It is only possible to use an 8 Ohm external cabinet at half power on the Twintone. This is because the power-soak inside of the amp is 8 Ohm. This one is connected to the 'Main'-output when there is no speaker connected to the 'Main'-output. If you connect an 8 Ohm cabinet to the 'Extension'-output and no cabinet to the 'Main'-ouput, the amp 'sees' two 8 Ohm cabinets parallel and their total load will be 4 Ohm. That is why you'll have to switch the speaker output to 4 Ohm to play on half power.

In the actual manufacturing process, the operation point of a tube is not influence able, and because of this each tube is different. For this reason all power tubes, after receipt by us, are measured to determine the operating point.
We have subdivided the operating points in 12 classes (bias classes). After measurement here at Koch, we label the tube with its bias class. Because of this labeling system we can supply matched sets of 2, 4 or more equal tubes for our amps, which will replace the old tubes without the need to rebias the amp.

Every active electronic device (opamp, transistor, tube) can generate an output voltage and current, the product of voltage and current is its output power. The difference is that some devices are designed to generate a high output power so they can drive something (a speaker, a motor, etc). But in the Pedaltone we do not use the circuit to drive anything (speaker), but only to generate tube power amp distortion. We choose the 12AX7 in the single-ended circuit because it's distortion came the closest to a 30W 4xEL84 (Vox type) power amp, and that was the sound character we wanted to add to the Pedaltone as a unique and special feature.

An amplifier sounds best when the tubes operate in sync, i.e. perform the same operations at the same time. For this the power tubes need to be equal (matched). Should this not be the case the tubes could obstruct each other, which will influence your sound negatively.

Don't! We will give you 2 reasons:

Tube amps operate on very high voltage (400 to even 700 volt), and that is why you'd better not open amplifiers yourself.
With most amps there is no biasing trimmer or not even a measuring point available, so this needs to be built in first. We are one of the few manufacturers to have these facilities built-in standard. But aren't there tubes by Koch, Ruby Tubes, Groove Tubes, Marshall, Mesa, etc.? That's correct, but all these are brand names, and not original manufacturers. Most tubes are offered as spare-parts for amplifiers. Amplifier manufacturers purchase their tubes from one or more tube manufacturers and mark the tubes with their brand name because the tubes, after a special test procedure, have been found to meet with their quality specifications.

All well-known tube manufactures like Philips, Siemens, Telefunken, Tesla, Mullard, General Electrics, R.C.A. or Sylvania have stopped tube manufacturing and started making modern electronic components (semi-conductors).
Tubes from these brands that are still for sale, come either from old-stock batches or are remarked tubes originating from present manufacturers.

Fortunately for us, musicians and sound connoisseurs, tubes are still being produced in a few large facilities in Russia (Svetlana, Sovtek), Slovakia (J/J), Yougoslavia (EI) and China (Sino).

For understanding this, it's easier to make a comparison with a car. When your car needs new spark plugs and contact points, the ignition time and the stationary rpm need to be adjusted to have your car set-up to the optimum. This is the same with your amp, it will only function optimally and sound the best, if the new tubes operate on their own unique operating point.

Yes, unfortunately there are big differences in quality. Partly in sound, but mostly in reliability. A tube is built up with a lot of small and sensitive metal parts which are, during operation, exposed to high temperatures. If you look at the conditions under which tubes are put to work in guitar amps: shocks, vibrations, electrical overdrive and temperature extremes, it's not hard to imagine that all these can cause the internal small metal parts to become undone and cause problems like microphony, crackling and even, if parts get loose, internal short-circuiting. The quality of the materials used and the preciseness of the construction and the manufacturing process, are determining factors of the quality of a tube.

TIP 1:
Tubes and tube amplifiers are products you should not throw around, always take special care in transport, and always let the tubes cool down before moving the amp. This will increase tube life considerably.

TIP 2:
Periodically check the tubes by tapping against them carefully with a pencil or a screwdriver. A bad preamp tube usually will give a squealing, peeping sound, a bad power-amp tube can resonate or crackle. If the latter is the case, switch off the amplifier to prevent short-circuiting and replace the tube. Should no spare tubes be available, and you have a 100 Watt amp with 4 power tubes, you can also play with 2 power tubes (the 2 inner or outer tubes).

The existing setting in your amp accidentally matches the operating point of the tubes and everything will function as it should.
The existing setting is too low. Now the idle current in the tubes will be too low which will result in an ugly sounding distortion on your clean channel. The whole amp will sound thin and squashed. The existing setting is too high. Now the idle current is too high, resulting in an extra fast wear-out of the tubes and a dull sounding amp. In extreme circumstances the tubes can get overloaded. TIP 3.: Overloaded tubes can be recognized in general by a red glow of the grey metal plate about midway inside the tube. If you see this, switch off the amp immediately !!! (a blue glow is normal).

This is absolutely normal, there is nothing wrong with your amp. The solution for this is simply mute your strings when switching.