BOOK

THE ENGINEERING AND DESIGN

When we started to design this amp there were certain criterions it had to fulfill. These were:
1. The amp must have different gain and distortion options suitable for a wide range of styles. The player would have to be able to customize his/her sound with controls on amp and also control the amplifier through picking attack and from controls on guitar, and/or from pedal board. Complex chords should be defined even at high levels of distortion, and single notes should have a singing quality even at clean settings.
2. Have only one channel made without compromises and with dedication to classic and immortal sounds of amps of yesteryear.
3. Be original and not a copy.
4. Have a tone control section extensive enough to allow the discriminating guitarist to fine-tune his/her preference.
5. Have output power enough to be used with high powered bands, and good enough tone for session players in small clubs, at levels appropriate to circumstances.
6. Have a loop to allow positioning of Echo-circuit after main distortion generation, with level options to allow use of valve driven tape echoes as well as transistorized analog delays.
7. Have selectable boost with as little as possible tone coloring. This very simple function is, though in demand, rather uncommon.
8. Use tubes available throughout the world.
9. Be made so that you can depend on it delivering exact tone under any circumstances. This being the one criterion a player would ask besides excellent tone. .
10. Be tested by working musicians for excellent tone before shipping.
11. Have the ability to produce preamp distortion as well as power amp distortion at both light and heavy settings.
12. Have a fast, dynamic response. To do this several dynamic limiting distortion mechanisms had to be restricted through design.
13. Do the hardest thing of all - be inspiring and addictive.

SETTING UP THE AMP

Always ensure that you have speaker connected before turning on the amp. This is a tube amp and as such it requires a load. It can temporarily withstand a short circuit and brief duration of non load, but correct load impedance is recommended for prolonged use and long life. Set Master to Zero at startup. Turn on power. Wait a while, to let bias build up, then turn on Standby. Get a view of what you want to do.

There are three types of gain structures available: ABNORMAL input has two (FET & TUBE), and NORMAL has one. Both inputs are controlled by the VOLUME control, which sets sensitivity of inputs as well as onset of clipping, depth of compression in preamp.
ABNORMAL input is also slightly brighter than NORMAL for the purpose of making string separation better at high levels of distortion. Use this input if you look after distortion beyond light breakup.
You will have pre EQ distortion making it possible to equalize distorted sound. This way you can scoop out mids or honk them at will, and that at dynamic or saturated levels of distortion. The two structures offered (FET & TUBE) will give different shades of distortion more than difference in distortion depth to allow different tones to be achieved.

NORMAL INPUT VIDEO: Low/High

ABNORMAL INPUT AND TUBE/FET SWITCH VIDEO: Low/High

TONE CONTROLS
These are designed to produce multiple functions and be used under all conditions. The tone section is of the passive kind, T-notch-type, as almost all famous amplifiers, but adapted for the occasion. FOCUS switch is one of these adoptions.

FOCUS SWITCH VIDEO: Low/High

TONE CONTROLS VIDEO: Low/High

The two open symbols are used for break up and power amp distortion, while the solid symbols are better suited for medium to heavy preamp distortion. TREBLE control is the other special feature. This control sets treble response only (unlike on most amps) - while BALANCE is more like the Treble control on most amps. BALANCE sets the Treble/Bass balance of the amp, the more treble the less there is bass and vice verse. Use this control to find the nice balanced tone for your purpose and then use the BASS, MIDDLE and TREBLE controls to fine-tune your tone. The whole tone-control section is affected by the setting of FOCUS.

PURPOSE AND USE OF POWERSUCK
This control acts like a kind of master volume located within power amp. The circuit has been tuned to avoid the treble loss normally associated with this kind of control, as well as good balance and retaining of tone. It can be bypassed and for each position it will change the response of power amp from bold and clean to easier and easier break up. At lowest setting NORMAL input can be used to overdrive power amp at conversation level. Power amp construction is such that main distortion contribution will be from the tubes and not from limitations in peripheral circuits. POWERSUCK can be used to color the preamp tone with power-amp at lower volume levels using the MASTER VOLUME. You can get both small combo or big amp response using this switch.

