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Tube Amp Doctor Plexi 150W Kit.

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About six months ago I decided it was time to dig deeper into tube amps technology. It all started a few years ago when I decided it was time to learn how to replace tubes. Since I'm an industrial electrician I thought I'd at least be able to do that myself. Next step was troubleshooting my Orange AD200B MK2 which had an intermittent crackling distortion problem (turned out it was something completely else, another story).

Finally I wanted to put together a nice amp kit. I have the Orange and a Hiwatt DR201 and love their concepts along with memories of gigging with an old Marshall Superbass which actually was my turnover point towards ditching solid state amps in favor of tube amps so I fell for the TAD 150W Plexi kit.

The preamp is based on the old Marshall Superbass with some components tweeked further towards bass use, along with using ECC81-tubes as second stage and phase inverter instead of ECC83's all the way. The power supply and power stage is based on the same type as the aforementioned British amps which use what some call dual rail supply for the plates and screens and 6550/KT88 powertubes. This kit comes with KT88's when ordering a complete amp.

The documentation included schematics and wiring layout. The wiring layout only showed from between which points the wiring should go, not where the wires would be run inside the chassis to avoid any crossover feedbacks/interference. I found that to be the biggest challenge.

First step was mounting tube sockets, transformers and capacitors:

Heater and B+ wiring:

Power supply:

Turret board:

Connect turretboard with the rest of amp:

Measuring voltages. Install tubes:


Ok. So what did this beginner manage to screw up? First of all, Tube Amp Doctor provided all the material except double adhesive  tape used to keep the rear plexi panel in place where there are no components keeping it tight to the chassis. I did'nt have any thin tape so I used contact glue. Big mistake which can be seen on the pic. The glue was too harsh for the gold paint so it bled through. Next thing was when inititially firing it up (no tubes installed of course) I had no bias voltage on the control grids of the powertubes which would've been not so good in case the tubes were installed. It turned out to be a resistor in the bias supply that got disconnected while soldering a wire. The wiring itself was on top and hid the fact it got unattached. Third screwup was that I heard only hum when connected to the low input on the normal channel. It showed out that I totally had missed to solder one connection on the input jack.

The end result is very pleasing. The Marshall sound is there though a bit more cream and headroom to it. There's no hum or hiss unless I dime the volume and presence like the originals. This is after all a quite faithful replica using for example old school carbon resistors known for being noisy. The specs says 150W but the powerstage hits 180W RMS clean just like my Orange (which otoh hits the roof in the preamp well before that at ~150W). The power supply is not anywhere near Hiwatt (I've measured mine to 210W RMS clean) which the much smaller lower weight transformers already had suggested.

Looks really nice!
Tube amp builds are great fun. Three blemishes along that way is nothing to worry about.
No matter how many amps you have built, the most important thing you learn is to sleep on it and spend a
couple of hours checking connections/solder points before you fire it up. Forming caps as well is recommended,
be aware that rebranders buy huge stock and it's not unusual to get caps that have been lying for years on the shelf.

Nice work!

Thanks. Yes I did use a variable transformer to be able to gradually raise the primary voltage. There's really high tension in these designs (600 and up VDC) it would be quite a bit nervous for me to switch on full blast the first time.


I dig it, and nice choice on the tolex color  :toast:


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