final build of my take on the Mitsumin Valvecaster.
Apr
05
2013
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 convinced the blue 1/2 watt resistors inside the amp are not really metal film despite their appearance. if you find yourself working on amp with these sort of issues, here is what i did to alleviate some of the nasty pops.
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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 fake metal-film resistors just to make sure they don’t start arcing. sorry 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
replace the speaker with just about any 12″ of your choice, and the thing cleans up pretty nice.
Mar
06
2013
NOAA 19 APT copy…
just a copy with a homebrew 2m ground plane antenna on the roof top with the FT-60. pretty surprising results considering the antenna isn’t even tuned for 137MHz.
Feb
25
2013
arduino temperature TV…
kind of a goofy experiment, but really useful. i’ve had a lot of LM335Zs laying around for a while. my christmas present this year was a new Arduino Mega 2560 board that i’ve hardly had a chance to do anything with. using the lovely Space Tinkerer’s blog and the good ol’ TVOut library, i turned a thrift store bargain into a fancy CRT monitor that reports the ambient temperature on the work bench.
Feb
22
2013
simple phase shift oscillator…
a practice in designing an oscillator. this particular circuit employs a 2N2222A transistor on 6 volts. i used this Electronic-Tutorials.ws page to design the circuit. there were many reasons for this experiment, but the most immediate was to act as a simple CW practice oscillator. the final circuit looks something like the following.
Feb
20
2013
light bulb current limiter…
homemade light bulb current limiter. i’ll add more to this later.
Feb
17
2013
sqrt() reverb…
this was the final build before it shipped out. circuit is a simple MOSFET (BS170) driver stage followed by two JFET (J201) recovery stages. i’ve included a sound sample of a similar reverb i built later without the second recovery stage and clipping diodes. it makes for a much more subtle reverb, but also tames the noise floor from the Belton module.
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this is the actual first build. the “dirty verb” comes in towards the end which just switching on a pair of germanium clipping diodes. you get more reverb for your buck with the second JFET stage, but i haven’t found a way to cool down the noise just yet. that’ll be revision 5… maybe 7.
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Feb
16
2013
Mistumin Valve Caster build…
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.
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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
Feb
16
2013
no. 5 re-appropriation…
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.
