I think my amp is broken

[MCT]

I have 6 year old Ampeg SVT-3 Pro and yesterday during practice the amp made a loud pop (unplugging your instrument cable type sound) and a really loud hum. I thought that I had unplugged my cable so I pressed the mute button on the amp and to no avail the LOUD hum persisted so I quickly turned it off. I waited about an hour and turned it back on. For about 15 minutes it sounded normal but then again POP HISS/HUM. I took it home and plugged it into another cab with a different bass and a different instrument cord but it did the same thing.

I came here and did a search but I couldn't find my specific problem.

In addition my fan is making a grinding noise.
I appreciate any and all advice. I will run it to the repair shop but money is tight and I was hoping it would be something that I could fix at home.

[calebbarton]

That does not really sound like a effects loop problem but just for poops and laughs, try plugging a patch cable to and from the send/return of the effects loop after it shuts off again. If it works after that, clean out the jacks.

[MCT]

I opened it up and where it says V3 in the pre-amp the 12AX7 tube is not lit while the other tubes are glowing orange. I think that's my culprit now I have to do some research on tube replacement. Should I replace all of them or just the one that went bad?

[Russell L]

Might wanna check out that fan, too, if it's grinding. Without it blowing properly the heat could build up too much.

[Jerrold Tiers]

I'd just replace the bad one, but you can do whatever. Note that some tubes don't light up very brightly, so it might not be bad just because you don't see light.

Grindy fans probably need, or will shortly need, replacement, but check for any wires or inhaled spooge that might be hitting the blades.

BTW, Ampeg units generally do NOT use ball bearing fans..... we found them to be noisier in general than plain bearing fans. They last longer, but they tend to make rattling noises even wen new.

[MCT]

I replaced the tube last night it's back to normal. The fan has gotten worse however now its squeaking during power down. I tried ordering a direct replacement but I could only find the ST24B3. I read that the only difference is that the B3 has ball bearings. In the long run I wonder how it will make a difference.


I'm glad its running again I've got a couple of gigs this weekend and I had no backup. I really need to figure that out lol maybe get a cheap (price) peavey for my backup.

[MCT]

The same thing happened again. I've replaced the fan since then but after a while of playing the same thing happens. I'm trying to find a place locally that will repair it. I'm so bummed.

[5port]

If the exact same thing happened again then either the filament transformer has an intermittent winding or the wiring under the tube socket has a bad solder joint. Either way your going to need a tech.

[MCT]

Well the tube I replaced is fine. I think it is a temperature thing since it happens whenever the amp gets nice and warm.

[B-string]

[quote=5port;11242695]If the exact same thing happened again then either the filament transformer has an intermittent winding or the wiring under the tube socket has a bad solder joint. Either way your going to need a tech.[/QUOTE]

This a very real possibility. With heat the connection is lost (POP then hum), when it cools the connection remakes.

[Jerrold Tiers]

I can guarantee that the filament winding is NOT bad, since the amp does not HAVE one.

The power amp tube filaments are in series in the fan circuit, and the preamp filaments are all in series along with a resistor across one of the power supply voltages.

By the way, that means that the tube you replaced is probably OK... it just didn't "show" as brightly, but it was getting current if any other preamp tubes were lit up.

if it happens when the unit gets hot, it may be a fault in the fan circuit, because that reacts to heat by increasing fan speed.... or it is SUPPOSED to....... BUT, that means the hum is probably just "kinda loud", not ear-splitting crazy loud.

You need to make a check in order to figure out where your problem is....

What you need to do is play it until it makes the hum. Now plug a signal into the power amp input. See if 1) the hum goes away, and 2) if any sound comes out.

if the power amp seems to be working OK, meaning that the hum goes away, AND signal is OK, then the issue is in the preamp.

if the hum does NOT go away, and/or the signal does not get through, then there is a different problem.

Now once you know what's the issue, it may not help much, but you can tell it to a tech, who will probably (if he has any sense) appreciate it.