School me on clipping

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School me on clipping

Postby sinner » 30 Aug 2014, 08:23

I'm working on new amp design. It's a vintage base with bit of modern twist. I'm going to give it to the DIY community very soon as an open source project. The problem is the diode clipping option I want to add to this amp. Usually we see electra type of clipping (signal patch to ground). Is there any other option I can try? BMP type of clipping is not an option I suppose, applying clipping from anode to grind sounds (literally) deadly to me. Switching it on/off seems to be challenging as well.

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Re: School me on clipping

Postby Nocentelli » 30 Aug 2014, 09:12

How far from vintage are you prepared to stray? I know nothing of tube amp design, but could you have a lower voltage overdrive circuit in front an use an opamp? The nicest clipping I've heard from diodes is something like the Timmy, i.e. pair of antiparallel diodes in the feedback loop.
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 09:39

I will post schematic as soo nas I can. Im on my phone today only

Basically it's classic plexi era Marshall topology + added switchable gain stage (two triodes) after stock first gain stage - check Cornford RK-100 schematic I've done it looks like that in modern mode

Spare triode will be used as an clean channel return stage, input stage is taken from OD's channel first triode's anode.

Few add ons like O'Connor's fx loop, independent presence, resonance and pre master controls. Maybe VVR. Will see. Rightnow I'm concentrated on preamp topology really.
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 09:48

I forgot to add. Im thinking to add diode clipping to the clean channel to kick it into the crunch mode only.
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Re: School me on clipping

Postby phatt » 30 Aug 2014, 09:55

Offset a triode stage is about the easiest way.
All the big names use that trick now, some go stupid but others are obviously ok.

Look at TrainWreck circuits for a very simple but very effective way to do it.
Notice the value of the *cathode resistor* on V3? :scratch: Forces halfwave clip earlier than normal. :secret:
Anywhere from 10k ~50k seems to be the go. Pick a value and let your ears be the judge.
be careful with layout it will squeal like a pig if you are sloppy. :twisted:

And don't forget the Reverb channel in fender circuits;
Just remove the reverb but keep the extra triode stage complete with that 3 meg mix resistor it does a lot for the mojo.
Mesa Vtwin pedal impliments that 3meg in the clean section but it actually delivers a beauitiful sweet OD. 8)
Have fun, Phil.
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 10:19

Thanks Phil. I was hoping you will chime in. I do value your opinions

I like this idea. The only problem is that I don't have a spare triode to do this ;-)

I want to keep OD channel's topology as is. I like the ability of this channel to stay true vintage or go wild modern with minimum hassle

My amp 'design' takes already 5 valves (with PI). I can take it down to 4 if I take ss fx loop but this will leave me again with unused triode ;-) of course I can squeeze it somewhere, make first gain stage parallel common anode, common grind, split cathode as I always wanted to try but I'm already going way to wild with this amp. Clean channel diode clipping seems to be easy and cheap compromise
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Re: School me on clipping

Postby Cub » 30 Aug 2014, 10:51

That sounds like a very exciting project !

I really like the diode clipping in later versions of the Split Channel Reverb JCM 800 and especially the Silver Jubilee. Do make sure you look at the schemos of later Split 800s if you're looking for inspiration, as the earlier ones are pants.
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Re: School me on clipping

Postby MoonWatcher » 30 Aug 2014, 17:13

phatt wrote:Anywhere from 10k ~50k seems to be the go. Pick a value and let your ears be the judge.

Or, use the standing consensus value of 39k. Just look at the number of amps that use it.

But it's important to pay attention to the voltage at the plate, and adjust its node from the b+ rail as is appropriate. It's also not uncommon for the plate resistor to get a small cap in parallel. While I've seen 470pf in some instances, 1nf seems to be more common.
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 17:41

Is that a cap to kill oscillation, right?
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Re: School me on clipping

Postby MoonWatcher » 30 Aug 2014, 17:52

Cub wrote:I really like the diode clipping in later versions of the Split Channel Reverb JCM 800 and especially the Silver Jubilee. Do make sure you look at the schemos of later Split 800s if you're looking for inspiration, as the earlier ones are pants.

I was going to mention the Marshall stuff.

I had an early split channel, and it had possibilities. It wasn't something I could use regularly for the long haul, and those early Marshalls typically had channel bleed issues, too.

