Where best to post about diff between Fender and Dumble?

[imo1]

Regarding cathode bypass caps, I’ve messed with that before a bit and the difference was subtle to me. I know this is a much bigger deal with higher gain amps. @whopperplate, I am very aware of the difference the caps” values make, especially coupling. I actively switch those around in my builds. What I was hoping to hear was some thought on experience with cathode bypass, as I haven’t messed with that as much and most of my thought on it is theoretical, based on frequency shifting.

As I already mentioned , 5uf across the board tightens bass response . For reference , far less extreme than a Marshall bright channel with .68 bypassing each cathode and . Here’s a quick and dirty calculator to give you more understanding.

https://www.ampbooks.com/mobile/amplifi ... alculator/

Woke up sick this morning, so not able to do much other than read and comment. I'm interested to dig into this some more, so don't take it as a disagreement, but rather an inquiry.

Yes. Those calculators are good. Thankyou for the link. I always forget about them.

The reason I asked you is that you brought up, I think, earlier in the thread, that the cathode caps, as used in fenders, had kind of a different effect on the bass beyond just adding more, which intrigued me. What I've been trying to get at is what that would be. You were insinuating that they aren't necessarily adding more bass but changing the way it reacts. That is where the technical knowledge, at least what I have taken in, starts to fall away, and understanding the reactance to how it responds to the guitar, and that murky area within, becomes front and center.

When I think of the main Dumble designs, I think of powerful, clear bass. That, to me, is the biggest difference I hear and feel when I play his designs, and amps inspired by him, especially in comparison to Fender designs, which often fall apart when you try to boost the bass too much. When I played my Super reverb, I often had the bass on 2-3, because much more that that would just cause things to flub out. With the Dumbles, it seems that it is power(wattage) and filtering that do most of that lifting. I feel like I understand those really well. I have enough experience with filtering and PT's( either too big or too small), to have a pretty good feel how those both effect low end. As well, the coupling caps seem to work in a pretty intuitive way. On the deluxe build i just finished, I greatly reduced the coupling cap that followed a modified marshall gain pre-amp section. I loved the way it tightened up the low end in a way that translated to more of a gainy design.

Its gonna take some time to understand better how both the cathode caps and the neg feedback work. I know I tend to like amps without the localized neg feedback. I think I'm attracted to the "hair" on the sounds, as opposed to the super clean(tight) feel that I associate it with. To me, at this point, neg feedback seems to make it harder to play legato on the amp. It seems that when the first stage is heavily treated that the notes tend to be kind of clanky. I could see this being good for super clean, chordal stuff, but when you are playing runs of notes, it makes it sound less fluid. My test here is to play the guitar acoustically and then plugged in. If it sounds more disparate than the natural sound of the guitar, to me its doing a disservice.

I'll try tacking in some caps. I'm always a little uncomfortable using clips in an amp with high voltage. Easy to do on the tone stack, but the cathode caps are sitting really close to some high voltage areas and i would hate for them to slip!

[WhopperPlate]

25 uf vs 5 uf . Fender vs dumble . 25 uf increases bass response . Too much and it gets flubby , like fender past 2-3 on bass. Insert the values into the calculator to see it on the graph . Marshall and Dumble decrease this value for more midrange and less bass . Helps tighten the sound . Tight doesn’t mean more bass. Tight means less flub.

Dumble also has increased power filtering , this also helps tighten bass , less sag more dynamics . All of this has been discussed ad nauseam and can be found with careful use of the search engines.

[WhopperPlate]

I'll try tacking in some caps. I'm always a little uncomfortable using clips in an amp with high voltage. Easy to do on the tone stack, but the cathode caps are sitting really close to some high voltage areas and i would hate for them to slip!

You can simply clip them on the board , no need to play operation on the tube socket . Plenty of space for safety , just make sure your clips are properly insulated

[imo1]

25 uf vs 5 uf . Fender vs dumble . 25 uf increases bass response . Too much and it gets flubby , like fender past 2-3 on bass. Insert the values into the calculator to see it on the graph . Marshall and Dumble decrease this value for more midrange and less bass . Helps tighten the sound . Tight doesn’t mean more bass. Tight means less flub.

