2. amps-preamps
  3. 183841-148959

Willsenton R300 Circuit Design Issue

So I was able to repair my Willsenton R300 after it died and made no sounds.

The dealer was good to work with me thru emails from China to troubleshoot the problem instead of shipping it to a repair center.

It turned out to be a diode in the preamp circuit that shorted out. The diode the builder used is a 1N5380B which is for use in 5V-200V circuits. The B+ voltage in the preamp is supposed to be 370V dropped to 120V by a power resistor adjacent to the diode. However I was getting 417V dropped to 140V. So within spec but would not take much of a current surge or voltage spike on startup etc to fry the little diode.

I replaced it with a 1N5408 rated to 1000V. (Less than $1 repair cost!)

So the reason I posted this is that I have read some reviews of users having crackling noise with the R300. It could be that diode.

Hopefully the builder updates the design with a sturdier component.

 

calieng

Showing 2 responses by calieng

calieng OP
144 posts
 

@gs5556 

Hey thanks very much for the info!

The instructions I was getting were a little unclear as english is the second language and we had a significant time zone difference that delayed replies. But I did confirm the diode was shorted with the Fluke 117 meter and the amp did work with the replacement. 

But I appreciate you letting me know the rectifier diode is not the correct replacement. 

I do have the circuit diagram but not allowed to share online. The diode is indicated 120V and in parallel with a 10uF to ground on the B+ that feeds the 6SL7 plate.

There is a 5W resistor prior to that to reduce the 370V to 120V so the diode is there to regulate the 120V in case of high voltage if I understand the purpose from what you mentioned?

I was also given the prior version R300 schematic and there is no zener diode in that amp. Nor shown on the photo of their amp in house....so I am leaning towards just deleting it altogether as it seems it might have been an afterthought to help better regulate voltage. I would not want it shorting out again.

Will confirm with the builder.

Thanks again! You learn something new everyday on these forums.

 

calieng OP
144 posts
 

Thanks for the advice. The resistor in the B+ is 150k.

I agree to remove the diode. Not sure how the 400V would be created though as the resistor already dropped the voltage to around 120V....I guess I need to look at the schematic again to try to figure out how it all works.

From what I understand when the diode shorted it grounded the B+ which is why I was getting a reading of zero volts on the 150k B+ resistor. That is why the preamp section died and no sound was produced.

I have built several guitar tube amps so am pretty comfortable working on this amp. Just not too up to speed on hifi design and had not come across a zener diode to regulate voltage in past guitar amp builds.

Thanks again!  I'll report back if the builder has anything else to add once I hear from them again.

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