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Here's my Amp Mods document (PDF
6.9M), 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. 3/12/22 -
Updated with a "new" Bassman mod that recently came into the shop
for maintenance, also slightly edited a few things for clarity and
fixed a gruesome overlapping image glitch caused by loss of image
attributes after doing an edit elsewhere (note to self always
check everything after every edit no matter how minor).
Here's a page about using JFETs, a
transistor type that comes close to working like a tube.
Here's an Electronics Calculations program
I made using Free Pascal.
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
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...
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.
6/5/20 - Here's the original schematic of the Pro Octal Amp...
This shows the voltage control connected to the 1st stage but
ended up moving it on the 2nd stage instead where it can better
alter the tone. Originally wanted to use it to set the first stage
gain but the operating point shift had too much impact on input
overload. There is no negative feedback in the power amplifier
section, making it possible to flip the phase by reversing the
power tube grids. Other than using old octal pentode preamp tubes,
there's nothing that special about the preamp design other than
the compensated gain I usually use - this time with variable low
and high pre-clip equalization. The post-clip tones are
traditional with an extra cap on the mid so that it doesn't also
boost highs as much. The real coolness is using light bulbs in
series with the output transformer supply feed for the low power
function. Surely someone has thought of it but haven't ever seen
an amp that does this. At first I used resistors but to achive a
useful power reduction they had to be in the 1K range and got
quite hot, so replaced the resistors with light bulbs. This is a
class-A amp so the full power and zero power current is similar
(in this amp full power current is actually a bit less) so there
isn't a whole lot of dynamic effect, would be interesting to try
this trick with a grid-biased class-AB amp.
Stromberg Signet 22 (SAU-22) Mod
This is a cool little amp from around 1960, puts out about 20
watts with a pair of EL84's, a 6U8/7687 pentode/triode gain
stage/phase inverter and a 12AX7 preamp tube. They're fairly
common and make a nice mod amp, so far I've done three of them
(two for Chuck, another for a customer who had one and heard the
first one I did for Chuck). The original amp is a simple PA
amplifier with two screw-on type mic inputs, a RCA ceramic
phonograph input, four knobs for the inputs and tone, and
terminal-strip speaker connections. Before even bothering with
doing too much I change out the filter and coupling capacitors
which are usually toast and make sure the basic bones of the amp
Here are some pics I made of the original rough mod schematic and
mods 2 and 3...
Neither mod matches the original schematic - every amp is
different and has different needs. Mod 2 on the top apparently has
no plate cap at all on the 2nd stage, and an extra cap on the
phase inverter output at the power tube grids. Also has no front
input jack. Mod 3 looks like it has a cap on the pentode plate
resistor, and also has a toggle switch to select whether the tone
control is after gain 2 or after the master. Also left in the
original pentode stage bypass capacitor. Basically when I do mods
I play guitar through it and experiment with various filter values
until I'm happy with the tone. On both these mods added an extra
filter stage for the EL84 screen grids (470 ohms plus 22uF), on
the original they were connected directly to the main output
transformer supply.. yuck. The extra resistance limits the screen
grid current to keep the tubes happy and lowers the hum level.
6/5/20 - A better schematic of the first two versions of this
The tone position switch on the 3rd mod adds a 220K after the
wiper of volume 3, then switches between the resistor after volume
2 and the resistor after volume 3. The preamp resembles my 3-stage
overdrive design but using a pentode rather than a triode for the
3rd stage adds a lot more gain - it needs the extra gain control
to tame it down and be able to get clean tones. Having the preamp
clip stage in the power amp feedback loop has the effect of
lowering the gain as the master volume is increased.
Modified Vibrochamp with a Solid State Phase Inverter
This has got to be one of the funniest things I've done to a tube
amp but it came out surprisingly well. Customer had a blackface
Vibrochamp and wanted it to be louder to take on the road... on a
good day a stock Champ output is maybe 4-5 watts into 4 ohms
(stock was 3.2 ohms) and it had an 8 ohm speaker in it so it was
maybe pushing 3 watts. Well sure, could go even 50 watts with
Bassman transformers but the replacement speaker had a huge magnet
on it, no way 6L6's will clear that so had to be something with
6V6's. At first was thinking Deluxe-like but before doing that and
ordering transformers and cutting holes and stuff in a vintage amp
thought I would try to see what I could do with the stock power
transformer and something more minimal and reversible. First
thought was to use the existing 6V6 to drive a 2nd 6V6 for push
pull but testing that idea in LTspice showed it to be flawed.
Simple idea but the 2nd tube will always be an inverse of the
first tube (once the gain is balanced) so no way to get class AB,
at best just doubles the output with the same (ugly weak)
single-ended distortion. Adding a proper Princeton-style phase
inverter fixed it in simulation, able to get about 15 watts clean,
but as I was already putting another 6V6 on the heater line wasn't
keen on adding another 12AX7 to the heater winding, plus the whole
punching a hole in a vintage amp thing. So thought just use a
transistor phase inverter. Which would have been awesome if I had
a 500V 1W+ high gain transistor hanging around. Got 160V 600mW
2N5551's though, maybe I could stack 3 together? Why yes I can!
The resistor values could probably be optimized further but these
were the components I had on hand and seem to work well. Using a
solid state rectifier (a pair of 1KV 1.5A diodes) and an output
transformer I had laying around was able to get about 12 watts
into 8 ohms and about 15 watts into 16 ohms, roughly equivalent to
a Princeton. Didn't touch the preamp section, it had already had
the negative feedback disconnected.
The following simulations show more about what's going on with
varying levels of drive (click for bigger images)...
The floating emitter/collector notes get a bit "spiky" under
heavy overload, not sure what's up with that but the spikes do not
appear on the phase inverter outputs and with the output tubes in
full saturation it isn't going to be heard anyway. Using the stock
2n5550 LTspice model here but it's the same with another 2N5551
model. Adding 1000pF or 0.01uF capacitors to the floating base
nodes mostly make the spikes go away by clamping the emitters to a
mostly constant voltage, but that actually puts more voltage
stress on the transistors. Letting the emitter and base nodes
float and do whatever they want provides better voltage
distribution, keeping the maximum voltage on each transistor to
under 160 volts even with the spikes (if they are even real).
Every node is current limited so even if there is some breakdown
it's not going to matter much. Here are the LTspice files
if interested, the tube models were found on the web. The idea for
using LTspice's uniform RC-line symbol for a potentiometer came
from "analogspiceman's" tube
amp simulation files, awesome idea that avoids having to add
a symbol file.
Terry Newton (firstname.lastname@example.org)