Ampeg SVT is shocking me!

[Cuff]

I am working on an old SVT head and I have taken the power amp and preamp section out of the cab. When I touch the preamp section and I am grounded in some way (like if I am touching the power amp chassis) then I get a heck of a shock. All measurements indicate that the preamp chassis is charged to about 450vdc. As you can imagine this is also measured at every component that is connected to ground therefore messing up the bias of the preamp tubes. Where does it come from? Would this be the case if the wire that grounds the preamp section to the chassis were not connected somehwere?

[Ray Ivers]

Cuff,

I'm glad this thread isn't named "SVT killed my friend".

A guy I knew pulled two SVT's off the curb, waiting for the trash man - I got one of them - with the same problem AFAIK; the ground wire in the power-amp-chassis Molex connector snapped. This would be the first thing I'd check (amp unplugged, caps drained, etc.), along with every other wire/socket/connection in that connector, its mate on the PA chassis, and the far end that terminates inside the preamp chassis.

Ray

[Cuff]

Once shocked, twice shy

Yeah, I am glad too. I was warned about the amp shocking people so I took it out of the cab and powered it up on the bench using a DMM to check out the voltages first but I did accidentally touch both sections at the same time once and it was "make a funny noise" time. I was hoping that was a Molex connector, I am going to replace it, some dumba** has tried to repair a wire that was pulled out of it and it is pretty much shot at this point.

[Don Symes]

Your safest move at this point would be to replace that whole harness - leaving NO doubt as to the security of each connection.

Molex connectors are cheap and available from places like DigiKey or Mouser

HTH!

[David Schwab]

A guy I knew pulled two SVT's off the curb, waiting for the trash man - I got one of them

I would have left them there!

Except for the B-15N, I really dislike Ampeg bass amps.

[drewl]

Do yourself a favor and hardwire a ground link to all chassis sections!

[Ray Ivers]

David,

Oh, man, you would hate my living room, then! If I played bass live, though, I would definitely 'roll my own'.

Ray

[Cuff]

I did

I have hardwired a temp ground line and the amp is somewhat working, now we are back to the original problem from before the grounding problem. The signal is distorted at all volume levels. No matter what, when I plug a P bass into it I get fuzz like distortion. All the tubes are new and the bias is well within tolerance on all tubes. I even adjusted the two bias pots on the back of the amp to the manufacturer's specs. If all the bias levels are correct and the tubes are new then I would assume that all of the resistors have to be good. So does that leave problems with a coupling cap maybe? All of the caps are original except for the caps in the power supply, as a matter of fact everything is original except the ps caps and the tubes. What gives?

[David Schwab]

David,

Oh, man, you would hate my living room, then! If I played bass live, though, I would definitely 'roll my own'.

I just never liked the way they sounded. I didn't like them in the 70's, and I like them even less now! They take any bass and make it sound the same. The Ultra-Hi and Ultra-Lo switches are annoying. The tone controls are almost as bad as GK's!

In general though I just don't care for most tube amps for bass... the response is too slow. Just a lot of mush. I guess if you can't hear the notes you play, it covers up sloppy playing! Must be why they are popular.

I did have this vintage tone infatuation for a brief period, and bought myself a Mesa 400+ ... it sounds OK, on the warm and mushy side, but you can't get any volume out of it without it breaking up, and still it colors the sound too much. I like distortion on bass, but when I want it, not by default.

It's currently sitting in my basement...

I actually dislike the majority of the amps currently on the market. I spent an afternoon in NYC trying out amps a few months ago. I think the big problem is way too much EQ in the preamp sections. I've gotten so used to hearing my bass through my home studio that bass amps just sound like a blanket was thrown over the bass.

The only amp I tried that day that sounded like my bass was an SWR LA 12 Combo (and not the 10 or 15, and not the regular SWR's). It's nice to see more 12" speakers being used for bass. 10's are getting old. I didn't get to try any Edens though. One very expensive all tube amp (name starts with an "A") was truly dredful! Just mud city! Their solid state version was just as bad.

I'm waiting to try some Mark Bass amps when they come to GC this August... But I think I'm going to build my own.

This is what happens when you build your own basses! Spoiled for life!

[Ray Ivers]

David,

Late last year I A-B'ed an SVT Classic with one 250W channel of a friend's old SAE hi-fi power amp; the direct-coupled SS amp just sounded so much clearer, crisper, and punchier than the SVT.

I hold out a lot of hope for these devices, which have recently become available:

http://www.lovoltech.com/Products/Products.htm

24V, 50A, 69W JFET's - definitely a new option for a solid-state output stage. I don't know if they have any dealers as yet, although I believe quantity purchases can be made direct.

Ray

[Rick Erickson]

24V, 50A, 69W JFET's - definitely a new option for a solid-state output stage.

Ray - How would you make a high powered Bass amp with these? It would seem that the 24V rating would severely limit these devices as audio outputs, even in bridge mode, unless you're driving extremely low impedances or using a step-up O.T. .

[Ray Ivers]

Rick,

Well, one way would be to use a high-voltage MOSFET as the top stage and the JFET's as the bottom stage in a power cascode arrangement, kind of like the Music Man RD100 etc. (which uses BJT's and tubes). I've read that the bottom stage dominates the transfer function, but FWIW the Music Man still sounds like a tube amp to me; the OT is a factor too, of course. An all-JFET 4-device cascode would be good for maybe 200+ watts @ 4 ohms; I agree, not exactly high power for a bass amp.

Ray

[drewl]

i have a Sound City that sounds good for bass.....Entwistle managed to get great sound with tube bass amps!

[Cuff]

So...

Does anyone have any thoughts on the distortion issue?

Personally I think a tube amp is great used in conjunction with a direct box, that way you can mix the tube tone with the punchiness from the DI box. Great live set up, but I play guitar and do sound occasionally, not really a bass player.

[David Schwab]

i have a Sound City that sounds good for bass.....Entwistle managed to get great sound with tube bass amps!

Those were great amps (I used to use Sound City strings!). And Hi-Watts too. Orange amps were another.

John Wetton got a great sound out of a Hi-Watt. Jack Bruce got a great sound from a Marshall. Dirty... but great. My Mesa 400+ sounds good too.

Any of the Fender clones will sound good for bass. A Fender Twin sounds good for bass! That's what Joe Osborn and Carol Kaye used on a lot of recordings.

I mostly remember Entwistle using SS Sunn amps with a zillion speaker cabs.

I just don't like Ampegs.