Willsenton R8: Must do Modification to Bias Boards!
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- Опубліковано 6 жов 2024
- In this video I show in detail the modifications to the Bias Boards, mainly by installing a very important bias "failsafe resistor" that will save the amplifier if the potentiometer goes open for any reason.
I also replace the coupling caps with some Mundorf Alum/oil caps which sound very good. Finally we install some output tube grid stopper 1K resistors. These are shown in the factory schematic, but didn't make it into the amp.
The schematics showing the factory bias board wiring and then my mod wiring are here:
www.skunkiedes...
This is a video series about the popular Willsenton R8 KT88 EL34 Push Pull tube audio amplifier. We will go through the repair this amp needs, testing and reviews of the amp as delivered, and then work on modifications to improve the sound quality and durability.
A great book for DIY amp building info!
Morgan Jones: Building Valve Amplifiers.
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You can buy this amplifier here
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The tools I use in this video include:
Hakko FX888D-23BY Digital Soldering Station
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Skunkie is wonderfully methodical.A superb instructor!
Thank you kindly!
On which temperature You do that de-soldering?
Thank you for the public service! I added the failsafe resistors today.
Glad to help
Another well thought out mod to this great amp. Good for you for explaining in detail about this. Nice job!👍
Solder suckers are pretty cheap these days too, so that helps with clearing holes on single sided (and a lot of doublesided) boards, and works out cheaper than cans of air :).
Great video. I saw the new ones recently to show Willsenton made changes in the new R8's, but there are some old stock ones around as well as many in the wild, so this video is still very relevant. Appreciate your advice.
Thanks for this vid, I have had two of these boards fail on me and red plate a KT88. I will definitely be doing this mod.
Wow! I have read several posts about this, but didn't know the problem was this widespread. Hard to understand why they would leave out a few cent resistor.
@@SkunkieDesignsElectronics I think it may be cheap potentiometers which they have used, but can't seem to find a decent replacement..
@@tonycallaghan4746 right, and trying to find a higher quality pot, that will fit the spacing on the board, physically fit the chassis etc would not be easy.
@@SkunkieDesignsElectronics Your protective resistor is very important, but if the machine fails, it only protects the tube. it is not easy for people to find the machine failure and continue to use it. It is important to check the current regularly.
@@SkunkieDesignsElectronics I have looked, but cannot find a suitable replacement. I think the only way would to create a bracket over the board for replacements to fit on??
Again, great job. I'm getting a lot out of this series. To clean the detritus from PCB's and other various areas, you can also use fiberglass scratch brush pens made for jewelry and watchmaking purposes. You can buy a pack of three from a major online retailer for around 8 dollars. The more you use them, the more you find uses for them.
Great tip!
I experiment to use braided shielded wires replacing the red and white wires. It eliminates some static and hum it sounded alot clearer
Great job identifying another short coming and fixing it, great vid!
Thanks! 👍
Another great video! Another mouser order placed! haha.
Excellent video very informative!
another great video... love the content. Awesome series.
Glad you enjoy it!
One more great upgrade job !
My R8 is wonderful. It doesn't need any mods. It's a new one, though. Feb. 2024 from Amazon.
That's what people say, until a bias pot goes open, red pates a tube and blows out the cathode resistor because there are no "safety" resistors on the bias boards...
Well done
This critical part could have been done (as stock by Willsenton) P-to-P on nice tag strips or turrets… So the 100uF lytics are the last caps in the Neg Bias Supply. Quality is really needed there..
Great job as usual !! no teaser on what's next? The grounds? heaters in parallel? Toggle sw for the feed back??
Heater + grounds! I need to get this thing finished, the owner is wanting it back :)
The modification on the PCB is very interesting, in my first amplifier (EL84) I used that system, but in the one I use now I use an IC 7318 and I have put two capacitors in parallel with two resistors, thus keeping the current constant around 36 -37 mv. What do you think of that system? Very good work!!!
This is a fixed bias amp, that controls the tube bias via negative voltage applied to the grid. Works really well on a push pull amplifier.
I was hoping for a video about the routing the strange grounding… are you gonna do that also? I almost begging you to do that. Think it’s important issue. Anyway million thanks for this series
Patience, I'm working on that next.
@@SkunkieDesignsElectronics ok sorry if i sounded like i stressed you. That was not my intention. I loved this video, i didn’t even noticed that 100K was missing😨 wow. Thank you 🙏
Great mod Stephe! I am going to attempt this one too. I know you said the safety resistors were 100k -1/4watt. What are the grid stopper resistors? I only heard 1k resistor? Thanks for all you do!
yeah 1/4 to 1/2W for the 100K is fine. I would use 1W for the grid stoppers, mainly because the smaller resisters have very small leads and given these are soldered to the end of a wire, it will give them some mechanical strength.
