selenium rectifier - UK Vintage Radio Repair and Restoration Discussion Forum

Ed_Dinning · 17th Aug 2008, 8:19 pm

Hi Bob, seleniums do run quite hot and you certainly know by the smell when they die. If you fit a silicon diode you will need a resistor in series of 1-200 R, it will get quite hot so a high wattage part is required.

Ed

Sideband · 17th Aug 2008, 8:20 pm

If you are replacing a selenium rectifier with a silicon, you MUST fit a resistor in series with it. A recent thread https://www.vintage-radio.net/forum/...ad.php?t=30836 explains all this.

Rich.

Station X · 17th Aug 2008, 9:18 pm

Bob.

I replaced the main rectifier in a Pye Mozart with a silicon bridge type. This was heat sinked to the chassis and I had no problems with overheating. Having said that these amplifiers do seem to run exceedingly hot.

It's interesting that your grid voltages are much lower than the 14V specified. I found this too. Some of these amps use cathode bias whilst others derive grid bias from the mains transformer and a seperate rectifier.

There are two versions of the preamp. One has a rectifier to derive DC for the heaters, whilst the other takes its DC from the main amp. If you use the wrong combination you end up with two rectifiers in series.

paulsherwin · 17th Aug 2008, 11:41 pm

The resistor will not be dissipating 15W unless there is something seriously amiss. It may feel warm but shouldn't be too hot to touch. 10R is a rather low value as well, though this isn't critical. What is the voltage drop across the resistor?

Is the transformer running cool?

Paul

paulsherwin · 17th Aug 2008, 11:50 pm

Do you mean you have fitted the resistor between the mains and the transformer primary? If so, this explains why it is running hot. You should add the resistor between the +ve output of the bridge rectifier and the reservoir capacitor. 100R-300R is a better value, and it should dissipate less than 1W.

Paul

Station X · 18th Aug 2008, 11:14 am

With a 100R resistor you're dropping 15 volts. You need to drop about thre times that, so try 300R. I'd be inclined to try something like four 1.2K wirewound resistors in parallel to see how hot they get.

Station X · 18th Aug 2008, 8:07 pm

I don't understand that. In post 11 you say that with a 100R resistor you got 296 volts, yet in post 13 with a 100R resistor you get 269 volts. Was it a typo or are you using another bridge with a MUCH higher forward volt drop?

Station X · 18th Aug 2008, 8:33 pm

This might be a suitable resistor:-

http://uk.rs-online.com/web/search/s...duct&R=0159792

17th Aug 2008, 8:19 pm	#1
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: selenium rectifier Hi Bob, seleniums do run quite hot and you certainly know by the smell when they die. If you fit a silicon diode you will need a resistor in series of 1-200 R, it will get quite hot so a high wattage part is required. Ed

17th Aug 2008, 8:20 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: selenium rectifier If you are replacing a selenium rectifier with a silicon, you MUST fit a resistor in series with it. A recent thread https://www.vintage-radio.net/forum/...ad.php?t=30836 explains all this. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

17th Aug 2008, 9:18 pm	#3
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: selenium rectifier Bob. I replaced the main rectifier in a Pye Mozart with a silicon bridge type. This was heat sinked to the chassis and I had no problems with overheating. Having said that these amplifiers do seem to run exceedingly hot. It's interesting that your grid voltages are much lower than the 14V specified. I found this too. Some of these amps use cathode bias whilst others derive grid bias from the mains transformer and a seperate rectifier. There are two versions of the preamp. One has a rectifier to derive DC for the heaters, whilst the other takes its DC from the main amp. If you use the wrong combination you end up with two rectifiers in series. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

17th Aug 2008, 11:41 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: selenium rectifier The resistor will not be dissipating 15W unless there is something seriously amiss. It may feel warm but shouldn't be too hot to touch. 10R is a rather low value as well, though this isn't critical. What is the voltage drop across the resistor? Is the transformer running cool? Paul

17th Aug 2008, 11:50 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: selenium rectifier Do you mean you have fitted the resistor between the mains and the transformer primary? If so, this explains why it is running hot. You should add the resistor between the +ve output of the bridge rectifier and the reservoir capacitor. 100R-300R is a better value, and it should dissipate less than 1W. Paul

18th Aug 2008, 11:14 am	#6
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: selenium rectifier With a 100R resistor you're dropping 15 volts. You need to drop about thre times that, so try 300R. I'd be inclined to try something like four 1.2K wirewound resistors in parallel to see how hot they get. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

18th Aug 2008, 8:07 pm	#7
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: selenium rectifier I don't understand that. In post 11 you say that with a 100R resistor you got 296 volts, yet in post 13 with a 100R resistor you get 269 volts. Was it a typo or are you using another bridge with a MUCH higher forward volt drop? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

18th Aug 2008, 8:33 pm	#8
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: selenium rectifier This might be a suitable resistor:- http://uk.rs-online.com/web/search/s...duct&R=0159792 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.