Hey everyone, nice to see this website redone and active.
Just to get the back story knocked out before I ask for help....
I'm a 31 year old who has owned and played nothing but MM since I was 14 years old and was hipped to them by a friend who was in a band called The Exploding Hearts.
My First was a 65reverb head I bought at a music go round for $200. This one is now owned by my best friend so I still get to see it.
After the 65reverb I scored an HD150 2x12 that I kick myself for selling. It was way too heavy and at the time you could still get one relatively cheap.
Fast Forward to the present. After selling off both MMs I tracked down a 75reverb head (2275-75) and a 1X15 cab from a pawn shop in Indiana for $250 in the summer of 2013.
__________
The problem....
From the beginning I noticed the amp was a bit quiet and the jewel light did not dim nor brighten when I switched from Hi to Lo.
Thought I new set of Mesa Boogie tubes would fix the problem but this wasn't the case. Opened up the sucker...
In the Hi position I'm getting around 375volts. In the Lo It's putting out 275v.
My OT trans is marked 5-75 EIA 606-024
Power Trans 100-65 EIA 606-000
Not sure if these are mis-matched.
I'n not too elec. savy, but I can follow instructions and I have friends who can help with what I can't do.
Any ideas as to where I should go from here?
I would appreciate any help/info, need this one to be tip top asap.
Problem?
Hi Xavier:
>From the beginning I noticed the amp was a bit quiet and the jewel light did not dim nor brighten when I switched from Hi to Lo.
This is normal behaviour in this model.
>In the Hi position I'm getting around 375volts. In the Lo It's putting out 275v.
Where is this voltage measured?
>My OT trans is marked 5-75 EIA 606-024 , Power Trans 100-65 EIA 606-000. Not sure if these are mis-matched
No, they're not.
The only way to find out if your amp is indeed putting out the proper power is to measure it with a reference resistor as load (8 ohms, minimum 75 watts). I would be very suprised if the amp is putting out too little power without any other sound degredation (distortion, lack of sound fidelity). Perceived loudness is a very subjective thing and has as much to do with the speaker, the environment it is being used in (the room you play in) and the programme material.
Sincerely,
Lars Verholt
Measured the On--Off-On
Measured the On--Off-On Switch.
375 dc volts at the switch in the Hi position, 275 in the Lo position.
I'm aware that the later models without a 12ax7 are less obvious to the human ears and more of a clean headroom issue than an actual perceived sound. That's why I'm asking with a multimeter in hand opposed to just stating that I can't hear a noticeable difference.
75
I'm a little confused by your reference to being "a little quiet". Does that mean it's not noisy or not very loud? I can't imagine a 75 not being loud. I question whether the use of a single 15 is the reason for the perceived low volume, if that is what you are referring to? Try a 2x12 or 4x10 and see if that perks it up. Also, the light not dimming means nothing. Mike.
I will try a cab as soon as I
I will try a cab as soon as I can. It seems insanely quiet compared to my old 65reverb. At full blast, I would honestly compared it to a Fender blues Jr 15 watter, possibly less.
Do my readings of the switch mean anything?
The 375 in the Hi position and 275 in Lo is what I got when I turned on the amp and placed my meter (set to read DC volts) with the red touching the Hi contact while on in the Hi position and the black grounded against the chassis. Same for the Lo reading.
I have the schematic on hand. If this is the wrong way to read the voltage, please let me know. I'm a little confused by the "reference resistor) comments.
Like I said, I'm an electronic noob, but I can follow directions.
The voltages are fine
Your readings at the standby switch are right on the money. This is the voltage that feeds the voltage doubler circuit (and the screen grids).
However, the important place to measure is at the 6L6 plates. In the 'Hi' position you would expect approx 715V, in the 'Lo' position you'd get maybe 585.
Sincerely,
Lars Verholt
Thanks Lars.
Thanks Lars.
Just making sure I do this correctly...Do I remove both tubes and then check at the 3rd pin hole? If so, does it matter which tube socket I measure from?
