Distortion v. Overdrive, Sustain v. Feedback

[greiswig]

(Originally posted on the technical discussion, where it garnered 0 replies...hopefully it'll do better here)

Hi, all,
I've got aBad Cat "Hot Cat 30," which has a pretty nice clean channel and good distortion on the overdrive channel. But it is a little less "organic" sounding than I'd like...I like to hear the string itself, rather than just the amp. So I end up turning the gain way down on the overdrive channel. It works to a point.

Then I hear something like Normster's great clips here:
https://tubeamparchive.com/viewtopic.ph ... light=clip

Sheesh, there's more sustain in that amp, and he gets it to feedback fairly easily, where I can't get feedback in the Bad Cat without going really loud.

What's the difference between distortion and overdrive, and what is their effect on sustain? I'm curious as to what aspects of the Dumble topology lead to nice, singing overdrive and feedback like Normsters, where the Bad Cat can't do anything close?

Thanks!

[ayan]

(Originally posted on the technical discussion, where it garnered 0 replies...hopefully it'll do better here)

Hi, all,
I've got aBad Cat "Hot Cat 30," which has a pretty nice clean channel and good distortion on the overdrive channel. But it is a little less "organic" sounding than I'd like...I like to hear the string itself, rather than just the amp. So I end up turning the gain way down on the overdrive channel. It works to a point.

Then I hear something like Normster's great clips here:
https://tubeamparchive.com/viewtopic.ph ... light=clip

Sheesh, there's more sustain in that amp, and he gets it to feedback fairly easily, where I can't get feedback in the Bad Cat without going really loud.

What's the difference between distortion and overdrive, and what is their effect on sustain? I'm curious as to what aspects of the Dumble topology lead to nice, singing overdrive and feedback like Normsters, where the Bad Cat can't do anything close?

Thanks!

I will take a crack at trying to answer what you're asking. Strictly speaking, whenever the output of a network is not equal to a perfectly scaled (up or down in amplitude) version of the input, which can be with or without a fixed phase shift, we say the network has introduced "distortion." In the case of tube amps like the Dumble, distortion occurs mostly in the preamp stages because at some point the input signal is too large for a subsequent stage to handle. Thus, the preamp tube will lose its self-bias on the positive cycle in the presence of a large voltage, and the output is "clipped"... This is "overdrive," i.e., hit something too hard and it will deform the signal for you.

There are a number of reasons why an amp has "sustain." In the case where there is clipping, the input signal will decay with time but since it is "way larger" than it has to be for the stage to handle all of it, it can take a “long time” for it to result in "quietness." This type of sustain is therefore caused by the "compression" that goes hand in hand with clipping – you cannot have one without the other, however dynamic an amp may feel, if it’s clipping it is compressing the signal somewhat.

There is something else that's going on, and that is that because there is so much gain present (i.e., the preamp is capable of generating very large signals from small inputs), the guitar pickup will be sensitive not only to string vibrations but to other sources of noise as well, such as acoustic noise. So, for example, you can put your mouth against a pickup and talk and you will hear yourself through the amp's speaker. Much in the same way, the output of the amp can be picked up by the guitar pickup, and you will get a feedback loop that way. If the guitar easily resonates, then you’ll be in feedback/sustain city. 335s semi-hollowbody types, for instance, are great for this because they will resonate nicely and lend themselves to feeding back, but they will not be as out of control as some hollowbodies are.

For this type of feedback to occur, room acoustics usually have to be "right," volume (or gain, or both) have to usually be pretty high, and you have to be strategically located with respect to the speaker so that the feedback is mostly in phase with the guitar. If you have ever seen Gary Moore or Santana on stage before, both of whom play a lot with feedback, you will notice that the stage floor is often marked with "X"s to indicate where they should stand to get a note to sustain, etc. At home, where volume is often lower, you can do something like touch the headstock of the guitar to the speaker cabinet and you will get some more sustain that way, as the output source is acoustically coupled to the input, etc.

There is yet another factor that plays a role. Imagine everything I have described above, but taking place inside of the amp. So, if at certain locations the output of a stage can be coupled (in phase) to the input of another stage, you can get a feedback loop that way and get either a very unstable amp or a very cool effect. If there is out of phase cancellation, the amp will sound dead or muffled. Often, when an amp is unstable you can find the “culprit” part of the circuit and change the amp from being super sustaining to sounding really bad by moving a wire around. As much as the Dumble's layout is very specific and plays a role, I really don't believe it's intended to cause any instabilities in the circuit.

