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240z4u 22nd April 2009 01:37 AM

Troubleshooting hum in 12b4 preamp.. finally almost done!!
 
I bumped my other thread but wanted to show pictures of this preamp that you guys helped me with in the other thread. Thanks Eli and Douglas in particular.

I am having trouble with hum in the output, and it is not changing with the volume control. I have everything grounded to a common spot in the preamp.

Thoughts on where to start?

http://i230.photobucket.com/albums/e...b4/12b4002.jpg

http://i230.photobucket.com/albums/e...b4/12b4006.jpg

So far, this thing sounds absolutely great. If I can get rid of the hum I will build a second one to keep (this one is for my dad) and sell off some of my other gear.

Thanks!

azazello 22nd April 2009 12:12 PM

Hi !
Where is the power supply /PS/?
You must put in PS a choke 5-10 H, after rectifire between first and
second elektrol. caps. And put parallel to the choke non elektrol. cap
1 uF/160 v.

240z4u 22nd April 2009 12:33 PM

It is in a separate enclosure.

http://www.diyaudio.com/forums/attac...amp=1191802078

That is a schematic of the power supply I used. I did use a battery bias'ed CCS that I got from another member that replaced the top part of that schematic (the part with the tube connections).

Evan

azazello 22nd April 2009 01:20 PM

OK !
Put parallel every chokes caps 1 uF/160V.
/I prefer You connect between ampl. and AC main, LF & HF filter
from PC PS 300-500 wts/.

Bandersnatch 22nd April 2009 09:23 PM

Quote:

Originally posted by azazello
Hi !
Where is the power supply /PS/?
You must put in PS a choke 5-10 H, after rectifire between first and
second elektrol. caps. And put parallel to the choke non elektrol. cap
1 uF/160 v.

Cap in parallel with the PS chokes? Why?
cheers,
Douglas

240z4u 22nd April 2009 09:35 PM

Hey buddy!

Bandersnatch is another character who helped me get this thing functioning.

Evan

Eli Duttman 23rd April 2009 01:56 AM

Evan,

By chance, did you forget to bias the 12B4 heater CTs off B+? If said bias is present, switching to DC heaters may be in order.

Don't forget the "routine" checks of lead dress and cold solder joints.

240z4u 23rd April 2009 03:31 AM

Thanks Eli and everyone else. Ill go back and check the basics.. it never even occurred to me until today that it may be a simple ground loop in my system. It will be a few days before I can get back to it though.

Thanks guys. I am enjoying the sound. I had a buddy over that was absolutely shocked at the change and asked if I was willing to build him one. I said sure, it will be ready in 2011 LOL.

Ill get back to this soon.

Evan

azazello 23rd April 2009 11:23 AM

Quote:

Originally posted by Bandersnatch


Cap in parallel with the PS chokes? Why?
cheers,
Douglas

Please, think!
The cap has 2 plates, if on the first there is +U. on the second - -U....

Bandersnatch 23rd April 2009 11:33 AM

Quote:

Originally posted by azazello

Please, think!
The cap has 2 plates, if on the first there is +U. on the second - -U....

Obviously...so why shunt all the chokes with a cap? Looking for a resonance point to block a particular frequency of interest? Probably best to do that on only one choke, and not the first.
cheers,
Douglas


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