SOB 100 watthead - huge hum

[es336td]

Used this head last Friday and it worked flawlessly all night. After 4 hours of sleep, loaded in for a Fall Festival (outside, a place I've played with this rig before with no trouble). Turn the head on to warm up the tubes; left it in standby while I set up everything else. Plugged in a guitar I knew worked with no noise. Flipped off standby and there was a HUGE low frequency hum. After eliminating the pedal board and rack and playing connected directly to the amp with a good cord; I discovered that the clean input was fairly quiet, but there was a faint hum. Plugged into channel 1 and the hum is back to huge. I get the hum even if nothing is connected. The ground switch does nothing. I am assuming that one of the pre-amp tubes is bad, but don't want to rip them out if that is a dead end. Any idea why I'm getting this? Thanx

 

[Authorized Boogie]

It sounds like either a preamp or power tube. Oftentimes hum like that is a sign that a power tube is soon to fail. How old are the tubes in the amp? We recommend new power tubes every 1.5 - 2 years of average use.

To troubleshoot your preamp tubes - simply take a new 12AX7 and swap it into each position, one position at a time, to help pin-point a suspect tube.

Be sure to troubleshoot with no instrument plugged into the amp, no fx in front of the amp or in the loop. See if it happens in more than one location, and pay attention to proximity factors such as fluorescent lighting, computers, fans, dimmer switches, etc - and of course incoming line voltage.

 

[es336td]

It sounds like either a preamp or power tube. Oftentimes hum like that is a sign that a power tube is soon to fail. How old are the tubes in the amp? We recommend new power tubes every 1.5 - 2 years of average use.

To troubleshoot your preamp tubes - simply take a new 12AX7 and swap it into each position, one position at a time, to help pin-point a suspect tube.

Be sure to troubleshoot with no instrument plugged into the amp, no fx in front of the amp or in the loop. See if it happens in more than one location, and pay attention to proximity factors such as fluorescent lighting, computers, fans, dimmer switches, etc - and of course incoming line voltage.

No idea how old. My amp tech did a cap job a few years ago and told me the tubes were fine. He had to fix something a couple of years ago... I don't remember what... the amp stopped working, but the power stayed on until I unplugged it. He said something about the way it was wired from the factory could have caused a bunch of trouble. If it's important I can look it up. When he did that repair, he said one of the preamp tubes was bad, so he replaced that. I guess the safe thing to do is take it back to him. Since I'm unemployed and depending on gigs, it may have to wait until I can afford the $65. an hour... I think I may have some 12AX7s around here that are good, or I could rob one out of my Blues Jr to do testing. More as I know it...

BTW... they aren't the factory tubes... the previous owner replaced them all with Groove Tubes.

 

[Authorized Boogie]

Even though its not under warranty, I'd highly recommend taking it to an Authorized MESA tech. You'd be surprised how little most techs know about Mesa amps - we take their calls daily looking for help. Thanks!

Music Technology
5418 Port Royal Road
Springfield, VA, 22151
(703) 764-7005

Universal Electronic Repairs
1112 East Market Street
Charlottesville, VA, 22902
(434) 974-1519

Audio Visions
8705 W Broad Street
Richmond, VA, 23294
(804) 346-4617

 

[es336td]

I sent it to my tech that did the cap job and fixed the switch when it died; Eric Sumer http://www.vikingamps.com If he can't find it, I will take it to Roger in Charlottesville. I just started a new job and sending it to a tech on the way to work just made sense. Roger is over an hour away. I will post Eric's findings when he's done.

Also, I noticed that the fan in there is not connected to anything. There is a two wire connector on the fan and a two conductor AC connector on the chassis near the transformer. I am assuming I need a cable from the fan to the connector. I can make one. I work on computers and have wired up these muffin fans for computers and amps. Before I go through the motions, are my assumptions correct?

 

[es336td]

Well, I need a new set of tubes. Since I started a new job, they will have to wait until I get paid.

I asked my tech about putting a set of Yellow Jackets in the low power position so they'd be the normal tubes used, and when I clicked on the 100 watt, the 6L6s would kick in. He said, "That might work OK. The 6L6s would swamp the el84s when they're all on tho" He suggested modding the amp to accept 6V6s, that it wouldn't change the tone very much at all. Probably tighten up the bottom end a bit. So... while I am waiting to get paid, I am trying to figure out what I want to do. I almost NEVER use the 100 watt setting; maybe twice since I've owned the amp.

Some time back, someone posted that it probably could be modded to be a Mark I. Don't know what that entails. Anyone know? Schematic?

Thanx

 

[Authorized Boogie]

I would NOT recommend using 6V6's in your amp...

FWIW, You can find your nearest AUTHORIZED service center by using the search, here: http://www.mesaboogie.com/US/Dealers/locator/zip_search_r.php

 

[es336td]

I would NOT recommend using 6V6's in your amp...

FWIW, You can find your nearest AUTHORIZED service center by using the search, here: http://www.mesaboogie.com/US/Dealers/locator/zip_search_r.php

I've had two or three others tell me this, so the 6V6 mod is out... I appreciate your wanting me to go to an authorized dealer, but I trust Eric with my amps. He did the cap job and when my power switch on the SOB shorted out, he ensured the hot side of the switch was fused instead of the neutral side... when it shorted, it shocked me numerous times and would not shut off... seems like a poor design to me. I had seen somewhere on here, where the SOB could be rewired to Mark I specs very easily. Eric told me that it would be cheaper to find a Mark I reissue or original Mark I, so that's out.

So, at this point, I will replace all the 6L6s with a matched Quartet. The fan connection was missing, so he's going to rewire that. He also suggested that he install a pentode/triode switch and change the Limit control to a Post Phase Inverter Master Volume. I believe that's the way I'm going.