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FDP Forum / Amp Mods, Repairs, and Projects / USA Blues Deluxe Hum

quindecima

U.S A.

Apr 27th, 2017 12:53 PM   Edit   Profile   Print Topic   Search Topic

I have a hum in my deluxe amp. most pronounced when all knobs are at #1 and I turn up the Mid and Bass pots. If I pull a pre amp tube it drops by 1/2. All filter caps have been changed and the work was double checked. Don't seem to be any bad solder joints

Hammond101
Contributing Member
**********

So. Cal. USA

Apr 27th, 2017 01:03 PM   Edit   Profile   Print Topic   Search Topic

Check for DC on the control pots.

Could be a failing power tube. Have they been swapped for known good tubes? If one is on it's was out the amp will hum due to mis-match.

quindecima

U.S A.

Apr 27th, 2017 01:30 PM   Edit   Profile   Print Topic   Search Topic

No, I haven't done that, I have no spares at the moment. I did remove each of them and the hum was still present. How do I check for DC? I also noticed that the Little square red light between the treble and drive flashes on when I turn the amp off.

(This message was last edited by quindecima at 07:21 PM, Apr 27th, 2017)

Hammond101
Contributing Member
**********

So. Cal. USA

Apr 27th, 2017 04:26 PM   Edit   Profile   Print Topic   Search Topic

You need to have a reasonable, well exact actually, knowledge of amp tech safety as it's required the amp be turned on (hot as they say)as you check each volume and tone pot, the pins that connect to the tone or coupling caps in the circuit, for DC due to a leaking cap in that stage.

You may be better off taking it to a tech if a power tube swap doesn't resolve the issue. This should have the bias checked when the power tubes are replaced anyway.

These amps also develop bad solder joints at the power tube sockets....where the pins solder to the pads on the tube socket board.

Does any of this change with a guitar and cord properly connected? Another simple thing that fails are the ground outs in the FX loop jacks. Connect an instrument cable between them and see if the hum vanishes.

quindecima

U.S A.

Apr 27th, 2017 05:29 PM   Edit   Profile   Print Topic   Search Topic

My guitar is at the Luthiers for a nut change. I have also replaced the plate load resistors, the power resistors, Input jacks, changed the Master volume to an audio pot, C1,8,5,6,7,. Other than a bad output tube maybe the xfmr? This anp is supposed to be self biasing. I'd like to install an adjustable bias but need a schematic.

(This message was last edited by quindecima at 07:31 PM, Apr 27th, 2017)

quindecima

U.S A.

Apr 27th, 2017 06:28 PM   Edit   Profile   Print Topic   Search Topic

So, I take everything apart again to check it out and make sure, even thou I have already checked it 4 times, that I have wired it correctly and all caps have the correct polarity, AGAIN I check all solder joints and grounds on the chassis, then as I am staring and crying up at the ceiling it dawns on me (as I look at my Florescent lights) that the problem isn't my work it's the darn lights I have in my computer room. I re-install everything and take it out to the kitchen and plug it in, total niceness Happy Days!

(This message was last edited by quindecima at 08:30 PM, Apr 27th, 2017)

stratcowboy
Contributing Member
**********
*

USA/Taos, NM

Apr 27th, 2017 06:50 PM   Edit   Profile   Print Topic   Search Topic

Now there's an easy solution. Jam in the kitchen...makes it easy when you're sailing along with buddies making great tunes and you happen to get hungry.

amphead4

Cincinnati, USA

Apr 28th, 2017 08:28 AM   Edit   Profile   Print Topic   Search Topic

Keeps you close to the refrigerator!

Hammond101
Contributing Member
**********

So. Cal. USA

Apr 28th, 2017 08:59 AM   Edit   Profile   Print Topic   Search Topic

Cool story.

FDP Forum / Amp Mods, Repairs, and Projects / USA Blues Deluxe Hum




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