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Topic: Session 400 Sensitivity: What does it really do? |
Paul Sutherland
From:
Placerville, California
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Posted 13 Aug 2012 7:41 pm |
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I've been playing Session 400s since the late 70s. I still don't really understand what the Sensitivity control does. It's apparently not a master volume control, but the amp does get somewhat louder as it gets turned up.
Is the sensitivity control wired into the power amp, or the pre-amp? Is it controlling the output of the power amp? Or, is it designed to work on the input stage of the amp to compensate in some fashion for the output of the pickup(s) of the particular guitar.
The Peavey manual's description of the sensitivity control does not clarify anything for me. Just curious. |
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Clyde Mattocks
From:
Kinston, North Carolina, USA
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Posted 13 Aug 2012 9:52 pm
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I've always assumed it was a pad so you could attenuate the signal of a too hot pick up. I've never had this confirmed. I always ran mine about 3 o' clock.
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LeGrande II, Nash. 112, Fender Twin Tone Master, Session 400, Harlow Dobro, R.Q.Jones Dobro |
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Bill Terry
From:
Bastrop, TX
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Posted 14 Aug 2012 8:11 am
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I always thought it was a way to decrease input noise for recording applications, i.e. lower preamp gain? I dunno? I DO know that on live situations, the amp is a lot punchier with Sensitivity up above at least 6 or 7, JMO...YMMV
_________________
Lost Pines Studio
"I'm nuts about bolts" |
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Paul Sutherland
From:
Placerville, California
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Posted 14 Aug 2012 8:57 am
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The manual reads as follows:
"The master sensitivity control (11) is a unique innovation which allows the player to "cut" or "boost" sensitivity as he desires. This allows the musician to cut out noise for a perfectly quiet performance during recording sessions."
I guess that means a person should use lower settings when recording or playing at low volumes, and higher settings to get the full power of the amp for stage work. But what is it really doing? I wish Peavey had explained this feature a little better. |
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Brint Hannay
From:
Maryland, USA
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Posted 14 Aug 2012 9:28 am
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I agree--that Peavey manual language is fuzzy at best. I hope one of the technically knowledgeable Forumites will chime in here. (Ken? Brad?)
I've always assumed it was just a fancy name for a master volume. (I bought my Session 400 used, so never even had the manual.) |
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Mike Kowalik
From:
San Antonio,Texas
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Posted 14 Aug 2012 2:17 pm
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Mike Brown....where are you????????
_________________
Cremation is my last shot at having a hot body...... |
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Ken Fox
From:
Nashville GA USA
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Posted 14 Aug 2012 4:51 pm
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| It is nothing more than a master volume, afte the preamp and before the power amp. Called post gian on the N400 and other amps |
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Bill Terry
From:
Bastrop, TX
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Posted 15 Aug 2012 10:58 am
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It sounds like it is a master volume, but it doesn't seem to work like a guitar player would expect a master volume to work, i.e drive the preamp really hard for overdrive/distortion, and then reduce the overall volume of the amp.
I guess the gain structure is set up with lots of headroom in the preamp since steel guitar outputs are pretty hot?
_________________
Lost Pines Studio
"I'm nuts about bolts" |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 17 Aug 2012 5:02 am
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Yeah, it's a master for the sake of dropping the noise floor for low-volume studio use. It's not a master for the sake of driving the preamp. In that way, yes, it's not like a "guitar" amp.
The term "sensitivity" is a bit confusing because it's NOT input sensitivity. But if you view it as "power amp sensitivity" it makes more sense.
B |
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Paul Sutherland
From:
Placerville, California
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Posted 17 Aug 2012 6:20 am
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| Thanks to all, particularly the experts, Ken and Brad, for the input!! |
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Lane Gray
From:
Topeka, KS
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Posted 18 Aug 2012 9:03 am
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Since I would not dream of contradicting Ken and Brad, who know these things and have worked on them for years, I'll accept their account. But why does it sound like it has an effect on tone?
Someone told me, years ago, that it did something to the input impedance, and it certainly seems to sound sweeter when at its higher values like I'd expect from such a knob.
Izzat psychosomatic, and I only hear it because that's what I am expecting?
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 18 Aug 2012 9:51 am
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No, you're not a psycho, Lane. 
The Sensitivity control is actually a gain control for the final stage of the preamp (last 2 transistors). It's in the feedback loop of that stage, and changing it does affect the frequency response a little...particularly in the very low and very high audio frequencies.
I don't have my bench equipment set up right now, but I remember back when I had my shop in Gnashville I did an analysis of the effects of that knob. It actually has a cool, but subtle, effect on tone.
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Best regards,
Mike |
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Ken Fox
From:
Nashville GA USA
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Posted 18 Aug 2012 11:13 am
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| Mike, as I recall Peavey in the early 70's called that "variable negative feedback" for the volume control on their early mixers. Every now and then the cobwebs clear and I remember something! |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 18 Aug 2012 12:34 pm
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 You and me both, bro!
Yeh, designs like that, "variable negative feedback", were rather popular back then.
Hope house life is improving for ya. Are you back to normal yet?
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Best regards,
Mike |
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Brad Sarno
From:
St. Louis, MO USA
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Lee Baucum
From:
McAllen, Texas (Extreme South) The Final Frontier
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Posted 18 Aug 2012 2:23 pm
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| Quote: |
| variable negative feedback |
I hate it when we receive that kind of feedback written on napkins... |
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Ken Fox
From:
Nashville GA USA
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Posted 19 Aug 2012 9:42 am
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| Mike, we are finally back in the new home after about a 7 month rebuild. We are taking a little time off now and visiting with some friends. Looking forward to retiring at the end of this year and maybe traveling a bit more as well! |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 19 Aug 2012 12:14 pm
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Excellent. I'll be retiring next July, myself. I hope to get my "lab" set up then so I can pursue some ideas I've had germinating on the back burner for a while.
I hope you'll still stay active on the forum, Ken. You're insight and knowledge is very needed here...even if you aren't actively doing repair work any more. You know..."positive forward feedback".
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Best regards,
Mike |
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