Sustain - Grid Blockin or Anode Clipping

[Merlinb]

What I found in my designs is that when I don't have at least a small CK, the more even frequency response tends to want to fight the pick attack in a noticeable and unpleasant way. There is too much "thud" and not enough "zip" on the very front edge of the attack envelope.

I agree that it isn't usually practical to build an amp with lots of unbypassed stages, it's just one of those things you have to balance out. Obviously you can perform bass cut and other frequency shaping using coupling caps / bright caps / tone stack etc. which go a long way to duplicating what a small bypass cap does for you. Personally I would tend to use bypass caps on the early stages, and try to avoid them on later stages (which get more heavily driven).

Still not sure I'm hearing any sustain difference in practice since I've experimented with the no-CK vs CK thing and nothing jumped out at me except the pick attack thud from the no-CK thing, but if I could get around that I might play with it more.

It's certainly not something that is easy to do A/B testing with! After all, to compare similar levels of distortion and gain from two amps, one with and one without Ck, you'd have to build completely different amps, and then you'd have a hundred other things that would make the two sound different.

[Steve Conner]

I think that when you crank up a classic old Vox or Marshall, you get lots of every possible kind of clipping you can imagine.

Generally, when the valve clips due to the grid being pushed positive, that's hard and looks like a square wave on the scope, and when it clips the other way by cutting off, that's a softer rounded edge. There's an art in getting a nice mix of both types in your high-gain design.

I like the smoother Larry Carlton/Robben Ford kind of gain myself, and people tell me they think my homebuilt amps sound a bit too dark. But I play Metallica when nobody's looking.

I tend to use Ck's on the early stages and leave them off the later ones, once the signal voltage has built up enough. I also use undersized coupling caps to keep the grid blocking thing under control. If you undersize everything and then apply a bass boost towards the end, to put the low end back in, that's how you make what Bruce Collins called the "wanker woofie drone tone", as modelled by James Hetfield, Dimebag Darrell and so on.

[anson]

I've had really good results with a series of 12ax7 stages cold biased, around -2.8v or so. Makes for a real nice creamy clean tone, and a not-so-harsh overdrive tone. I'm very happy with the sustain too. I use 6 stages, in a series, slightly clipping at each stage, all cold biased, and then a PI.

[wizard333]

Steve - What are you using to add the bass boost at the end if not a big CK? I agree with the small coupling caps; a big mistake I see often is people trying to push too much low end through gain staging and ending up with indistinct flubb.

I've also never really understood the use of cold biased stages in a drive channel. Could someone that uses them and understands why clue me in? What are you getting from 6 stages of cold bias you can't get from fewer stages? I typically only use 3 actual clipping stages in a gain channel and I've never been dissatisfied with the results. None of them are cold stages.

[defaced]

I've looked at alot of high gain schematics and I can't say I've ever seen one with six cold stages, of the schems I have on hand, there is at max one cold stage. The VHT, Recto, and SLO schems are two normal stages, one cold stage, DC coupled cathode follower, TS/MV, PI. The 5150 schem has one cold stage and a boat load of normal stages with a plate driven TS. The JSX schem has no cold clipping stages (100k to 150k / 1.5k), and unless you unless 100k / 3k cold, neither does the Uberschall.

To the original point, I would say all of these amps have sustain (the only one I haven't played is the Uberschall, but Youtube is evidence enough). Could it be as simple with high gain builds that it's just that there's so much gain that the amp has no choice but to sustain?

[wizard333]

Add enough gain and the guitar will sustain through controlled feedback. Even with the angry mosquito tone. Add more midrange, it requires less gain to happen. All those amps you mentioned have very high gain capability.

But I think the overall question was, is there something you can do in the design to get more sustain WITHOUT a ton of gain; say 'clean' tone sustain or mild gain sustain.

I still don't get the idea behind the cold stage.....whats it for? I can only understand it in terms of adding another stage for tonal shaping options or adding a clean boost or something, but I don't get how it helps with the basic gain channel function of clipping.

Larry Carlton has more control over his sustain than anyone I've ever seen, with very little overdrive, and I've seen him from 5' away in a small room where all I was hearing was what was coming from the cabinet. And no, there was no compressor in his signal chain, just a wah. Dumble uses CKs on his amps on pretty much all stages and no cold stages anywhere if the schems I've seen floating around are accurate, so I have a hard time understanding why either of those 2 things would be the magic bullet.

[Steve Conner]

Steve - What are you using to add the bass boost at the end if not a big CK?

Well, when I started out playing with tube amps, my favourite players were using Boogies with an EQ, either built in or in the FX loop. So that's what I used, an EQ built into the amp. I played with various designs and eventually came up with a 2-band parametric, a hybrid circuit that uses a 12AX7 and a bunch of op-amps.

