Loud Buzzing When Engaging the Drive Circuit on LSS

[fox5150]

Hi Guys,

I've got an irritating problem with my LoneStar Special. When I engage the drive circuit on channel 2 I get this loud buzzing. The buzzing is not present on Channel 1, nor on Channel 2 without the drive circuit engaged. It's not the 60Hz cycle that we all know and love, sounds more like the sort of noise when you've got a lead plugged in with no guitar on the end of it. I assumed it was one of the pre-amp valves - most noises are, right - so replaced all the preamp valves. Unfortunately the buzzing is still there.

I've tried different guitars and different leads to check that there's nothing wrong with that and the noise persists. I've got the amp plugged into a Furman Power Conditioner so the supply shouldn't be an issue. Just in case, I've tried it straight into the mains at different locations and the noise persists.

My knowledge with amps was used up when I replaced the pre-amp valves. Does anyone have any ideas what it may be?

As always, your help is really appreciated.

Simon

 

[ellem52]

You will want to look at the 12AX7’s. Specifically the V2 and V3 positions. You can locate their locations via the manual.

 

[Octavarius]

Try this:

SOUND “DROP-OUTS” / INTERMITTENT SIGNAL

One Possible Cause


Most (but not all) Mesa/Boogie amplifiers have one or two “cathode follower” tube stages in their preamps. Tube selection is CRITICAL in these stages.

Specifically, in a 12AX7 tube used as a “cathode follower”, the voltage difference between that present at the cathode, as compared with the heater voltage, can be withstood or tolerated by certain types of tubes, whereas other tubes will fail. The failure of a “cathode follower“ tube will cause sound dropouts or signal loss.

For the past few years, Mesa has been using two types of 12AX7 tubes: ones originating in Russia (Sovtek EH), and ones originating in China. The Russian (Sovtek) tube is NOT reliable as a cathode follower. Of the tubes we are using today (March 2008), ONLY THE CHINESE 12AX7 IS RELIABLE AS A CATHODE FOLLOWER.

In conclusion, if you are troubleshooting for signal dropout in a Mesa/Boogie amp,
suspect a cathode follower tube, and try replacing it with a Mesa 12AX7 that is marked as “CHINESE” (silk-screened on the tube itself).


Below is a partial list of Mesa amps and cathode follower tube locations:


GUITAR AMPS: BASS AMPS:

Lone Star & LS Special: V3 M-Pulse: V2
Stiletto: V3 & V4 Venture: V2
Road King I: V3 & V4 Big Block 750: V4
Road King II: V3 & V5 Titan: V4
Roadster: V3 & V5 M2000: V2
Dual & Triple (2ch or 3ch): V3 & V4 Bass 400+: V2
Tremoverb: V3 & V4



ANOTHER POSSIBLE SCENARIO may occur in the “SPONGY” (or on some models, “TWEED”) power setting: the reduced filament voltage may cause very low output from a RUSSIAN preamp tube. Again, the recommended fix is to replace the “sagging” tube with the CHINESE type of Mesa 12AX7 - which are more immune to this type of failure.

 

[user 9654]

Did you ever find resolution of this?

 

[fox5150]

Did you ever find resolution of this?

It went back to the UK Distributor (Westside) for checking. They stripped the whole thing down and couldn't replicate the problem. Since I've had it back, it's not been making the noise. It is now out of warranty and I got them to agree to extend the warranty to cover that element, should it reappear at a later date.

Simon