Ampeg SVT Tube Bass Amp -Mods

Hey guys I just picked up a Ampeg SVT Tube Bass Amp. Does anyone know of any cool mods that can be done to this thing. How about replacement tubes? This thing sounds good, Hums a bit. Overall a great amp, but as always I want to tear it apart and make it better.

Thanks Guys,

Danny

These amps are running the outputs pretty hard. The next step up for outputs are transmitting tubes, and you're not likely to be able to match the load impedance even if you could fit them in the cabinet. These amp rely on carefully matching the tubes and critical biasing / balance to minimize hum from the output stages.

[quote author="dmusic101"]Hey guys I just picked up a Ampeg SVT Tube Bass Amp. Does anyone know of any cool mods that can be done to this thing. How about replacement tubes? This thing sounds good, Hums a bit. Overall a great amp, but as always I want to tear it apart and make it better.

Thanks Guys,

Danny[/quote]

Hi Danny....those amps are beasts...when you get done horsing that thing around, you'll feel like you just rebuilt a 454 V-8 without an engine hoist. there is a very good article wriiten by Gerald Weber in his tube amp book called "Getting rid of those SVT Blues". I could sent you a copy of it if you need it.....just PM me your mailing address (I have no way of scanning it and sending you a file)....be prepared to spend about $300us for a set of power tubes for that thing plus replacing quite a few parts (mostly resistors and some diodes). The hum is probably coming the hum balance control being blown out (a common problem in these) or a shorted tube(s) or even bad filter capacitors. It'll be a project but well worth it if you can get the beast running well as there is nothing that quite sounds like an old
SVT running at full tilt through a good cabinet....
Good Luck
TP

to spend about $300us

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If you haven't already, make some room in your budget for a good set of earplugs. That stuff sounds addictive, and while modding & debugging & testdriving you'll be getting a lot of SPL I figure.

And another obviousity(...): check your whole chain. When the hum in that amp is gone, then perhaps something on/of your bass is the best next subject for upgrading. And if it didn't came with one, be sure to use a good speaker cable. And/or replace the speaker-connectors with more sturdy ones or other types.

Bye,

Peter

what vintage is it? if its a newer one you can make them a lot more road worthy by removing all of the molex connectors and hard wireing the connections. Wil

Wilebee

The bass player in my old band had one. If I remember correctly it had a hum balance pot in it. You may want to wiggle that a little before you try and do things the hard way.

cheers,
kent

> Ampeg SVT Tube Bass Amp. Does anyone know of any cool mods

There is no way to "improve" an SVT. It is a true classic and should not be messed-with.

If you make it any "hotter", it will just burn-up on the first gig.

If you try to "tame" it for better reliability, it will just be a soul-less back-breaking lump.

Ampeg really did it right. Good balance of balls and "portability".

Fix what is wrong. Figure out that hum: it will never be dead-silent but odds are you have sour caps and mismatched output tubes. A set of cheap 6550 is expensive and you probably need very-good 6550 in an SVT. I'd check with guitar-amp tube suppliers: the SVT works the tubes very much harder than a hi-fi or industrial 6550 amp would dare. (You really want a set of low-mileage TUNGSOL 6550s, but a genuine set of those would cost as much as a new Honda.)

Just thinking about the SVT makes my back hurt.

A SVT I worked on had a burned hum balance pot.

Like PRR said leave it be.

Make sure to feed it good tubes and it should stay happy.

Make sure you have a proper load on the outputs at all times, and you don't have loose / intermittent speaker cables. These tube sockets are just waiting for the chance to arc across the pins. I had to replace a number of sockets once because the customer had a bad speaker cable.

Hey guys I just picked up a Ampeg SVT Tube Bass Amp.

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Your back has my sympathy.

PRR is right-on: the SVT is the sound of rock 'n roll bass. Fix it if it's broken, but don't try to redesign it.

I saw Black Sabbath last summer. Geezer Butler was playing a Fender bass, fingerstyle, through SVT heads and SVT cabinets loaded with EV drivers. If that wasn't a damn powerful bass sound, I don't know what is.

Thinking about SVTs makes my back hurt, too, but perhaps in a more literal sense. I f*cked up my upper back/neck area several years ago while picking up an SVT head to place it on my workbench. I woke up the next day with all my back/neck muscles locked in spasm: I literally could not move! My back has gotten better since then, but it's really never been quite the same since. Those amps are heavy mo-fos.

you picked it up to your bench????????

wow!

I worked on the svt on the floor.

Danny;
We are proud of you and your back doctor will start making a killing.

I have 1.5 of them. They are the ticket and do not need much
modding. I would like to see that article though.

Remember one thing and one thing only which turns out to be two things.

The SVT has 695 VOLTS on the plates of the 6550 tubes!
I will say that again 695 VOLTS!

Treat his amp with a lot of respect if you do not it will be the last amp you work on. Be very carefull you do not get tangled up with that B+.

The second thing is since they run the tubes with that much B+ you will
have a hard time finding tubes that work in that amp.
Be prepared for tubes to arc over very wild to watch. Tubes run fine then one of the six will glow bright purple arc over for a minute or so till you kill the power or the 5 watt plate resistor is blown into a million pieces.
You will repeat that cycle many times.

