After 18 years, Dutch manufacturer Koch has revised its Classictone combo, which was then equipped with four 10″ speakers. However, the cabinet of the Classictone-II Twenty is no less spacious despite the single 12-inch speaker. With otherwise identical features, Koch offers the amp with either 20- or 40-watt power amplifiers.

As a special feature, the amps offer a recording output section, each with a jack and balanced XLR connector with frequency correction. In addition, an integrated load resistor (dummy load) allows the on-board speaker to be switched off even under full amplifier load, but should definitely remain connected.​

IMPOSANT
The mighty cabinet and the 22 kg live weight of the Classictone-II Twenty lead us to expect anything but a 20-watt combo. But we know that the cabinet and its volume influence the amp sound not insignificantly. 15 mm thick birch plywood forms the frame, in the corners of which four 4×4 cm square timbers provide increased stability. The tightly fitted, screwed rear panel and front were made of 13 mm birch plywood, each with trapezoidal openings.

Behind the front sits the actual baffle with the textile covering and the custom KochVG12-60 speaker, which is screwed four times from the rear and whose feed line is connected via plug-in shoes and amp-side via jack plug. The large spring reverb system was mounted in a tolex pocket on the bottom of the cabinet, the high-quality send and return cables meticulously laid with cable clamps and connected to the amp via RCA plugs with gold contacts.
​The cabinet, front and rear panel have been carefully covered with dark brown Tolex and visual accents have been set using black and gold piping. Eight screwed corner protectors made of sheet steel, four large rubber feet and a solid leatherette handle complete the case equipment. Velcro straps are available on the inner side panel to secure the footswitch and power cord during transport.

Shielded by aluminum foil, the amplifier chassis, which is bent from 1.6 mm sheet steel, hangs vertically from the rear panel on four threaded screws. Thus, tubes and transformers are aligned to the front, which requires dismantling the rear panel along with the chassis to change the glass bulbs, since the speaker magnet also obstructs access.

​The cabinet, front and rear panel have been carefully covered with dark brown Tolex and visual accents have been set using black and gold piping. Eight screwed corner protectors made of sheet steel, four large rubber feet and a solid leatherette handle complete the case equipment. Velcro straps are available on the inner side panel to secure the footswitch and power cord during transport.

Shielded by aluminum foil, the amplifier chassis, which is bent from 1.6 mm sheet steel, hangs vertically from the rear panel on four threaded screws. Thus, tubes and transformers are aligned to the front, which requires dismantling the rear panel along with the chassis to change the glass bulbs, since the speaker magnet also obstructs access.
​All pots and connectors, standby switches and fuse holders are screwed to the chassis, four mini toggle switches are reliably soldered to circuit boards. The exemplary workmanship underlines the high manufacturing standards of the manufacturer. Briefly, I am confused, because Koch speaks of a three-channel amplifier, but the layout of the controls suggests a two-channel. Between the Clean and Gain controls I discover the small Gain/Boost switch, which allows the selection of crunch or high-gain sounds in the OD channel.

​Gain/boost and channel change can also be accessed with the included dual footswitch, suspending the function of the Clean/OD and Gain/boost switches. Two sturdy footswitches and bright red (Clean/OD) and green (OD/OD+) status LEDs are used in the robust sheet steel housing. The permanently connected supply cable with TRS jack plug is generously dimensioned at 5 meters.

The amp chassis offers three control surfaces, namely top, rear and bottom. On top we find input, channel select switch, clean volume, the OD channel with gain/boost switch, gain and volume pots, a classic passive 3-band tone control with bass, mid and treble for both channels (post-distortion) as well as reverb control, level and standby switch and a red pilot lamp as operation indicator.
​
Level switch? This three-way rotary switch, located directly in front of the power amp, replaces the familiar master volume pot with the positions Home, Studio and Stage. The rear panel provides a connection for an external speaker (8 ohms), a speaker on/off switch (dummy load), a balanced XLR DI out with ground lift switch and a jack line out (both with speaker simulation) as well as FX send and return (serial).

You have to bend down very deep - or put the amp on your face - for the lower connection panel, where Koch placed the sockets for on-board speakers, double footswitch and power cable, the holder for the HT fuse and the power switch. I would have liked to see the latter on the top, or at least on the back.

