Revisiting my first build and finding mistakes!

[Aaron Walker]

I haven't built an amp in quite some years and while waiting on parts for my upcoming Princeton Reverb (reverb & tremolo delete ala Rob Robinette) build, I starting looking at an old Octal Fatness build (6SJ7/12AX7/ single-ended EL-34 output).... maybe my 2nd of 5 builds I've done. I have done some intense reading in my Kevin O'connor TUT books and reputable sites to remind myself of all theory I used to have memorized and have some deeper knowledge than when I put this contraption together.

I saw some "mods" I had done over the years to the Octal Fatness, and read up on why I probably did them..... some weird RC filter at the input (maybe to fix noise?), some series and paralleled resistor pairs (probably cause I didn't have the right values), but the strangest thing is a mistake I found. I have a couple 100ohm 2W Metal Oxide resistors going from the heater pins (2&7) of the EL-34 to the cathode pin 8 of the same! Ostensibly, I was probably using these as a ground reference for my heaters, and meant to connect them to Pin 1, which is currently going to ground.....and I'm not sure if that's right either.

What I'm wondering is how this thing has worked all these years and sounded fine..... I'm planning on correcting it, but I'm really interested in what this actually did to the circuit and components.

Here a reference for the EL-34 pin connections to explain the situation better:

Pin 1 - Ground
Pin 2 - Heater to 100r to Pin 8
Pin 3 - OT Primary
Pin 4 - 1k to B+
Pin 5 - Two 4.7K resistors in parallel (Think I was missing a 2.2K for the grid at the time)
Pin 6 - NC
Pin 7 - Heater to 100r to Pin 8
Pin 8 - Both 100r resistors and Cathode Bypass/Cathode Power Resistor (25uf/330ohm)

PXL_20231024_212604567.jpg

[rangdipkin]

nothing wrong with that... just an elevated ground reference via fake center tap for your filaments. Might help with noise!