Ampeg SVT4-Pro Problems

[doombilly]

ok, let me start off by professing my ignorance of most in depth electronic gadgetry. I know enough to be dangerous.

My wife/bassist plays a P bass, flat wounds, with her fingers. (So fairly low output passive electronics). She's been using a (purchased new) Ampeg SVT4Pro head for about two+ yrs. We've replaced the pre-amp 12AX7 tubes about FOUR times. This is because after speaking with Ampeg (LoudTech) early on, they suggested it. The amp seemed to have lost it's punch. Each time the problem seemed to correct, but the duration between the need to replace the tubes was less each time. Now, right after the most recent replacement it just does not have the oomph to compete with my 15w guitar+cabinet played at the same setting we've always done. The cabinets (GK BLXIIa 4x10; SWR 1x15) are running dual mono full range. Attached by neutriks. I've tested all the speakers. And they are working. We just took the head to a shop and they tested the outputs of both channels and got 288w (8ohms) from each output (A, B).
After doing some reading I've found that there can be some problems with other SVT hybrid heads wherein they may require some biasing of (what I am guessing is) some part of the solid state power amp section.

Obviously we would want that done if there was any logic to it. Can anyone recommend any tests a novice without a meter or other implements can check?

Please advise. I'm new here, my apologies if I did not format this correctly or have crossed any bounds.
Eric in Charlotte NC USA.

[52 Bill]

Welcome to the board. I'm confused, but that's not hard for me.

If the amp in the shop put out 288W into 8 ohms, I don't really see a problem with the power amp biasing.

Are you saying the amp is weak sounding with no tonal oomph? Or is it just quieter than it used to be?

[doombilly]

SVT4-Pro

52Bill, I would say both. If we turn the gain up where it approaches what it sounded like when it was working well, the gain light comes on. If I use the
-15db pad I have to crank the gain wide open and the volume wide open to get close to a good sound. I am going to try another bass guitar, cables, speaker cab cabling etc... tonight. The speakers work, but I guess they could be not working as well as they once did.

[52 Bill]

Also try plugging in a spare patch cable into the FX loop.

[doombilly]

why?

Also try plugging in a spare patch cable into the FX loop.

Why? into the in or out of the fx loop?

[oc disorder]

Into both... Efx send to Efx Return

Why? because this is a very basic common fault.

The Efx loop usually sits between the pre-amp and the power amp.
With nothing plugged in to the efx loop the pre amp and power amp
are connected together by a primitive switch inside the efx return jack.
ie a couple of metal strips touching.

When you insert a lead the pre-amp signal has to come out from the
efx send to the effect unit and has to go back in to the power amp
via the efx return socket as the normally connected switch is open.

Often a bit of grit, dust or sometimes oxidation can get between the
internal switch contacts in the jack socket.
This can affect the signal , dropping out ,crackling etc.

By putting a patch lead from send to return you are bypassing that "internal switch in question" so if its causing the trouble you will hear it as the signal will sound cleaner as its now traveling through the lead rather than the internal metal contacts of the switch.
Hope that clarifies the issue.

[Enzo]

Preamp tubes last for years. If you have already replaced them once, then the tubes are not your problem.

Make sure the power amp switch is set to mono. Both outputs work either way, but in bridge they will be out of phase and the sound will cancel - A LOT.

Plug the guitar into the power amp A input jack, and then the power amp B input jack. Is the sound strong in the associated speaker for these tests? You are bypassing the preamp this way, so the controls won;t do anything, and the guitar can;t push the amp to full power, but you shouold be able to hear whether the thing still sounds anemic or if it sounds like it might be healthy.

Connecting FX send to FX return as Bill suggested is a good test.

And you can connect a cord from the FX send to the input of some other amp. How does that preamp sound coming out of some other rig?

There is about nothing else you can do without a meter and the knowledge of its use. That is why Loud suggested trying a new tube - it is the only thing the consumer can do on his own.

You may have a weak sound coming out of the amp, but it is not going to be because it can;t make power. Your local shop may have run a signal into it and gotten 288 watts from the power amps, but that doesn;t help you if the drive signal doesn;t reach the power amps.

[doombilly]

MONO?

Make sure the power amp switch is set to mono. Both outputs work either way, but in bridge they will be out of phase and the sound will cancel - A LOT.