​NOT A BIT QUIET
For the first listening I turn the (master) level switch to the home position as a precaution. The Clean Channel shows plenty of headroom, because with PAF pickup types and powerful string attack, the first tightening (break-up) can be detected on the scale of 0-10 at about 6, and that at an impressive volume. What does Koch mean by "home level"? If you turn up Clean Volume further, the distortion ends with a very subtle crunch, characterized by powerful but tight, never boomy basses, warm mids, crystal clear, airy highs and pleasant brilliance.

In general, the Clean control works extremely precisely and increases the input level completely continuously over the entire control range. The amp's impulsive response and outstanding dynamics allow you to turn Clean up full and leave any further tone shaping to guitar volume and/or attack. More effectively than initially expected, the passive tone control intervenes in the action, the individual sections even interact a little. Thus, the most diverse guitar types can be balanced without any problems.

In terms of distortion, the overdrive channel starts where Clean ends, namely in gain position 1 with a subtle crunch. From here, the distortion intensity increases with almost astonishing continuity over the entire gain range up to heavy rock rhythm or singing blues rock lead. As expected, the amp shows increasing compression, but even at the highest settings it still reacts extremely dynamically and conveys a pleasantly sensitive feel.

​But it gets even better: I turn OD Volume to 1 - yes, to ONE - and the Classictone-II delivers almost the same characteristics and dynamics at bedroom level! Okay, not with the pressure of higher volume settings, but voluminous, fat and yet lively and transparent. Gradually the concept of the master level switch makes sense to me. Amps with conventional gain/master control sound at room volume - i.e. gain fully up and master down to emission compatibility - mostly characterless and expressionless, floppy and meaningless. Here, however, the power amp is always included in the sound by the master switch, since the master level cannot be lowered lower than "home".

The boost switch of the OD channel activates the third channel of the Twenty and at the same time adds a clearly audible boost of gain, even when the pointer of the gain control points to higher realms. Result: Creamy, homogeneous lead distortion, healthy compression, precise, detailed resolution, high assertiveness, sustain until the day after tomorrow, all this with manageably limited dynamics and, last but not least, surprisingly low ambient noise.

In view of the home level, I hardly dare to select Studio and Stage. But this is absolutely recommended when testing this combo - preferably with band level - because then the amp shows its qualities to the fullest. The reverb pot mixes the effect signal of the large spring reverb with the direct signal, i.e. it is connected in parallel. Thanks to the even control characteristic, the effect component can be finely dosed until it produces an intense, natural, homogeneous, warm and spatial sound with a healthy portion of brilliance and clean signal at full output.

The long decay time of the large system may not suit everyone's taste, but can be compensated by lower reverb settings. With its nominal level of -10 dBV, the non-switchable serial FX loop is compatible with both pedals and 19″ processors and is exceptionally faithful to the signal. Both recording outs, the balanced XLR as well as the jack output, deliver appealing speaker-emulated DI sound that requires at most slight (taste-related) corrections at the mixer. The DI sound deviates somewhat from that of the on-board speaker, as it doesn't sound quite as lively and airy. I can always live with it, and the recording and FOH personnel certainly can as well.

The Verdict
With the Classictone-II Twenty, the Dutch amp manufacturer Koch once again makes a successful contribution to the popularity of small tube amps, which has been increasing for years. Although, in view of the housing dimensions, there can be no talk of small here, here the adjective "power-reduced" would certainly be more appropriate. And so it happens that the 20-watt combo not only looks like a big one, but also sounds like one. With voluminous yet differentiated, transparent and lively clean to high-gain sounds, the Twenty is tonally very variable and displays impressive dynamics and sensitivity at no less impressive volume.

On the other hand, the amp also masters the discipline of bedroom levels without losing its vital characteristics. Practical line/recording outputs, serial FX loop, very good sounding spring reverb and master level switch complete the equipment of the impeccably manufactured three-channel amp, which is also offered at a fair price.

PLUS

● Sounds & tonal flexibility
● Precision, response & dynamics
● Harmonic distortion character
● Master level switch concept
● low side noise
● Quality of components & workmanship
● price/performance

MINUS

● Access power switch
● Inconvenient tube change

 I turn up the volume further, the volume increases completely evenly, to end - I hardly dare - at 10 with a fat crunch. Ouch, that's not really possible without ear protection! Now it sounds really fat but still defined and even with full chords with fine resolution. The subtle compression supports the sustain of the guitar, and the impulsive response gives the Koch Ventura the finest dynamics. It gets even better.
 