THE POWERSUCK VIDEO: Low/High

LOOP LEVEL
This switch has two positions controlling effects loop level. There are two level options. One for low-level effects, driven by a 9V supply, and one for line level effects, like tape echoes and certain studio or rack gear.

BOOST CONTROL AND FUNCTION
BOOST is made as a mere volume boost with as little as possible tone coloration. BOOST level can be set to 0dB - +15dB (no boost to five times) depending on settings used. The purpose of this control is to provide a level change for e.g. lead work at same level as vocals. Though designed for pure clean boost it can also be used for driving the amp into power-amp distortion when power amp is at the edge of distortion. BOOST has a pedal jack brought out on backside of the amp. Any ordinary shielded guitar cable and one pole shorting switch can be employed to operate function should original foot switch be lost, stolen or forgotten, including foot switches supplied with amps of different makes, should such a need occur.

PRESENCE CONTROL
This tuned control sets overall brilliance and provides a resonant peak located higher than TREBLE or BALANCE for exacting control over the sensitive treble part of sound. With distorted sounds, it can be used to control feedback from pickups and add bite to pickups lacking that. You can get saturated sounds out of a single-coil guitar pickup with out high frequency squealing and sting from a jazzy neck pickup. The soul of guitar amp construction is to get control over treble and distortion. This amp has ample control over both. Even at essentially clean sounds there are distortions covering difference in sound. The question is not whether or not distortion, but detail of contents of distortion.

MASTER VOLUME
This is your main output level control but you can also use this to overdrive the power-amp with the POWERSUCK for yet an other dimension of overridden tones.

MASTERVOLUME AND NORMAL CHANNEL VIDEO: Low/High

TUBE SWAPPING

You can use many kind of tubes with the Mad Professor amp. The mains transformer has build so it can deliver power to all possible tubes, so you can use 6L6, 5881, EL34, KT66, KT88, 6550 .... or even 6V6 if you find good ones that can handle 500V plate voltage.

The preamp section is also tunable with different tubes like 12AX7/ECC83/7025, ECC81, ECC82, 5751, 12AT7,12AUT ....
The Mad Professor design is such that you will hear the tone of the tube, there is no transformer distortion or sag, the distortion and sag comes from the tubes and it is fully adjustable.
We have tested quite a few tubes with the amp and all sound different but wonderful.
It's fun and easy with the Bias Test points and Bias Switch and Pot.

All you need is a Voltmeter and potholder from the kitchen (for hot tubes, you can't wait for them to cool, it's too much fun)
The EL34 sounds great with this amp, lots of power and solid fat tone with nice treble response. You can get great clean tone with these tube also. The best tube was Mullard EL34 but we were happy with both Svetlana and JJ too. JJ EL34L was the loudest EL34. The amps comes stock with Svetlana EL34 if you don't want something else.

6L6: More open mids, bit less power but super nice clean tone. Distortion was very sweet, not quite as fat as with EL34 but more into upper mids. Bluesy.

KT66 by Ruby Tubes, this one has the best of both worlds and is on my amp if the Mullards need to cool down. Singing like a bird, the tube for the Robben Ford, Carlton tones or fat Texas Blues.
If you need power put in a pair of KT88, it made the amp very loud with a lot of headroom and rock solid distortion, this is the tube for loud Rock'n' Roll.
And the preamp tubes, the 5751 in the first position with the TK66's wow!!!
ECC81 and KT88, super clean and loud and so on.....

BIASING THE MAD PROFESSOR AMP

This amplifier was designed around the now extinct Mullard EL 34 power pentode, but well in the know of this, it should be able to take tubes available today including all tubes that have the same socket connections and heater current draw in range, plate load characteristics in range. Therefore it is possible to run the amp with 6L6, EL34, KT66, KT88, 6550 and in case extra heavy duty 6V6's can be found, also those, but extra caution must be taken that these can handle in excess of 500V of plate voltage.
Idle current of the EL34 power pentodes recommended is chosen at 20 to 25mA, which may be considered low for the tubes in question and higher idle currents are possible if requested. The reason for choosing this low idle current is to gain as much power from the EL34's as possible at very low crossover distortion. Increasing idle current will result in easier compression and a little less output, sometimes preferred but not totally gaining most effect out of amplifier circuit.

