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Mesa Blue Angel Questions


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8/3/2000 4:31 AM
Dave B.
Mesa Blue Angel Questions
Howdy All,  
 
I've just aquired a Mesa Boogie Blue Angel Head and I would like to ask a few questions to those that have some knowledge of this and and of tube circuits in general. The Folks at Mesa Boogie were of no help at all.  
 
Input stage: input#1 has 1M grid stopper; #2 is 3M. I'm thinking of changing #2 grid stopper to 68K or 100K.  
V1a cathode resistor is 3uf; maybe change to 5 or 10uf.  
V1b is a strange circuit. It's wired to V1a exactly like a cathode follower except the cathode isn't the output; the plate of V1a (tied to V1b grid)is. Is V1b serving as some kind of load for V1a?  
 
Preamp: Customer service at Mesa said the Blue Angel was meant to be a clean preamp/power tube distortion amp. If so why are we using two 12ax7's in the preamp? Wouldn't one accomplish the same purpose? two gain stages after the tone controls and it seems like we're trying to get some preamp distortion aren't we?  
 
Mojo Module: Anybody have a clue what it is? It seems to be a solid state device between the preamp and the E.loop with two -15V supply rails.  
 
Power amp: seems like a great setup four el84's two 6v6's. My only thought here is make it a true simulclass power amp and make the 6v6's fixed bias instead of cathode bias. I have a nice circuit drawn up that taps off of the tube rectifier plate syupply with bias set voltage for each tube.Any opinions on this?  
 
Mods: anyone have any tried and tested mods they'd like to speak of?  
 
Last thought: small value caps; they're all over the place in this amp. three in the preamp, three in the phase inverter(between the plates and bypassing both plate resistors)any thoughts about removing any of these? (I know they prevent oscillations but six seems to be alot)  
 
Thanks for your kind input,  
 
DB
 
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8/3/2000 6:16 AM
Brian

Dave;  
 
quote:
"Preamp: Customer service at Mesa said the Blue Angel was meant to be a clean preamp/power tube distortion amp. If so why are we using two 12ax7's in the preamp? Wouldn't one accomplish the same purpose? two gain stages after the tone controls and it seems like we're trying to get some preamp distortion aren't we?"
 
 
Just because there are two 12AX7's in the preamp doesn't mean that there will be a lot of preamp distortion. It depends on a lot of different factors. Especially with the (what sounded to be) direct coupled stage after the first.  
 
I have heard the term anode follower (no not cathode follower) before, but have never seen the circuit for one. I would imagine that it would not provide much gain. Maybe an expert has something to say about this.  
 
as for the "stopper" resistors you mention, these sound like values for the grid leak resistors, not stoppers. Do they run from the grid to ground or input jack to ground? Or do they run from input jack to grid? Usually stopper resistors are smaller than 10k. The grid leak resistors change the gain by dropping some of the signal to ground. The 3M should have a higher gain than 1M. I doubt it would be that much more though -- 68 or 100k will give you less gain at the first stage.  
 
Dink with the circuit values all you want. If you like it, then keep it, otherwise change it back. You can't really hurt anything. That said, major changes in circuit topologies and such can be a big headache, so think about those a bit more if you attempt them.  
 
BT
 

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