DOD FX69B Grunge insanity mod…

dod_fx69b

this “insanity” mod makes this pedal sound much more insane that the original version. i dropped in a couple of germanium clipping diodes between gain stages for a warmer, beefier tone and decided to mess with the active filters that control the two-band EQ. the filters now allow each band to be driven into cutoff to make more PWM type wave forms when in the lower positions. if the player doesn’t want 8-bit computer sounds, he or she can bring the filters up to the horizon level (about 90 degrees on the left side of the straight up postition) to acheive cutoff when no signal is present from the instrument. i also maxed out the gain for a ferociously loud effect. the “loud” knob under the halfway mark about 7dB over the clean signal. both audio samples are recorded direct with no effects whatsoever.

original sound:

modded sound(s):

this item is for sale on eBay.

mellotronium 2.0: rebuild

mellotronium_2

after our move to Oregon two months ago, i’m finally getting back to a spot where i can do more building and experimentation. that being said, my arduino-based sampler/synth is getting a rebuild to make it more capable and road-worthy.

the LED segment display is currently showing voltage out what should be 5v DC. i think the power adapter might be limited on the amount of current it can provide which is creating a voltage drop. either that or the 3.3v and 5v pins are reading the load from the BJT stage which stabilizes and adds sonic color to the PWM output. also, i’ve dropped in an ATMega2560 board and removed the 328p for added storage and memory for more sample time. with some code revisions, i’m hoping to keep the processor for locking up when adding lots of modulations.

solid-state tremolo for tube amps…

opamp_LFO_trem_mod

i’ve have a CBS-era Fender Bantam Bass in for repair that looks like a bowl of spaghetti inside the chassis. it’s a slew of yellow wire spread from one side to the other that resembles the web of a drunken spider. the amp has been modified to include a tremolo, reverb, and an effects loop that replaces the bass channel in the amp. the tremolo circuit was originally a Weber kit, but someone ripped a couple of the pads off the PCB while trying to modify the mod. i just redesigned a tremolo circuit based upon a simple dual opamp LFO using the Weber’s rectifier as a guide to get voltage from the heater filament supply. i attached the output to an “Intensity” pot and wired it into the cathode of the second half of the initial 12AX7.

bantam_layout_01

this is the layout as i was laying my new designed tremolo into the existing circuit.

bantam_layout_02

this is the part of the original Weber tremolo where the last tech (careful with that soldering iron, Eugene!) attempted to modulate of the negative grid bias. though a sound idea, the circuit itself wound up acting more like a compression effect by changing altering the bias when signal was present. also, a solid-state tremolo like this won’t have much of an overall effect on negative grid biasing due mostly to it’s weak output. rails on the LFO is roughly +5.5V resulting in a good 2.25Vpk (1.5Vrms) signal which is hardly enough to be noticeable. i also tend to avoid using trem circuits that re-bias the output stage simply because it does seem to put undue stress on the tubes and surrounding components. it seems more efficient and makes more sense to me to modulate signals while they’re still small.

bantam_layout_04

cleaner and a little more manageable.

bantam_layout_05

and with a fine custom-made panel, the old Bantam looks and sounds more like a ’65 Super Reverb than ever before. granted, not all steps were taken to black-face the amp, but a few value substitutions were made to achieve more of a black-face tone.

Ampeg Super Jet SJ-12T repair…

I’ve seen a lot of the same complaints on various forums regarding these amps. Basically, the amp starts to get noisy and the occasional arcing can be heard through the speaker. The problem seems more exaggerated when using the tremolo circuit which is just a phase-shift LFO modulating the negative grid voltage. I’m not so sure that the blue 1/2 watt resistors inside the amp are not really metal film despite their appearance (**UPDATE** I’ve recently seen some evidence that arcing tends to happen more often in metal-film resistors. I’m not sure about how correct this is, but it would make sense in this amp). If you find yourself working on amp with these sort of issues, here is what I did to alleviate some of the nasty pops.

sj12t_01

On the main board, change R34 from 475k to a 1M 1/2 watt resistor. Also change R8 (pictured below) from 221k to 270k. This will lower the the overall voltage swing on the grid preventing so much stress to the power tubes.

sj12t_02

On the second board (the one that has the tube sockets), you’ll probably want to change out the metal-film resistors just to make sure they don’t start arcing. You can always tap them with a chopstick and listen for any pops in the speaker to determine if they are bad or not. I apologize for the lack of a picture, but there aren’t many components on the board. You should be able to find them all right in the middle grouped together.

R15 & R16: 100k
R17 & R18: 220k
R19 & R20L 22k

For this amp, the speaker was replaced with a 12″ Jensen C12N. The thing cleans up pretty nice and has a sound not unlike other true vintage Ampegs that I’ve heard.