Modular Tube Amp

Bernhard Walter, September 2008
a.k.a Heinz

This page is currently under construction


1. Motivation
2. Concept
2.1 Module Basics
2.2 Chassis Mounting
2.3 Amp Design
3. Modules
3.1.1 Triode Preamp
3.1.2 Pentode Preamp *New*
3.2 Reverb *New*
3.3 Fender Tone Stack
3.4 James Tone Stack *New*
3.5 Tremolo
3.6 Power Amp *New*
4. Example

1. Motivation

If you look at a guitar amp circuit you will notice that all amps are made of larger building blocks with specific functions. A very simple amp has a preamp section, an optional tone stack and a power amp, while more complex amps can have reverb, tremolo, multiple channels, switching circuitry or effect loops. These sections are mostly independent circuits with only a few connections to the rest of the amp. The basic idea of a modular amp is to extract these circuits and pack them into modules.

Although most classic guitar amps have their very own sound characteristics they build on only a handful of circuit topologies. While component values and types may vary, the basic circuit topology (e.g. a grounded cathode preamp stage or a PP power amp) remains mostly the same. A variety of different topologies can be covered with jumpers on a carefully designed PCB. This makes it possible to build virtually any preamp stage (Fender, Marshall, Soldano, etc) on a single PCB simply by adapting component values and placing jumper wires. All other module designs follow the same pattern. So a module is a basic topology that can then be configured to become an instance of a specific amp component.

This modular approach opens up many different options for experimentation. You can build the modules for a specific amp and then swap other modules in and out. By doing so the system can be used as a high-level prototyping system. The very same modules can then be used for building the amp, once you are satisfied with the results. This is a major advantage over a prototyping board. Besides, existing modules like preamps or power amps can be used while developing new circuits and only the new functions need to be prototyped.

In a nutshell, a modular amp system offers the following advantages:

2. Concept

2.1 Module Basics

The T7 is a collection of modules for building guitar amps. Each module is a functional unit that utilises one or more tubes. All modules have a similar layout and consist of two single sided PCBs, a large PCB for the circuit and the smaller tube carrier PCB. The tube carrier is optional. It does not contain any components except the tube and can be replaced with a chassis-mounted tube socket (some tube carriers actually carry components like blocking caps, but these need to be close to the tube and would normally be soldered right to the tube socket). The tube carrier board acts as an adapter from the circuit board to the tube. Some modules (e.g. the preamp module) can be operated with a variety of tubes. Each of these tubes has its own tube carrier board while the circuit boards remains identical. The tube carriers can be jumpered for series and parallel heaters.

Using tube carrier boards or chassis-mounted sockets is a matter of taste. I personally prefer the tube carrier boards for several reasons:
The main disadvantage of the tube carriers is the extra effort needed to design and build them.

Simple modules can be built with a single tube (e.g. preamps) while more sophisticated modules like power amps can use several tubes. However, tubes are never shared among modules. This may leave parts of a tube unused (e.g. a triode in an ECC83/12AX7) but it is essential for easy repair, interchangeability and experimenting.

Each module has a set of inputs, outputs and controls. Power supply connectors are located in the upper left, signal inputs in the lower right and signal outputs in the lower left corner. The modules basically take an input signal on the right side and transfer it to the left side. Control inputs/outputs (for pots, switches, lights) are located at the front to minimize the distance to the front panel. Terminal blocks are used for all connectors to ensure secure connections while retaining flexibility.
Some modules need additional inputs/outputs (e.g. the reverb module). These can be placed wherever appropriate.

Basic Module Layout
Single tube moduleTwin tube moduleChassis mounted tube

All modules are the same length but may vary in width, depending on the complexity of the circuit, the size of the required components and the number of tubes. Tubes are fitted on the extra tube carrier board and face downward. The tube sockets are therefore soldered to the solder side of the board. Tube spacing depends on the size of the tubes, the heat dissipation and the space needed on the tube carrier board for routing the traces.

Screw holes should be placed around the tube sockets, so that each tube tube is framed by four screws. This arrangement is solid enough to withstand the forces of inserting/removing tubes. Four screws are usually enough for the circuit board unless it carries very heavy or large components.

Module design
Top viewSide view flatSide view angled

2.2 Chassis Mounting

The modules can be mounted flat to the top/bottom of the chassis (e.g. in an amp head) or bent to an angle and fitted to a corner of the chassis (e.g. in a hanging chassis with the tubes sticking out of the back as in an 18Watt). The tube carrier boards are only 40mm high and will fit very flat chassis.

