Diving into a vintage SVT

[nateflanigan]

I feel like I've been circling these things for the past 20 years and last week I finally picked one up.  I played it a bit by myself, seemed cool, but today when I got to use it at practice it was kind of blah.  My 250w Galien Krueger 400b really put it to shame.  Seems unthinkable, right?   Before I bail on it I'd really like to open it up and see if I can get it tuned up.  Any advice?  Anything specific to look for?

[sluckey]

Don't fix it if it ain't broke.

[woolly]

well , tube vs sandstate....  the SVT needs an 8x10" cab to tighten it up...

[Willabe]

Don't fix it if it ain't broke.

Plus, an SVT has enough B+dcv and B+dcv current to fast fry a couple of horses. This is not an amp to cut any ones teeth on.

I still would not feel comfortable poking around in an SVT. This is not an amp "to dive into."


             Brad      

[birt]

it's either not healthy, or not your taste.

any time i've played a vintage SVT i've had this "oohh yeah" happy feeling after a couple of notes.
so if you are REALLY up for it and you know what you are doing... check if this oldie is still in shape. or maybe try a known good one to make sure it sounds really different.

[nateflanigan]

Hi, I hope this clarifies things a bit...

Don't fix it if it ain't broke.

Well, I'm thinking it's broke.

the SVT needs an 8x10" cab to tighten it up...

I normally play through 2x15's.  But for the occasion I borrowed an Ampeg 8x10 from a friend.  I was jamming with a drummer, and one guitar player playing a bf bassman through a 2x10 cab and could barely keep up using the SVT/810.  That's what made me think this can't be right.

Plus, an SVT has enough B+dcv and B+dcv current to fast fry a couple of horses. This is not an amp to cut any ones teeth on.

I still would not feel comfortable poking around in an SVT. This is not an amp "to dive into."

By dive into, I didn't mean that this would be the first amp I've ever worked on.  Largest for sure.

it's either not healthy, or not your taste.

any time i've played a vintage SVT i've had this "oohh yeah" happy feeling after a couple of notes.
so if you are REALLY up for it and you know what you are doing... check if this oldie is still in shape. or maybe try a known good one to make sure it sounds really different.

Totally possible I just don't like it, off the top of my head I don't know any one else that has one but that'd be a great resource if I could find one. 

I can't see any harm in recapping it, checking the bias, going through the calibration procedure etc. 

[Jack1962]

if ANY GK sounds better than ANY Ampeg somethings wrong with the Ampeg . However this really isn't a amp to dive into if you are not familiar with them. If you really , really want to work on this amp , check the tubes first this is most likely your problem . If this is a vintage Ampeg SVT it's built like a tank . Contact Ampeg and get a schematic for this amp they are not all the same. I work on tons of Ampegs they are not easiest amp to work on but if you take your time and are careful you can do it .

[terminalgs]

I feel like I've been circling these things for the past 20 years and last week I finally picked one up.  I played it a bit by myself, seemed cool, but today when I got to use it at practice it was kind of blah.  My 250w Galien Krueger 400b really put it to shame.  Seems unthinkable, right?   Before I bail on it I'd really like to open it up and see if I can get it tuned up.  Any advice?  Anything specific to look for?

blue-line SVTs are the magic.   what year SVT do you have?

[nateflanigan]

Ok, spent some time at the practice space after work checking out the amp.  So first things first, the original schematics were still in the cabinet!  They're dated 1975.  So that's cool.

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PreAmp_.pdf
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVTV9.pdf

- The filter caps are nichions, couldn't say how old they don't look extremely new.

- Tubes, Svetlana 6550's, with tubestore decals on them. 

- Preamp tubes,
v1 raytheon 12w7
v2 tung sol 12ax7
v3 jj something, couldn't read
v4 jj something, couldn't read
v5 jan-phillips 6c4

- On to the biasing procedure, step 2a checked out fine.  Step 2b however was way off, so I adjusted VR2 to as close to 0 as possible.  This was a big improvement in the output distortion. 

- I didn't have the gear with me to do step 3, but I'll get that together.

- Still switching back and forth between the GK and the SVT, the low end was pretty murky on the SVT and it was hard to match volumes with out the SVT getting really blown out sounding.

- So, I thought I'd try running the GK pre out to the SVT power amp in.  Holy Moly, sounds AMAZING and LOUD.

Now I know the power amp is working, so that's good.  But I guess I gotta figure out if I just don't like the Ampeg pre, or if it needs some attention.  At least the pre-amp tubes aren't too expensive.

[birt]

...Holy Moly, sounds AMAZING and LOUD.

that is what we are looking for

[SoundmasterG]

There are a couple filter caps in the pre amp section also that you should check. While you're in there the switches should be cleaned as replacements are not available anymore.

Also, make sure each power tube is working and the diodes aren't burned out. An SVT can blow out a couple power tubes in each side and still work...but if some are blown out the power will be down.

Have fun with that beast!

Greg

[nateflanigan]

Thanks Greg, I think next pay check I'll order some replacement pre amp tubes, then I'll take the chassis out and look around. I've never checked a diode in circuit before.  Would a bad diode test as open using the continuity setting?

The action on switches is pretty loose. They function fine but maybe they could be cleaned up. Replacements are available from fliptops.net but they're $30 a piece.

[SoundmasterG]

Thanks Greg, I think next pay check I'll order some replacement pre amp tubes, then I'll take the chassis out and look around. I've never checked a diode in circuit before.  Would a bad diode test as open using the continuity setting?

