Mesa-Boogie Dual Rectifier farting

[punkykatt]

Hello all,  I hate working on these amps, http://schematicheaven.com/boogieamps/boogie_dualrectifier.pdf. but its for a friend of mine.  Using Dougs listening device I traced the signal all the way to the PI. Thats where the trouble starts. I get good signal on the PI tube grid pin 7, but almost no signal grid pin 2. Schematic plate voltages call for 280v on both plates. My readings showed 358v on pin 6 and 203v on pin 1.  Schematic cathode voltages call for 48v. My cathode voltages showed 29.2v . The plate load resistors R value read spot on. All tubes in the amp are known good tubes.  All the other PI circuit resisters R value were spot on except R214 1M read half its value. I replaced R214 and still got the same reading as before. All voltages on the power tubes look normal. Can bad coupling caps be the cause? How about the bypass caps on the plate resistors? This amp is a nightmare to work on.  Any help or advise is much appreciated.   Punky

[phsyconoodler]

Quote: "All tubes in the amp are known good tubes"

 Doesn't matter if all the tubes are 'good' Boogies need to have chinese only tubes,especially in the cathode follower positions.Othe tubes will not work.
  Also,the channel switching matrix is a huge problem on those amps.There is a 175 J-Fet that causes muting if it fails,so you either get a low,distorted output or no output at all.I'm assuming yours has no output.
  The previous tube controls the signal through the effects loop.Nasty way to do things and pretty dumb IMHO.
Try a chinese tube in the previous position to the PI and then look at the J-Fet. J-175.
 

[simonallaway]

... Boogies need to have chinese only tubes,especially in the cathode follower positions.Othe tubes will not work. 

Why is that? Is the Chinese manufacturer's implementation, of say an EL34, so bad that Boogie relied on it and designed themselves out of using other manufacturers?

[Jack1962]

Quote: "All tubes in the amp are known good tubes"

 Doesn't matter if all the tubes are 'good' Boogies need to have chinese only tubes,especially in the cathode follower positions.Othe tubes will not work.
  Also,the channel switching matrix is a huge problem on those amps.There is a 175 J-Fet that causes muting if it fails,so you either get a low,distorted output or no output at all.I'm assuming yours has no output.
  The previous tube controls the signal through the effects loop.Nasty way to do things and pretty dumb IMHO.
Try a chinese tube in the previous position to the PI and then look at the J-Fet. J-175.
 

then why are Mesa tube relabeled Russian tubes??????

[phsyconoodler]

All the CATHODE FOLLOWER positions of ALL mesa amps now use only chinese tubes.There was a bulletin out about that.
  Russian 12AX7's cannot handle the high voltages on the cathodes in those positions.They fail on a regular basis.
Yes,boogie has had a LOT of Russian 12AX7's and many fail in the CATHODE FOLLOWER positions and work fine everywhere else.
   I'm a Mesa warranty tech.(doesn't mean I'm a pro at fixing them!)

[punkykatt]

"Also,the channel switching matrix is a huge problem on those amps.There is a 175 J-Fet that causes muting if it fails,so you either get a low,distorted output or no output at all.I'm assuming yours has no output."

Phsyco. thanks for that info. Yes, no output just farting. How can i find that 175 J-Fet on the board? And how can i test it?  I dont have any China tubes on hand. Wouldnt a Russian tube work just for a little while? Did I say I hate this amp.  Punky

[jojokeo]

Until p-noodler gets back (and I see you're online) I'll attempt to assist - I would be it's near your circuitry where you're working on and you know solid state when you see it in a tube amp? 
Anyway, these have diodes in them and using your diode/ continuity position of your VOM, check each pin one way and then the reverse. Like any normal diode you'll see a reading one direction and nothing in the other as normal. If you get any low continuity reading or direct short, then that's your problem. It should read infinity then 1.47k or so but don't get too hung up on the exact reading. This is my fast way to check these things. Sometimes you "might" have to lift it out as other circuitry like caps can get in the way of proper readings but hey, if it's suspect - you'll have to unsolder it anyway if you're going to replace it. My VOM has a transistor tester and then I'd confirm it that way after it's out. Still that diode check works 95% of the time.

[phsyconoodler]

Quote:"Did I say I hate this amp"

  I hate them too.The J-Fet should be near V-4,when the chassis is out and the pots are towards you.
Or at least it may be near the last tube before the PI tube.That may be the issue,but get a couple of Chinese 12AX7's to try before you go any further.The Rssian ones may die or not work at all.

[punkykatt]

Thanks guys,  Until the Chinese  tubes get here I might as well check out this J-FET.  I did a search for J-FETS on the net and it looks to be a half circle D shaped body with three leads. I dont remember seeing one of those. It might be hid under a silicone blob. I will look closer tomorrow. There are plenty of the square four lead LDRs. Some are stacked three and four on top of each other. Can those be tested the same as the J-FETS?
I also checked the schematic for a J-FET symbol. Didnt see any?? They show plenty of LDR switches. Can you tell me where on this schematic(see first post) the J-Fet is suppose to be located?  Thanks. Sorry to be such a pain. 

[navdave]

Dual Rec's fart out in channels 3's modern mode because there is no negative feedback loop. The bypass caps on the PI's plates resistors are for tone shaping you can remove them.
My dual rec had all Mesa labeled Electro Harmonix tubes. These amps sound best if you don't mess with the bias scheme, leave the power tubes running cold all the dirt in these amps come from the preamp. Before I sold mine I used nothing but channel two in vintage mode it's as close to a Soldano SLO these amps can get.
That's my 2 cents.

[Jack1962]

All the CATHODE FOLLOWER positions of ALL mesa amps now use only chinese tubes.There was a bulletin out about that.
  Russian 12AX7's cannot handle the high voltages on the cathodes in those positions.They fail on a regular basis.
Yes,boogie has had a LOT of Russian 12AX7's and many fail in the CATHODE FOLLOWER positions and work fine everywhere else.
   I'm a Mesa warranty tech.(doesn't mean I'm a pro at fixing them!)

Thanks bro I wasn't aware of that.

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[phsyconoodler]

I have a much better schematic,but I'll have to figure out how to get it to you.Maybe I can fax it to you?
  It shows 3,yes THREE J-175 J-Fet's in the muting circuit.It's very complicated.
Nasty,silly circuit.

[Jack1962]

There's a schematic on schematicheaven.com for it , they sure do like those J175's lol lol lol


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