You can ask some SVT experts what tubes are good enough
for that voltage I assume Svetlana tubes may be the only
ones made today that may cut the mustard. I do not know for sure
so ask someone before you spend that much money on output tubes.
Yes Tung Sol will work in there great as maybe real Gold Lion KT88
no not the reissues from China the real older ones. But both of those
options you will need to sell your house to buy those six tubes.

Have fun.
Let us know how you make out.

I'd say leave well alone in the modding dept! i've never heard a bad one yet! If it ain;t broke, don;t fix it. Instant panacea on any session - Hire in an SVT. It just sits the bass in the mix SO well! Only other Bass rig i like is the 1x15" Portaflex. My mate Jo has one that he maintains came from Motown. He left it with me for a couple of years . . . . Then we had Julian Cope's SVT for a while . . . . . (sigh!)
Seriously, The biggest difference I have found is with an early one with the original tubes(6146's???) They are a lot tighter sounding, and work like a demon with the (un-usual) 1x18' Cab(Same size as 8x10") that I used to use at a (mostly) Reggae studio. The 6550-tubed Head they had was a bit "flappy" sounding, v.v undamped. I would have a guess that changing to these tubes would tighten up a regular 8v10" rig as well. I believe that Groove tubes published the changes necessary in their tube-amp book. I can't help you there, mine literally fell to pieces.

Bouf!

Andy P

i´m not aware of your experience so here´s a tip. set the bias to the highest negative voltage before you turn it on with new tubes. some new tubes can short (svetlanas and electoharmonix for instance).

the last svt (i think it was a pro 300) i opened really was´nt well built considering the 85* C caps being exposed to heat. most of them were bubbly and needed to be replaced. after that and a tube change, the hum was alot lower. i also noticed there was a mechanical hum when the head was sitting on a speaker. it was quite fun testdriving it but it weighs a ton. but what´s a broken back compared to a cool sound anyway?
good luck

I`m with you folks on this one. Its difficult to make this one sound better.
I know plenty of people who say "try this bass amp, the tone controls are great". It seems like many other bass amps rely on the tone controls to get any kind of a sound at all. With an AMPEG it doesn`t matter how the tone controls are, you know you will just have serious in your face BASS.

The bass player in the band I`m in uses a mkII, & I had another friend who had a MKI.

The mkII had the valves replaced with Sovtek 6550`s & it worked fine. The worst thing about it are those white polythene connectors inside the mkII.

Definately don`t run it without a speaker or dummy load connected.


The other good thing is the 8 x 10" cabs. The way they are split into 4 separate 2 driver sealed cabinets. If you gently push one driver in the one next to it moves out !!

Hi again Danny...take my word for it as I have worked on dozens of these....you have to make sure that everything is perfect on this before you fire it up for the first time after fixing it. This a very unforgiving amp if anything in it is out of whack as it is very powerful and very high voltages and currents floating around in that thing.....if something starts to go when you flip on that standby switch, it will happen so fast you won't have a chance to shut it off before the damage is done (and possibly blown up a $300 set of matched 6550's)...BTW these were also made using 6146B tubes which are literally transmitting tubes (very rugged and some people actually prefer these to the 6550 models). I have used "Winged C" tubes which are Svetlana tubes I guess (it's hard to keep up with who owns what anymore) and had very good luck...I always buy them as matched sextets....you don't want one of them hogging more than it's fair share of current.....PM me your address and I'll drop that article in the mail to you or if there is someone on the group that can accept the article, scan it and put it up somewhere, I'd be glad to do that as well......let me know
Cheers,
TP

The practical problem with SVTs is that they're unreliable. I don't think there's anything intrinsically wrong with them, but they're so heavy they get dropped, they get thrown around, and generally abused. When I used to play live a lot I avoided them like the plague because they always crapped out. It's a shame because they are so good sounding, but unless you're prepared to cart it around yourself they get trashed.

The practical problem with SVTs is that they're unreliable.

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The bass player in the band I`m in has been using the same SVT for the last 14 years & it has never gone wrong since he had it serviced when he first bought it.
I agree that you want to treat it well which can be difficult with its weight, but when you look inside the thing, to me it seems to be made a lot better than some of the Marshall, fender etc amps on the market.

I've worked on a half dozen of them, the two sections often end up occupying the family kitchen table for a few days ... they are fantastic !!

Ken Fischer, who used to work at Ampeg, has a great section on servicing SVT's in Aspen Pittman's book ... I just wanna echo a few things he says there ...

As per Ken's recommendations I replace all the 22-ohm/1watt screen "fuse" resistors ... check the 10 ohm / 5watt plate resistors also - if they look suspect change them ... use new matched tubes and bias them at around 24mA per tube ...

I measure bias two tubes at a time (one on each side of the tranny of course) and do tranny shunt testing with an Ammeter, I don't trust the cathode resistors ... then I do it again with all tubes operating ... yep - be careful !

Ken also wrote somewhere else that the SVT actually has the 4ohm output tap wired to the main speaker jack instead of the original 8ohm one - apparently people were blowing speakers too much when the head and high roll-off 8x110 cab design first came out ... you can rewire for full power but I've never done it for some reason ... :roll:

The only mod I would bring to an SVT or a V9 is to move the audio feed from the preamp away from the rest of the wiring harness which is carrying AC power to the power switch on the front of the preamp panel and back ... that makes the amp way nice when hum is totally gone ... just run a shielded feed to the other end of the preamp and down from there ... a jack/plug arrangement on the power amp chassis works well ...