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RE: Selenium Rectifier Lowdown

Posted by coffee-phil (A) on January 22, 2017 at 20:44:33

In Reply to: RE: Selenium Rectifier Lowdown posted by Interstage Tranny on January 21, 2017 at 15:43:48:

Hi Interstage Tranny,

Please don't shoot me. I have one of those fine vintage preamps (at least I think it is) of which you speak. It is a Harmon Kardon Citation 4. It had a selenium bridge followed by an RC filter feeding the 6 12AX7 heaters in a series/parallel arrangement. The B+ supply was a 1/2 wave selenium rectifier feeding an RC filter network fairly typical for preamps of the day (~1960). When I got it the heater supply rectifier bridge was on the way out as the heater voltage was low so I replaced it with a silicon bridge. The dial lamp was part of heater supply filter. As tubes were heating the lamp would be very bright. I took a notion to use a 24 Volt three terminal regulator for the heaters as there was plenty room with the selenium bridge gone. The lamp was across the resistor between the filter caps and now across the three terminal regulator. With that arrangement the lamp would start dim and get brighter as the tube heated. All was well except that when the fridge came on my subwoofer went nuts. I decided that I needed to regulate the B+ supply. After the second design iteration I ended up with a MOSFET pass device controlled by a TL430 via an opto-isolator. I replaced the selenium rectifier with a silicon bridge and set the regulator to the input of the OEM RC B+ filter to the value on the schematic. The TL430 got its power from the now regulated heater supply. All was well and my subwoofer and fridge could co-exist. A few years later the twist lock high voltage electrolytic caps started to go South. During that re-fresh I used modern electrolytics mounted on copper clad board and replaced the incandescent dial lamp with a green LED. OK, not OEM but I liked the look. The heater supply electrolytics were still good so I left them.

That was all good for few years until the preamp developed an intermittent hum. A move was on the horizon so I had no time to deal with it so I got a Hafler DH100 off eBay. All sand but it seems to be a nice preamp.

I have recently dragged out the Citation and started troubleshooting it. I found one of the heater supply twist lock electrolytics to come in and out as I stressed its leads. Since the heater supply now also powers the TL430 that could account for my intermittent hum. Also I discovered that the three terminal regulator was coming out of regulation at ~105 Volts. I can't have that so I found a state of the art low dropout regulator from Linear Technology. Now both my B+ and heaters stay in regulation down to about 95 Volts AC input.

The front panel was getting rusty so I spent silly money to get a graphics file generated, the panel sanded out, chrome plated,and silk screened.

I'm closing in on it now, there are some OEM electrolytic caps as cathode bypass caps. I want to replace them, run some signals through it, and check the RIAA accuracy on phono, then I can listen to it.

Phil

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