In all cases, just about everything will be a player in achieving sustain: the guitar you use (hollowbodies and semi hollow feedback much more easily than a solidbodies), the pickups in the guitar (the higher their sensitivity, the more likely feedback will be), acoustics, the amp’s bandwidth (whether it filters out higher frequencies), gain, volume, what kind of tubes you’re using, etc., etc.

You ask why your amp doesn’t feedback the way the D-clone clip does? Well, it could be a number of things, starting with the guitar itself – I think the clip used a semihollow. What I suspect is also happening is the following: in an amp like a Dumble, the overdrive sounds really good and not too much like “distortion” really, so you can dial a goodish amount of it and control the “grind” by hitting the strings softer of backing off on a volume pedal, etc. But, the amp will be very “sensitive” all will tend to feedback easily. In your amp, you said you don’t like the character of the distortion too much, so you turn the gain down… That’s probably the biggest culprit right there; the distortion sounds too much like… well, distortion, and dialing less of it will decrease the tendency of the amp to feedback.

Hope this helps some,

Gil

[greiswig]

Hi, Gil,

Thanks for the response. FWIW, I still have one of your original "Smooth & Slim" units I bought for my old Boogie.

I can crank the gain on my Bad Cat, and it still won't feed back like that. You may have nailed it that my guitars just aren't as resonant: I'm using a mid-80's PRS custom with Harmonic Designs Z-90's pickups (very hot!) for this test, but maybe the body is just not that resonant.

I'm mostly aware of how a distorted signal leads to sustain, and how gain and volume can cause feedback. What I'm curious about is how the distortion in a Dumble circuit can be so...transparent, or not-distortion-like. In the clip I mentioned, Normster talked about the feedback occurring at "low volume." That's a relative thing, I suppose, but I gathered that his point was that the acoustic/volume part of feedback was pretty much out of the picture for that clip. I've heard this described as overdrive, as though it is different from distortion, but I'm not clear on the difference. Previously, one differentiation I'd heard was that overdrive is what happens in the power section. But as you say, in the Dumble circuit, the distortion tends to occur in the preamp. It amazes me that the same preamp tubes can distort so differently in different amps, but still apparently have the same overall gain?

[Normster]

One of the tests I perform when I complete an amp is to check harmonic feedback. I set the volume at 3:00, tone controls at 12:00, gain at 3:00, and master at 10:00. I sit about 4 feet from the amp and just press the strings (one at a time) to the 3rd fret. If the amp is performing correctly, it will begin to feed back with a fairly nice harmonic, except for the B string which has a bit of squeal to it. I usually do this with my Strat but occasionally use a 335 type as well, albeit, a bit further back.

[jaysg]

Thinking about the X. I find that it has to be closer to the amp in a band situation. All the extra sound gets in the way.

[Bear]

I'd have to imagine that as a point of comparison the Bad Cat has fewer gain stages delivering the high gain. A cascaded gain stage amp like a Dumble has the potential to get a greater cumulative compression-sustain effect with less apparent distortion. I'd guess that's a lot of what's happening here.

Bear

[ODwan]

From my experience it's mostly the mid to high mid frequencies which create feedback. Comparing the Hot Cat to the ODS the HotCat has a more mid scooped sound due to the Voxy tonestack. The ODS on the other hand is very midrangy (esp. the non HRM models). Try to set the tone controls on the HotCat very low. This yields th lowest relative attenuation of the mids.
Greetings,
Timo

[greiswig]

From my experience it's mostly the mid to high mid frequencies which create feedback. Comparing the Hot Cat to the ODS the HotCat has a more mid scooped sound due to the Voxy tonestack. The ODS on the other hand is very midrangy (esp. the non HRM models). Try to set the tone controls on the HotCat very low. This yields th lowest relative attenuation of the mids.
Greetings,
Timo

Interesting...I would never have described the tone of the Hot Cat as "scooped!" To me, it has LOTS of mids, particularly high mids. (Especially with a Celestion Blue in it). I've tried turning the bass/treble controls down all the way, and what seems to happen is the low mids come out more, and overall gain is reduced.

[heisthl]

What makes these amps sound so good? It's the Ibanez tube screamer we've all been hiding under our circuit boards.....