We've tried the mono bridge stereo switch. It sounds louder with the switch in the out (stereo) mode. We are not bridging but running dual mono - full range into 2 cabinets. I will try the FX loop thing when we get back. (gotta run out for a few hrs). But what it sounds like you are saying is it should be louder with that mono/stereo switch IN in the MONO mode, even if we are not "Bridging."

EDIT. I just a/b tested it again. It is noticeably louder with the Mono/Stereo OUT in the STEREO mode. The FX loop did not make any difference we could note.

Also we have heard so many conflicting things on the Gain and Master volume levels. It sounds a LOT better with the gain set higher. The shop we went to suggested we NOT let the gain light come on (stay w/ gain turned lower to not light it up). Loud has said conflicting things. 1st they said it was no problem. Then they said we damaged the pre-amp allowing it to light up. She is playing a passive Precision bass, flat wound strings. So fairly low output. It was very much more driven when I played through the pre-amp than inserting into the power amp inputs with the guitar.
I think the box is better since the re-tube but we have gone through a lot of tubes for a pre-amp. I'm just trying to head off a problem with this batch.

[Enzo]

I am not really saying either way. It is just something quick and simple to do that may or may not reveal something of the operation of this amp. But what I do say is that if you have it in bridge mode but conect the speakers for dual mono, then the phase issue will bite you.

[doombilly]

phase

I follow what you are saying. But my ears tell me it comes out a lot louder with the switch out in Stereo. Could it be wired backwards? Hmmm. It seems like last year I tried to follow the instructions for mono bridging just to see what it would sound like. And I could not get it to work. It is possible I missed something in the set up. But now I am wondering if there is a problem there that would explain why I'm not getting a weaker signal (due to phasing) in the Stereo position.

From pg 12 in the owner's manual for DUAL MONO:

In the example shown below, the SVT-4 PRO’s two internal power amplifiers will each power a set of full range cabinets.
Set the Stereo/Mono Bridge switch OUT (Stereo position) and connect the system as follows:

[doombilly]

this is our set up

sorry, poor imaging editing on this compy.


except our bottom cabinet is a SWR 1x15.

Right now I am just trying to figure out if she can set the gain where she wants and what we need to watch out for as far gain peak light. Also why the heck we keep going through tubes. And if it is not the tubes, why it seems to get better after I change them.

[oc disorder]

Perhaps one of your speakers is wired reverse polarity.
Quick test
Using a 9 volt battery connect +'ve to tip of jack plug and -'ve to sleeve
and observe if the speaker cone moves out or in.
Likewise connect the 9 volt battery to the other speaker.
If one moves out and the other moves in they are cancelling each other
with a large noticable loss in bass response.
If not its back to the problem!

[doombilly]

ok, hmm

I'm at work now. I did test the cabs with a battery to see if they moved, but did not notice if one moved in and the other out. I will check that when i get home.

[doombilly]

ok tested 2 more variables

I did the batt test on the speaker cabs. Both the 4x10 and the 1x15 push out. So they are at least not wired out of phase.
Also I got a multimeter and attached a cable to the inputs on the speakers and checked the ohms. Weird, I got like 6.8 on one and 3.5 on the other. Maybe that is not the best way to test the impedance but I did not want to really dig around with the assBay ayerPlay looking over my shoulder. ;-)

[oc disorder]

Well that eliminates any speaker problems.
From your readings it appears you have 1 x 8 ohm load and 1 x 4 ohm load.
This means the amp should be putting out 790 watts rms (300 + 490).
"the amp in the shop put out 288W into 8 ohms" This sounds an accurate
reading before clipping .
The manual states" 2 x 350 Watts @ 8 Ohms (300 Watts Continuous) "

Just wondering if a fresh pair of ears might be worth pursuing.
Another bass player with a different guitar.
Other than that I see no option other than taking it to an authorised
Ampeg repair centre.

The fact that you have changed the pre-amp tubes 4 times and noticed a difference is unusual.
A P bass with flat wounds, played with fingers is a low signal to begin with
(which you acknowledge) as compared with say Flea poppin' and slappin' !

" Now, right after the most recent replacement it just does not have the oomph to compete with my 15w guitar+cabinet played at the same setting we've always done."
This seems very "out of order" for 790 watts to not be oomphy enough to keep up !
My first investigation woud be with the pre-amp section -power supply-
problem with the sockets etc.
I think a trip to an experienced "Dr Ampeg" is called for here.
There is not a lot more that you can do apart form hiring/borrowing another preamp to confirm if its the pre-amp section.