I want to build this preamp but I am suspicious about some values. First 2x470 kohm on grid 2 seems to much in comparison to what I have seen on other circuits. Second the lack of feedback loop bothers me. Do you have advice on how to modify this or another better circuit suggestion?
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R15 just gets rid of some signal. Depends on how high you want the volume control turned up. I find R12 and R13 a concern, usually 100k up to 220k. What is the supply voltage?
Here's the source: TOMATO guitar preamp
You could probably nitpick a few things, but I would think it's perfectly fine as a project.
You could probably nitpick a few things, but I would think it's perfectly fine as a project.
For "Hi-FI"...
C10 is much too small for "flat"; it gives a 6dB lift from 1.6KHz to 3.2KHz. C9 is somewhat small; ~~80Hz. These are typical fixed tome corrections for electric guitar: reduces bass boom and lifts string overtones.
The 470k "greed" resistors insert some loss between stages to balance overload for a layered distortion flavor.
Yes, it will work as a guitar preamp.
C10 is much too small for "flat"; it gives a 6dB lift from 1.6KHz to 3.2KHz. C9 is somewhat small; ~~80Hz. These are typical fixed tome corrections for electric guitar: reduces bass boom and lifts string overtones.
The 470k "greed" resistors insert some loss between stages to balance overload for a layered distortion flavor.
Yes, it will work as a guitar preamp.
As PRR says, those two resistors throw away a little gain, and allow the first triode to be driven a little harder, which should produce slightly better (richer, more complex) distortion.
However, both C8 and C7 are "bright" capacitors, which is odd. The circuit might get shrill at low volume settings (i.e. when R17 is turned down.)
For electric guitar, I've never seen global negative feedback in a valve preamp. We want the preamp to create a lot of distortion for e-guitar, and to create it progressively (the amp is "touch sensitive"). Using negative feedback damages both those qualities, so it's not usually used in guitar preamps.
I would reduce R1 to 10k or 12k. 68k in this location will generate more noise than the actual valves!
It's usually easier to find 33k resistors than 32k resistors. This will not change the sound or behaviour of the circuit at all, so I suggest using 33k for R12 and R13.
There are sound clips posted on the page Leadbelly found. Have you listened to them? Perhaps you'll like the preamp exactly as it is?
If you haven't listened to them already, the sound samples are at the bottom of the page: TOMATO guitar preamp
-Gnobuddy
Ampeg VT-40, front-panel model with "distortion" knob.
(Note that the factory schematic has a key error.)
Classic 2-triode with NFB from 2nd plate to 1st cathode. *And* a loop through a couple of diodes for dialable "distortion".
The *other* input is a plain triode gain stage with a hair of NFB in cathode resistor. Even to my tin ears it sounded "organic", the high-NFB input sounded either super-clean or rock-crusher racket.
This beast also had a high-NFB middle stage for EQ; pretty extravagant.
I'm sure there are others, but that's the only one comes to mind, and they sure are un-common.
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