a close-up picture of the guts of a JJ EL84 with all it’s grids exposed. such a seemingly odd and fascinating device.
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.
a tube overdrive build for Anthony of the The New Diet and The Regular Fucked Up People. it’s my take on the Mitsumin valve caster with the values i tend to use and a Telefunken 12AX7. excuse the ugliness for this one is just the prototype.
four sections of clean vs. effected total.
[audio:http://abrammorphew.com/notes/wp-content/uploads/2013/02/tube_overdrive_sample_long_02.mp3|titles=Tube Overdrive sound sample]
1) high volume, no tone roll-off, and low gain
2) mid volume, high tone roll-off, and mid gain
3) mid volume, mid tone, mid gain
4) high volume, low tone, high gain
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.
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.
and what a lovely voice it is. even through a Crate speaker (i’m saving the better speakers for when the testing phase is over). overall, i’d say a success for cheap transformers, Radio Shack parts, and a few pieces taken from a busted computer power supply.
audio notes: it was late, and i was annoying enough playing at midnight. the mic is sitting just above the speaker which was laying face up in a milk crate. i’m currently using a 100k audio pot, and i think it would fair better with a 1M. overall though, the amp is surprisingly quiet at mid volume. i still need to drill out the hole for AC connector, but it will come in due time. for now, it’s making sound and that’s a huge first step for me.
like Dr. Frankenstein, it’s about time to bring this little fellow to life… or at least try. the schematic i have is a random luck of the draw internet find, and i have no idea whether or not it will work. however, testing will occur on the work bench soon enough to determine what needs to make this single-ended 5watter amplify.
I’m hoping this will clear things up for the night. Practical matters have brought about the decision to build a tube amplifier. In this case, I’m going to get a WeberVST kit as not to go it solo on the first attempt. The winner of much debate as to which circuit to start with is the ever popular 5E3, a Fender ’57 Tweed Deluxe circuit.
Unfortunately, this circuit isn’t quite ideal. The goal for our prototype is going to be about balancing loudness with clarity and transforming its bluesy rock tone into something a little smoother and less aggressive. We’re going to attempt lowering the gain in the preamp by using tubes with a lower gain factor and increasing wattage in the output stage which will include substituting some 6L6 variety for the standard 6V6s. The use of two separate volume controls seems to be pointless for our purposes as well. I think I’ll probably wind up setting up a treble and bass tone stack by dumping one of the volume pots.
Anyone care to buy a Marshall 2×12 4102?