Spacers and screws should be chosen as needed for the specific chassis.

Chassis mounting

2.3 Amp Design

The complete amp consists of a number of modules which are chained via their inputs/outputs. This ensures short signal connections and easy access for experimenting and troubleshooting. For more complex amps (e.g. with channel switching) the chaining method will not work and a more sophisticated module layout with longer signal connections will be required.

If a module does not contribute to the signal path (e.g. the tremolo module which only wobbles the bias of the power amp module) it simply connects the input and output terminals to maintain the structure.
Each module has its own power supply terminal and a separate ground terminal that can be wired to a star ground.

Basic amp layout

3. Modules

The following section discusses the modules that I have designed so far. The schematics were drawn with JSchem and the layouts with ExpressPCB. You will need to download and install these programs in order to view the schematics and layouts.

The schematics of the modules described here are available in a single file here. Please note, that the component values in this file and in the following descriptions represent a configuration for a Marshall-type amp. The modules are a platform for amp design and do not a have component values per se.

3.1.1 Triode Preamp Module

The triode preamp module is a highly configurable module. The schematic may initially look confusing but it is basically only two simple grounded cathode stages with a number of jumpers that offer the following options:

Single channel, double stage---XX-XXuse Input1/Output2
Single channel, single stage, double triodesXXX-X---omit all passive components of the lower section
use Input1/Output1
Single channel, JCM800-style double stage with volume control at second stage input-----XXXuse Input1, Input2 & Output2
Dual channel, single stage----X---use Input1/Output1 and Input2/Output2
X=installed, -=open, *=don't care

The module allows for several components that may not be needed in all configurations (e.g. the cathode bypass cap C403). These components can then simply be omitted or bridged as needed.
There is an RC-filter for B+ on the module to ban all power supply noise. The addition of this filter has made my prototype perfectly quiet.

Triode Preamp module
Preamp module schematic, configured for a Marshall JCM800Preamp module layout

3.1.2 Pentode Preamp Module

This is a pentode preamp module as seen in the famous VOX amps. I have used an EF85 here (because it is cheap) and it performs quite nicely. The EF85 can be used for parallel and series heating and does not show any signs of microphony. Changing the tube to an EF86 does not change the circuit topology, so you only need a different tube carrier board and have to adjust some component values (e.g. the cathode resistor).

The brilliance switch is an optional feature that allows you to switch between 4 values of the coupling cap. You can choose between the default value (switch open) and the three switchable values which are paralleled to the default cap. If you do not need this feature, you can choose a suitable value for C703 (e.g. 10nF) and omit C704-C706.

There is an extra RC-filter for B+ on the module to smooth out all power supply noise.

Pentode Preamp module
Pentode preamp module schematic, configured for a VOX AC-15Pentode preamp module layout

3.2 Reverb Module

The reverb module is a transformerless reverb that is my own design. It uses an ECL86/6GW8 or the series heater equivalent PCL86/14GW8. The power section of the tube drives the reverb and the triode section, which is very similar to an ECC83/12AX7 triode, recovers the reverb signal. When used with an Accutronics 4Fxxx (high impedance) reverb tank this circuit is capable of producing very deep and vintage sounding reverb.
The initial design used a power resistor as plate load for the pentode. This works to some extent but wastes a lot of power for a limited amount of output drive. This can be very much improved by using an inductor as plate load. Since regular chokes are much too big for mounting to such a small PCB, I used the primary winding of a tiny PCB-mount 3VA transformer. This solution is very effective, both in performance and cost.

The module has two controls: drive and level. The drive control sets the signal voltage for the reverb driver and the level pot controls the mix of the returning reverb and the dry input signal. Having both controls can be redundant since both modify the volume of the reverb signal. However, the drive control also slightly modifies the tone of the reverb. If control panel space is an issue the drive control can be omitted, permanently applying full signal to the reverb driver.

Reverb module
Reverb module schematicReverb module layout

3.3 Fender Tone Stack Module

This module is a classic Fender tone stack topology that can be fitted with varying component values according to taste. It has one ECC83/12AX7 tube which is used as a gain stage and cathode follower. An optional master volume pot can be used if required. If the mid pot is lifted from ground it will disable the tone stack and give the signal an additional boost.

There are a lot of other tone stack topologies that might be worth trying. These could be designed as new modules and would be a nice addition to the module pool. Such a collection of tone stack modules would be handy for a comparison in an otherwise unmodified amp.

