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Here's my Amp Mods document (PDF), it
details a bunch of guitar amp mods and rebuilds I did, mostly from the
'90's while
working for Shiloh Music but some a little more recent. Back then it
was economical to get a silverface Fender from a pawn shop for cheap
(didn't matter much if it was broken) and gut some or all of it and
create a new amp, typically with an overdrive channel and an effects
loop.
Here's a page about using JFETs, a
transistor type that comes close to working like a tube.
Stuff about my home gig designing
surface mount electronics.
The Smokin Tone overdrive
12/13/17 - This is a JFET-based overdrive preamp that can be dialed
from almost clean to heavy distortion. The design has three gain stages
with a compensated gain control between the first two stages (at low
gain it boosts the highs, at high gain it cuts the lows), a fixed high
boost between the last two stages, and tones after that - inspired by
my amp mods and not coincidently it sounds a lot like a high-gain tube
amp. The stock design has a true bypass switch but can be wired to
leave the unity-gain output buffer in place even when bypassed. Very
low current drain, less than 2ma and most of that's for the LED.
Here's an early one in light purple (can't get that case
anymore) and later ones in blue and dark purple cases...
A view of the insides...
Eventually I'd like to get someone else to make the cases
pre-drilled
with professional silkscreening but for now just making them myself as
needed and labeling as requested or however I feel like. Order from the
SmokinTone page, $179+tax.
Customizations are available, including case color and doodling, more
or less gain,
optimizing for bass (more lows, more output, less gain) and
always-buffered output.
The Smokin Tone "Professional Octal Tube Amplifier" Head
12/13/17 - Recently I built a compact tube amp
head for Chuck Kotlarus...
Specs for this particular amp...
A cool thing about this amp is the low power switch that inserts two
11 watt bulbs in series with the output transformer center tap.. the
bulbs pulse with the signal and because it's a class-A amp, they dim
when it goes into clipping. I've seen a lot of schemes for reducing
output power (usually done by switching to triode wiring or varying the
screen voltage) but never tried the obvious - just put a resistor in
series with the output transformer feed. That worked but a resistor
there gets very hot, so used light bulbs instead.
Scope shots for normal, round and low power settings...
Overdrive with different gain and voltage settings...
Cool stuff!
Planning on making another one just like this one and maybe some
other variations, I love the chassis. Possibilities include using
common 12AX7 preamp tubes (6SJ7's are rare and often noisy), gain or
channel switching, and a class AB configuration for 40 to 50 watts
output power. It's very hard to predict what another musician will want
in an amp - some like two or three knobs, some want independent
channels and output power needs vary widely. At Skully's Saloon where I
play and run sound even a 15 watt amp can be too loud, but a downtown
country gig might need 100 watts of clean. For what I do a simple gain
switching scheme where it just boosts the preamp gain and drops the
output level works for me. Others need full channel switching or no
switching at all because they use pedals for that. So for now treating
it like an amp mod... customer tells me what they want then I make it.
Terry Newton (wtn90125@yahoo.com)