Tag Archives: modification

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.