Rumor has it that Michael Schenker used the early split channel for some stuff.

I think the later version 2205 is actually a bit more clever with how it uses diodes for clipping, but I think some people view it as a shift towards the JCM900 stuff, and the negative light that may come with that.

Tom Morello has used the later version 2205 for years and years, and I think his tone is pretty cool.

The Silver Jubilee is probably the crowning achievement of all of this. I think it uses both silicon and LED types of diodes for the distortion modes.

It was brought to my attention that Carvin has snuck clipping diodes in quite a bit of stuff in their tube/valve-based line. I think even the little low-watt tweed amps have it. And I seem to remember seeing a schematic of their variation of the Mesa Rectifier-based designs, and there were indeed clipping diodes in at least one of the non-clean channels, probably both of them. I don't know if Carvin did anything clever or different though. Going on memory from what an owner of their little 16 watt tweed appearance amp had to say, he wasn't really pleased.

Something else to possibly consider - maybe a hybrid approach? Maybe both valve and solid state-based clipping stages? I heard that the amp will have effects loops, so some of that (circuitry) could be used for the solid state bits.

Getting back to Carvin, their solid state amps seem to have some clever diode clipping stuff happening. So if going for the hybrid thing, that might be good inspiration. I think the guy who designed it for them said Carvin was dumb not to put a patent on it.
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Re: School me on clipping

Postby MoonWatcher » 30 Aug 2014, 18:05

sinner wrote:Is that a cap to kill oscillation, right?

I guess the purpose is twofold - handle the potential oscillation issue, and bring down harsh clipping in the treble range.

There's been some criticism that too much treble content might typically be removed in the process, so it might be worth investigating using a high-cut filter closer to the preamp's output. I think something like what was used in the Matchless Lightning is cool for that, but I never put it in anything that would be considered super high gain.

I think the SLO either doesn't use any of those plate resistor caps, or it's just one. I know the Bogner Ecstasy and Carvin Vai Legacy are both chock full of those caps. I think both of those are on the bright side, so if you like the tones from them, they might lend clues. Again - pay attention to those plate voltages at each particular node. It can sound like serious shit if all that you're doing is putting a bigger cathode resistor on a stage. While I'm thinking about it, I think that the Soldano Atomic 16 or the equivalent amp by Jet City might be kind of clever with how the cathode resistor values were handled.

Aside from something like the Atomic 16, I usually don't see a lot of variation from what is done with cathode resistor values. They're usually 820r, 1k5, 1k8, 2k7, 10k, or 39k. And it's not uncommon to see the 1k8 and 39k combination only, with much of the Rectifier-like amps. The 1k8's will typically get bypassed with a 1uf capacitor, which gives the same frequency shelf as the Marshall 2k7 & 680nf combination.
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 18:51

I have never paid much attention to Carvins before. I think I will right now. Can you remember that SS model name? BTW, I have just realised that our forum favourite Electra Distortion was a part of the guitar amp circuitry

According to M.Blancowe paralleling bypass cap to plate resistor should be avoided. Setting small cap from plate to ground deals better with paralistic oscillation. It prevents high tension noise to be applied to anode

Can you give me a math how to calculate cathode resistor/bypasd cap value to freq?

Thanks man
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Re: School me on clipping

Postby MoonWatcher » 30 Aug 2014, 19:28

sinner wrote:Can you remember that SS model name?

It's their SX series. I think all of them use similar techniques with the unusual clipping diode setups. And I think even Craig Anderton had positive things to say about them, many years ago.

sinner wrote:According to M.Blancowe paralleling bypass cap to plate resistor should be avoided. Setting small cap from plate to ground deals better with paralistic oscillation. It prevents high tension noise to be applied to anode.

That makes absolute sense. I've never had an instance of one of those caps working to my satisfaction. I seem to recall that the Marshall 2203 & 2204 had a cap to ground on a critical stage.

Like I mentioned - I think the high-cut circuit works a LOT better. If your amp has a long tail phase inverter (which it sounds like it does), the high-cut is VERY easy to add - it's just a series pot and capacitor that goes in place of the 47pf "fizzy cap." The typical cap value for high-cut is usually at least 2n2, and I think some are 4n7. It makes it very easy to precisely remove unwanted treble, or remove none at all. Trainwreck amps were mentioned, right? That uses the high-cut, but I don't know the exact specs. I just use the one from the Matchless Lightning and it's good enough.

sinner wrote:Can you give me a math how to calculate cathode resistor/bypasd cap value to freq?