Utilizing different tubes such as the beam power tetrode's KT66, 6L6, KT88 or 6550 will present no problems and idle current may be chosen higher, within respect to plate and screen grid dissipation, and heater supply will suffice for use of the hefty KT88's. Guidelines as set by tube manufactures and other technicians may be followed without problems, though it is recommended from designer that as low as possible idle current be set, to gain most dynamic output.

In regards to what class amp is working in, it is not that important, because by definition a class B amp would have zero distortion or be class C. It would then be more of a question of efficiency, applied power to power gained. The most important would be the sound and to what kind of circuit is used would have little importance to user as long as correct sound is produced.

The circuit of Mad Professor Amp, model CS-40, is designed to take the most advantage of power applied in the sense of power output, and limitations to this has been as much as possible reduced for the benefit of maximum dynamics and as such would be most unusual.

Where most designs would optimize for getting deviations of peripheral circuits to coincide with the whole of the amp for the absolute optimum and there really are some amps capable of just that, at a beauty seldom heard the MP was designed to as much as possible rid amplifier of limitations instead of getting the optimum within the limitations. Therefore some unusual measures were taken to ensure this, one being the operation point of the power tubes an other the capacity of power supply.
You may punish this amp severely before it goes up in smoke!
B.Juhl

EFFECTS LOOP

This is where you connect your Reverb or Echo unit (many other effects work well here to - please experimentation). There are two sets of levels to allow use of 9 Volt driven units to Tape Echoes or Reverb add-ons requiring line level. It's of the series kind, but made as an integral part of amplifier for best possible tone. This sets some demands on effects connected into the loop but sound is not compromised within amplifier.

Other uses for the loop include preamp driving other amp or amps in a multiple amplifier setup, or Receive jack being used as instrument input for real early amp sounds or for Slave purposes.

Loop level switch affects both send level and receive sensitivity keeping overall level of amp the same.

MAINTAINS

This amp is designed and built by servicemen to last and therefore shall not need any service of components located inside chassis within our lifetime. Expected tube life is 800 - 1500h for the power tubes, even 1500 - 3000h with high quality tubes. Change the preamp tubes every second time you change the power tubes.

Here's a list of possible tube failures and other finer points of tube usage: A shorting power tube will blow HT fuse, prompting you for a new fuse and new set of output tubes.

Bias is set through bias pot and measured with standard VOM or DVM. Only back panel needs to be removed to perform this service.

Excessive hum and/or squealing is usually a sign that one of the preamp tubes gone bad. This is most often tube 1 located closest to input. Changing this tube 9/10 times restores normal operation.

Amp produces no sound regardless of position controls. Most likely candidates are tubes 4 and 5. Change these tubes in sets. Use matched side's version for tube 5 for best results.
Tubes 2 and 3 are intermediate stages. These will be the least prone to failure, but will need to be changed now and then. As these tubes grow old, impact on sound will be less bandwidth, easier distortion at the cost of a little higher noise levels.

If you have a bin full of old Mullards, Brimar or Siemens used, but still good, you can use them in these positions for tuning purposes when you want the effects of a worn tube and can live with the extra noise. Tube 2 is the most sensitive for this.
Sockets 2 and 3 can use ECC 81/12AT7 or the ECC82/12AU7 for these purposes although the ECC81/12AT7 is considered a non musical tube originally designed for computing purposes. The ECC82 trades voltage gain for power and its use in this amplifier would be to lower overall gain. Because of different tube parameters distortion characteristics will be different from the ECC83/12AX7/7025. Sound produced will sag on attack - in a way emulating the response of a tube rectifier amp driven to sag. The mechanism is totally different but sound nevertheless reminiscent. There is a certain overtone structure produced.