My Les Paula has a conventional 50s Wiring, normal CTS pots, current Bumblebee capacitors and no-Treble bleeds. Nothing special, then. Turning down the guitar volume, I get down to 1, and the level decreases steadily without any significant treble loss. I am amazed.
Basically, I'm fine with the previous EQ setting, but I still correct Bass to 4, Mid to 6 and leave High at 5. Mind you, the controls interact, and if I turn them all to zero, nothing more is heard. Traditional passive tone control.
 
At about Gain 2, the distortion intensity of the Overdrive channel is equal to that of the fully turned up Clean channel. With identical EQ settings, the OD sounds a bit leaner overall and leaves more room for upper mids. If you turn up the gain further, not only does the distortion increase evenly, but the sound becomes fatter, punchier and richer in sustain with hardly any reduced transparency, yet always retains assertiveness and dynamics, all this with surprisingly low ambient noise.
 
Even with the gain fully turned up, whose tonal event I would like to call a hard rock or mid-gain lead, OD Volume regulates completely continuously over its entire range. A Strat with vintage single-coils delivers no less pleasing sound results on the Ventura if you always set the Volume and Gain controls one or two strokes higher. In any case, however, the individual sound character of the guitar is always preserved.

Effects Loops Explained by Dolf Koch

Nowadays, an effects loop is an integral part of almost any musical amplifier. Although it just looks like it’s no more than just two jack inputs, a “Send” and a “Return”, there is so much more behind it!

The History

Tube amps made in the 50s and 60s have no effect loops simply because there were no effect units. The only two effects available were reverb and tremolo and top of the line models had one or both effects built in. Later, when effect units became available (wah-wah, delay, chorus, flanger), they were always connected between instrument and the amplifier input. Because amps didn’t have distortion channels this worked very well. However, when the first amps with overdrive channels came on the market, it turned out that having effect units in front of an overdrive channel increased the level of hissing noise rather drastically. Besides that, chorus type effects almost dissapeared and became inaudible when the distortion level was turned up high.

In recording studios, mixing consoles had so called “Inserts” in each channel as well as in the master section. This is a stereo jack with a combined input and output which made it possible to insert an effects unit in the signal path (serial connection). If you wanted to have better control over your effects, the unit’s output could be fed back to a (free) channel input (parallel connection).

In the 70s, these Inserts became a feature on guitar amps as well and got the name: “Effects Loop” or “FX Loop”. The output was called “Send” and the input “Return” and they are always placed between the preamp and the power amp section. This is the so called serial effects loop.

It might seem simple to build an effects loop in an amp, just cut the wire that connects the preamp with the power amp and place two jack plugs instead. Yet nothing could be further from the truth, especially in tube amps. I’ll try to explain why.

Problem 1: Tubes operate on very high voltages from 300-500V. Therefore, signal levels inside a tube amp are very high.
Effect units, which are mostly battery operated, operate on 9V. Therefore, signal levels in an effects unit are very low. So if we would connect both by feeding the very high signal of the tube amp through the effects unit, it will be completely overloaded.

Problem 2: Tube stages have very high impedances while solid state stages have rather low impedances. This means that if an effects unit is connected, the signal level of the tube amp will drop drastically.

Problem 3: Most effect units in combination with the connecting cables will affect the sound of your amp, mostly because they have low quality components inside.

The Solution: 

A good Effects Loop has to include an electronic circuit that:

1)     brings the high signal level of the tube amp back to the low signal level of the effects unit which goes to the Send input on         the amplifier

2)     brings the low signal level of the Return jack back to the high signal level of the tube amp

3)     matches the impedances of the tube amp to those of the effects unit.

4)     does not affect the sound of the amp (!!!)

Such a circuit can be made with tubes, with opamps (solid state) or with a combination of both. All our amps have a combination of both, except for the Powertone-III which has an all-tube driven Effects Loop.

In my Effect Loop designs I always pay much attention to all four design criteria.With the right combination of good design and the use of quality parts our Effect Loops do not affect the sound of our amps. All our Effect Loops operate on a signal level of -10dBV which perfectly matches the signal level of all battery operated effects units. I always use a “buffer” circuit to drive the Send output. This buffer circuit brings the high impedance of the tube amp down to a very low output impedance. Therefore, our Send outputs will drive any effects unit on the market with minimum signal loss due to cable influences.

Serial vs. Parallel Effect Loops

In a Serial Loop the signal of your amp is completely routed through the effects unit, which passes the signal through while adding an effects signal to it. It is obvious that the effects unit, if it is not of the highest quality, could negatively affect your sound. The advantage of a serial loop is that you can connect several effect units (in serial) to one loop.