Dumble also has increased power filtering , this also helps tighten bass , less sag more dynamics . All of this has been discussed ad nauseam and can be found with careful use of the search engines.

Yes. To me its mostly the increased power filtering that defines these beasts. I think that is a HUGE part of the sound. Again, I understand the basics of all of the stuff discussed here. I am trying to bridge a gap that I understand, and possibly is getting lost in translation. I don't want to be rude or dismissive in any way, and I am deeply respectful and awed by many people's knowledge of circuitry and building. I am trying to find a path that is maximizing the circuit for players like myself, and I think it has a value that I haven't seen much on the forum. I apologize if my angle is frustrating you and appears flippant.

I have read a majority of what is up here. I am not posting out of a lack of time reading, searching or researching. I am looking for something beyond what I've found, which is why I'm posting this topic. Please know that my intention is sincere, and I know it has value. It's what I've been looking for in other topics and posts and I'm assuming there will be others that will understand as well.

Cool?

[imo1]

So, here is what I've found with a short testing session. I will say as a caveat, that when I am testing an amp build, I really need to have it in a bunch of different rooms. What works in a bedroom often doesn't translate to a club, and bigger venues, etc will really change how an amp responds.
I tried the tone stack mod, the bypass of the negative feedback and the cathode cap on the first stage.

-The biggest immediate improvement to my ears was bypassing the negative feedback on V1. To my ears this makes the amp sound more open. There is more gain, and typically we react to higher gain in a positive, but i think beyond that the amp sounds sweeter. To my ears the negative feedback on V1 reminds me a little of that Supertwin sound that I never could vibe with.

-The tone stack mod shifted the frequency in a way that was familiar with me..It bit more, which really was nice with a litte bit of a boost doing the Albert King big bend thing. The tone stack as it is, sounds a bit more Claptony. All of those great recorded Fender sounds are super bright coming out of the speaker. The Austin trick(I attribute to Richard Mullen) was always to put the 57 at an angle on the cone so you get that gain in the treble without it totally taking your ear off. I don't know if you ever stood in front of Stevie's amps, but they were painfully bright. Thats how he got that intensity that translated so well to tape and live. The plexiglass was needed.
I liked the sound with the bass pot shorted to the top of the mids, but found the lower mids to be a little hollow sounding if I was playing a lead with my "clean sound". Because the amp has so much bass and such a defined treble sound, I find that I'm always going back and forth with the mids. For chords, especially anything with some dissonance like the Hendrix E7 chord, I like the mids paired back a bit, but as soon as i start doing single note stuff, I find it gets a little hollow on the lower strings and I find myself wanting to boost the mids. I would say that as it stands, I probably prefer the tone stack as is, though I feel like I need a littld back and forth.

-cathode cap- I found that(again with a quick test) the most impactful spot to increase it(15U in parallel for 20U total) was on V1B, after the volume pot. It was pretty subtle with everything else in the amp. I could go either way, but that would be where I would increase it if I did.

the other thing i did, a few weeks back, was to increase the 120k mixer to 220k(where it meets with the 68k before the master). I like that change for this amp

[imo1]

25 uf vs 5 uf . Fender vs dumble . 25 uf increases bass response . Too much and it gets flubby , like fender past 2-3 on bass. Insert the values into the calculator to see it on the graph . Marshall and Dumble decrease this value for more midrange and less bass . Helps tighten the sound . Tight doesn’t mean more bass. Tight means less flub.

Dumble also has increased power filtering , this also helps tighten bass , less sag more dynamics . All of this has been discussed ad nauseam and can be found with careful use of the search engines.

