Help debugging my mesa boogie studio preamp

Digamma · 15th May 2008, 12:18 PM

Hi All,
My new(used) preamp has a problem and I need some help. I have a little experience/knowledge in electronics but mostly non-pratical from Uni. It is not a bad tube as I have re-tubed all positions already. Here are the symptoms:

- day 1, gain dropped out on lead channel temporarily
- day 2, same thing but it stayed like that for a while.
- day 3, changed tubes. Now noticing a lot of intermittent noise(chirping/interference) on lead channel until finally gain dropped out permanently. No more noise. Just and low output with no distortion/gain. While experiencing this interference, I noticed a hissing sound coming from inside the amp.

So my clean channel is fine. My lead channel now is weak and has no gain at all. The lead gain control doesn't even make any difference at all. All tubes light up. I have the schematic.

http://www.tubefreak.com/studio.gif

I'm guessing the problem is around V3 since this is the tube that provides gain on the Lead channel. I'll start by measuring the resistors around v3 specially the plate resistor. I have no idea what else to check.

Any help would be appreciated! Thanks

Miguel

mbrebes · 15th May 2008, 09:46 PM

The first thing I would do is pull the tubes that are used for the Lead only and, with something small enough, push in the edges of the individual sockets in the tube socket so that the tube pins will make tight connections. If that didn't fix the problem, did you swap that tube with a tube that is used in the clean channel, to verify it's not that tube? Next after that I would be suspicious of the switching circuit, can't remember if it's Vactrols or relays.

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15th May 2008, 12:18 PM	#1
Digamma Gear nut Join Date: Dec 2004 Posts: 78	Help debugging my mesa boogie studio preamp Hi All, My new(used) preamp has a problem and I need some help. I have a little experience/knowledge in electronics but mostly non-pratical from Uni. It is not a bad tube as I have re-tubed all positions already. Here are the symptoms: - day 1, gain dropped out on lead channel temporarily - day 2, same thing but it stayed like that for a while. - day 3, changed tubes. Now noticing a lot of intermittent noise(chirping/interference) on lead channel until finally gain dropped out permanently. No more noise. Just and low output with no distortion/gain. While experiencing this interference, I noticed a hissing sound coming from inside the amp. So my clean channel is fine. My lead channel now is weak and has no gain at all. The lead gain control doesn't even make any difference at all. All tubes light up. I have the schematic. http://www.tubefreak.com/studio.gif I'm guessing the problem is around V3 since this is the tube that provides gain on the Lead channel. I'll start by measuring the resistors around v3 specially the plate resistor. I have no idea what else to check. Any help would be appreciated! Thanks Miguel

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15th May 2008, 09:46 PM	#2
mbrebes Gear addict Join Date: May 2006 Location: Northridge, CA Posts: 329	The first thing I would do is pull the tubes that are used for the Lead only and, with something small enough, push in the edges of the individual sockets in the tube socket so that the tube pins will make tight connections. If that didn't fix the problem, did you swap that tube with a tube that is used in the clean channel, to verify it's not that tube? Next after that I would be suspicious of the switching circuit, can't remember if it's Vactrols or relays.