Ac volts after selenium rectifier help needed

Steve Waltz · From: USA

I have an old 50's Magnecord preamp that I use. I replaced every cap and many if not all resistors with metal film and wire wound. It sounds good and I use it. I'm trying to track down the last little bit of noise. The noise is in proportion to how much you increase the gain without anything plugged in. I've tried switching out of the tubes and the same sound is there. This same sound is also on the other Magnecord that I have. With an LDC mic you really don't hear much noise but with a ribbon on a low volume source the noise floor of the amp is kind of there.

The first preamps are 5879 and then a 6sl7 and they have DC heaters. There is a selenium recitifier for that. I used my MM set to AC to check for any AC after the rectifier thinking that AC should not be there. I get the right DC power but AC shows 1.5 volts AC.

Could this be a noise source? Is my method of checking for AC correct? I've seen others repacle the selenium for new Diodes, should I?

Thanks,

Steve

Here it is before all of the work in case you are interested and one of the front

Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

Nice amp Steve. I have one of the earlier versions from an old AM station. Worked with two of them on the air in Atlantic City in the 60s. Normal for the multi-meter to show a reading when set to AC Volts since it's designed read both halves of the cycle. 5879 is one of the best low noise mic preamp tubes out there. Pull the 5879 and see if the noise drops. Might be the tube getting ready to retire, or possibly some resistors or caps in the front end of the Magnecord. The amp is certainly worth going over. If you can get a scope, the noise should be an easy find. Do you have a schematic diagram?

Let us know how you make out with it Steve. Happy hunting.

Sincerely,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024

Steve Waltz · From: USA

Craig,

Thanks for the info. I don't have a scope. I really should get one since I've been trying to learn as much as possible about this stuff.

I have about five 5879 tubes with a few being brand new and although some are more quiet than others, the noise is there. Same for the 2nd preamp tube, the phase splitter and the 5881's. I tried new 6l6 tubes and that dropped the noise to where it is now.

I have not replaced the small square brown capacitors that you can see in the picture. I didn't think those kind went bad often. I think they are in the .005 range.

I do have a schematic but it a slightly different than this amp.

All capacitors are new. I even used Jupiter caps. All resistors on the first two preamp tubes are wire wound. The rest are metal oxide or metal film.

I guess I'll take it to the local guy that works on tube recording equipment. It usually takes a week or two but I guess I'm not that patient and I like to learn it on my own, but at least he get's it done. Hopefully I can copy what he does on to my other ones.

Thanks,
And here they are. Altec 1592b is out of the picture.

Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

Steve,
If your local amp guy knows you are sincerely interested, he may be generous with his knowledge. Let him show you how to use the scope. At this point, you should probably be looking at the B+ and filament supplies as the noise may be riding on one of those lines.

Best regards,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024

Stephen Cowell · From: Round Rock, Texas, USA

Selenium rectifiers need to hit the trash... and those old paper filters are not helping. Probably a lot of those old resistors have drifted... if you want a collector item, don't touch it.. if you want a studio workhorse, rebuild it and blueprint it to original specs. We redid my buddy's Hammond B2 and 21H cabinet, amazing what decent modern components can do!
_________________
Too much junk to list... always getting more.

Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

Stephen,
You're absolutely right regarding those selenium stacks. They will fail one of these days, and when they do you can smell them half a mile. It's a nice unit and worthy of a complete rebuild.

Sincerely,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Steve Waltz · From: USA

Donny, I wasn't sure if I should have measured for ac after the rectifier or after the cap right after it. I knew there was ripple but I didn't know How a mm would see that. I do understand now that I can't measure ac with a mm like I tried to do. Thanks for the help everyone.

I posted the new pictures to show the caps and resistors that have been replaced. The mutisection caps on top I did as well. I think I am stuck until I can get someone to scope this. Thanks for the help again.

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