Ampeg SVT or other 200-300W bass amp?

[Cliff Schecht]

I've got a buddy that is dying for me to build him an Ampeg SVT clone. Looking at the schematic shows nothing that is too daunting but I'm wondering if this is the best amp to build. I'm not very well versed in old Ampeg gear, let alone old bass amps, but I'd like to build him something that has a versatile EQ and isn't one dimensional (i.e. works well in multiple genres). Since we are playing mostly older rock and blues I'd like to make something that will nail a "classic" bass sound but he's also got acoustic/semi acoustic basses, Fender basses and many other odds and ends which I want to work well with whatever I build him.

I'm not a big fan of the Marshall 200W amps simply because they have the lame passive tonestacks. I've read great things about the Hiwatt bass amps but I haven't seen anything for one of these yet.

So in essence I want a 200-300W power section, a well rounded EQ and lots of PUNCH. We are building a 1x15 or 1x18 and a 4x10 cab to compliment the amp. It has been made clear that money isn't an issue but I'm still looking at an Antek Inc toroid PT and a Hammond 240W output transformer (the biggest one in the 1600 series PP OT's). I've got a chassis and most of the parts for him already, we just need sockets, pots and other little details (like the tapped inductor in the SVT which can be BS'ed with 8x100mH inductors).

I appreciate any recommendations you guys have. Post some schematics if you have them too!

I'm really excited about this project! From August until late December I didn't build a single amp and so now I'm putting in a crazy amount of bench time. In 1 week I've built a Vibrochamp (in an old Heathkit case with copper chassis!), a 6G15 for the guy I'm building the bass amp for, started working on my own 6G15 with built in vibrato and started building head shells for all of the damned chassis I have laying around. I also finally got my Rocket build to a point where I can sell it to another friend and put the finishing touches on my 5E8A build (which sound KILLER!!). Next up on the bench is a Vibroverb clone and a 2204 Marshall build.

We also put my 4x12 on the table saw and ripped into two 2x12's.. Damn that one felt good . Finally have a house for my two ancient Jensen's that have been dying to make music for much too long.

[LeftyStrat]

I have such a build on my list, but I only plan on cloning the power section (which has a lot in common with the mythical SSS).

The ampeg tone stack used in both guitar and bass amps was quite versatile. Basically a baxadall tone stack with a tapped inductor for three different ranges for mid boost or cut.

Unfortunately those tapped inductors are hard to come by. There's a solid state variant using gyrators in "The Stomp Box Cookbook" by Nicholas Boscorelli, but long out of print.

My leaning is to go with a standard Baxandall Bass and Treble, and augment it with a couple of parametric EQ stages.

I have the Boscorelli book, so I could scan the article if you're interested.

[sharkboy]

If sound is the goal, I've never heard an amp that could compare to a good one. It's my dream to someday put two of them together and play my 4003 through them in stereo.

[9pins]

put a ampeg "B" pre on a hiwatt "STA" 200 inverter with kt88/kt90

and see what happens with the right power supply

marshall jcm800 1986 bass has a very simple "slope" control thats worth exploring too

[LeftyStrat]

We definitely want to see pics of your progress.

[Cliff Schecht]

Alright fellers.. I have a host amplifier to work with for the bass amp, a Silverface PA100 head. It's a great candidate for changing to a bass amp and already is modified so I don't feel bad tweaking on it. There is lots of working room inside the chassis and an excess of channels if I want to do something else with a different voicing later.

What I'm thinking is a modified TMB control for the first new channel (essentially takes up what was two channels before). I tweaked around some ideas in simulation last night and came up with the tonestack I'm attaching to this post. Making the slope resistor a pot and adding a bypassable cap to ground seem to be enough to shift the bass and boost/cut the mids effectively (respectively). Since this amp is loaded with 250k pots I'm trying to reuse as many of these as possible, hence the typical Fender values for most of the tonestack. Also none of these values are final but I suspect there won't be a lot of tweaking necessary with this setup.

I'm thinking to keep the tonestack from being too lossy I will drive it from a cathode follower ala 5F6A. The order of the stages will be a preamp stage-->gain control-->preamp stage-->buffer-->tonestack-->master volume-->PI. This channel will be mixed like like the typical Normal channel on a Blackface (i.e. no effects) and the rest of the amp will get some tweaks to make it more bass friendly (larger coupling caps and the like). I'm also considering the presonance control Martin posted but most likely need to tweak it a bit for bass.

I'm also thinking about robbing the other two channels and putting in an Ampeg style Bandaxall with a simple RC-based mid cut circuit. Whether I do this or not depends on the effectiveness of the first channel. I think it would be cool to have both a Fender style and Ampeg style tonestack in the same amp though. If I do this I might dump the reverb and use the extra holes in the chassis for more controls. We'll see how the first set of changes go first..

[Structo]

Unfortunately those tapped inductors are hard to come by. There's a solid state variant using gyrators in "The Stomp Box Cookbook" by Nicholas Boscorelli, but long out of print.

My leaning is to go with a standard Baxandall Bass and Treble, and augment it with a couple of parametric EQ stages.

I have the Boscorelli book, so I could scan the article if you're interested.

I have that book as a PDF file.
It's over 80MB so a bit large to upload but if you can tell me the page or article I can post it.

EDIT:

Is this what you are talking about?

[Firestorm]

I think it would be cool to have both a Fender style and Ampeg style tonestack in the same amp though.

+1 on this. The bass amps I build have a 5F6A channel and a Baxandall/James sort of channel, in phase. The 5F6A side has huge bottom, the Ampegy side is more midrangey, so mixing them together to taste with the channel volume controls gives a pretty large tone pallette.

As to the SVT: something I never noticed until I read it in an O'Connor book is that the SVT preamp is only about tone shaping -- almost no gain. The input to the power amp has a pair of antiparallel diodes to ground in front of it that clip any signal above 600mV. So you only need a maximum signal of about half a volt into the driver/PI.

Not saying that's the way to do it; that's just the way Ampeg did it.

[Structo]

Actually the Stomp Box Cookbook has a big section on Tone Control.
Really thorough documentation for a stomp box book of passive as well as active tone control circuits.

Just a bit of trivia, but SVT stood for Super Vacuum Tube.

[Firestorm]

Just a bit of trivia, but SVT stood for Super Vacuum Tube.

Thought it was "Super Valve Technology." Eh, who knows?

[Structo]

I guess when the bottom feeders want to move a lot of air their isn't much that compete with a 810 cab.

The SVT 810AV

[img:331:483]http://basscentral.com/basscombo/ampeg/SVT-810AV_LG.jpg[/img]

[vibratoking]

For older rock and blues, the SVT is the absolute icon. There isn't much that amp can't do, although it is one loud and heavy mofo.

The handicap moving bar attached to the back of most of the 8x10 cabs is a great idea and a little bit funny. I have noticed that the SVT just doesn't sound the same without that cab.

[Reeltarded]

Ditto. Itsa MUST do.

[LeftyStrat]

I have that book as a PDF file.
It's over 80MB so a bit large to upload but if you can tell me the page or article I can post it.

EDIT:

Is this what you are talking about?

That's the technical discussion of simulating inductors. Look for one of the Tone-O-Matic projects, maybe second or third one. Don't have the book with me, I can check when I get home.

[LeftyStrat]

Page 46, Project 13 Tone-o-matic: "Duplicates the active and passive tone controls of the Ampeg SVT."