Today I have finished the build phase of the amplifier, and it's working... kind of.
There's a problem however – one of my KT88 output valves is no good. It has a short-circuit between the control grid and the cathode, which only manifests when the valve is heated.
So this means I have three functioning output valves. So until I can source a replacement (and these things should really be bought as a set) then I can not use this amplifier other than one-sided.
This wasn't the only problem I hit during the construction of the output stages either. It turns out my valve sockets for the output valves were a bit less than ideal... when powering up, two of them were drawing no current at all. Some of the pins were not connecting properly. This necessitated procuring and installing replacements, with all the attendant re-soldering and swearing that entailed.
However, each channel now works perfectly with a functioning pair of valves. Watch as I power the amp up, feed some signal through it and listen to it through a tiny little monitor speaker. Then stick around for a tour of the internals
After this successful test I re-connected the dummy load and connected the output to the oscilloscope and ran the signal generator into the input. Then I increased the volume until THD reached 1%
The result is that this amplifier will deliver 80 watts into a 4 ohm dummy load at 1% THD. I am very happy with this result.
A few photos of the completed internals
Insides all completed now. This has taken a long while to get this far!
Preamp stage all tidied up and with the negative feedback in place
Interior looking tidy(ish)
Close-up of the final stage and bias adjustment circuit
Worth mentioning one item was left to experimentation - the negative feedback. By luck I got the phase correct the first time around, and I used a potentiometer to determine the optimum level of NFB, while watching on the oscilloscope. The correct point became very clear in short order: too low resistance and the feedback was visible. Too high and noise waveforms were visible. Just right and neither were present. That Goldilocks point came at 100K, so that's the value of the NFB resistors I used.
There's still some work to complete before the amp is completely finished... specifically, the input selection LEDs and remote sensor need to be mounted properly, the way they're set up at the moment is a bit rude.
Also, there's going to be a brass nameplate on the front, that needs to be designed first. Once these two parts are done, the amp can go the right way up and migrate to the living room
But first, before any of these steps, replacement output valves are needed!