Bass 400+ Help

Nightlyraider · #1 12-03-2011, 02:17 PM

While screwing around some in the basement I decided today might as well be a fine one to move the cabinets around some to re-arrange; and then play a while.

After playing for a little bit I realized that I had only plugged one of the cabinets back in on the cab side after moving them. Soooo the amp was running an 8 ohm cab out of the 4 ohm tap with another cable plugged in.

I turned the amp into standby and off and plugged in the cab again, but now I think there is more "hum" noise when you switch the amp out of stand-by and into on than ever before.

I did a few internet searches and couldn't find anything.

The amp is dead silent in standby, but if you flip the switch a constant humming starts. Where should I look next?

RickenBoogie · #2 12-03-2011, 02:41 PM

I don't think any harm was done for such a short while, but it should remind you how important it is to have everything connected properly BEFORE powering up. Not sure about the humming, but if it's a bother, it's always best to take the amp in to a qualified tech for servicing.

BassmanPaul · #3 12-03-2011, 02:43 PM

For a local tech?

Have a look at the output tubes and in operate see if any of the tubes plates are glowing red. sounds like you've lost one.

Nightlyraider · #4 12-04-2011, 07:45 AM

Thanks for the input.

Pretty sure there is a capacitor leaking somewhere in the power supply chain and not a tube problem after further research.

The amp is over 20 years old now and all the caps are original (opened her up yesterday and discovered birthday's signed on both transformers (5-xx-90).

Is it unusual for a cap to go in an amp that has bajillions of them and is just about as old as me?

Thanks very much for the input!

SurferJoe46 · #5 12-04-2011, 08:12 AM

The first thing I'd do is check if the unit is still noisy if you move it back where it was plugged in and operating before the move.

Unless something was just old and waiting to go smoke-o on you at the next opportunity and with the impedance mismatch you created, you just brought it's failure on a little earlier.

The single most common reason for hum or noise is usually something getting into the amplifier from a bad ground, RFI or some other electrical effect.

If it still makes noise after you move it back where it was - they you've got troubles.

LiquidMidnight · #6 12-04-2011, 10:03 AM

Even if you can't get rid of the hum by moving the cabs back to their original positions, I doubt it's a major (i.e., expensive) problem. Take it to a respected tech and your amp should be as good as new.

**B-string** · #7 12-04-2011, 11:56 AM

20 to 30 years is average life span on electrolytic caps depending on usage. The new position of the cabs might make you more aware of some hum that was there before though.

Caca de Kick · #8 12-04-2011, 12:30 PM

Back in the day, just about every Bass400+ I ever saw in any random GC had the cab plugged into the wrong tap (just about always an 8ohm cab in the 4ohm tap). These heads are very tough.

Yeah, I'd move the head back to where it was to see if it was just picking up RFI. My jam pad is a noisy building for amps and single coil pickups.