It’s funny, but trying to use adjectives is tricky at a certain level. I would say that fender bass in general sounds “round” but most of the designs get flubby at a certain point. To me, the big Dumble bass, especially with the deep switch sounds huge, but somehow not as round?? Maybe because the frequency goes so low. It’s like in mixing a kick drum where you will go up 200 or 400% to get the harmonic of the initial kick hit, and that will often be the dominant kick frequency in the final mix.
I’m tempted to order a bunch of 15u caps(I’m out) and see if it would get flubby or if it would hold together. As much as a love the deep switch, I have found that for live use it’s just too much. Hmm

[martin manning]

Great, thanks for the report. So you would say that the bass-middle short shifted the overall tone in the Fender direction? If so, that supports the hypothesis that you were missing the deeper mid scoop. Seems like this experiment went a bit too far, so instead of a jumper try a 10k resistor and see how that sounds.

And to be clear, the middle pot is wired as a variable resistor, and you may or may not have the wiper shorted to the top (CW) lug. If not, this shorting jumper or resistor would go to the wiper.

[imo1]

Great, thanks for the report. So you would say that the bass-middle short shifted the overall tone in the Fender direction? If so, that supports the hypothesis that you were missing the deeper mid scoop. Seems like this experiment went a bit too far, so instead of a jumper try a 10k resistor and see how that sounds.

And to be clear, the middle pot is wired as a variable resistor, and you may or may not have the wiper shorted to the top (CW) lug. If not, this shorting jumper or resistor would go to the wiper.

I would say so, but because of the enormous disparity with bass with this design, when you shift away from that mid forward sound I find there is a hollow spot in the low mids that I don’t find in fender designs. I hear it clearest with single note runs. The A and D strings any where around the 5-7th frets don’t have the same quality. It makes the sound a little uneven.

I think this amp design probably sounds best with that general mid forward sound. Again, it’s an amazing sound, just that if I were building the perfect rig for myself, I would probably mix it with a super reverb or something.

I’ll mess around with the 10k. It’s really interesting to see where you can push the design when you are really testing its limits. I do think the amp is perfectly designed for Mayer, and though he mixes his amps with others live, his style would fit on this one amp.

[WhopperPlate]

I have read a majority of what is up here. I am not posting out of a lack of time reading, searching or researching. I am looking for something beyond what I've found, which is why I'm posting this topic. Please know that my intention is sincere, and I know it has value. It's what I've been looking for in other topics and posts and I'm assuming there will be others that will understand as well.

Cool?

Of course everything is cool , no one thinks you are flippant , but I am just talking to you as if I was talking to myself learning. I can still find old posts on the internet where I was running around like a chicken without a head, and in hindsight the answers I wanted were available with just a bit more patience and reading.

Dumble circuits are far from perfect for my own tastes , and I tend to agree with most of your preferences described. just increasing the cathode caps and removing the local nfb on v1 gets me closer to where I want, that’s with the modified classic tone stack. Don’t really like the typical sky liner, I prefer the classic low plate 250pish treble 100k .05 .1 bass mid.

I think you might also like the older generation dumble preamps .

[imo1]

Probably not Martin, though there seems (or should I say is for clarity) to be some common ground between different types of stacks Dumble used. #94 has a stack that I would define as a classic stack, but with a 150k slope resistor. Might be half an update to a Skyliner stack? Who knows? Maybe Had built it that way and couldn't care less about what name we would apply to that stack.

But it is beside the point really, IMHO. imo1 made the point that Dumble's and derived designs do not do the Fender Funky stuff that well and they have got a lot of low end. I read his posts a bit like: I like the Dumble amps, but is there a way to make them suit my style of playing without giving up the Dumble goods? If I understand him correct that would mean a bit less low end, a bit more scoop, a little less direct/more give in the feel of the amp. Please correct me where I misunderstand.

One way to do this would be, IMHO, bring the stack more to Fender values, but also keep Dumble possibilities. One way I tried to do that is lower the treble cap a bit to 330pF so with the mid boost cap you land more in the direction of 250pF when the mid boost is off. Also tried a 100k logarithmic pot instead of linear in a Dumble stack. 250k J-taper for treble, 250k log for bass, other caps 100n and 47n, 100k slope. With the mids on 3 this gets very close to standard Fender stack like in a AB763 Vibrolux.

