another set as well. I may go to 6L6s, the OT and PT can handle them.

10. It could also be a phase or a loading issue with your reverb section. I have not built this design so this is purely speculation!!

I don't think so, I've used this single tube reverb in several builds with no phasing issues.

BTW. I don't think the resistors your using is necessarily alone causing the amp to sound sterile or harsh. Just contributing to it
This is why I only mentioned them.

Hope this helps and good luck!

Tony

I'm beginning to think this is the every challenging PO issue. It had me running in circles in the beginning, but that was audible. Out comes the scope and the search

Yes, it's a big help and I appreciate your time.