He has always pushed the design boundaries for amps. His 1971 350w per channel Coffee Can Amp that he made for that years Macintosh Amplifier Clinic tripped the Clinic locations circuit breakers due to it drawing so much current. It didn't have a power transformer inside the amp as it was designed to somehow utilize the mains transformer located on the nearest power pole.

😂

I got this FU50 model (timber fascia - no Bluetooth): https://www.aliexpress.com/item/1005004910994491.html Here are the instructions and completed amp:

You could try one of the tube amp kits from ebay or ali, These obviously won't perform to the same standard as Stephe's designs but the have all the parts and a fabricated chassis. I built one a couple of years ago and it was a fun project. It also gave me the confidence to carry out a few Skunkie mods to my Willsenton R8.

Have you checked if your speaker cables are shorting?

Another informative video: Tube Amplifier Repair, and what to look for. https://www.youtube.com/watch?v=mE7SoZXX1FQ

This makes fixing the problem easier. What brand/type of KT88s do you have?

I have arrows pointing towards the caps in the audio path. If the tubes are good, hopefully the problem has caused a visible issue to the component:

Did you try swapping all the L/R channel tubes? Try the front 6SL7s, if no change swap the 6SN7s, still no change then swap the KT88s. If the squeal switches side then swap just one KT88 to determine which tube is the problem. If swaping tubes achieves nothing, then it must be something internal failing - I would look at the Willsenton branded caps first.

I have noticed that these new tubes maintain their bias. The originals would need adjusting quite regularly.

While it looks nicely built, the fact that none of the power supply cabling has been twisted is somewhat surprising.

I got the TAD tubes because they were approx 1/3 the price of a set of Gold Lions and despite being made in China, they get tested and matched in Germany by TAD so, fingers crossed, they should be reliable.

I should have added that when I fitted the new TAD tubes and switched on the R8, I did a quick check before biasing them and found socket closest to being already correctly biased was V3, the others were way off.

I suspect a higher rated OT would provide more headroom than one rated just above the tubes output. Don't forget that the specifications are often based on constant test signals rather than the dynamic shifts in music.

I am in the a similar situation. I completed a motor mechanics apprenticeship back in the early 80s then promptly got an office job in our public library service. Now I have retired, I decided to spend some time building stereo gear in the hope I will learn something new. So far I have only built some kits and I still need to learn how different components in the circuit can affect the sound. My first big build was an FU50 tube amp kit that works quite well. I have since fallen into the rabbit hole of the John Linsley Hood (JLH) 1969 Class-A solid state amp after finding literally dozens of different kits and completed amps on Aliexpress. Unlike many Ali solid state amp kits, the JLH kits are based on a design published in Wireless World in 1969 specifically for amateur builders to build so there are no copyright issues with these.

Yes.

Yes, you can see this on this schematic:

4.7k if it has the same markings as this image.

I thought the red plating was due to failures on the bias boards, particularly if it only occurs with one tube. I would have thought a problem in the power supply section would affect all tubes. Note, I recently had my R8 open to perform some of the mods. I had a good look at all the resistors while I was at it and only found one with a very small chip on it's surface. This is a photo of the same area as you have shown and all the resistors look fine. Mine does seem to have a different brand of capacitor on the right compared to yours and the one Stephe modified.

I realised I forgot to mention the general standard of components in my R8. They are essentially the same as those in the video series except that I could only find one resistor with a chipped coating (the green resistor on the right channel 6SL7 tube circled below). Here is my R8 post reliability mods: