TT PX45

You want a modern Plexi? No problem. The TT PX45 is a combination of JTM45 power amplifier and PX18 preamplifier and convinces beside the popular Plexi sound with full 30 tube watts.

Since a “master volume” is one of the most popular and most demanded modifications for Plexi style amps, we have already included a master volume behind the phase inverter (PPIMV), so you can fully exploit all the possibilities this amplifier offers – which are many.

The pre-amp is identical to the one of the PX18, and thus corresponds to the “Plexi”-known pre-amp, consisting of two separate channels, which differ in the frequency response of the highs. Each channel offers a Hi and Lo input and a shared 3-band tone control for Bass, Mid, and Treble. Both channels can be operated independently, but can also be patched and mixed together, creating a well-defined, dry rock sound, which these amps are famous for. The amp covers many different styles easily, like country, rockabilly, blues, stoner rock and so on…

The design and of the power amplifier is based on the JTM45 and can be operated with either KT66 or EL34 (when using the Tube-Town JTM45 transformer, only the primary winding needs to be re-soldered). Since 30 tube watts can be very loud, such non-master concepts always ask for a master volume control, which is understandable and justified. The PX45 is equipped with a master volume, which is located behind the phase inverter, which allows even more possibilities of different sounds, since the PI can be fully integrated into the overdrive behavior of the amplifier.

The layout of the amplifier is done in a classic way on a turret board, which simplifies later modifications (if you want) or possible needed repairs (unwanted and hopefully not the case). Surely a big advantage, even if it wiring gets a bit tighter in the chassis. But whether turret board or pcb: clean and careful working are the guarantee for a successful completed build.

Files

Schematic as PDF (2 Pages, DIN A4)

Layout as PDF (1 Page, DIN A3)

Bias-Settings

Bias adjustment is done with the trimmer Tr50.
It is recommended to use a measuring adapter to determine the quiescent bias current.
The recommended quiescent bias current is around 30 mA per tube.

You can get the kit at the TT-Shop.

Mod1: Level Adjustment

If the input level is quite high and the amplifier is operating at near full load, it is recommended to adjust the level as shown in these schematics. In this case, only three components must be replaced. Marked red in the schematics.
schematics (PDF)

Warning!

Care is necessary during construction! The voltages used in this amp are potentially fatal. This amp is NOT a beginner's project!
A successful build requires that you can read (and understand!) a schematic diagram, you know how to use various measuring devices, and you have some experience working with voltages over 60V.