Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: paintballnsk on February 02, 2021, 08:18:31 pm
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Hey guys,
I'm an admitted novice, but I do tinker a lot.
I'm trying to make myself a quick reference guide, mostly from a practical perspective to the tone, clipping, and output gain of a 12ax7 gain stage.
I have an 18w Soldano Atomic 16 clone that I tinker with from time to time. Mostly trying to get a mid to high gain distortion, with punchy bottom end without being muddy, and I'm designing it with a TS808 on the front end in mind. And with EL84's in a 4x12 cab, it's been quite a struggle, but it's super close.
Would one of you experts take a look and tell me what could be improved or if some of these are wrong? (See attachment)
I'm also not really sure what Ro is doing, and I'm not sure what it's called. I see it on my schematics, but I'm not finding an explanation for it in the amp calculators or other tutorials out there. Is it just reducing the output without having to mess with the Plate Resistor?
Rg - Grid Stopper - Prevents high radio frequences from bleeding into the amp. Limits the current applied to the grid and allows the grid to bleed back to ground thus avoiding electron build up.
Rgb - Grid Leak - Blocks input signal to ground. Lower values lower V1 distortion and output. Common spot for a 1M “Gain” pot in V2.
Rk - Cathode Bias - Changes the bias point of the troide. A higher value raises the impedence to ground. If un-bypassed, a lower value will raise the output gain of the stage.
Ra - Plate Resistor - Regulates the amount of gain going to the next stage. A higher value will push more voltage gain to the next stage. Typically between 100k and 220k depending on Va.
Ro - ???
Ck - Cathode Bypass - Creates a High Pass filter. Lower values will raise the High Pass knee frequency, thus reducing low end. If Rk was previously un-bypassed, can add a significant amount of gain pending the balance with Ra.
Ca - Plate Bypasss - Creates a Low Pass filter. Higher values will raise the Low Pass knee frequency, thus reducing high end. Good starting point is about 1000p
Co - ???- Commonly referred to as “The Soldaon Mod”. Creates a Low Pass filter. Higher values will raise the Low Pass knee frequency, thus reducing high end. Can introduce a nasally sound if pushed too far.
Cc - Coupling Capacitor - Allows AC signal to pass to the next stage while blocking high DC voltages. Most commonly around .022u with little to no affect on frequency curve. Can be pushed down to .0001u for cutting additional low end, but probably not recommended.
Thanks!
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IMHO -- to many capacitors, working against one another. Ck is really small, boosting only higher frequencies. But Ca & Co are working against that. Ca & Co are optional, and usually only used to cure a problem, or to work with a specific topology in the next gain stage(s). (Some builders do like Ca's, but the value must be selected based upon what the am p sounds like with or w/o it.)
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This is just a reference chart, I understand some of these components are optional.
I just want a clear understanding of what each of them does.
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Co and Ro are a treble peaker. not sure who decided to give credit to ol mikey but marshall amps has being using these for a minute
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This is just a reference chart, I understand some of these components are optional.
I just want a clear understanding of what each of them does.
You need to start at the start and learn the theory. You're going to spend way more time taking wild stabs in the dark like this than it would take to just learn the theory. Go to Valve Wizard and download the Fundamentals of Amplification chapter. Don't move on to the next sentence until you fully understand the sentence you're on. If you can't understand the sentence you're on, then ask the experts here.
People tacking their name or getting their name tacked on to something that they have nothing to do with always seems to annoy me. Like with master volumes and a whole slew of things with robrob tacked on them. This Soldaon (sic) guy had nothing to do with what is more commonly called a "treble peaker" circuit.
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> Co - ???- Commonly referred to as “The Soldaon Mod”. Creates a Low Pass filter. Higher values will raise the Low Pass knee frequency, thus reducing high end. Can introduce a nasally sound if pushed too far.
I don't think this is right; at least not expressed clearly.
The effect must depend *totally* on what comes after. ("Look at the WHOLE circuit.")
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So I guess I'm unsure what Ro actually does. Pretend Co doesn't exist (in my 18w schematic it's not bypassed, so I'm ignoring Co anyway). I've attached a snip of V1 and V2 from the original schematic.
And good point on the Soldano thing, It's just what I've seen it referred to out on the interwebs. So I'll remove that.
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The highlighted resistors are simply part of a voltage divider. Usually seen in high gain amps. Not so common in low gain amps.
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This helped me.
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This helped me.
This is excellent! Thanks for posting!
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And if you're really interested, Merlin Blencowe's "Designing Tube Preamps for Guitar and Bass" is a great read. Available directly from the author at Valvewizard.co.uk -- lots of good information there.
Merlin is worth supporting . . .
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... I'm trying to make myself a quick reference guide, mostly from a practical perspective to the tone, clipping, and output gain of a 12ax7 gain stage. ...
This was done decades ago in a writeup by Ken Fischer. Check out pages 44-45 of The Trainwreck Pages (https://robrobinette.com/images/Guitar/Trainwreck/The_Trainwreck_Pages_searchable.pdf).
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Attached is a diagram of a triode gain stage with common notations for the components. The ones in black are what I typically use.
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A treble peaker circuit has a lot of variables, like the output impedance of the stage that is driving it, the coupling capacitor value, the values of the three components in the peaker circuit, and the input capacitance of the following stage. Attached is an example of a common treble peaker circuit and a graph showing what happens.