#1
I mentioned in my other scratch build thread that I had another preamp design that I came up with that I wanted to LTSPICE before continuing with the other one. This is kindof my first bash at a preamp that can do anything you could possibly want it to do, whether that be clean, blues, rock, metal, whatever.


Front end (V1 - 12AX7):




Middle (V2 - EF86):




Back End 1 (V3 - 6SN7):




Back End 2 (V4 - 6SL7):




*waits for inevitable "HOLY **** THAT'S THE MOST COMPLICATED PREAMP EVER"*

Before asking too specific of details, non of the component values are set. This is just a general "idea" right now.

The first stages (1a and 1b) are a 12AX7 cascaded, but incorporating negative feedback, which can be used to great effect to control the input gain of the amp (these first two stages). It can pass a LOT of signal, not much, etc, but it doesn't rely on voltage dividers to do it like normal. It's a neat trick that has been used to great success on making high-gain amps sound good clean. Bass control is here, up front for maximum effect.

The second stage is an EF86 pentode with some "fun" circuitry around it. "Drive" up front determines the amount of signal going into the pentode. The rest of the circuitry is taken from Merlin's tube amp design book. I've used it on another amp before, really really really liked it. Basically, in one position it's a straight up pentode, which has a "grittier" sounding overdrive than a typical triode because it generates odd or harmonics. Great for a gritty blues tone or a rock tone with more bite. At the other extreme, the pentode is effectively a triode, which gives it a very stiff sound. This also pretty much doubles the gain of the stage. Extra gain + stiff = great metal tone. In the middle of the control setting it becomes a "squishable" pentode...meaning, it compresses some if you drive it hard enough. Great for clean and blues tones.

The third pic is the third and fourth stages with a bridged T mid control thrown in. The mid can be controlled up and down and making the resistors in the T variable makes the frequency of the cut adjustable, so it's a parametric EQ. Adjustable for whatever tone you want...American, british, modern, etc.

Fourth pic is 5th and 6th stages that have a treble control thrown in between them. the treble control might be relocated to the bridged T earlier, dunno.

The gain control in the backend is a dual gang, so that 1 knob affects both the 3rd and 5th stages. Right now, I have a 6SN7 and a 6SL7 in there (they have gains comprable to a 12AU7 and 12AT7 respectively). I wouldn't be surprised if they both become 12AU7s though.

The idea is that the front stages generate a "gritter" tone, with the 4 stages at the end are the more "traditional" amp circuit, generating most of the distortion, hence calling it the "gain" control. The pentode would add more "grit", so it's called drive.

Thoughts on how the settings would be used:

Clean tone: Drive low, character to pentode, gain low - passes through clean
Sparkly clean: drive mid, character to squish, gain low - bit of compression for sparkle
Blues: drive high, character to pentode or squish, gain low - grit from the pentode, clean through the rest of the preamp
Rock: drive mid, character to pentode, gain mid - grit from pentode, gain from backend
Hard rock: drive mid, character to triode, gain mid - same but more
Rock Solo: Drive max, character to squish, gain mid - compression for "singing" sustain
Metal: drive max, character to triode, gain high - stiffest, "smack you in face" setting


Thoughts anyone? I know it's really complex, but it might be worth it in the end, dunno. This one is a project or two away, thought I'd put it out there for now.

Parting thought: according to SPICE, with it maxed out, it's a gain of 160dB, or 100,000,000x voltage gain.

Quote by kcdakrt
DLrocket89 makes my ug experience better!


Member of the official GB&C "Who to Listen to" list

Kit Amp Building Tutorial
#2
Are there any mass produced amps that use a preamp pentode? I've been considering incorporating one into an amp, but I'd like to have an idea of what I'm getting into before I do it.
#3
A preamp that can do anything....I like the sound of this. I am very interested as to how this will turn out.
Question though, will starting this project lessen the updates and work done on the other one?
#4
Quote by end_citizen
Are there any mass produced amps that use a preamp pentode? I've been considering incorporating one into an amp, but I'd like to have an idea of what I'm getting into before I do it.



Yeah, a fair number of boutique amps do, I think Vox or someone else "bigger" did so as well.

The main problem is that they just copy the old cricuits from the 60s, which SUCK. Too much gain...pentodes are noisy, so if you make them high gain they are very noisy. This circuit is pretty lox gain for the pentode, all things considered.

The "character" control is in the public domain, but I haven't seen any amps use it.
Quote by kcdakrt
DLrocket89 makes my ug experience better!


Member of the official GB&C "Who to Listen to" list

Kit Amp Building Tutorial
#5
Quote by Baby Joel
A preamp that can do anything....I like the sound of this. I am very interested as to how this will turn out.
Question though, will starting this project lessen the updates and work done on the other one?


I like it too. No, it won't delay it further, other than the delay of me posting this thread, just wanted to get it down on paper so I don't forget.
Quote by kcdakrt
DLrocket89 makes my ug experience better!


Member of the official GB&C "Who to Listen to" list

Kit Amp Building Tutorial
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