TT66 (engl)

After nearly 8 years and countless builds, in 2011 it was time to renew the TT66. Although the amp is a simple design, several obstructions had to be overcome.

The primary goal in the second version of the TT66 was removing these obstructions without degrading its tone. Since many people who built TT66s had a problem with hum, the revision above all reworks the rectification. SE power stages are much more prone to hum than PP power stages, therefore the powers supply and filtering has been improved, and a choke added, and the result: the hum problem has been completely solved.

A further change is the PC board construction. Admittedly not very authentic for a vintage-oriented amplifier, it simplifies the build and increases the chance of success.

Neither point-to-point nor PCB construction have any inherent advantage as far as the amp's tone is concerned; often it is more a matter of taste, and in this case the advantages of a PCB more than outweigh the disadvantages.
Clearly, PCBs do have some disadvantages. Mounting the tube sockets directly to the PCB can cause fine cracks, which may cause malfunction or even the complete breakdown of the amp. For this reason, the sockets in the TT66 are not mounted on the PCB but in the chassis.

Additionally, the negative feedback was completely removed. Why? Many builders had the impression that without NFB, the output stage was a bit more open and free. Nevertheless, the PCB allows for including NFB if desired.

Finally, the tone stack capacitors were changed from Orange Drops to Mallory. Although one would think any 0.022uF capacitor should sound like any other 0.022uF capacitor, in practice this turns out not to be true. Compared to the Mallorys, the Orange Drops were a bit muffled, and so they were replaced with Mallorys, which sound more open and fresh.
However, there is enough room on the PCBs to use Orange Drops or othter capacitors, as desired.

Otherwise, everything is as it was in version 1. We did not scrimp on the components, as in such a simple amp, with a very direct signal path, the components chosen have an enormous affect on the tone.

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Build notes

The amp is simple, but is nevertheless not a beginner's project. Knowledge about the function of the components is necessary,
as is the ability to read a schematic as well as how top work with high voltages. Furthermore, it is strongly recommended to know how to hook up a mains ground, and how to measure and test.

The voltages noted in the schematic are measured with 235V mains voltage, with an 8 ohm load and no signal. The voltages can vary due to variations in components and mains voltages.

Tubes

Although the TT66 is designed for a KT66, other tubes, such as the 6L6, EL34, and 6CA7 can be used. In the case of the EL34, it is important that pin 1 is also ground.

If a 6V6 is to be used, the cathode resistor must be increased to 470 ohms, otherwise the 6V6 can be damaged.

Negative feedback

As mentioned, negative feedback is not necessary; quite the contrary. If it is, however, desired, it can be done in two ways:

a) without a pot: the 4 ohm tap of the output transformer must be connected to 'R-FNB'. A 2.7K resistor is the recommended value for R-FNB.

b) with a pot (also known as the presence pot): in this case, the 4-ohm tap is connected to the point 'PRS' on the PCB and a 5K linear pot (Pot position 'Pres' on the PCB), and a resistor value 1K for R-FNB.

Tone Stack

The tone stack is the same used in Fender amps (e.g., model 6G6-A), with a fixed resistor replacing the mid pot.

The tone stack is often the starting point for modifications, at that is also the case here. This version of the TT66 yields a bright,
Fenderisch tone, and works well with Strats and similar guitars. But other variations are possible.

If the mids are to be emphasized, a classic Marshall tone stack can be used.

But it can also be radically different, for example, for bass. Of course, the TT66 is not useful for high-power slap bass, but it can be used to produce a solid bass sound for home or studio. If this is desired, the following values can be used:
C2: 390pF
C3: 0.1 uF
C4: 0.1 uF
R6: 8.2k

A Spice simulator or Duncan's Tone Stack Calculator are useful here, to investigate the effects of different values quickly and without great effort.

That's it for now, have fun with your build.

PS: Kits, components, and chassis are of course available in the the ->SHOP

And some pictures...

Please note: the picture anove are showing the prototype of the TT66-MkII. Some details which did not make it to the final version of the TT66-MKII are defaced. Also the shown PCB could differ from the final PCB but not very much. The schematic and layout itself is for the final MK-II Version and up to date.

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