Repurposing an old tube amp, part 3

Hello, steemians, and welcome to my page, eh!

It took me a while to get back to working on this amp after the previous work session, but I finally made some more progress on it. In the last post, I had gotten the power supply all assembled and was ready to move on to the next step. I had to mark and drill holes in the back plate for the standby power switch, the speaker impedance switch, and two speaker jacks.

The next step was to mount the switches and jacks.

Then came the wiring to finish that part of it.

After making sure everything was connected that needed to be connected, it was time to test the power supply. I decided to use my variac to power up the amp the first time instead of just plugging it in and turning it on. Doing that can be hard on new power supply capacitors. A variac is a variable transformer that plugs into the wall, and you plug your amp's power cord into it. It allows you to adjust the AC voltage going into the amp to anywhere from 0 volts to full voltage so that you can power it up slowly instead of just flooding the amp with full voltage. Once you bring it up to full voltage, you leave it there for several minutes, and check the voltages of the various sections in the amp. If everything checks out OK, after that you can just plug it in and turn it on without any problems.
After that procedure, I plugged the amp into my isolation transformer. The isolation transformer gives full voltage, but disconnects the amp from direct connection to the power line. That gives you some measure of protection from shocks. It limits how much current you get if you put your hand in the wrong place in the amp. It's always a good idea to use an isolation transformer when you work on live equipment, especially tube amps with their high voltage circuits.
I hooked the voltage meter up to the amp and turned it on to see the voltage at the first section of the amp.
476 volts DC, that's about what I expected to see at the first stage. This is the voltage with all the tubes loading the power supply, with no load, it runs about 520 volts or so.

So, with the amp turned on, it was time to see if the tube heaters were glowing. If you look closely, you can see the orange glow of the heaters. They're a bit like the filament in an incandescent light bulb. The tube has to be hot in order to work.

Since everything seemed to be working properly, I decided to test the amp. First I used my audio frequency generator and a load resistor, and used the oscilloscope to make sure the amp was passing the audio signal from the input all the way through to the speaker outputs. That all seemed to work, so I decided to plug in a speaker and try the amp with my guitar. I used my single 10 inch speaker cabinet to try the amp, just to see what it sounded like.

I discovered a couple of things, one, it works, and two, it needs more work on the preamp. I couldn't get much volume without distortion, which might seem ok except that it should have a lot more volume without distortion. It definitely got loud, my ears were ringing after I got done messing with it. It has serious distortion when you turn it up, which is fun to play with, but it needs to be more adjustable. I'm going to add a distortion circuit to it once I get it to work properly, I want to add a footswitch circuit to be able to switch between distortion and clean on the amp once I get everything else working. And so the project continues.

Well, that's all I have for this post, I hope you found it interesting!

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