DIY Ampex 350 PSU

[schmidlin]

Hi Gang, I did a search and couldn't find what I wanted so here goes.

Inherited the 350 sans power supply.  I can handle everything except for the +- 12.6 VDC.  What AC should I be starting with?  One guy on here suggested a plain 6.3 VAC, but I just did a quick test with a spare filament transformer, hooked it up the same way Ampex does and I get no VDC.  Confused.

Also, the seleniums seem to get the thumbs-down so what are the recommended diodes?

TIA.

Schematic:
http://www.electronaut.info/public/Ampex350PWR.gif

 

[shabtek]

you have the power supply, and what is it to power? A 350? or a DIY project...I would guess there is additional CRC filtering downstream of the selenium. Silicon diodes will have less voltage drop, so you may have to up the R...but it depends on what wants the 12.6, and if it is forgiving.

 

[schmidlin]

Yeah, I'm building a PSU for the Ampex 350.  There is a 2000uf cap inside the unit itself to clean up the 12.6.  And it will be supplying heater voltage to a whopping 3 12SJ7's.

 

[PRR]

> 6.3 VAC, but I just did a quick test with a spare filament transformer, hooked it up the same way Ampex does and I get no VDC.  Confused.

Did you use the rectifier??

You won't get 12.6V DC without a big cap (probably on the amp chassis). But with the rectifier you should get some DC V.

6V AC is far too low.

Agree: no Selenium. 1N540x rectifiers are cheap and robust.

I figure 18.7 Volts AC Center-Tapped (AKA 9.4V-0V-9.4V). Anything within 10% is entirely in spec, and 18V CT should not be a tough find. 

Alternatively: figure the heater load, go to Jameco or other supplier for a 12V DC supply with enough Amps. It will probably be regulated, a bonus. "12.6V" preamp heaters will be quite happy on 12.0V.

 

[Mike Cleaver]

It's not DIY but there are plenty of cheap and good regulated dc supplies out there, cheaper than you can build your own.
Evilbay has lots of sellers, many with free shipping.
I use several of these for various projects.
Even with high gain pre-amps, they are extremely quiet.

 

[schmidlin]

Thanks Gents!
Two very solid replies and just what I was looking for.
(man, I love this place!)

Thanks Again,
-Schmidlin