Off course it means less mids going to the OD section when you do not use PAB. So it's a compromise. Unless you turn the mids up to 7 or 8. Then it does the same thing the Dumble stack would normally do.

Another tweak point would be the grid stopper on V1b. I haven't experimented a whole lot with it, but maybe Martin can comment on the effect of that mod. I'd think the 220k grid stopper would drastically alter the overdrive character of V1b. Before overdriving V1b the 220k/470p combo is pretty transparant; there is not much alteration of the frequency response.

Next Fenders typically use 25uF bypass caps, where Dumble's and derivatives use 5uF. This should give less low end. I think in the preamp Fenders have more low end than Dumbles, at least the components and tone stack should lead to that.

Increasing your slope resistor a bit to 150k like in ODS #94 would also give a bit less low end. Something like 2 or 3dB. Not much but very noticeable.

So if you want less low end from your Dumble and make it a bit more Fender and Funky, these are the points where I would look. Changing that mid pot to log goes a long way and is where I'd start. Attached is plot comparing both stacks/preamps. Mids at 3 in this simulation.

@rootz I must have been traveling when you posted. I didn't see it. My apologies. I appreciate your post. I am using the Amplified Nation JM100 as my source layout. It the midboost is off, doesn't that put the .001 in series with the 330pF, and bring the "treble cap" value right to 250p? With it engaged, you have the .001 going directly to the treble pot high side. As far as pot type, for now I'm willing to move it around just to see if I can get in the right area before I sub that out. I don't have a PAB circuit on this build. Its a simple circuit that I use gain pedals in front of for any additional gain

[imo1]

I have read a majority of what is up here. I am not posting out of a lack of time reading, searching or researching. I am looking for something beyond what I've found, which is why I'm posting this topic. Please know that my intention is sincere, and I know it has value. It's what I've been looking for in other topics and posts and I'm assuming there will be others that will understand as well.

Cool?

Of course everything is cool , no one thinks you are flippant , but I am just talking to you as if I was talking to myself learning. I can still find old posts on the internet where I was running around like a chicken without a head, and in hindsight the answers I wanted were available with just a bit more patience and reading.

Dumble circuits are far from perfect for my own tastes , and I tend to agree with most of your preferences described. just increasing the cathode caps and removing the local nfb on v1 gets me closer to where I want, that’s with the modified classic tone stack. Don’t really like the typical sky liner, I prefer the classic low plate 250pish treble 100k .05 .1 bass mid.

I think you might also like the older generation dumble preamps .

I may try upping the cathode caps to see. Thankfully I can just tack them on and take em off if I don't like them. I found a few axial 15u so maybe I will try that tomorrow.

I have a nice studio with sound treatment, but as I said earlier, i never feel like I can really get into an amp with a solid impression until Ive had it on the road in a bunch of different scenarious. So many things will sound good in one environment, and then you change it and everything falls apart. To me, the mark of a really good amp is one that consistently sounds good in all kinds of different environments, so I always take changes and my opinions of them with a grain of salt. I like to rough stuff in, and then live with it for a while, before making a final decision. The tricky thing for me is that i live in the Seattle area and my touring rig is in Austin. I do have a studio there so I can make changes if needed, but I'm always in a hurry and don't have all my tools, so I kind of need to have whatever choices I make dialed in.

My touring amp right now is a 50w version. It's crazy how loud my rig was when it was 100w. Im playing theaters and pretty big rooms. Its my gig, so I don't have to keep it down for a singer, and people are coming largely for the guitar, and even with all that, I'm always struggling to not kill the crowd and piss off the soundman. I carry a plexi shield and have an attenuator. When I play up here in the PNW i don't even bring these amps out. My vibrolux is way too loud for most of the rooms up here. Its crazy! Really hard to be a guitar player playing tube amps and doing the big guitar thing.

