Replace Selenium Rectifier with Silicon Diodes & MOSFET Voltage Dropper in Tube Radios #pcbway#

This solution is practical and easy to follow, whether you’re restoring a classic tube radio or simply looking for a modern and safe replacement for the selenium rectifier.

NOTES:
The Tab of the Mosfet is at the '-' potential, so the bolt connecting it to the PCB and then to the Chassis must be a nylon one if the chassis is not a common GND.
Before applying power to the board, set the Trimmer Pot to about half-way to prevent the Mosfet from being OFF when power is applied. This will prevent the circuit from seing the full B+ across it, which may damage the Trimmer Pot.

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[SimonB]

I’m just refurbishing a Grundig Mandello GB and decided to try this to get B+ at the correct voltage. The PCBs have arrived this morning so as soon as the MOSFET arrives I’ll get the job finished. Thanks for this 😊

[matambale]

Again, very smart design (downright clever!), clear explanation, thank you Manuel.

[techobaz]

A very simple, smart design Manuel. Will solve many issues I feel, beyond the Selenium application as well !
Thank You !

[JonRig]

Yet another slice of EO&N genius!!! Excellent work and really great having the detailed description. Well done Manuel!!

[SBWings]

I'm just in the process of placing my 1st order with PCBWay. Thanks for this!

[tubeDude]

Another EXCELLENT EO&N design! 👍 Thanks, Manuel!

[ericrawson]

Clever design. I was watching for the voltage to change as the FET warmed up, but it didn't. I thought, doesn't the Vgs/Drain Current characteristic change with temperature? I looked at the datasheet for NTP067N65S3H which I recently used, and for a given current, Vgs changes by as much as half a volt between 25C and 150C. This would be magnified by the potential divider ratio of the trimpot. I guess it's more than accurate enough for a valve radio supply, and a very economical design. Edit: by the way, I used Pcbway on your recommendation and can recommend them. Quick delivery and perfect product quality.

[fredrikviklund]

Great idea and well explained and executed as always! One thing I'd love to see in a follow-up is a failure mode analysis. What happens if the MOSFET fails short, if the pot is shorted etc. Are there any circumstances where this circuit could add a new risk or is not suitable? A systematic discussion of any potential issues would be a great addition.

[BKGoldDetecting]

Another great design, I’ll try this on my next radio. Incidentally, do you do anything to address the heater voltage being too high when you modify a 220v radio to 240v. I’m getting close to 7v on my Freiburg even though I’ve got the B+ correctly adjusted.

[anandarochisha]

Brilliant work, Manuel. You could be mistaken for a very talented Electronic Engineer. 😊

[poormanselectronicsbench]

Nice little board design! One thing you can do to create some more heat dissipation for the MOSFET is, move the AC2 and + output through holes closer to the diodes / trimpot area, then, do a copper fill on both sides of the PCB, and then drop some 1.5 ~ 2MM via's outside of the MOSFET's mounting area to create some more cooling surface area. You would just have to make sure that the mounting tab area does not contact anything else when mounted, but according to the Vishay datasheet that tab is not shown as connected to the D-S or G pins.( If you say it is a DRAIN connection, then no harm or foul as it will be chassis potential anyway, as you stated)

[goosgitaar]

Gone try it Out !
Had no succes with the zener version a terrible hum apeart
In the guitar amp

[Phil-Webb]

You said that you couldn't add the circuit to your Braun radio. Would this be an improvement, or is it not possible to replace a half-wave rectifier with a full-wave one?

[murilomonconill]

Perhaps for diodes D1 to D4 microwave oven diodes could also be used, such as the HVR-1X-4 or similar, which has a voltage drop of 10V and a current capacity of 500mA.
No additional circuit would be necessary, except that it would not allow linear adjustment of the voltage drop.

[alainblazit]

A MOSFET and a Zener can make a voltage regulator, to avoid B+ overvoltage during startup.

[bofor]

I like the design of this as it simplifies the adjustment of the B+ difference and it can be setup with a lower input voltage. This does of course bias the whole chassis above earth and in some cases this can interfere with the bias setups already exisiting, so this would be unusable. Also you would have to be careful not to add a mains earth or ground earth to the chassis. I can see the point about about the risk of high potential contact if placed in the B+ line, but if needed could this be potted (allowing access to trim pot) or non metallic bolts used to reduce contact risk? I know if extra heatsink is required, this would rule out the chassis.

[georgebliss]

I like the simplicity of the series zener diode.
To reduce the wattage of that single one, would 3 x 6.8V or some other combination in series do the trick?

[mackfisher]

Manuel, I want to add to the Praise of others.
If PCB way marketing department is reading the comments might I suggest they send you a Christmas bonus.

[hahaagi]

FET GATE 와 SOURCE 사이에 PROTECTION ZENER DIODE(20V)를 넣는게 더 좋지 않을까요?

[DavidSmith-zxwz]

How did you get so smart! Are you self-taught, or ? Thanks for what you do.