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Issue with used big block 750?

M

melloncollie

Nov 27, 2016
782
854
35
#1
I got a used Mesa boogie big block 750 from guitar center (which may have been my first mistake) and it doesn’t seem like I’m getting any signal when using the direct out into my interface.

If I flip the pre/post to pre switch I get plenty of signal but when I flip it to post, seemingly nothing.

Any trouble shooting tips?
 
#2
AFAIK the OM does not provide a block diagram or a description to say specifically where the Post DI signal is taken from. Most likely it's after the Gain and Tone controls, but it could also be after the Master .

If it's after the Master, the level in the DI will vary when you adjust the Master. You may have to crank the amp fairly hard to get a Post DI sign.

Note there is a level adjust for the DI on the back panel.

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Try cranking up the Direct Output Level. Also try cranking up the Gain, Tone, and Master.

One possibility. The Post DI could be tube driven. Although unlikely, a bad tube could be the source of the problem.

If the Post DI is totally dead, decide whether you will return the amp or ask for a repair.
 
#5
I was able to get a good amount of volume but I had to crank the gain and master on the amp and the gain on the interface I was going into which seemed to introduce a lot of noise.

I don’t have a cabinet currently but I’m going to rake it to the store and run it through a cabinet there to see if I notice the same noise and see if it’s a tube problem.
 
#6
Are you playing the head by itself with no cabinet? These amps require a load or you’ll destroy the output section. Modern amps typically have a simulated load which allows you to run the amp with no cabinet but this is not a modern amp. Do not operate without a cabinet!

-EDIT- I’m totally wrong on this one. I’ve played tube amps my whole life and the BB750 is my first amp with a solid state output. I’ve never heard you could run solid state powered amps without a load so I’ve been overly cautious apparently lol!
 
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#12
So I have noise on my BB750 too. There is an oscillating zap or something that is audible, even over playing. It happens regardless of the amp settings and is not affected by an increase or decrease in gain or master volume. Here are a couple of tests; one with the noise floor only and the other with bass playing on top. Is this what you're hearing too?

This noise is not audible through a cabinet.

My big block is a v1 (no passive/active switch) fwiw. I'm at least the second owner and I think it's probably due for service. Bought it from a friend a couple months back.

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-EDIT- Noise goes away when I toggle to "Pre" mode. I probably just need to take it to Slemmons in Dallas for service
 
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