A few questions about a Boogie I just picked up

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AdamGian, Jan 3, 2008.


  1. AdamGian

    AdamGian Member

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    I recently bought a Boogie SOB off someone on this board. Great amp, just like I remembered them to be from the mid-'80s. While these amps don't seem to appeal to some people, I like it for its blues tones. This one has the 100w option, reverb, and the Limit circuit (that some people don't like). A friend of mine had one in the '80s that had Simul-class, reverb, and the Limit control and his amp simply ripped a hole in anything that we put next to it.

    Anyway, here are my questions:

    Much seems to be made of the idea that an amp needs to be properly matched to the load. Mesa states over and over in the manual for this amp that impedance matching is not critical and that the user should experiment with mismatching. Why the divergence from general opinion especially coming from the manufacturer?

    Can you remove the two inner power tubes if running the amp in the 60w mode?

    This amp was built in 1985 or so. I would assume that it needs a cap job, but it seems that the caps used from the '80s onward did not dry out as quickly as the ones used in the '50s and '60s. So, is a cap job really needed? (No hum.)

    When I got the amp it had a set of JJ 6L6s in it. They were only warm to the touch after the amp had been on for quite some time which leads me to think that they were biased too cold. I replaced them with a quad of new GT 6L6s I've had on the self for 20 years that are rated H4. These warm up much like they should and sound good too. I know a measurement is a better technique though.

    Can I use a set of Yellow Jackets in this amp?

    Is there an attenuator that seems to work well with Boogies? Just to knock it down a little in 60w mode. (I don't hear much mention of people using them with Boogies.)

    Here is a copy of the schematic for this amp. Can someone tell me what the Limit circuit does and why it seems to be replaced with a Presence control on many of these amps?

    http://www.schematicheaven.com/boogieamps/boogie_sonofboogie.pdf

    Thanks for any help.
     
  2. pfflam

    pfflam Silver Supporting Member

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    I have an SOB from 1983. I thought that the SOBs from that era were just either 100 or 60? and I never heard of one with reverb.

    The Limiter should act like an attenuator - no need for one, use it like you would a Master Volume or attenuator.

    These are great amps that, IMO, deserve to be seen as something unusual in the Mesa line. Mine is, as I thought all SOBs from that era were, just straight forward AMP! no frills, or whistles
    2-channels of tone-full ultra loudness!!
     
  3. AdamGian

    AdamGian Member

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    Right, no frills, that's why I like the amp too.

    They came with a few options if you asked for them when the amp was ordered. I know my friend asked for Simul-class and reverb when he ordered his. Maybe most of them were not built to order and came more stripped-down as they were meant to address that part of the market for Boogie.

    Yes, the Limit does lower the overall volume and serves to roll-off the highs and thicken it up at lower volumes - only used for dirt settings. It's a more versatile amp than one might assume by just looking at the front panel. Mine has no middle control.
     
  4. John Phillips

    John Phillips Member

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    Some manufacturers over-spec components (especially OTs, tube sockets etc) more heavily than others, and they are less likely to be damaged.

    Yes - in fact it's better to do so if you're going to run it like that all the time, otherwise you're in danger of 'cathode poisoning' in the unused tubes.

    IMO, no.

    JJs do seem to run cold compared to most other tubes.

    Yes.

    I found a Hotplate worked well. It does give you more control than just using the onboard knobs, and as with most amps I prefer the sound of attenuation from not the fully-cranked sound, but a bit below it as a final volume adjustment, not an extra distortion generator (if you see what I mean! - ie with the power section of the amp not distorting... and yes, I know that this is contrary to why most people want to use attenuators :).)

    The Limit control is a sort of headroom control for the phase inverter. The diode is used to create a constant bias voltage, and the pot controls the signal level. It's very unusual (I've never seen it in any other amp), and I'd assume it was replaced because people didn't like it... or just maybe because it was perceived as 'diode clipping' (although it isn't), and Mesa didn't want that.
     
  5. AdamGian

    AdamGian Member

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    As always, thanks John.
     

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