By changing different tubes for position 2, and at some extent position 3 you can get different kind of amplifier response and different shades of distortion and bandwidth can be tuned. Always employ a low noise good quality tube for tube 1.

In the output section you can use different sets of output tubes, e.g. 6L6, KT88 but EL 34 is the preferred type - Mad Professor amp was designed around the characteristics of this tube for excellence in both clean and distorted tones. Test results of different tubes are pending further investigation. Information will be supplied of use of other tubes of different / lesser performance.

HOW TO GET DISTORTION OUT OF THE MAD PROFESSOR AMP

The design goal of this amp is to get good control over treble response and distortion, these being the most tone determining ingredients. Here are some examples of settings to give a view of how the controls work.
1. Medium to heavy distortion with dynamics: Use ABNORMAL input, set desired distortion depth with VOLUME, put POWERSUCK off and control output level with MASTER VOLUME.
2. Medium to heavy distortion with power amp compression: Use ABNORMAL input, set desired distortion depth with VOLUME, set MASTER VOLUME at middle position or higher and control output level with POWERSUCK.
3. Almost inaudible distortion to light: Use NORMAL input, set MASTER VOLUME on full, then back off a notch or two to get quietest operation, set desired distortion depth with VOLUME and control output level with POWERSUCK.
4. Light distortion: Use NORMAL input, put VOLUME at three o' clock, POWERSUCK off, set MASTER VOLUME for desired output level. You will need some power here to get the most out of this setting.
Notes: Setting of tone controls will effect distortion mainly when employing power amp compression. This can be used to dial in sounds where treble strings distort sooner than bass strings. This effect is most evident when using No 3 settings.

MASTER VOLUME will appear to have little effect when using NORMAL input. Its use here is to swap out preamp noise. POWERSUCK and VOLUME are your level controls here. MASTER VOLUME is however essential when using ABNORMAL input where it takes on the role of output and compression control.

POWERSUCK is intended to control output level and set distortion characteristics of power amp. LOOP LEVEL settings will also change the gain structure of the amp even if there is no effect in the loop, on HI settings the signal will hit harder into the power amp.
WORDS ABOUT SETTING THE CONTROLS:
The recommended way of getting familiar with the controls on this amp is to first decide the type of distortion (barely audible to massive) you'd like to use.

FEW EXAMPLES

(named with matching dressing)
SUIT:
NORMAL input, VOLUME about half way, MASTER a little less than half way POWERSUCK: CLUB or PUB. Neck pick up, humbucker preferred.

BELLBOTTOM:
NORMAL input, VOLUME halfway to full, MASTER almost full, POWERSUCK set for desired volume level. Bridge pick up single coil or humbucker preferred.

HAIRY BOOTS AND HIGH HATS:
ABNORMAL input, Tube, VOLUME slightly passed halfway, MASTER near halfway, POWERSUCK: ARENA. Neck pickup single coil preferred.

LEATHER JACKET (with or without rivets): ABNORMAL input, Tube, VOLUME full, MASTER below halfway, POWERSUCK: ARENA. Humbuckers preferred.

SHOULDER-PUFFS: ABNORMAL input, FET, VOLUME past half way, MASTER about halfway, POWERSUCK: ARENA. Bridge or pick up combination single coils preferred

SIDEBURNS:
NORMAL input, VOLUME just opened, MASTER near full, POWERSUCK: ARENA. Preferred setting for pedal board

THEN SET THE TONE CONTROLS

A good starting point is BASS, MIDDLE, BALANCE, TREBLE, PRESENCE in mid position. Then select FOCUS. Open symbols on left gives an open, lighter tone; Solid symbols on right gives bolder, thicker tone. Fine-tune to cabinet and preference.
MORE EQ USE:
MASTER on full gives deeper bass
ABNORMAL input cuts some bass
FOCUS: Treble boost selectable on both open and solid symbols.