Issue with biasing my Willsenton R8 amplifier

[Jon S]

I just noticed a peculiar issue with my Willsenton R8... I noticed that when adjusting the bias of either power tube on the right channel results is a scratchy sound emanating from the right speaker. This never happened before... I doubt it's an issue with the bias pots as both pots' noisy adjustments surfaced at the exact same time. Any ideas? Is it something to be concerned about?

 

[vinylkid58]

Is the noise coming from the pot, or just when you touch the meter probe?

jeff

 

[TriodeLuvr]

If both pots on the right channel started doing this at the same time, and especially if neither left channel pots does it, then I would be concerned. It might be a sign of oscillation or a failed component in the right channel bias circuit.

Jack

 

[Jon S]

The scratchy sound emanates as soon as I touch the bias pot.

I think I will need to find someone to take a look into it...

 

[Aerobat]

Did you do the Skunkie mod to add resistors to the bias boards? If those pots go bad it will take out the power tubes.

 

[Jon S]

My amp is the "Mark II" version which already have the safety resistors... I don't know about these safety resistors... I also have a Muzishare X7 which has the safety resistors built-in. One of the bias pots went bad (it would not maintain the bias point) and it actually red-plated a Gold Lion KT-88 on me. I should have replaced the bias pots earlier, I noticed that the pot was binding, getting difficult to adjust... I think I am noticing my pot on my R8 seems to be tightening up a wee bit compared to the other three pots. I'm planning to replace the pots next week... Kind of surprising because the pots are new ones from Bourns and are rated for 50,000 cycles of use. I guess you can always get a bad pot.

 

[Oasis8]

Did you try cleaning the pots?

 

[Jon S]

They are sealed

 

[Aerobat]

I haven't replaced mine and they seem OK, although I rarely change the power tubes. I guess I'll swap in some EL34s and see how it goes.

 

[Jon S]

Well, I waited too long... My 6CA7 tube in the right channel in the back red-plated... I opened up the amp and the bias pot burned the board. I tried replacing the pot but there was no continuity. I inquired with China-Hi-Fi for a replacement board. I hope they are still in business.

On a side note:
A lot of shops in China have shut down because of Xi's economic policies which forced a lot of manufacturers to exit China and go to Vietnam, Indonesia, India and other countries. China wants all intellectual properties of companies products if companies want to do business in China.

To show you how rampant IP theft in China is, they bought a couple of Airbus A320s several years ago. Airbus noted that only one was registered as flying in China. The Chinese tore down the second plane and copied everything possible. They then released the Comac C919 airliner as China's own designed airliner to compete against Boeing and Airbus. Upon closer inspection, the C919 turned out to be a clone of the missing Airbus A320. China is trying to sell their C919 airplanes to international carriers but hit a road block. The European and Americans have refused to certify the plane to fly in their countries.

France and Airbus just made a big trade deal where China will buy a large number of Airbus planes. The stipulation is that Airbus must build the planes in China... That is not going to turn out well. China can just take over the factory and build there own planes once they know how to build them. Airbus is cutting their own throats.

 

[TriodeLuvr]

France and Airbus just made a big trade deal where China will buy a large number of Airbus planes. The stipulation is that Airbus must build the planes in China... That is not going to turn out well. China can just take over the factory and build there own planes once they know how to build them. Airbus is cutting their own throats.

I've lived in China. I can tell you that the entire West has cut its own throat.

 

[gadget73]

yeah there is a whole long story there but the current situation is the consequence of choices made long ago.

anyway I'm not remotely surprised at the cloning. Just look at how many knockoffs there are of basically everything. Imitation electronic components, hardware with bogus certifications, right on up to apparently airplanes now. This can of course happen anywhere and its far from a new thing. Just space-racey type stuff, the CIA tore down a Soviet Lunik probe to see what it was all about, and if you've ever seen the Buran its pretty clear it was a rip-off of the US space shuttle. If copyrights and trademarks and all those other fun legal things are not able to be enforced, they become less useful than toilet paper.

How the CIA 'kidnapped' a Soviet moon probe during the space race

"And the Soviets were none the wiser. Sound like something from a movie script? It really happened."

www.space.com

 

[Jon S]

China-Hi-Fi has sent out my bias board from China to their warehouse in the U.S... Once it clears customs it will be forwarded via Fedex to me. They said to expect it to take 21 days for delivery...

 

[Jon S]

Heads up! I got my bias board from China... Got here pretty quick. Got confused about the six leads attaching the board to the amp... The color coding of the leads going to the bias board were different from what I saw in other posts. I finally figured out that the Willsenton amp do NOT standardize their color codes on the leads. One picture shows the white leads going from the power tube socket, another picture shows the lead as a red color. I also noticed other leads show different colors in the amp. If you are working on the amp, make sure that you takes tons of pictures or you may wire the amp wrong...

This picture shows the white lead running from the power tube socket to the top right corner on bias board in my amp...



This second shot shows the same channel bias board with a RED lead from the bias board...