Don't remove the tubes.
I would measure on the tubes in place. While you're in there, check both tubes.
Sincerely,
Lars Verholt
Got ya. Dumb question, where
Got ya. Dumb question, where do I place the red tip of my meter to measure the plates? with them in place, obviously someone from the inside, just not sure whear. I have the 2275-75 schematic.
?
The 6L6 plate is Pin #3,
The 6L6 plate is Pin #3, clockwise from the guide pin when looking at the bottom.
Be really careful in there... those voltages can kill. Always securely clip the negative (usually the Multi Meter black wire) to the chassis when testing. I never want to measure those voltages using both hands.
Thanks for the replies
Thanks for the replies everyone. I'll be measuring the 3rd pin shortly.
You guys say to measure with the tubes in place. On the old archive I read that you should stick the pin inside the hole with the tube removed to check the transformer.
So, If I'm not suppose to remove the tubes, should I measure from inside?
I've included a pic. Each solder joint has a number. My read stabilo is pointing to the 3rd pin solder/contact joint. Is there where I grab my measurement? Tubes in place instead of removed.
Sometimes it's out, sometimes it's in
Sometimes it is appropriate to measure without the tubes in place. In case of an amp that blows fuses, it's a good idea to measure with the tubes out as to rule out a tube short being the culprit. In your case the amp seems to work fine, so the appropriate method is to measure with everything in place.
You could check your bias voltage while you're at it.
Sincerely,
Lars Verholt
OK, so check with them in
OK, so check with them in space at the 3rd pin from above. Will do.
How do I check the bias?
Sorry for all the noob questions guys. I appreciate every bit of help you guys have given me.
Alright. I've check the 3rd
Alright. I've check the 3rd pin with tubes in place.
Hi position is putting out 694 dc volts
Lo position 465 dc volts
Looks ok to me
Those voltages are right on the money.
Sincerely,
Lars Verholt
Yea I guess it's just the cab
Yea I guess it's just the cab robbing me of headroom.
I still wish the light would dim when I cut the wattage,
Oh well.
Now to find me a MM 2x12 cab in Chicago.
The lamp comes on with line voltage!
Don't know why the lamp would be affected by the HV selection.
I **wish**
"I still wish the light would dim when I cut the wattage".
I never said it affected the way the amp operates.
This isn't a "pickguard on or off for better tonez" type of post.
Every other model I ave owned, the light would dim when cut from Hi to Lo. I simply stated that I wish my lamp did the same. That's all.
The lamp is not fed from line voltage!
After looking at the schematic, I see the neon is fed from the PT secondary, however the lamp circuit is not switched with the high/low/standby switch. I see no change in my RD One Hundred when changing voltages.
Why the exclamation marks ?!
Weird. To be fair, it was only really noticeable on my old 65reverb with the PI tube. I can probably even make a video (my best friend now owns it) if no one believes me. Not a big deal. Maybe they changed it for the later models.
I honestly don't know if you are cyber-yelling at me with the use of exclamation points, but whateves.
I cranked my amp up as loud as it gets, and yea I do notice a difference in the Hi setting. Oh well, I learned the hard way. I've tracked down a MM 4x12 cab locally. $700 for the cab with an HD130 (no reverb/tremolo) head, $400 for just the cab.
I really, really, REALLY wanted a 2x12 as I love the look of the cab. But I may be able to buy the stack and trade the head for a combo or a guitar.
Some dim, some don't
The older MM amps with the hi/off/lo switch on the front actually switched the primary to the main power transformer. This made the neon lamp glow stronger (along with the tubes!) in the 'Hi' position. The neon lamp was fed from its own winding on the power transformer. Later designs with the solid state driver circuit had the hi/standby/lo switch that switches between different windings on the secondary side of the power transformer. In this type the neon lamp is fed from the 'lo' winding regardless of the switch position.
Sincerely,
Lars Verholt
Great reply - that just about
Great reply - that just about "nails it".