Bogner Alchemist Head

After making some new connections I started to acquire music equipment to repair on a more regular basis. This lead me to learn a lot more new stuff about electronics. I haven't had much time for my own projects, now I was handed an interesting problem.

A customer had brought a Bogner Alchemist head to the local music store because it had mysteriously stopped powering up. I took t

he amp in and after a while I had to admit that my skills weren’t up to the task. The power switch had no effect on the head itself but the lights on the foot-switch lit up. This meant that power was running somewhere so all hope was not lost yet.

Autopsy

I was baffled. I took the amp out of its casings and started to eye over the circuits. At some point I noticed that the secondary coil of the power transformer was connected to a wrong place on the circuit board. Moving these two leads to the correct spot brought some life to the nearly dead patient; the power amp tubes started to glow and warm up. Pre-amp was still flatlining and all the lights on the front panel were dark.

After pondering the situation for a while I decided to let the customer know that finding and repairing the fault would take so long that it wouldn’t make much sense since I was working on an hourly rate. I offered to buy the head from him for a  reasonable price knowing that I might be buying just a pile on spare parts. At least this way I wouldn't have to keep looking at the clock all the time while working.

I spent many nights searching for information and studying the circuit board. In terms of efficient working time it must have taken dozens of hours, but all that time I kept learning new things about the inner workings of a tube amplifiers. After a while of going through the amp with a DMM and studying the schematics I noticed the the pre-amp wasn't getting the voltage it needed. The pre-amp tubes require an AC voltage off +3.5V/-3.5V. My measurements showed only +1.6VAC/-1.6VAC going in.

At this point it was again time to take out the schematics and take a good look at where the current flows on the power board section. The circuit feeding the 12AX7’s had two resistors of equal values on both the positive and negative sides of the power feed. These two resistors were connected to each other via potentiometer that controls the voltage going to the tubes.

After this discovery I connected the DMM to one of the sides and started adjusting the potentiometer until it showed about 3.5 volts. As the voltage rose to the desired value, all the lights on the front panel turned on.

The head found its place in a new home

I re-assembled the head and plugged it into my trusty 4*10 bass cabinet the I had designed and built some time earlier. Everything worked just as the user manual said it was supposed to. I was ecstatic since now I had in my hands my very first own tube amp. I was also extremely tired since this project had kept me up for numerous nights in a row. Soon after getting the Bogner fired up, I got a 4*12  guitar cab to pair it with so I could actually hear what it sounded like. I kept this amp for a while as a part of my tool arsenal before eventually trading  it in for something getter suited for my type of music.