[imo1]

Great, thanks for the report. So you would say that the bass-middle short shifted the overall tone in the Fender direction? If so, that supports the hypothesis that you were missing the deeper mid scoop. Seems like this experiment went a bit too far, so instead of a jumper try a 10k resistor and see how that sounds.

And to be clear, the middle pot is wired as a variable resistor, and you may or may not have the wiper shorted to the top (CW) lug. If not, this shorting jumper or resistor would go to the wiper.

Well, turns out the cold is covid, so I'm isolated for a bit. Good news is I feel ok and can spend the time messing with the amp.

I really appreciate all of this! I feel pretty confident with the blocks of fender design. Feel like I understand what they do. I sometimes get tripped up with all of the Dumble amendments, especially around the tone stack.

So, with the prior tone stack experiment, if I were to try to emulate a Fender tone stack, with mid 100k pot, if I had it at 10k I would be at the same place as the Fender tone stack with mid maxed out?

I know I keep stabbing at this, but this is the area where the technical, data driven stuff isn't getting me there. I'm sure there is a way to do all of this through technical study, but I don't see it. My theory is that because the circuit, due to its much larger filtering, and usage of its power(which exponentially takes more for low end emphasis) which has so much more low end than a Fender design, it shifts the balance of the tone stack, so a traditional Fender stack just doesn't work as well. I mainly hear this in the mids. When you scoop them like a Fender design, it messes up the evenness of the gain across the EQ. This is why when I'm playing single note runs I feel a hollowness in the low mids. It's like in a recording where there is too much 50h and 100h for example. It will stop the other frequencies from coming out. Low end is like pacman. It eats everything up if its not carefully tended.

I am gonna keep experimenting with this. I'll try the 10k and see what that does. I'm also gonna tack in all the 15u on the cathodes(outside of the reverb driver).

[imo1]

Allright, allright, allright.

So, the cathode caps do a really cool thing(at least to me) with the low end! I could see why Dumble would try to keep them tight, because they are looser, but because of the filtering(at least in my mind) and the speaker, they are able to hold together way better.

I approach amps and playing more from a single channel vibe. I tried early on to go the EJ multi amp route but it was too planned out, and I felt shifted the playing to a more technical approach. To that regard, my clean sound isn't clean by the more "rock" way, but it is how many of my peers and I approach it. If I play lightly and/or turn down the volume 2 clicks, I have that sound..This is getting me pretty damn close to where I'm wanting to go.

I also tried the 10k but, as I supposed in the earlier post, the mids start to sound anemic when you pull them back more.

Anyway, here is a quick Iphone recording of straight in, and then one of light heavy hand(tube screamer like)

IM100 light heavy hand.m4a

IM 100 clean.m4a

[martin manning]

Well, turns out the cold is covid, so I'm isolated for a bit. Good news is I feel ok and can spend the time messing with the amp.

Let's hope it remains that way!

So, with the prior tone stack experiment, if I were to try to emulate a Fender tone stack, with mid 100k pot, if I had it at 10k I would be at the same place as the Fender tone stack with mid maxed out?

Correct, around noon if your mid pot is log taper.

I know I keep stabbing at this, but this is the area where the technical, data driven stuff isn't getting me there. I'm sure there is a way to do all of this through technical study, but I don't see it.

There are so many interactions in the human-guitar-amp-human loop that it's very hard to isolate cause and effect. Everything is confounded. The best you can do IMO is to use known (simplified) relationships to guide experiments.

I am gonna keep experimenting with this. I'll try the 10k and see what that does. I'm also gonna tack in all the 15u on the cathodes(outside of the reverb driver).

Here's a plot of the effect of cathode bypass caps 4u7, 10u, and 22u on a standard 100k Ra, 1k5 Rk 12AX7 stage. All of the action is below 100Hz, and even at 40Hz (open 4th string on a 4-string bass), it's only 2dB. As you go lower, the direct effect becomes much less audible, but might cause other issues that are. If those notes get distorted, then the higher order harmonics generated become very audible.

[martin manning]

PS, nice tones! You live in that low mid zone, so I can see why carving that out would not be good for you.