Triple down!......MEDIC!

I have a 3ch triple. I gigged it saturday night and it was slamming the whole 40min set, no problems what so ever. Today i open it up to take some gut shots to show a couple dudes in the "pre 500 tone" thread and i noticed a burned spot under the 4 diodes between the v3 & 4 preamp tube sockets. I fire the amp up and it sounds really bad, loss of top end and really muddy whoofy low end. No sign of swelled caps, burned resistors or anything else out of the ordinary. Swapped all tubes out with known good ones and no changes. Thoughts? Unfortunately i am in a situation where this amp could be sitting until next tax season before i can afford to have it looked at. This sucks the big one!

:shock: **** dude that really sucks!
I just looked at your pics you sent me and that looks like those rectifier diodes ran hot and burnt the trace below it. :?
If your still getting output then I believe the traces are not damaged.

It still has great output but is sounds like butt. Tone controls aren't as responsive and it's overly bass heavy, loose and mushy.

Post your pictures publicly so that other people can get a look, someone else might have an idea that will point you in the correct direction.

I would love to, but my internet is down at home and all i got are my cell phone pics(good cell phone). Maybe JC can post them? I sent them to him via email from my phone.

Best i can guess from the schematic, and it's been a long time since I've had to try and read one, is that its the rectifier for the low voltage +\- 3v supply to the preamp tubes. My preamp tubes get pretty hot to the touch now as well.

Burned area in question is in the second pic.

Thanks JC!

Well I broke out the multi meter tonight after looking over the schematic and suspecting a problem with the power transformer. 1 of the 2 yellow wires that supply 5vAC to the rectifier tube pc board has 0v output. So i know it pretty much needs a new tranny. For poops and grins i flipped it to diodes and pulled the gz34's out hoping it would at least sound better and be playable....it wasn't. All I got was louder mud and ***** tone. Off to the tech it goes. At least he said he could hold the amp as long as it takes for me to get the money to pay him for the work. As much as I hate to, I might have to take a real hit on my Splawn to get it to sell.

Whew, I'm glad you singled out the problem, but it's going to suck being boogieless
Crazy how that part of the board blackened up like that though.

well i already knew i was pushing things running 6550 w/ GZ34 having the plate voltage at 450v and pulling 42mA of plate current. Also, that filament supply was still providing a steady +/-3Vdc to v1.

Another freakish measurement.....v2-5 should have 3vAC on pins 4,5 an 9 according to the schematic.....V2 has 3vDC on those pins.

One of the tubes' heaters are run on DC to mitigate hum/noise. I thought it was V1, though. Check the schematic and look just above the relay coils coming off the 6.3VAC feed. There should be a heater connected across it after the rectifier.

Yeah according to the schematic only v1 has the dc filaments....2-5 are supposed to be ac but v2 measures dc only. Dropping it off to the tech tonight but he's gotta hold it until i sell my splawn to pay for the repairs and i can't give that f'ing thi g away right now.

good news: mesa is not broken

bad news: 1, 6550 + GZ34 will fry a mesa....eventually. Tech says I need to run them with 5U4's and bias as cool as I can get them.

2, I am an F"ING IDIOT! When I hooked my pedals back up i screwed the chords up big time. The send of the MXR noise clamp goes to the input of the TS9, the out of the TS9 goed to the return of the MXR....giutar input to MXR, MXR output to the amp.....I had the TS9 going back in to the output of the MXR and the Send of the MXR going to the amp.......made it bassy and muddy as hell!!!!


So now...back to our regularly scheduled discussion in the pre500 mods thread!

Whew, great news.
I think after you get done with those mods, you probably wont need the boost and noisegate anymore, it really makes that much of a difference.

yeah if i could only find those 100k resistors to remove and jump, i am going to remove that cap here in just a few.

Hey JC....can you look at the 3ch schematic and verify this for me?
v1 pin 3 cathode 100k resistor
v2 pin 3 cathode 100k resistor
v3 pin 8 cathode 100k resistor
v4 pin 3 cathode 100k resistor

and if these are the correct ones, how, in the name of Zeus' butthole, do i find them in the Sea of Resistance, aka the ginormous resistor splattered circuit board?

bye-bye tone sucking v2 capacitor! ahhhh I had no idea there was an invisible blanket over my speakers...the size of a small orange disc.

R_ADKINS80 said:

Hey JC....can you look at the 3ch schematic and verify this for me?
v1 pin 3 cathode 100k resistor
v2 pin 3 cathode 100k resistor
v3 pin 8 cathode 100k resistor
v4 pin 3 cathode 100k resistor

and if these are the correct ones, how, in the name of Zeus' butthole, do i find them in the Sea of Resistance, aka the ginormous resistor splattered circuit board?

Click to expand...

Sorry for the late response, I was gigging out tonight.
I don't think you'd remove the V4 pin 3 100k resistor since that's the loop send/return, and I spotted V3A pin 3 100k, V3 pin 8 cathode 100k is I believe a biasing resistor, you would leave that in place.

The rest are correct.