In a Parallel Loop the signal of your amp is NOT routed through the effects unit but it stays inside the amp. The effects signal on the parallel input (Return) is mixed by a (Level) potmeter with the signal of your amp inside the amp. Therefore, the sound of your amp can not be affected by the effects unit. The disadvantage of a parallel loop is that you can connect only one (multi) effects unit to each loop.

The Applications & Final Thoughts

In our Twintone II &III, Powertone-III and Supernova amps we offer combined effect loops with two return jacks, which can be used to connect your effects unit either serial or parallel.

Apart from your choice of effects units, all our amps have effect loops which get the maximum out of these unit(s).
We hope this article helps you with your quest to find your dream sound, please leave a comment with your thoughts or questions and be sure to check out some of our other articles.

Voltage, Current, Impedance of guitar speakers

In this article I will explain the basics of guitar speakers such as the Voltage, Current, Impedance, Series and Parallel connection of speakers.


Voltage is the height of the audio signal that an amp offers to a speaker. If you compare it with a tap water installation, voltage is the height of the water in a tank or the water pressure in the water system.

An amplifier sends this audio signal as a current to the speaker. This current flows through the speaker and pushes it to create soundwaves. In a tap water installation, the current is the water flowing from the tap.

Impedance or resistance is the matter in which the speaker resists the audio signal. In a tap water installation resistance is determined by the tap. The more you open the tap, the lower the resistance gets for the water and the higher the water current becomes, and vice versa.

So remember: higher resistance = less current and lower resistance = more current.

Impedance

Since the introduction of the first “Radiola” moving-coil loudspeakers in 1926, we use the word “impedance” to define how much a speaker loads an amplifier. The value of this impedance is specified in “ohms”. But why in ohms? Isn’t ohms used for resistors? And isn’t a speaker a totally a different thing than a resistor?

This is all true… let me try to explain.

Ohms & Frequency

A resistor has the same ohm value throughout the whole frequency area, this ohm value is called resistance (R). A speaker on the other hand has an ohm value that depends on the frequency, therefore this ohm value is called AC resistance or impedance (Z).

The picture to the left shows how the impedance of an average 4 ohm speaker varies with the frequency.

In the graph you can see that the 4 ohm (3.99) impedance of this speaker is an average (or nominal) value. The lowest value is 3.47 ohm (at 400Hz) and the highest is 12 ohm (at 20 KHz). Also, you can see that there’s a peak value of 6.5 ohms at 100Hz, this is called the resonance frequency of the speaker.

Source: Wikipedia


Impedance vs. Output Power

As you might know, the output power of solid state amps varies with the load resistance of the speaker. A solid state amp generates a constant ouput voltage and therfore if the impedance of the load increases the output power decreases, this happens in a lineair way. Normally, full power is given at 4 ohms, half power at 8 ohms and a quarter power at 16 ohms. This means that when a speaker loads the amp, the power which the amp generates depends on the frequency, because the impedance of the speaker varies with the frequency.

However, with a tube amp this works differently. Especially tube amps for guitar do not generate a constant output voltage. The output voltage increases when the load decreases. If a tube amp gives full power at a 4 ohm load, it might generate even 70% or more of it’s power at 8 ohms (instead of 50%). This is one of the reasons that tube amps sound different than solid state amps, they react different to the speaker’s dependence of frequency.



Tube Amps & Speaker Output

As described above solid state amps lose their output power in a lineair way: the higher the impedance, the less output power. If the speaker is disconnected, there’s no load and the amp’s output power is zero.

With tube amps this is totally different. A tube amp tries to maintain its output power and as a result of that it raises its output voltage if the impedance of the load gets higher. So if you play a tube amp and disconnect the speaker the load gets unendlessly high and the amp will raise its output voltage to very high levels (thousands of volts inside the amp).

Parts such as output tubes and output transformers are not able to withstand these high voltages which often results in severe damage to those parts. So keep this in mind if you have a tube amp; always connect a speaker or a dummy load to your tube amp, never play it without speaker or power attenuator/dummybox connected.



In Practice

Our ATR series of hybrid amps the Jupiter, Startrooper and ATR4502 (see also the blog article about these amps), are designed in such a way that they react the same way to speakers as tube amps. Although they operate with mosfets to drive the speaker, their ouput power does not fall back to 50% if the impedance is doubled. This is one of the reasons why our ATR series of hybrid guitar amplifiers sound like real tube amps.

Although their speaker outputs behave similar to tube amps you can safely disconnect the speaker and play them without a speaker or load connected. If you do that you will not damage anything.