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.
this is the layout as i was laying my new designed tremolo into the existing circuit.
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.
cleaner and a little more manageable.
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.
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.
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.
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.
this is a re-appropriation of a Fender Frontman 15-B. it’s one 8″ speaker in a closed cabinet primarily designed as a really cheap bass amp to be included in starter packs. having gutted the amp a few years ago and replaced it with the contents from the No. 5 tube amp project, i got some time to rework the circuit into something more usable.
the additions include a tone stack, second gain stage, and a stand-by switch. the power amp is now wired as a pentode instead of a triode, and i didn’t feel it necessary to have a switch between the two modes. it still needs a little work to keep it from self-oscillating at high volumes. thanks to those guys who run AX84.com for their P1 project. it definitely helped the way i thought about redesigning the amp.
insides of a Fender 15B solid-state bass amp that i gutted and converted into a simple tube circuit. i constructed this several years ago, and it’s not been very functional as of late. it definitely needs a better chassis ground, and i’d like to add in a switch for triode/pentode operation as well as a tone stack. i’d also like to play around with some of the filtering design as a practice.
And here we have a solid 160W solid-state bass amp with an RCA input on the front. this amp was brought to me with the complaint of really low volume on the RCA input. upon further inspection, this definitely seemed to be true. a reasonable listening volume could be obtained with the volume knob all the way up. definitely not a practical option if attempting to play live.
looking over the schematic (this one being Revision I) you’ve got 22k resistors at each input (left and right) mixed together for mono delivery into a 33k resistor all before the first gain stage. that’s definitely a hefty signal drop. to boost the signal, i simply removed the 33k SMD resistor on the preamp board pictured below and replaced it with a small piece of copper wire between the pads resulting in a significant volume boost. cranking up the tweeter pad on the back basically turned a mediocre bass amp into a portable DJ amp.
and if you’re not familiar with solder surface mount devices, you should really check out this extremely informative video.
here’s a 300 watt Ampeg SVT-CL that i had on the bench today. a much different kind of amp from the 70s models that i’ve been more accustomed to working on in the past. this one had no sound or output and the fault light would stay on constantly. i assumed right away that it must be circuit related. turns out that it was actually just drift in an old set of tubes that got so bad it caused the amp to fault. i switched the tubes around so that they could get a different set of voltages and then adjusted the bias until it stopped faulting. also replaced the 12AX7 phase inverter since it was showing signs of age. i figured it would help to at least have one known good in one of the most important spots. after that, i gave it a couple hours of play time with some solid signal just to make sure it didn’t drift again. a new set of 6550s should square out everything for another year.
well here’s the first draft of the simplest medium powered solid state design i could muster. TL082 can easily be substituted with a TL072 which is probably better anyhow (i just have all these TL082s laying around). the power supply i’m using more like 14-15V. both ICs should easily be able to handle up to 20V and that would allow for louder operation, but i think 15V is safe. all the resistors are 1/8W minus R20 which is 1/4W.
an older video, but one i’ve been wanting to post for some time. this is Alex Wilson of Son Cats playing on one of the several cigar-box amps i’ve constructed. this one, in particular, had one of the best sounds during the more work-with-what-is-available days. i thought the video was a great demo of an application best suited for a small 1/2 watt amp. thanks, Alex.