Stuff about Audio and Electronics

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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.

All information here is provided as-is and without warranty, use at your own risk. Tube amplifiers use lethal voltages, do not attempt to build these circuits unless experienced in high voltage safety procedures and are familiar with building tube amplifiers. These are not construction plans, many details are omitted. Rather these are ideas to inspire and to document some of the things I make.

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.

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 are good.

The mod replaces the mic connectors with 1/4" jacks (high gain and low gain), turns the controls into gain (high), gain (high+low), master volume and tone, and adds a 1/4" speaker connector and an ohms selector switch. Basically it puts the preamp stages in series and puts the master volume between the pentode gain stage and the triode phase splitter stage. When the high gain input is used both 12AX7 stages are in series with the first gain control between the two stages and second gain control between the 2nd stage and the pentode gain stage. The two gains have different value treble boost networks so different tones can be achieved by different combinations of the two gains - gain one has more mid-range boost at lower settings and gain two has a higher frequency boost at lower settings, more like a conventional bright capacitor. When using the 2nd input jack the 1st stage is disconnected and only gain 2 is active for cleaner sounds. In the original mod the tone control was after gain 2 and before the pentode but it also works after the master volume and before the triode phase inverter (with a resistor between the wiper and the tone circuit).

With some old amps I can series the preamps without a lot more other than compensating the gain(s) so they cut bass when up and boost highs at lower settings, and add small-value caps across the preamp plate resistors to tame the harmonics (also supplemented by the tone control and further stages). If done right the high cut on the clip stage and preamp stages is balanced by the high boost on the gain control so when set clean it's still reasonably bright. With this amp it's not quite that easy, the mic preamps are of the "grid leak" variety with grounded cathodes - that might work with a low-level signal like a microphone but they have totally unusable overdrive characteristics, as in they overbias and cut out when clipped. The 12AX7 circuitry has to be almost totally rewired to convert the stages to conventional cathode bias.

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 mod...

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.

An LTspice simulation of an entire tube amp

This simulation borrows ideas from analogspiceman's tube amp simulation, the control and tube models are adapted from the Fender5E7Bndmstr.asc file, the rectifier tube models are from Duncan Amps, adapted to use a triode symbol to avoid having to make a new rectifier symbol, so the schematic looks a bit funny. Most aspects are simulated - power transformer, ripple, filter capacitors, sag - but the transformers are "perfect" other than winding series resistance. That's fine by me, in my opinion if one can hear transformer effects then it's either inadequate or being overloaded.

Here's the LTspice schematic...

The simulated amplifier uses three 12AX7 tubes, a JFET for driving the effects loop, two 6L6 tubes, and one 5AR4 or 5U4 rectifier tube. The preamp is a three-stage design with a gain control between the 1st and 2nd stage and a drive control between the 2nd and 3rd stage. The gain control has both low and high frequency compensation, the drive control has low frequency compensation. The preamp is followed by a cathode follower driving the tone stack and volume control, which feeds a unity gain N-channel JFET buffer driving the effects loop send. Choice of JFET isn't critical, doesn't even have to be a JFET, an NPN transistor works just as well. The preamp design is typical of the kind of stuff I make but haven't built this exact design (yet), usually I just have a single gain control in conjunction with a clean channel or some other way to drop the overall gain. In this single-channel non-switching design the additional drive control provides a simple way to clean it up or go full-on dirt.

The extra circuitry around 3rd stage V2a and cathode follower V2b is to provide more symmetric 2nd-order filtering - high frequency filter C10 is connected directly across V2a so its effect is asymmetrical, R13 and R12 attenuate the signal to drop it to effects level (after passing through the tones/volume), and C11 provides additional more symmetrical high frequency filtering. R45 and R46 shift the DC down to avoid exceeding the 12AX7's cathode-heater breakdown voltage, C26 bypasses R45 so that the full AC signal is applied to the cathode follower.

The power amp (from the effects return) has a 12AX7 gain stage V3a with a rather large plate resistor for maximum voltage gain (to reduce the effects loop level), feeding a single stage unity gain phase spliter which feeds the 6L6 power tubes with fairly large (56K) grid resistors to avoid too much asymmetric shift when overloaded. There is no overall negative feedback. Resistor R35 between the OT supply and screen grid supply is moderately high (1.5K) for a bit of sag compression, can be smaller or a choke for tighter response. 1 ohm resistors R32 and R33 are for setting the bias, Rpca and Cpca are only for the simulation, for measuring the average current through V5. Rload represents the speaker, Rf1, Rf2, Rf3, L10, L11, C30 and C31 are only for the simulation to mimic high frequency speaker response, although a basic compensated line output could have a similar design. Originally I tried the simulated speaker model from the 5E7 simulation, although probably more accurate it distorted the output waveform making it hard to tell how close the output was to a real tube amp - I'm used to using a pure resistive load when bench-testing amplifiers. With the resistive load it looks pretty much identical to what I'm used to seeing on the scope.

The power supply is pretty much like a typical tube rectifier amplifier - ignore the grid in the 5U4 symbols and both sections are in one envelope. Rrectfil represents the rectifier tube filament, Rheaters represent the 12AX7 filaments. D2 and D3 in series with the rectifier tube plates is a common trick for keeping the rectifier from sparking/shorting from power surges. TR1 is the bias control. V3, V6, Ddc1 and Ddc2 are not part of a real circuit, they are only to briefly pre-bias the simulation for setting a useful DC operating point so that static node voltages and component power dissipations will be useful, the initial pulse voltages are set to be similar to the zero signal supply and bias voltages. The GigaOhm resistor Rleak is only for the simulation so that it doesn't complain about a floating node, there should be no connection between the AC primary and ground. The power switch, fuse, standby switch and pilot light are omitted from the simulation schematic. Although the simulation is mostly complete, it is not intended to be plans for making a real amplifier - do not attempt to build this unless experienced in these things and you know how to fill in the missing bits.

Here's are low gain high volume transient simulations with 5U4 and 5AR4 (GZ34) rectifiers... (click images for bigger)

The 5U4 rectifier has lower output and more sag under load, whereas the 5AR4 delivers more power and a tighter response. In the simulations the 5U4 is clipping at about 24V peak/17V RMS, equivalent to about 36 watts into 8 ohms at clip, and the 5AR4 is clipping at about 28V peak/19.8V RMS, equivalent to about 49 watts into 8 ohms at clip. Thereabouts.. output power in a real amplifier is highly affected by the power and output transformer specs.

Here's how it responds to an actual guitar signal at low gain (with a 5AR4) and at higher gain (with a 5U4)...

Nice. Here's the low gain simulated output, and the higher gain simulated output (converted to MP3 files). Not exactly what the amp would actually sound like but close, at least about what it would sound like into a dummy load with a simple 7K LP filter "speaker emulator" output. Here's a zip file containing the LTspice files and the original input sound file.

Terry Newton (