Fender tone stack module
Fender tone stack module schematicFender tone stack module layout

3.3 James Tone Stack Module

This module is a James- or Baxandall-type tone control that can be used as a replacement for the Fender-type module. It has only bass and treble, but no mid control.

James tone stack module
James tone stack module schematicJames tone stack module layout

3.3 Tremolo Module

The tremolo module is a typical bias tremolo. This module was designed around a PCF802/9JW8 tube simply because I had some. The LFO uses the pentode part of the tube, the triode acts as a buffer and amplifier for the tremolo signal. This circuit needs at least 220V of well-filtered B+ to operate reliably. If the voltage is too low the oscillator won't start or might die off slowly. If the power supply noise is too much you will hear ghost notes.

The PCF802/9JW can be replaced with a ECC83/12AX7 (with a different tube carrier). R504, R505 and C504 can be omitted in this constellation, other values may have to be changed.

Tremolo module installation
Tremolo/power amp module combination

This module is designed to be a right-side companion of the power amp module. Since it does not contribute to the signal path the output terminals are simply connected to the inputs. The power amp module has a bias input at its upper right side that has a corresponding output on the upper left side of the tremolo module.

Tremolo module
Tremolo module schematicTremolo module layout

3.3 Power Amp Module

The power amp is another highly configurable module. It is a cathode biased push-pull amp with a long-tail phase inverter. The module offers the following options:

Single channel inputX*****R116 and R118 can be omitted
Separate cathode resistors*****-useful for highly unmatched tubes
Top cut control*XX***see VOX circuits
Post-phase-inverter master volume*X****
No presence control***X**add the value of the presence pot to the long tail resistor
External bias tremolo****-*apply tremolo signal to T102
X=installed, -=open, *=don't care

The module has a dummy load resistor on board and does not required any external conponents except the output transformer.

The original design uses two PCL82/16A8 (or ECL82/6BM8) tubes and produces around 10 watts of power. I have designed additional tube carrier boards for a pair of ECL86/6GW8 (or PCL86/14GW8, about 13W power) and one for a pair of EL84/6BQ5 and one ECC83/12AX7 (about 18W power). However, I have not been able to build and test these.

Power amp module
Power amp module schematicPower amp module layout


This is a modular amp that is loosely based on a Marshall JCM800 (2203). The preamp section is mostly original with only some minor mods, the power amp is my own design. The amp has the following features:

The power stage drives a Dynacord 12" speaker through a Deluxe OT in an open back combo. It has a nice clean tone but lacks some of the Marshall grind when overdriven. The bright switch is very effective when used with single coil pickups, while the preamp-boost is more suited for humbuckers and overdriven sound.

The prototype uses series heating (63V dc/300mA).

You can download the schematic here.

From left to right: power amp, tremolo, tonestack, reverb, preampDetails of the reverb module (the metal standoffs were later replaced with plastic)

I designed the modules, one by one, on a breadboard and made a PCB as soon as I was satified with the design. Then I prototyped the next module and used the finished module(s) for testing. The first module was the power amp, the preamp was next, then tonestack, tremolo, reverb.

Chassis components

The chassis is a simple construction made of two pieces of aluminium U-profile and an aluminium sheet from the trash, screwed together. It is very solid and can be disassembled easily. Drilling the sheet part is much easier than drilling a 3D chassis.

Modules installed, front viewBack side

The "faceplate" is a sheet of transparent film. I designed the faceplate with Front Designer and laser-printed it (mirrored) on transparent film. The film is then folded around the edges of the chassis with the paint on the inside of the film (between film and chassis) and glued to the chassis. It is held in place by the pots, jacks and switches.
Looks pretty professional, but I have no idea how long it will last.

Front panel
Faceplate and front panel controls installedFront view

The power supply is located on the outside of the amp, because there was no more room inside. The transformers are much too large but they were free (so who cares).

Finished amp front
The power supply still needs a coverAmp wired and ready to rock

The chassis is designed for vertical mounting (18watt-style) with the tubes facing downwards. Since the chassis is a bit too narrow, there was no room left on the back side for the speaker plug and the fuse holder, so I moved the fuse to the inside and the speaker plug between the PTs and the OT.

Finished amp back
Tubes protruding through the back profileThe wiring of the front controls

Here's a close-up view of the modules and their connections. I can take out any module for repair (tweaking or replacing) by unscrewing the terminals and the mounting screws. Un/mounting could be even easier if connector assemblies were used instead of terminals and for connecting to the tube carrier. I'll try that in my next design.

Module details