• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

pre-amp design, just curious about your thoughts here.

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I'm working on redesigning an old amp.. thread here http://www.diyaudio.com/forums/showthread.php?s=&threadid=117944
So, I've been scanning over guitar amp schematics and noticed far more variations than I expected. I thought there was an excepted science to the placement of valves with respect to volume and tone pots. What I've found really made me pause for a moment.

First, can you really put a Volume pot inbetween the two triods in a 12ax7? Will that cause a problem? I saw this in a MesaBoogie amp. It also had a 3 tone stack following the 2/2 of valve 1. Valve 2 was the gain makeup, V3 was after a reverb insert and also the PI. I was under the impression this tone/stack arrangement would somewhat degrade the signal with signal going through so many paths. Most amps I've seen just have a single "tone" pot. What's your take?

Second.. I found a tone stack on an Ampeg Jet after the PI and right before the power tube stage. That flipped my lid.

I've also seen the "master volume" in a place between the PI and Power stage. Is this somewhat common if a master volume exists?


Pardon my newness to the subject, but all of my reading has lead me to examine vintage designs that are mostly simple in concept. I realize every design has it's compromises, but are these examples I presented extreme in anyway? I might try to use a more advanced design in my homebrew amp.
 

m6tt

Member
2007-08-08 12:12 am
when I scratch together amps, I often leave wires hanging where the tone stack "goes"...I'll try a few different combinations and styles until I get the sound I want. You can have a triode-coupling cap-voltage divider (pot)-triode for a volume control. Just don't forget the coupling cap! With cap values, I also experiment, you can get a good variation between .1 and .0047. Also, the later the tone stack in the signal chain the more effect it will have on not just the instrument sound but also the amp's distortion. Same for volume control. The RCA tube manuals which can be found online have tables for looking up the 'ideal' cap, but your mileage may vary. Some of the things that hifi nuts will claim are no good actually are just fine or even great for guitar or instrument amps. Hifi amps are supposed to be relatively 'clean', but guitar amps are supposed to add to the sound of the instrument.
 
Thanks Matt. Ya, I only just now realized that all the specs I've been looking up.. are safe HIFI radio benchmarks. I was aware that guitar amps "pushed the envelope" further. But I also thought this was somewhat risky and outside tolerances. I guess not eh :whazzat: I'm getting closer here on adapting a schematic for my use. I'll remember to keep a looser approach on experimenting.. good advice on your part :D
 

m6tt

Member
2007-08-08 12:12 am
well, I don't know about pushing the envelope *too* much, not that old amps didn't--the screens glow bright and pulse to the music on my old mcgohan PA. It's more that it's important to ignore a lot of the advice you'd hear for hifi amps. An unregulated power supply, bad output transformer, commercial caps & resistors, and the like resulted in some of the most famous guitar amps ever (a bass amp might want a regulated power supply, gives more concise bass). I guess I mean that you have a lot more room to experiment and still come up with something good in an instrument amp than an amp that's supposed to sound *exactly* like what was recorded.
Instead of tone controls, some of the really old amps had switches which would parallel in or out 'extra' coupling caps, changing the low frequency response without much insertion loss. Might sound cool, easy to try (call it a 'fat switch' if you will). Two diodes back-to-back and in opposite directions (say 1N4001s, LEDs or germanium ones) between signal and ground can make for a great fuzz effect between gain stages. Put a switch between signal and the top of the diodes to be able to turn it on and off.
I definitely recommend trying to use just a low-pass filter, and setting the high pass with the size of the coupling caps. Not much insertion loss and depending on the design it can sound really good. One of my favorite amps right now came from a 1940s stand-up radio that was in bad, bad shape. The antique guy sold it to me for $20-after removing most of the radio, I added a 6sj7 gain stage in front of the audio amp. I still have about 9 tube sockets left unused on the chassis, all octal. It only has a low-pass filter tone control, but it sounds great. I'll have to add some crazy effects to use up all those sockets! Happy hacking!
 
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