SWR bass 750 issues... advice?
 

I bought a SWR bass 750, hybrid bass amp a few months ago and have been rather happy with until tha last show where it, was real muddled, and even cranked on 8 was barely punching thru the mix..we could barely hear the bass when whole band was playing… I had to use my back up hartke for rest of show… then at the next rehearsal, same problem…and it had a distorted sound coming thru her cab….
Not sure what the problem could be…could it be the preamp tube has blown, or is it something more serious? Any suggestions from those of you in the know would be much appreciated…
:help:

Hard to know. Since you were using the same cab, bass and cable, at least you know it is the head.

The first, simple thing to check is the preamp tube, which is the most likely suspect. Easy, cheap fix that you can easily do yourself. UNPLUG that amp and let it sit a bit before you pop the top. Just replace the tube as a fist step.

thanks man.. thats what is was thinking..

Just be safe and careful. Don't touch anything in there that you don't have to.

A super easy thing to try first would be patching a guitar cord between your effects loop jacks.
If this instantly corrects the problem, you just have dirty switching contacts in your return jack.

+1 Forgot about that. Often an issue. +1 Always check the stuff that doesn't require popping the top first.

I've never used these jacks....I'll try that tonight when i get home.. but i won't really be able to tel until band rehearsal on Sunday.. thanks, i really appreciate the advice guys!

+1 in that 'never using' those jacks is a direct cause of the potential issue.

I have been using a digitech effect pedal with the amp.. But i have been running straight thru the pedal into the passive input on front of amp... i have never used the effects loop jacks on back of amp... please forgive my ignorance on this, but please do explain why this might have caused my issue, and why i should indeed utilize the effects in/out jack on the back for this..

Effects jacks can sometimes get corroded and interfere with the operation of the amp.

No reason to use the effects loop per se. You aren't doing anything wrong.

The issue is (and I'm being a little loose here with the explanation, since I'm not an EE) that if you don't use those jacks, over a long period of time, you can get some oxidation in the internal components that can cause some crackling and even failure. Easy to fix, and usually just a little spray cleaner (check with the experts, since there are different kinds, some that work, and some that can actually do damage), sprayed on the tip of a jack and inserted a few times will keep you going for years!

Worth a try. Basically, putting an instrument cable between the two takes the internal circuitry out of the signal chain, so it is a nice way to see if that is your issue.

here is the paragraph on effects loop from the manual. i took from this that i could do it either way.. however, after reading it again, I see that perhaps not.. perhaps i was doing it harm, because the pedal has volume, and gain level controls, and by sending it straight in, it is sending a "hot" signal into the tupe preamp...not sure, but sound slike that isn't a good thing.. my weakness is in the tech side.......obviously..

From the Manual:
EFFECTS LOOP
The Effects Loop should accept any effect such as a chorus, flanger, etc. It is designed as a “side chain”
function and works exactly like that of studio consoles. Some effects units have an input level adjustment
switch. Whenever possible, set it at 0 dB. If that’s not an option, +4 dB is fine. The level going to your effect
is controlled by the Gain control on the front panel of the Bass 750.
Use of the Effects Loop should greatly reduce the noise generated by effects units (as compared to using the
effect between your instrument and the input jack). This is because the loop is after the preamp gain stages.
Note: The effects loop must be used in conjunction with the Effects Blend control on the front panel.
When that control is in the “dry” position, no effects will be heard.
All patch cords used with the Effects Loop should be as short as possible and should be routed as directly as
possible. Running patch cables over the top of the Bass 750 (as with any amplifier) can induce hum in the
cables.

10/4.. i'll check this out as soon as i get home.. i have some contact cleaner, as long as it is not solvetn based it should be good to use..

Nope... you aren't doing ANYTHING wrong. It is just that not using the effects loop MIGHT cause some issues, but that is easily fixed. No reason to change anything you are doing, and often, an effects unit designed as a pedal does not even work properly in an effect loop, since it doesn't have the ability to handle that hot of an input without distorting.

Another example of this issue was with the Mesa heads having a similar problem with their second input (the active versus passive input thing) since most players only used one consistently. So, they took the second, padded input jack out of the design and replaced it with an active/passive switch.

Again, no user error here, just can cause issues sometimes. My money is still on a bad preamp tube!

thanks kjung.i'm gonna try and pick a tube up on the way home today..

Nah, you're good. I've never ran an effect in a loop in my life and none of my amps ever suffered for it.

Well, I replaced the tube, and it didn't make hardly any difference.. a little but not much... damn.

Have you made sure the "effects" are set to DRY or looped the effects jack? BTW the contact trouble is called fretting and can/does happen to any low level (voltage and current) mechanical connection.

Bummer. If that didn't do it, and cleaning/bypassing the effects loop doesn't do it, then I guess it is off to the repair shop for you. Always good to eliminate the cheap, easy stuff before you pay an 'evaluation fee' at a shop.

Good luck.