Plexi Details
Build notes specific to this amp from Mitch.

This amp is based on the classic JTM 45 Plexi and the Model 1987 lead amp with a few changes.

It has a push/pull power amp operating Class AB. It uses two KT66 tubes like Clapton's Bluesbreaker amp. These are premium tubes made by Gold Lion in Russia. Power output is 45 watts.

The preamp has two channels, normal and bright. Each have their own volume control. There are also Bass, Mid and Treble controls and a Presence control. The preamp uses a couple of 12AX7 tubes. A third 12AX7 is used for the phase inverter. All three are made by TungSol in Russia.

The amp has Alpha pots and Cliff jacks.

It has an 11 gauge aluminum chassis with gold Plexiglass front and rear panels. The panels are similar to the originals. That's where the "Plexi" name came from.

I began by installing all the tube sockets, pots, switches, transformers and jacks on the chassis.

Plexi

Plexi

I wired up the AC, the power transformer, the heaters, the output transformer secondary, the jacks and the pots.

Plexi

Now that the power supply was wired I could give the amp its first test. No tubes were in yet, of course. I double checked all the wiring I had done so far. Then I plugged in the AC cord and flipped on the power switch. The pilot light lit up. I measured all the AC and DC volteges and all were good.

Plexi

I started working on the turret board next. I installed all the wiring and the flyoff leads on the bottom side, then I installed the resistors and capacitors on the top side.

Plexi

All that was left now was to attach the flyoff wires coming from the turret board to the tube sockets, pots and jacks.

Plexi

The amp was now finished and ready for final testing. I checked my wiring very carefully. First visually, then I used an ohmmeter and the schematic to make sure everything that's supposed to be connected actually was connected. I did one more check to make sure there weren't any shorts and the polarity was right on all the electrolytic capacitors. All was good, so I applied power.

It passed the smoke test. It also passed all the voltage checks. I plugged in the tubes, first the preamp tubes, then the power amp tubes and a dummy load. They all glowed normally. There was no signs of red plating on the KT66's and all the voltages measured good.

Plexi

The amp has variable bias for the output tubes. I used the "bias probes" I made to measure the plate voltage and the cathode current and set the bias to an appropriate value for the KT66's.

Plexi

I plugged in a guitar and a speaker for the final test. I fired it up and it worked like a charm!

Time now to work on the cabinet. It's made from 3/4" Baltic birch plywood. The tops and bottoms are fastened to the sides using dovetail joints.

Plexi

Plexi

I covered the cabinet with black Tolex with some gold piping to give it a vintage look similar to the amps it's based on. I also installed feet and a handle.

Plexi

Plexi

Plexi

Tolex corners on cabinets with compound rounded edges can be challenging if no corner protectors are going to hide them!

Plexi

The front panel is covered with Tolex as well. It has gold colored piping around the edges and across the middle.

Plexi

The rear panel is sprayed with three coats of black hammertone paint. It also has a large vent grill made from perforated sheet steel that I spray painted black.

Plexi

For more information please contact: message@markinamps.com

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