[heisthl]

Sorry I couldn't resist - IMHO the "secret" to the sound is an amazing clean sound with lots of harmonic overtones (think of the best Fender Blackface sound you've ever heard and imagine it being even better) coupled with an overdrive that seems to introduce "character" outside of the normal overdriven tube compression. How does the amp accomplish these sounds? For one, the midrange is really special on these amps and manages to "sing" without sounding midrangey. Little things like the 220K/500pf pair to CL2 taylor the balance of lows to highs. the .001 across the bass pot lifted 10K from ground. The plate/cathode ratios and the magic of 200volts on the first pre amp stage. The OD trim that lets you adjust the clipping onset at your preferred preamp volume level. All things that were calculated and/or or trial and errored for the guitar tone that gets heard in the back of the room even at low volumes. When I let people play through these amps they can't make them sound bad no matter how they adjust the controls and even more amazing is the amps can do Blues, Country, Jazz and most Rock equally well.

[ODwan]

Sorry I couldn't resist - IMHO the "secret" to the sound is an amazing clean sound with lots of harmonic overtones (think of the best Fender Blackface sound you've ever heard and imagine it being even better) coupled with an overdrive that seems to introduce "character" outside of the normal overdriven tube compression. How does the amp accomplish these sounds? For one, the midrange is really special on these amps and manages to "sing" without sounding midrangey. Little things like the 220K/500pf pair to CL2 taylor the balance of lows to highs. the .001 across the bass pot lifted 10K from ground. The plate/cathode ratios and the magic of 200volts on the first pre amp stage. The OD trim that lets you adjust the clipping onset at your preferred preamp volume level. All things that were calculated and/or or trial and errored for the guitar tone that gets heard in the back of the room even at low volumes. When I let people play through these amps they can't make them sound bad no matter how they adjust the controls and even more amazing is the amps can do Blues, Country, Jazz and most Rock equally well.

Right you are!!!
And comparing the HotCat and ODS is not really the thing to do. They are differnt kind of animals. The HotCat has some interesting engineering under it's fur for sure and it's surely an amp meant to rock hard! Whereas the ODS is for the more filigran playing. I've played a HotCat30 2x12 combo about a year back in a heavy prog rock context and it really shined against the Peavy 5150 of the other guitarist. Exellent amp I say! Has to be played loud, though!
Timo

[ayan]

Hi, Gil,

Thanks for the response. FWIW, I still have one of your original "Smooth & Slim" units I bought for my old Boogie.

I can crank the gain on my Bad Cat, and it still won't feed back like that. You may have nailed it that my guitars just aren't as resonant: I'm using a mid-80's PRS custom with Harmonic Designs Z-90's pickups (very hot!) for this test, but maybe the body is just not that resonant.

I'm mostly aware of how a distorted signal leads to sustain, and how gain and volume can cause feedback. What I'm curious about is how the distortion in a Dumble circuit can be so...transparent, or not-distortion-like. In the clip I mentioned, Normster talked about the feedback occurring at "low volume." That's a relative thing, I suppose, but I gathered that his point was that the acoustic/volume part of feedback was pretty much out of the picture for that clip. I've heard this described as overdrive, as though it is different from distortion, but I'm not clear on the difference. Previously, one differentiation I'd heard was that overdrive is what happens in the power section. But as you say, in the Dumble circuit, the distortion tends to occur in the preamp. It amazes me that the same preamp tubes can distort so differently in different amps, but still apparently have the same overall gain?

Thanks for the S&S...

Anyway, back to this topic, overdrive occurs whenever the input of a network is greater than the network can handle, whether it occurs in the preamp or power amp section. When you turn the amp's volume (master) up, the best sound IMHO, you will couple some power amp coloration to the preamp overdrive and you get a really nice sound. [ Also, the more overdrive that occurs at the power section, the less effect the preamp will have on the sound... So if you have an amp dimed all the way up, for instance, chances are the tone controls will do little if anything at that point. ]

I think the beauty of the Dumble design is that he kept the number of stages to a minimum (4 in overdrive mode, compared to like 7 in a Boogie Mark IV), which preserves quite a bit of the dynamic range in spite of the clipping that occurs, so you retain the touch sensitivity and therefore the feeling tha the signal is not too distorted. And then, the amp is voiced nicely so it won't sound bad even if you can turn the gain way up, and when doing that you increase your sustain, etc.

If you're up for the task, build yourself one. In the 8 or 9 years I have been dabbling with this stuff I only know few people that decided the Dumble type of sound was not for them.

