Amp head power issue?

JBMysteron · #1 12-10-2010, 06:00 AM

Currently running an Ampeg SVT3Pro with a Sansamp RBI patched through the effects loop into an Ashdown ABM810

I find this to be generally quite noisy, I'm thinking this may be due to the fact that I'm running a head that runs at 450W@4ohms yet my cab is 1200@4ohms

Would this have any effect on my tone?
When I play at reasonable volumes/dig in relatively when playing finger style/playing with a pick often I find a distinct distortion coming out of my speakers.

Have considered selling the Ampeg to buy a new power amp yet I'm beginning to think it may be more beneficial to upgrade my cab in quality or downgrade in size?

Thoughts/assistance?

qball26 · #2 12-10-2010, 06:33 AM

I actually had the same set up, but an Ampeg 810 instead. Try running the the RBI into the pre amp in on the 3 Pro, that way you will completely bypasss the pre amp on the 3 pro. No sense in running 2 pre amps. Never had any noise issues running it that way.

EX.: Bass > RBI input > RBI out into 3 Pro pre amp in

I think you get the idea. And this way is saves you a little bit of cash before the holidays.

Nowadays I just run a Shuttle 6.0 into the 810.

JBMysteron · #3 12-10-2010, 06:48 AM

I just tried that, still got a bit of distortion going on. Same deal when I remove the Sansamp RBI from the chain.

Sounds like it may be my cab then?

Jerrold Tiers · #4 12-10-2010, 06:57 AM

Depending on the type of distortion.....

1) the "tube drive" is set too low (low is more distortion)?

2) amp bias is "off" slightly ?

3) active bass/bad battery?

4) bass overdrives RBI?

JBMysteron · #5 12-10-2010, 07:05 AM

Quote:

Originally Posted by Jerrold Tiers

Depending on the type of distortion.....

1) the "tube drive" is set too low (low is more distortion)?

2) amp bias is "off" slightly ?

3) active bass/bad battery?

4) bass overdrives RBI?

1) does this regardless of the "tube gain" level
2) ???
3) Passive bass
4) RBI has been taken out of the chain and issue still remains

Narrowing down to Ampeg power-amp not handling something or dodgey cab?

qball26 · #6 12-10-2010, 12:09 PM

Blown tube in the 3 Pro maybe??? Or blown speaker in the cab? Try using your head with a different cab and vice versa, that should narrow it down somewhat.

JBMysteron · #7 12-10-2010, 10:00 PM

All 8 speakers in the cab distort so I feel like it is the Ampeg. Borrowing my brother's bass rig this weekend to try out different combination etc

Jerrold Tiers · #8 12-10-2010, 11:44 PM

SVT-3PRO are somewhat notorious for mosfet bias problems, which are easily fixed by a tech making an adjustment.

The symptom is distortion that is worst as the note dies away. basic "crossover notch" distortion.

The problem is on 3PRO because that amp has no 'feedback" in the power amp. "feedback" is usually used to reduce distortion, and without it, the amp does produce better tube distortion, but at the cost of being much more sensitive to mosfet bias.

JBMysteron · #9 12-11-2010, 01:53 AM

Hahaha I do wish I knew this before I bought the SVT3Pro, thanks for the info anyway Jerrold!

Jerrold Tiers · #10 12-11-2010, 08:20 AM

Quote:

Originally Posted by JBMysteron

Hahaha I do wish I knew this before I bought the SVT3Pro, thanks for the info anyway Jerrold!

it's not a "permanent problem" that is a serious issue....... it's just something you get adjusted and it's done...... Many of these units are 10+ years old, and stuff happens.