@@SkunkieDesignsElectronics Thank you so much! I love your videos!
Hello @@SkunkieDesignsElectronics - thanks very much for your vid series on the R8, highly educational and valuable videos! I know I'm late to the party here, but I have a question: I'm thinking that the output tube grid stopper 1K resistors are missing from your schematic, wouldn't they be connected directly to Pin 5 of the KT88's? I'm a solid state guy learning tube basics, and I feel I'm pretty solid with reading schematics, but maybe I'm missing something? Thanks!
Plasticine or blutack could help here!
Thanks for much for these videos Skunkie. Do you have a sense of the subjective changes in sound quality with the modifications that you're doing?
So far I would say the previous mod gave more sparkle/air to the top end. Not bright, just clearer. I suspect these mundorf caps will add richness and extend/tighten up the bass. That's what they have done in other amps depending on the caps they are replacing.
Another excellent video Steph! Is that the last mod or do you have something else? Can't wait to se the BOM and order parts.(gotta finish my VTA ST-70 first !)
More to come!
Thanks great detail I want to attempt this mod myself where is the best source to get these resistors and caps
Mouser is where I buy most of my stuff.
@SkunkieDesignsElectronics Thanks looked there didn't see the Mundorf or audio note caps
You aren't gonna find all the parts in a single location. Madisound has Mundorf caps and parts connexion has both mundorf and audio note.
@@SkunkieDesignsElectronics Thank you for the help!
Just finished this mod as well as replacing the output resistors and the regulator capacitors. Huge improvement! Much fuller bass and lower midrange and cleaner sounding. Next up are the power supply mods including replacing the choke since I run KT88's. Any idea what the diode value is? I would like to replace it too if I have the correct value in stock. Thank you again for putting together such great mod videos!!
No I don't, you should be able to read the number. I would think a 4007 would work fine there.
Thank you. I’ll get out my magnifying glass next time I have it open and will post my findings here.
@@SkunkieDesignsElectronics It looks to be a 4007 which is what I have in stock. 👍
watched the video and cross referenced against your schematics on your web page… am i correct that the “grid stopper” (1k resistors) are not shown on yr modified schematic ?
You caught me!! Someone always seems to be able to study this stuff.. Obviously I hope you will know that was a mistake since I show installing them.
Hello Stephe ! Thank you so much for your excellent videos ! I am fixin to perform the bias bd mod and have a question about the resistors. I understand the reasoning for the 100 k fail safe resistor but I am wondering about the 1 k grid stopper and what it's function ? It did not exist as a original component. I figure it's needed because of the fail safe 100 k resistor being installed on the bias bd ? Just trying to understand the logic for it's need and what it dose. Thank you so much for your work !
The grid stopper is there to prevent unwanted oscillations in the output tubes. It would be recommended with or without the bias board safety resistors.
Thank you !@@SkunkieDesignsElectronics
Nice video, I like the upgrades that you did, but I am wondering, if the pots are problematic, why not change them out with better units? Are they an odd style or something that is hard to find in a higher quality piece?
Trying to find something with higher quality, the same lead spacing so they will fit these PC boards, are mechanically the same size and the same value... And honestly, if you just install these resistors, it solves the danger caused by the pot wiper going open, and most high quality amps, even with the best parts, use this backup resistor.
Could you explain the need for the grid stoppers? What will the benefit be to the audio?
They help prevent oscillation in the output tubes.
Does anyone know of a tech in the los Angeles area who can do this mod ? This is not something I want to try. Thanks.
That safety resistor is a great idea. Very easy mod for great piece of mind. Just a note, your schematic doesn't match the mod you made. You soldered the 100K resistor between the 1.2K resister and the potentiometer. The schematic shows it connected directly to the negative power coming into the board.
It doesn't really matter which point it's connected, but I went back and reedited the schematic for people who might get confused.
Skunkie, can you send what Component to upgrade all Willsenton R8?
The parts list is on my website.
Helio What yoi website。
skunkiedesigns.com
Hi Skunkie - Love your videos, and not just because I own the R8. When I purchased my Willsenton I sprung for the upgraded psvane tube compliment. After 10 months of use (almost daily) one of the EL34's red-plated on me. I have no clue what caused it. Also, I don't know how often one should expect to manually bias the tubes in this amp but I was doing this about twice a month. Is that "normal"?
It could easily have been caused by this missing "safety resistor" I cover in this video. Tube loses bias = bad stuff happens.
