Bass amp problems! Ampeg BA-115HPT

tjw101 · #1 03-10-2011, 06:59 PM

Hi All,

I've done some fault finding but can't get to the bottom of this problem, hopefully someone can help!!

recently my Ampeg (Tube pre-amp, solid state power amp, 15" woofer + hf horn) started making a distorted noise when the bass is played through it. here is my list of observations and things I've ruled out:

1. Its not the Bass guitar (new batteries, and tested elsewhere)
2. Its not the pre amp tube (changed the tube)
3. Its not the circuitry between the pre amp & the power amp (completely clean sound from the DI which according to the circuit diagram is the last bit of the circuit before the signal heads to the power amp).
4. I don't think it is an earthing problem (not a constant hum, only get the distorted noise when a note is played on bass)
5. The distorted noise remains constant, so when the amp is run at low volume it is overwhelmingly annoying, at high volumes you can only just hear it as the clean noise of the bass takes over. but you have to run it at like 75%+ volume to get to that point - too loud for most situations. (would suggest its not the Speaker cone as it would get worse at high volumes I think)

I figure its a problem with the power amp or transformer... but I haven't been able to find enough info to be sure and don't want to spend out on more parts that won't fix it!

any ideas anyone?? I'm stumped.

Thanks in advance!
Tom>>>>

vin*tone · #2 03-10-2011, 07:12 PM

Hows the headphone out?
It might be a speaker/cabinet issue as well.

tjw101 · #3 03-10-2011, 09:09 PM

Hi Vin*tone.

Headphone out is fine.

The circuit chain according to the manual is as follows:

input ->Tube -> Gain -> EQ/FX -> Mute -> Master Vol -> DI/Headphones -> Power amp -> Speakers

I just tested the amp with the HF horn off and then on full, distortion is present both times (ie through both speakers) and gets worse as I turn the HF horn up, suggesting that the problem is happening before it gets to the speakers as it seems very unlikely that both speakers have blown (and anyway, does not sound like the sort of dirty noise you get from a blown speaker)

Hope that helps. any ideas?

Tom>>>>