cross-over bulb staying on?

[winning hard]

Hola folks,
My cab is an ampeg 410 with a horn and the horn has an attenuator on the back that I always have turned off. Part of the cross-over circuitry for the horn is a light bulb that glows if the horn is turned off, or whatever.
My head is SVT CL.

When I run my pedal board through the fx loop (which is actually pre-amp out/power-amp in or whatever), everything works fine. If I unplug the pedal board and run just my bass, the cross-over bulb in my cab lights up and it makes a pretty loud awful hiss, my bass sounds like garbage, and I have to crank the gain and master to get any kind of usable volume.

What do you think is going on?

 

[Dave W]

Are you plugging the bass into the FX loop? Or into the front of the amp? Which input are you using?

 

[winning hard]

plugging my bass into the front. From Pre-amp Out to my tuner pedal (beginning of chain) and from my final pedal (a ditto looper) into the Power-amp In

As for which input, I typically have a bass plugged into both. I have a fretless with an active piezo pickup and it's louder than my stingray, so the fretless is usually plugged into the padded input and the ray goes into the normal input

I was going to stop using the "fx loop" though, and run everything through the front, and that's when I encountered this behavior

 

[B-string]

The bulb is lit because of the excessive hiss (high frequency noise). You need to start from the absolute basics, one bass then the other into the input on the front only (nothing in the effects loop on the back). If you have the hiss (and lighting) with just a bass only in the front input then you most likely have a bad preamp tube. If no hiss or lighting, then add things one at a time back in to find where the noise is being injected.

 

[winning hard]

yeah, I started from the basics. No pedal board, no pedals, just my bass (one at a time, like you said) plugged into the instrument input. With master and gain turned at any level (from all the way down to all the way up), the bulb comes on and stays on. When I add my pedals via the pre-amp out/power-amp in, the problem goes away

 

[B-string]

Try just using an instrument cable or pedal patch cable in place of your pedal board on the back. If that also eliminates the trouble then you have dirty contacts inside the power amp in jack. If not chances are back to a tubes as the pedal board is providing a different load than a straight through connection.

 

[winning hard]

Try just using an instrument cable or pedal patch cable in place of your pedal board on the back. If that also eliminates the trouble then you have dirty contacts inside the power amp in jack. If not chances are back to a tubes as the pedal board is providing a different load than a straight through connection.

good idea! I'll try that out tonight. Thanks!

 

[Dave W]

I would also take the tuner and the Ditto out of the loop. There is no reason for them to be in it.

EDIT: I see that's what you're trying to do. Good

 

[BassmanPaul]

A thing to also remember is that Effects Loops are usually intended for line level effects units. Stomp boxes should be situated ahead of the amps input jack.

 

[BassmanPaul]

The bulb is lit because of the excessive hiss (high frequency noise). You need to start from the absolute basics, one bass then the other into the input on the front only (nothing in the effects loop on the back). If you have the hiss (and lighting) with just a bass only in the front input then you most likely have a bad preamp tube. If no hiss or lighting, then add things one at a time back in to find where the noise is being injected.

i agree fully with Mr B here but I'd like to add the possibility of high / ultra high frequency instability. The protection lamp should not be lit during normal operation. It's only there to protect the tweeter from damaging signals.

 

[12bass]

Yeah... I wonder if the amp is unstable. A pretty healthy voltage is necessary to light up the protection bulb.

 

[winning hard]

so to address a couple misc comments above... my amp has been in and out of the shop lately. First it was because it was faulting (shutting off and then showing me the flashing red/green light on the front). That had something to do with the preamp power switch or some such, don't remember what the tech told me exactly. Then I had an issue with my caps or something like that. I don't remember. But my tubes are all healthy. The power outlets in my rehearsal space are somewhat questionable, but not a lot I can do about that.

Anyway, last night I plugged a patch cable into the send/receive of my fx loop and ran my pedal board through the front and everything worked fine. So thanks for the input, everybody!