Weird hum/buzz. Ampeg svt cl

[FletcherML]

Hey friends,
I recently moved my ampeg svt cl to my new apartment. I'm running it through a power conditioner and into my roommates Orange 115 as opposed to my ampeg 810. For the first 15-20 seconds of turning on the amp, it makes a dirty hum sound. I've swapped the cables and recently re tubes it. My first impression is that it's bad power in the apartment. Or maybe the cab? Anyone have any ideas what I'm working with?

 

[JimmyM]

Does it stop after the amp warms up? Is the Orange cab 8 ohm or 4?

 

[beans-on-toast]

You could have a bad tube. Even though they are new, one of them could have a problem. It might be a good idea to check that all the tubes are seated properly in their sockets.

 

[BassmanPaul]

If you replaced the output tubes did you have the amp re-biased to suit them?

 

[FletcherML]

The orange cab is 4 ohm. It only lasts about 15-20 seconds at most once i take it off standby. Even if i let it sit on standby for a long time. I removed the tubes and made sure they were seated well. I still haven't tested the tubes at all. But the bias is solid.

 

[BogeyBass]

if it goes away after 15 to 20 secs then it just sounds like warm up time.

power up in stand by, wait for 30 seconds then take off stand by.
see if that helps.

otherwise remove the power conditioner and see if it goes away.
hmm that does not sound right does it?

well try it, some power conditioners will cause a line voltage drop
10 to 20 volts should not cause to much of a problem could increase warm up time.

likewise better to power up in stand by, wait. then take out of stand by. 15 to 20 seconds just sounds like enough time for everything to stabilize from a cold start.

most likely no problem. constant hum is more of a problem.

 

[FletcherML]

The problem did seem less prominent when I remove the power conditioner. I'm starting to think it's the power in the apartment. I move into the practice space tomorrow. Hoping it goes away.

 

[FletcherML]

So I got moved into our new space and I'm still getting the same problem. I don't really know what to do from here:'(

 

[B-string]

If you retubed do you still have the old ones? If so start swapping one at a time as you probably have a new tube that is causing this.

 

[FletcherML]

Brilliant! Luhyoo!

 

[rickdog]

If you retubed do you still have the old ones? If so start swapping one at a time as you probably have a new tube that is causing this.

That's a good troubleshooting technique for the preamp and driver tubes, but may not work well with the output tubes. Even assuming the old output tubes are still matched to each other, they may not be matched to the new ones. You might find that you can't even bias the amp with a mix of old and new tubes. You might have a little more luck swapping out a triplet of output tubes (the three leftmost or three rightmost, not the front and back rows).

Push-pull output stages depend on good matching between the two sides to cancel out hum. My first guess of what is causing the problem is that the new tubes are well matched to each other once they have been on for a while and stabilize, but they aren't when the high voltage first hits them and the plates and screen grids are still cold.

 

[JimmyM]

You might not be able to bias it to spec but you can find out which power tube is bad with this method. And then you can know for sure before you go buying three more power tubes and have it not fix the problem. Can't say I ever found hum with mismatched power tubes to be a problem. I suppose it could be but I'd think something else would be wrong with them, too. OTOH I'm not a tech so what do I know

 

[B-string]

Mis-matched output tubes does not "cause hum". Plate, screen and bias (control grid) are filtered DC supplies. Hum would come from heater supplies (AC) inside a tube or induced from the heater supply (proximity wiring).
Matching tubes provides maximum power output and lowest distortion.