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Re: O'Connor Preamp

4/7/1998 11:18 AM
Greg Re: O'Connor Preamp
The transformer is a brand new Hammond 270AX. I tried swearing at it and that didn't help....
4/7/1998 11:43 AM

I thought I had a good fix for ya, but  
I just looked up the specs on the 270AX  
and it's only got a single filament tap  
(6.3V 2.5A).  
Otherwise, your could've hung a bridge  
rectifier and a 4700uf cap off of the  
5V (rectifier filament) tap and just  
about nailed the 6.3V filament voltage,  
and with DC to boot....  
So your best bet is to follow Bruce's  
excellent enhanced description  
of what I suggested re. resistors inline  
with the filament supply...  
FWIW, I went through all this with the  
O'Conner book too, and learned a bunch in  
the process... and you're already learning  
how to swear at transformers, which I  
believe is an old HAM radio technique that  
actually works if you know the right  
cuss words :-)  
... Whit
4/7/1998 11:49 AM
Dang! I only know two cuss words and dang is one of 'em! Guess I'm gonna have to go hang out at the local high school and pick up a few more! : )
4/7/1998 2:03 PM

You just reminded me of a possible solution. Rectify the 6.3 winding with a full wave bridge, a capacitor, and use a 317 or 350 regulator adjusted for 6vdc. This would prevent hum pickup, also.  
4/7/1998 2:16 PM
Steve M.

Hi Doc,  
Thanks for the cool tip, I had wanted to try this in a project, but I have a question: what are the maximum current ratings for these two regulators? I was looking through the Mouser catalog for high current 6V regulators, but there are about 5-10 pages of semiconductors, most of which I'm unfamiliar with.  
4/7/1998 2:56 PM

I'm a little rusty on this solid state stuff, but as I recall the LM-317T is a to-220 package with current rating of 1 amp, and the LM-350K is a TO-3 device with a 3 amp rating. Someone please correct this if I'm wrong.
4/7/1998 2:20 PM
Yeah, I'm just trying to avoid that at this point as it's gonna cause me to adjust the layout of the chassis a bit to squeeze it in and I really don't want to drill any more holes. Also, I have a feeling that even if I do that, the hum is still going to be there. Pessimistic I guess. Seeing as how so many amps were run off AC filaments, it just seems I'm missing something. I understand that w/ 6.3VAC, I might expect SOME hum, but, man, this is a REALLY obnoxious hum. As a matter of fact, it's exactly like holding the end of your guitar cord w/ your finger. AAAAAGGGGGGGGHHHHHHHHHHHH! like that : )  
What if I play, oh say, Marvin Gaye's "Let's Get It On" and light some candles and give it some flowers?......
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