Cheers,

Gil

[greiswig]

Thanks for the S&S...

Anyway, back to this topic, overdrive occurs whenever the input of a network is greater than the network can handle, whether it occurs in the preamp or power amp section. When you turn the amp's volume (master) up, the best sound IMHO, you will couple some power amp coloration to the preamp overdrive and you get a really nice sound. [ Also, the more overdrive that occurs at the power section, the less effect the preamp will have on the sound... So if you have an amp dimed all the way up, for instance, chances are the tone controls will do little if anything at that point. ]

I think the beauty of the Dumble design is that he kept the number of stages to a minimum (4 in overdrive mode, compared to like 7 in a Boogie Mark IV), which preserves quite a bit of the dynamic range in spite of the clipping that occurs, so you retain the touch sensitivity and therefore the feeling tha the signal is not too distorted. And then, the amp is voiced nicely so it won't sound bad even if you can turn the gain way up, and when doing that you increase your sustain, etc.

If you're up for the task, build yourself one. In the 8 or 9 years I have been dabbling with this stuff I only know few people that decided the Dumble type of sound was not for them.

Cheers,

Gil

Yup...when Brown Note sounded the alarm that they were suspending production of kits, I knuckled under and got one of the last D'Lite 44 kits. I have to tell you ALL, one of the few reasons that I would even venture into this is the community you have right here: it really impresses me how willing people are to share their hard-won knowledge with beginners. I hope that I have a trouble-free assembly and tweek, but knowing that there are people on this forum who can help troubleshoot a bit is really encouraging. Hope I can repay the favor somehow...knowledge is for sharing, after all.

Best,

[Darkbluemurder]

Sorry I couldn't resist - IMHO the "secret" to the sound is an amazing clean sound with lots of harmonic overtones (think of the best Fender Blackface sound you've ever heard and imagine it being even better) coupled with an overdrive that seems to introduce "character" outside of the normal overdriven tube compression. How does the amp accomplish these sounds? For one, the midrange is really special on these amps and manages to "sing" without sounding midrangey. Little things like the 220K/500pf pair to CL2 taylor the balance of lows to highs. the .001 across the bass pot lifted 10K from ground. The plate/cathode ratios and the magic of 200volts on the first pre amp stage. The OD trim that lets you adjust the clipping onset at your preferred preamp volume level. All things that were calculated and/or or trial and errored for the guitar tone that gets heard in the back of the room even at low volumes. When I let people play through these amps they can't make them sound bad no matter how they adjust the controls and even more amazing is the amps can do Blues, Country, Jazz and most Rock equally well.

Right you are!!!
And comparing the HotCat and ODS is not really the thing to do. They are differnt kind of animals. The HotCat has some interesting engineering under it's fur for sure and it's surely an amp meant to rock hard! Whereas the ODS is for the more filigran playing. I've played a HotCat30 2x12 combo about a year back in a heavy prog rock context and it really shined against the Peavy 5150 of the other guitarist. Exellent amp I say! Has to be played loud, though!
Timo

Timo is correct on that one. The Hot Cat can do some things quite well, i.e. a clean tone at moderate volume and a unique high gain OD tone at almost any volume. What it doesn't do well is a crunch tone at moderate volume. Either you take the clean channel - then the volume isn't moderate by any definition - or you try to back off the gain on the gain channel, ending up with a middy sound without any harmonics. The pure opposite of an ODS. IMHO the Hot Cat's gain channel only sounds good with the gain turned up - then feedback sounds should not be any problem.

[stelligan]

grieswig,
heisthl rebuilt a MusicMan to a non-HRM clone for me last winter. To say I am hooked on this amp style is gross understatement. I have since built and tweaked a D'Lite kit (you're going to love it!) and am halfway through a pseudo homebrew HRM build as I write this. I began spending many hours playing with speaker choice, preamp tubes, power tubes, etc.. I began to spend so much time with these amps in my studio space that I thought my impressions of how spectacular these amps were could not be real. So, I went to the local boutique amp store and played everything in there - for only about 5 seconds each! I had to hurry back home to my crack pipes that are my clones.... Matchless, Victoria, Rivera, /13, Bad Cat, Mesa, Vox, whatever. My non-HRM clones can do big box jazz, country, blues, fusion, hi-gain. Love the amps, love this forum. If I had one complaint - it is that I have become so obsessed that my wife has been asking "So, are you gonna solder tonight, or can we do somethin'?"