The pre-Francesca build (Circuit A1A) used the Hammond 272FX PT and a Hoffman 50 watt OT (4.4K primary). I had mounted them on opposite sides of the chassis, so the OT was above the preamp circuit board. Tubes were in a straight line along the back and the circuit boards were hand-done turret boards. In that amp I tried tube vs. ss rectification, fixed and cathode bias, EL34, 6L6, 6V6, and 7591 tubes. Too many cap and resistor values went into that attempt, and none really showed me the "path to amp righteousness."
In a nutshell, it was loud, buzzy, hissy, and harsh. Shrill is another good word to describe it. It almost didn't matter which output tubes were in it, as they all sounded ratty and unfocused. I couldn't "dial it in" past a certain point and I knew I was shooting in the dark, so it got shelved for a while.
Then I read the post "Approach to Harmonics." Look it up if you haven't read it. That led me to believe that if you didn't get an Express about 70-75% there upon initial start-up it would be a loooong road to tune it in. So I knew I was missing some vital information! Also, I took it to heart that everything matters. Thusly, there were too many variables to consider in diagnosing the trouble, so...
Then came the Francesca pics, and I realized my transformers were NOT placed correctly. My circuit boards were too wide. My tube sockets were placed wrong. Wiring was incorrect.
Sigh. So I gutted the amp, and redid everything to get as close to a real
Express as I could. I placed the PT a little farther forward to accommodate a new OT, the Mojo 6.6K one. It's a bit smaller than the Hoffman, so I was good to go with the space I had.
I made completely new circuit boards, actually a bit smaller in scope than the Francesca boards mainly because the chassis I have is a bit shorter. This was a great move, as it forced me to understand the component spacing of the preamp board and adjust it about 1/2" or so shorter.
I placed the preamp tube sockets just slightly different from Francesca, to shorten more wires, and I think that worked splendidly. They're still angled the same, just adjusted along the length of the board for more optimal wiring.
The 47uf power supply caps are radial-lead small ones I got from Digi-Key and they're great. All the filtering in a smaller package. The 22uf caps on top are almost twice the length! Caulk, anyone?
And then there's lead dress. Lead dress. Lead dress. One of the first things I did was to "chopstick" the amp and see which wires were the most microphonic. Tap, tap, tap. Anything that rang out I made note of and did something with, be it a move or a replacement with a different type of wire. This helped stability something fierce! Not that the amp was howling upon initial start-up, but it definitely was "on the edge" and anything I touched could potentially send it off on a high cutting freq that would tear the top of my head off.
I moved the wire between treble cap and treble pot 1/2" and it fundamentally changed the character of the high end harmonic structure. Unbelievable! I moved the corresponding mid & bass wires farther apart and the mids got cooler, more juicy. I redid the top connecting wires on the pots to bend toward the chassis instead of running in a straight line. That removed some microphonics and shrillness. This confirms for me all the stuff KF was talking about.
I still went through many cap swaps, cap turnarounds, choke and no choke, oddball 1K 25W to "proper" 1K 25W, etc....EVRYTHING changes the amp!
So I had decided early on that I would call a change "good" if it:
1. Made the highs smoother
2. Made the mids "growl" more
3. Tightened up the bottom end
4. Added a little "hair" to the distortion content
5. Made the feel of the amp "bounce" more
So now there are Sprague 6ps caps, Mojo Dijons, a Black Cat .068, and carbon film resistors. I couldn't believe how harsh the metal film resistors could sound. Even the 100K plate load resistors were adding a harsh high end content that went away when replaced by carbon films.
Tubes are: V1 New Tung-Sol (Russian) 12AX7
V2 Amperex/Mullard 12AX7
V3 Amperex/Mullard 12AX7
Power: JJ KT77
These tubes are the best configuration so far for the least amount of hiss/noise with the most amount of cool mids, great distortion, and cleaned-up tones. I really, really like this amp now.
I promise there will be pics, just haven't gotten them done yet, and once I get the proper cab I will record some clips as well.
MANY PROFUSE THANKS TO ALL WHO POST THEIR WORK! I AM INCREDIBLY IMPRESSED WITH THE BEAUTIFUL BUILDS I HAVE SEEN POSTED HERE!!
VERY INSPIRING!! Shermock
