Build name: Tube Reactor
I’ve been planning to build nice tube preamp for a while and on summer break I got a chance to do it. My first choice was to take AX84.com High-Octane amp, build preamp only but with the cathode follower converted to the gain stage and with source follower after that. The sound was pretty disappointing. It sounded allot worse than my Firefly with cascode boost on. Also there was squeal with gain set on more than 3/4 even I used shielded wire for all signal leads.
So I decided to rebuild it without true bypass switch that was originally there and I started with Matchless Hotbox schematic. This time I didn’t use shielded wire that much, but I took care about layout more. Bright cap on clean channel pot is 220pF instead of 180pF, that’s the nearest value I had. Also, I made different power supply with two 47uF and one 22uF filter caps. I have higher voltage to start with, so resistor in first filter stage is increased giving even better filtering. Instead of 6.3V AC powered heaters I used regulated and heavily filtered 6V DC.
To avoid any interference I separated power supply with actual circuit. Power supply is packed inside old computer PS unit I had lying around. Four thick speaker wires and SpeakOn connector are used to transfer power to stompbox. One is for ground, one for 6V heaters and two carrying B+1 and B+2. All these changes should provide nice flat current for noise-free tone. As always, I was trying to get the best parts possible. Coupling caps are Mallory 150’s and Mylar poly caps are used for cathode bypass and Silver Mica for all other caps in pF range. All resistors except in power supply are metal film. Some of them are 2W, some 0.6W and one is 0.25W (I didn’t have anything else at the moment 🙁 ). For wiring I was using combination of great #22 gauge solid core wire (a gift from AmpMaker.com) and shielded wire for some leads. Originally I used two new JJ/Tesla 12AX7 tubes.
Of course I had to mod a thing or two 🙂 :
- changed tone stack to Marshall style Bass-Middle-Treble
- change second stage plate resistor from 200K to 100K
- use 1uF for second and third gain stage for more bass
- added switchable 0.47uF cap (actually it’s two 0.22uF) to the first cathode bypass cap. Switch acts as a bass boost
- 6VDC heaters
I tested all combinations with NOS GE 12AT7, EI 12AX7 and JJ/Tesla 12AX7 and I noticed that 12AT7 as cathode follower gives about twice volume compared to 12AX7. I used 12AT at second position for a few months but I finally settled with JJ/Tesla at first position and EI at second (I heard that JJ’s don’t last long when used as cathode followers).
Previous version had 470pF cap going from last stage anode to ground to smooth tone a bit. I put it because dring first test sound was to fuzzy and bright. After playing with it for a few months I realised that preamp was not the reason for fuzzy sound and 470pF cap only reduced distortion and made sound little dull. The last version does not have smoothing cap.
Center stomp switch is for channel selection and the one on the left for switching bass response of the first triode. That way it affects both clean and dirty channels. For both of them there’s LED indicator between tubes. You’ll notice two different knobs on the right of enclosure. I put the different one because it’s clean channel volume control. All the others are related with dirty channel so I was thinking it wouldn’t be a bad idea to be different. Besides, I didn’t have six knobs of the same kind 🙁 .
You’ll notice large brown caps. Those are 250V Mylar caps I got when I ordered set of caps and those are way too large for small stompbox so this was great opportunity to get rid of them. Also, there are 3 of 0.22uF Mylar caps on the first cathode. I have ordered some 630V caps for there but there were too large to fit (god knows why I ordered expensive 630V caps for cathode bypass when I have pack of 100 Mylar caps in those values)
Here’s first tube wiring. I put grid resistors directly to tube socket to reduce noise and save board space. Input jack is very close so there was no need for shielded wire going to grid. I used shielded wire for second grid because it’s rather long lead going from Gain pot. In my first version all other leads were shielded which cause real mess inside and more noise, oscillations and whatnot. By the way, tube sockets are mounted on aluminum holder which is screwed to the enclosure (yeah it’s aluminum, just it’s anodized to look like brass) and circuit board is glued down to it with epoxy.
Second tube is wired without shielded wire. There was no need for it because one grid lead is short enough and the other is wired directly at the socket because second triode is a cathode follower. Also note two resistor in parallel in tone stack. First version used stock 100K resistor but when I switched to Marshall style tone stack I put 47K in parallel which gives about 32K – close enough to 33K. This photo is taken before I removed that ceramic snubber cap from pin 1 to pin 2 and put 470pF cap from third stage to ground. It’s mounted directly on tube, from pin 2 to pin 9.
This photo shows pot and switch wiring. On the left side there’s bass response switch which toggles two 0.22uF caps in parallel with stock 0.22uF cathode bypass cap. It was very tricky to do the rest of pot/switch wiring because space is really tight. I used shielded wire on leads which could interfere with near leads. Clean channel is done without shielded wire. Those two leads are near some dirty channel leads but when clean is engaged there’s no signal in “dirty” leads so my clean is very noise-free. Dirty channel is also very quiet, noise wise.
And finally the result: clean channel is great. I’m glad I put switchable cap(s) on first triode because both settings sound nice. 0.22uF is tight and bright, while 0.68uF is slightly warmer and fuller. Dirty channel is now tweaked the way I like. Smooth, warm, without excess distortion – great for rock. With TS808 in the front it can do heavier stuff as well, such as Satriani, Vai and even Dream Theater.
Click on a thumbnail to play the video on YouTube.