@@SkunkieDesignsElectronics Thanks for the reply. I had a pretty good feeling it wasn't the tubes, which were fairly new. Question... Can you damage an amplifier by leaving it on without having a signal playing through it? I sometimes turn it on and wait a few minutes for it to warm up before playing anything.
Hello, how are you, ask you sell kit for upgrade willsenton R8?
No because different amps need different things done to them.
Hi Skunkie, I bother you again.There are two kind of audio note 100uf 100v , polar and non polar(bi-polar) . Which are good for the modification?
All the best
Either will work, given these don't have much of an impact on the sound, I'd just but the cheaper polarized ones.
Just curious if there would be any benefit in replacing the potentiometers for higher quality ones or are they just as likely to fail?
The problem is finding better quality ones that are mechanically and electrically the same. Installing these safety resistors at least avoids a melt down :)
@skunkie Designs Electronics Any chance of a list of the components you use in each video description - Not capable of doing the work myself but would like to get the components required to provide to the tech who will do the work. I see this mod as a necessity along with the prior choke mod. TIA
I plan to put a BOM together at the end of this series.
@@SkunkieDesignsElectronics awesome thank you
Thank you so much. I have learned something new with N.B. I have 1 amp doesn't have 100k and 100uF which I can do like this mod. And my 300B PP has 30K instead of 100k without 100uF, should I change to 100k and add 100uF? Does 10uF work instead of 100uF?
Without seeing that particular schematic and studying some data sheets, I wouldn't start changing anything. BUT if it is a fixed bias amp and the bias supply doesn't have a "Safety resistor" from the bias supply to the center of the bias adjustment pot, add one. www.valvewizard.co.uk/bias.html
@@SkunkieDesignsElectronics Thanks for the link. 1 amp doesn't have R3 and C2. Another one has R3 but no C2. I will work on it. 👍
What type of resistors did you use for the grid stop resistors? They don't look like the Vishay PRO2 resistors. I haven't seen resistors quite like that.
Those were some random 1W vishay metal films I had laying around I wanted to use up. You guys don't miss anything!
Hey Stephe, the Audio Note caps are out of stock. Is there another brand I should look for besides Nichicon?
The nichicon audio grade caps work fine. I've used the gold ones before and they are good!
@@SkunkieDesignsElectronics Thanks again for your help.
dear friend how much watt you use in small resistor 100k in board? please ...
1/2W is fine.
@@SkunkieDesignsElectronics thank you dear.
any thoughts on the muzishare x7 amps?? are they good? better than this one?
Sorry, never seen one. Honestly, this is the best made in china tube amp I have seen so far lol.
have you made and design your own crossovers?
I honestly am just now getting into working on DIY speakers.
I'm having an issue setting the bias on V3 after completing the changes to bias circuit boards. I had to turn the control nearly fully clockwise to get it in the zone. The others are fine and the amp sounds great (the Mundorfs do make a difference). But what could be going on with V3's bias adjustment?
Double check the value of the safety resistor?
@@SkunkieDesignsElectronics I replaced V3's safety resistor because the outer casing looked like I touched it with my soldering iron. But it still measured 100k ohms. Looking closer, the V3 bias pot had cold solder joints so I reheated them and added a little solder. Bottom line, the bias adjustment is behaving now.
I'm not done yet. I've replaced the main filter caps and the choke (the original choke measured 1.4 H, not 4), and done the bias boards, including the Mundorf caps. They make a real difference. Listening is really nice. But I can hear a tiny bit of hum with my ear to the speakers with volume up, so I think I'll redo the grounding scheme next, after I take a break. The resistors show no deterioration and measure right, so replacing them is lower down on the list.
Thanks for doing this.
@@carlespy7470 I have the same problem after adding four 100K fail safe resistors all 4 tube bias are low even when it turned to max position.
Hi Skunkie, if I replace the capacitors with mundorf at 0.47uf 450v instead 0.22uf 450v, is any problem?
The only issue could be amplifying subsonic frequencies, and they recommend using as small a cap as you can without creating a high pass filter for "clarity". I would stick with .22 or maybe try .33uf if you have speakers that can reproduce super low frequencies.
@@SkunkieDesignsElectronics Thank you for your quick answer.I wish you a nice day
L I K E 👍👍 👍 👍👍💯🤍 💯🤍💯😻😻😻 😻😻😻😻
Next mod: replace the output transformers 😂
IKR? At some point you have to limit what you are dumping into these cost wise not knowing really what kind of OT you are working with. Without having the exact same circuit with different OT, you really don't know if they are limiting the performance or not. I will say looking at the distortion vs frequency response, they are at least in the very decent category.