The original idea was to boost bass with one band and scoop mids with the other, but the idea has proved to be flexible, in my old age I now use it to boost mids and cut bass, which cuts through better at gigs.

I'm sure Larry Carlton's Dumbles help with his sustain. If you design a tube amp to give that kind of smooth distortion, then there can be a lot of clipping and you don't really notice it as actual distortion, it's more like compression. But it's probably mostly his playing technique and choice of a guitar that sustains well.

[lowell]

Wizard333,
A cold biased stage adds crispness and air around the note. That's the best I can put it into words. In my custom design my OD channel was quite dark and lacked definition, when I added a cold biased stage it came to life. If one is finding that tweaking Ck's and or coupling caps/bright caps is not providing enough frequency shaping, try a cold biased stage to brighten things up.

[anson]

As far as I understand things, sustain in an amp comes from gain stages being clipped, so that while the amplitude of the signal may decay at the guitar, there is still quite a bit of the waveform in one or more of the gain stages that has yet to "unclip". For example, the voltage swing going into the grids of a second 12ax7 stage may be 40v or 4v, but that stage will still (for the most part) either way produce an output swing of 100v or so (assuming a gain of 50 and a bias of -2v). So even though the guitar is getting quieter, the second 12ax7 stage is still producing a strong voltage swing.

[lowell]

seems a great way of looking at it, makes sense.

[BackwardsBoB]

There is an example of lots of underbiased stages with no cathode caps used as a drive channel. How about 11 triodes?

http://www.carvinservice.com/crg/sch...03C10Mar92.pdf

The gm of a tube increases with cathode current, and, conversely, decreases with decreasing cathode current. Plate resistance increases at lower cathode current as well. Negative going inputs get compressed, for a more gradual cutoff clip, and positive going inputs get expanded, right up until grid conduction. the gm follows a curve, so that the compression is greater than the expansion, and you can achieve compression without hard clipping. See page 4 at www.drtube.com/datasheets/12ax7-sylvania1955.pdf for curves. This is a great plot to stare at.

The Carvin design uses "standard" 100K plate resistors, so when they boost the cathode resistors to 10K, the first approximation of stage gain drops to 10, creating significant negative feedback, linearizing the tubes, and minimizing the stage gain variation due to gm variation from tube to tube. Wiring the stages for high gain with a larger plate resistor, or installing cathode caps, normal variation in tube gm from tube to tube would make the resulting cascade gain after 11 stages highly unpredictable.

The complete design includes a noise gate, and it needs it.

Note that the 10K and 4.7K resistors alternate, and the big interstage attenuators occur after the 4.7Ks, yielding lots of asymmetry in the results.

Carvin failed to conquer the world with this design and convert everyone to separate tube preamps, but the Quad-X is a favorite of some users.

[wizard333]

Now thats one crazy schematic! 11 stages.......I bet it needs a noise gate!

Seems silly though. I bet it doesnt sound as good as a 3 or 4 stage design, uses a lot more tubes, and has a big old noise floor.

[Chuck H]

There's been a lot of Q about what advantage, difference, usefulness there is to the cold clipping stage used in high gainers. Hang in there because I'm no EE, but... IME the cutoff only clipping adds more third order harmonics. Probably due to it's sharper/flatter clip than anode clipping (grid current limiting if you prefere). It also introduces a round, or nearly round wave form on the positive swing, unless the wave form is already clipped, in which case it's at least still accurate on the positive swing, but the negative wave form has a sharp, flat clip. Combining the two phenomenon in the same gain stage definitely seems to have a specific sound. Smooth yet gritty and defined. I don't use this type of circuit in my amps because "I" don't care for the effect. But A LOT of builders (Soldono, all the Soldono'ish amps like the Peavey 5150, TrainWreck and others) have done very well with it. I say if you "don't understand" why/how this makes a difference, then try it for yourself. It's no myth to a lot of people making $$$ on the concept. And for myself, I have experimented with it quite a bit and I do hear a particular character that is the "cold clipping stage" or "cold biased stage".

JM2C

Chuck

[lowell]

Can we clear some things up?

Are there 2 ways to clip the grid?
1 being a an input signal whose positive cycle exceeds Vgk.
2 being a colder biased stage, or, larger Rk.
(is grid current limiting the same as my number 1 above?)

Is there more than 1 way to clip the anode?
Other than exceeding the supply rail voltage?

[poorboybrad]

A little pre warning here I don't know what I'm talking about... But if your looking for clean sustain it seems like local feedback like the blackface bassman has may be something to look into... From what I understand it compresses your signal as its going through that gain stage.

On the note of ridiculous amounts of gain. Is there any way to get say... a 5150 sound with a couple fewer gain stages? like... is six gain stages REALLY neccesary or is it a marketing thing perhaps?