Willsenton R300 Circuit Design Issue

May I respectfully suggest that you do not turn on that amplifier until you are sure of what you did.

The 1N5380B is not a diode, it’s a zener diode and its breakdown voltage is 120V. That means it either maintains 120 volts (regulator) or it drops 120 volts. In order for the zener to function, there cannot be a high current draw bypassing it. So that tells me that it regulates the B+ voltage to a preamp or driver tube, which is designed for a plate voltage of 120 volts.

The diode you replaced, the 1N5408 is a 1000V zener. Since the breakdown voltage of 1000V is much higher than the 400 or so volts of the B+, it cannot reduce the voltage and instead passes the full 400V to the plate of the tube. One of two things can happen: either the tube performance changes or the tube will be destroyed if it cannot handle 400 volts at the plate.

Without a schematic, I cannot be certain. But the reason you got 140 volts instead of 120 volts is because the change in the B+ voltage changed the zener current and in turn changed the breakdown voltage from 120V to 140V. The cause of this is usually a change in the wall voltage. Variations in B+ voltages (and heater voltages) are a fact of life with tube amp ownership and most of the time these variation do not harm the amplifier.

I would replace that 1000V zener with the original. But call that tech first and read to him  what I wrote. There may have been a miscommunication between the two of you.

@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.

Now I see what’s going on. First, the 6SL7 will be damaged with 400v because its max plate voltage is 300 volts.

This is a simple common cathode amplifier. The 200K plate resistor and 400V B+ at 120 volts yields a plate current of about 1mA per side on the 6SL7 so a 125K dropping resistor is needed for 370 to 120 volts (2mA times 125k = 250V). I can’t tell the color codes of the resistor because the red circle covers the bands. That resistor is shunted to ground by the smaller electrolytic, creating an RC filter which reduces power supply noise. A better way of dropping voltage than a zener.

Using a 120 volt zener will work, but it’s not ideal. Zeners hunt for their voltage and that creates noise on the plate, which greatly increases distortion. The solution is to bypass the zener with a 1,000uF 400V capacitor. You have the ground bus right there covered by the bottom of the red circle if you want to go that route.

The best solution is to use two dropping resistors of 62K each, each with a 47uF shunt capacitor to ground, that is two RC filters and better noise reduction. But the space for that is very tight.

Get rid of the 1000V zener, it’s nothing but trouble. And please, ALWAYS use one hand when probing around tube amplifiers (put your other hand in your pocket) and never let any part of your body touch the amplifier chassis because those voltages are lethal and linger long after the power cord is unplugged.

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.

Any more feedback on the efficacy of this repair? I may be ordering one of these amps, and want to make sure the repair you implemented is ok to do.

Thanks.....

That Zener diode should not have failed on its own. I would be suspicious of the filter capacitors that it feeds.

Does anyone have a schematic of this amp? I'm about to dive into one of these to review and do a video series on and can't find one!

TIA

Skunkie