I always used just an old graph to manually plot it out (3 columns of resistor/capacitor/frequency), but I just found this nifty little thing:

http://www.ampbooks.com/home/amplifier- ... capacitor/

That is better, because it includes whether or not the next stage has a grid leak resistor. In the case of Marshalls, it's usually 470k (for what I think we are talking about).
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 19:38

Ampbook calculators... of course! :-)

Now I know what kind of cut control you talking about. It's the one used in old vox acxx's

I have added that thing to my work mate 50w marshall replica last month. He didn't like the fact that PPIMV fizzes on low volumes (as you know post pi master fucks with NFB)

Works like a harm ;-)

I'm on hunt for sx schemes right very now

Thanks a lot mate
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Re: School me on clipping

Postby sinner » 30 Aug 2014, 19:39

Btw. Are you guys sloclone forum members?
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Re: School me on clipping

Postby deltafred » 30 Aug 2014, 20:01

sinner wrote:Btw. Are you guys sloclone forum members?

I'm not, I already spend (waste?) far too much time on the 4 forums I regularly visit.
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Re: School me on clipping

Postby MoonWatcher » 30 Aug 2014, 21:05

deltafred wrote:
sinner wrote:Btw. Are you guys sloclone forum members?

I'm not, I already spend (waste?) far too much time on the 4 forums I regularly visit.

I'm not either, for similar reasons. I'm also actively trying to resist the urge to build any more amps, as I don't have the time to play the ones I currently have.

I did have fun tweaking a 2204 a few years back, though. And some of the stuff with the SLO Atomic 16 came in handy with that. I just wanted something that was basically (traditional) MV Marshall with a few twists and nothing else.
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Re: School me on clipping

Postby deltafred » 30 Aug 2014, 23:48

MoonWatcher wrote: I'm also actively trying to resist the urge to build any more amps, as I don't have the time to play the ones I currently have.

Likewise. I have far too many amps, cabs, guitars, and basses.

Trouble is I get a new gear but never sell the old.
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Re: School me on clipping

Postby MoonWatcher » 31 Aug 2014, 15:34

deltafred wrote:
MoonWatcher wrote: I'm also actively trying to resist the urge to build any more amps, as I don't have the time to play the ones I currently have.

Likewise. I have far too many amps, cabs, guitars, and basses.

Trouble is I get a new gear but never sell the old.

I guess that's me, too. I used to sell some old stuff off, only to want some of it again. I also used to trade stuff with friends for awhile (as in permanently), or just give them gear outright. I figured if it was going to be used regularly, they were the true owners.

I've got 3 guitar bodies with no matching necks that will sit forever. None are anything special, but all would make great players' guitars, especially for the young budding types. I won't let them go because I know they will probably just end up in the trash pile.

The pedals don't bother me so much, because they are easy to tuck away in a closet. And interestingly, I will re-visit something a year or two later, and end up strapping it to the pedalboard for at least a little while. I've got an old Reverend Drivetrain 2 that's like that - really good for some things that it does, and they aren't so easily found any more (the older non-Garage-tone ones).

The ridiculous part with owning too many amps is when I finally have played through them enough that it's time to replace valves/tubes, and how expensive that's become. At least none are anything like an Ampeg that used something strange and unavailable. I did get some 7581a-types that sound great, but the audiophile-types seem to have found out about them (the old ones - not the new stuff being marketed as the equivalent) and driven the prices through the roof. And why they are compared to a kt66 is beyond me.
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Re: School me on clipping

Postby Cub » 31 Aug 2014, 17:50

Glad to see I'm not the only one who likes these old Marshall beasts. :applause: On a different note, mister MoonWatcher, could you please have a look at your PM inbox ? :mrgreen:

If my memory serves me well, I believe some bloke bridged the diodes on his Peavey Triumph and turned it into really a nice rock amp. This might be worth looking into as well for a rock/metal gain boost.

Dr. Tube has quite a collection of Marshall schematics: http://www.drtube.com/en/library/schematics
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