ampeg voicing

[clankenstein]

i have an ampeg svt2 preamp.i like it a lot but sometimes i wish it wasnt quite so focussed on the low mids.does anyone know if there is circuitry in there to do this or am i imagining it.
 

[sniper]

wow, i was just reading about this last night. PBG is the one to talk to but i will try and make a stab at it, as i am understanding it.

SVT's use what is called a Baxandall (Bauxandual correct spelling i think?) preamp circuit. basically the circuit was originally desinged to use no mid preamp pot. it simply wasn't designed that way. this stabilizes the circuit by leaving a mid-tone adjustment to the treble or the bass circuits but actually does not move the midtones a lot on a graph if one were to put it on an ociliscope (sp?).

well, Ampeg did it = put a midpot in.

one thing a person has to realize is where it is put in and why. again basically 1) it is hard to put a wide sweep midpot in because of the initial design and 2) where one puts a tone circuit in, affects the overall tone a lot. i'll explain below:

there stages of gain to a preamp be it one tube or more as there are two sides or more to a tube. if one puts the tone circuit in after the first gain stage then it tends to drive the distortion higher as it amplifies or more correctly modifies the first signal and lets it go to the rest of the amp to be amplified.

if one puts the tone circuit in after the secondstage, either after the second half of the tube or after another tube depending on the design, then one is modifying the signal after it is already amplified hence you get a cleaner signal to send to the power section of the amp. this one fact is prolly the biggest difference in a nice bass amp and a guitar amp as i understand it. along with power because of the inherent power needed to drive low frequencies cleanly.

Ampeg in the SVT design used a Baxandall circuit as it is a very good design and they put it in after the first gain stage but before the second gain stage was complete if that makes sense. they put it between the second stages of the second tube. they then let the Bax circuit do its work (watch the magic crystal and be quiet so it can do its work!) and then they put a midpot in after the second stage tube and before the cathode driver (another magic thingy) almost bypassing what one might think of as a third stage gain circuit (a third tube in the tone circuit) again before the cathode driver. in effect they tried and to a good part successfully so, added the midpot to where there was none meant to be and it worked to a degeree. prolly not as well as if they had put it in the first part of the circuit but they used it to tie two channels together as well.

Fender did a different design by putting it all after the first stage (again basically a Bax circuit) with some mods on the treble end and adding a midpot there while they were doing it. not a lot of adjustment either but some as it basically raised the whole curve and not just altered the mid range. Fender designed their tone circuit to reflect a mid scoop as Leo and his engineers thought that mids in the pickups of the day were too midrangy to the point of making too much distortion. they probably were to some extent. we look at it as "vintage sound" now.

Fender never did figure out this placement or at least they didn't practice it a lot (they later got into channel switching = more magic keeping their guitard distortion!) and Ampeg married apples and oranges. PBG will tell me if i am wrong, but i am almost positive i am correct in this and i think this should answer some of your question.

it is built in. a Fender is not a Ampeg is not a Sunn. a Marshall is a Fender knock off to some (major) degree.



i enjoy apples, oranges and watermelons although not at the same time unless they are in a fruit (think boutique amps with everyones twist on things mentioned above or Uwe's analogy to the nature of a lion posted elsewhere) salad.

[gearHed289]

Sniper, are you talking about an SVT II pre amp, or an original SVT pre amp? I have both a '74 SVT and a '96 SVT III Pro (same pre amp as the II and the SVP). One thing I've noticed is that the tone controls on the new ones do very little compared to the vintage ones. Changes are DRASTIC on an old SVT, which is a good thing. I definitely have to go to further extremes with the new one to get the sound I want. It's a great sounding amp, but the vintage one just smokes it when it comes to tone and versatility. I've even tried running the III pre amp into the power amp in on the vintage head. Still no comparison.

[Psycho Bass Guy]

The SVT II is pretty much the same as the old SVT preamp, but with an added s/s graphic EQ. I find the comments about modern Ampeg preamp voicing puzzling because in my experience, newer Ampeg preamps still sound like variations on old Ampeg preamps, including the SVP series. Sniper covered it nicely, and as to modifying the sound of an existing SVT II, it's not worth the amount of work, time, and the amount you would be reducing the preamp's value, not to mention the potential to damage it. The low mid focus is 'the Ampeg sound,' and if that's not working for you, you either need to employ some sort of active EQ before the pre or consider a different pre entirely.

[sniper]

what i would do if it were me and i recognized i had a prob is this ... take this with a grain of SAFETYsalt. matter of fact use a whole dose of SAFETY salt!

i used to own two computer shops and was pretty good at diagnostics and software glitches but i never was good at programming as i am an ex truck driver with almost no college or tech school training. yes the remark about dyslexia is true because i flunked out of college because of it; but anyway, part of being a diagnostician is knowing there is something amiss in the wood pile as you do.

back to being you/me...i would open that sucka up and look. you know that great little soldering iron you have LEAVE IT ALONE!!! but you might grab mommas big looking glass and try to see if anything looks burnt (could be factory or other) and do this without unsoldering anything. nuts and bolts and copius notes only!!!

optional only if you have a discharge stick and know how and why to use one!!! if you find a burnt spot, check it with an OHM meter. if you can't answer the first part of this paragraph, then avoid this paragraph altogether because there is enough residual voltage in there to knock you dinky down under dead!!!

i found a lot of boards with a small cap or resistor blown that in first glance looked okay. usually meant a lightning strike or other gremlin esp on modem boards but not limited to them. them suckas can be small!!! not to mention printed circuit interlayer damage from said lightening strikes, surges or

second choice is to find a good Ampeg tech ... rotsa ruck.

[clankenstein]

wow lots of good info ta.i have had my preamp apart because when i got it the mid control did very little-bad solder connection to the board.it all looks good inside.very interesting about the mid control.

[clankenstein]

the way the mid control is hooked up to the cathodes seems like it might be in part some sort of negative feed back semi active arrangement?i dont quite understand it.tricky buggers.

[Psycho Bass Guy]

the way the mid control is hooked up to the cathodes seems like it might be in part some sort of negative feed back semi active arrangement?i dont quite understand it.tricky buggers.

That's a variation on the original SVT mid control, which was the first use of active EQ in an instrument amp.