Build name: Tube Reactor

Background

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.

Mods

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.

Schematic – click to open full size

Pictorial

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Result

And finally the result: clean channel is great. I’m glad I put switchable cap(s) on first triode because both settings sound nice, with both 0.22uF and 0.68uF. It adds very nice tube warmth to sound without adding noise or something, so I don’t really feel the need for true bypass. 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.

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