The action on switches is pretty loose. They function fine but maybe they could be cleaned up. Replacements are available from fliptops.net but they're $30 a piece.

The diodes I am talking about are in the power tube circuit....and they tend to blow when a tube goes south. Checking a diode in circuit....I think you probably have to pull one leg to check accurately.

The switches used to not be available....but they aren't the best switches anyway so constant cleaning of them is a good idea whenever the amp is serviced. I had a vintage MTI era SVT that I sold awhile back and it was miswired from the factory on the back of one of those switches. They forget a jumper wire and all of the signal on channel 2 went through a .022uF cap instead of a .22uF cap...which killed the bass response.

The preamp tubes may still be good....if you inject a 200 mV signal into the input, the first stage should output about 3V for a 12AX7...and you can check all the tubes through the preamp circuit by adjusting the input to be about 200mV and checking the output....but you also need to keep in mind that a 12AU7 or other such tube with lower gain will not output the same amount of signal. It is a useful way to test tubes for gain if you don't have a tube tester to check function.

Greg

[nateflanigan]

Hey Greg, thanks for the advice

The diodes I am talking about are in the power tube circuit....

I was pretty sure that's what you meant

I had a vintage MTI era SVT that I sold awhile back and it was miswired from the factory on the back of one of those switches. They forget a jumper wire and all of the signal on channel 2 went through a .022uF cap instead of a .22uF cap...which killed the bass response.

More and more I'm learning you really can't take anything for granted with this stuff.

That's a great way to test tubes.  I'll have to figure out what a 12DW7 should be putting out.  I have my suspicions about the V5 6c4, both channels are pretty close volume wise, so maybe a volume drop is happening there.  Taking your idea further, I think I'll measure the out put of one of the external preamps I've used and compare that to what's coming out of the SVT preamp.

[SoundmasterG]

Hey Greg, thanks for the advice

The diodes I am talking about are in the power tube circuit....

I was pretty sure that's what you meant

I had a vintage MTI era SVT that I sold awhile back and it was miswired from the factory on the back of one of those switches. They forget a jumper wire and all of the signal on channel 2 went through a .022uF cap instead of a .22uF cap...which killed the bass response.

More and more I'm learning you really can't take anything for granted with this stuff.

That's a great way to test tubes.  I'll have to figure out what a 12DW7 should be putting out.  I have my suspicions about the V5 6c4, both channels are pretty close volume wise, so maybe a volume drop is happening there.  Taking your idea further, I think I'll measure the out put of one of the external preamps I've used and compare that to what's coming out of the SVT preamp.

The 12AX7 half of the 12DW7 should be the same as any other 12AX7. I haven't checked a good 12AU7 to see what it should output.....but it all depends on the circuit too. What I said works with the typical Fender style preamp stage...but might not be the same with something that is grid biased for example.

I think your idea to compare preamp outputs is a good idea.

Greg

[nateflanigan]

Cool, I'll measure around when I get into it, but I know how that sort of thing can get you even further into the weeds.  I've been searching for some service documentation or something that would have some specs on what the preamp output should be, but I'm coming up empty.

[SoundmasterG]

If I still had my SVT I could tell you but its been gone about a year now. I don't think you will find any info on the preamp out gain anywhere....it wasn't a spec anyone bothered with I think. I'd just check over all aspects of the amp thoroughly and make sure it is working as it should. Most of the MTI era amps were about 240 watts RMS due to slightly lower voltages in the power amp....some of the early ones like the one I had used the same transformers as the Magnavox era ones like the one you have, the Magnavox ones were a bit more...maybe approaching 300 watts RMS, and the early blue line amps with the 6146 power tubes were 330 watts RMS. You could use KT88's and bump the screen voltages up and get close to 400 watts out of an SVT but that requires some transformer changes. The cabinet is very important for the sound of an SVT and the earlier the better as far as sound. They can't handle the power though so you need multiple cabs. I sold my SVT and kept my Sunn 2000S as I think the Sunn sounds better.

There was some stuff on the SVT in one of the Vacuum Tube Valley magazines from about 10 years ago....but nothing really technical per se. They did have a couple different schematics, but the Ampeg book has more useful info on these amps....though again not super technical.

Greg

[nateflanigan]

Good point about the holistic approach.  I think I need to dig out my scope, man where is that thing?  Also, any recommendations on making a dummy load that can handle 300 watts?

Those Sunns are great, I made a Sunn-ish guitar amp using bassman transformers that I love.  Come to think of it a friend has a 2000s that needs some attention.

[SoundmasterG]

I have some huge 200 watt ceramic resistors that I used. (The kind that look like a ribbed sex toy...haha) They are 16 ohms each and I can run a few in parallel and get 4 ohms and like 800 watts....they weren't even warm after an hour connected to my SVT. You can find them at surplus stores, or new online, but I would hit up the surplus stores myself. I actually have a bunch of others here of different wattages and values. Too many probably....if you wanted some I could make you a deal. If interested, send me a PM and I can let you know what I have and how much they would cost.

I use the Sunn 2000S for bass only and it is great. It is an early one with the Dynaco transformers and Conrad Sundholm ( a friend of mine) tested his brother's 2000S which is similar to mine and it amplifies cleanly down to 10Hz! They're great for bass, and the cabinet is well liked too. I still need to rebuild my 2000S one of these days and plan to use Solen polypropylene caps to replace most of the electrolytics. They are HUGE caps but the 2000S has the room and I think a bass amp would benefit from those caps.

Greg

[pullshocks]

It has been a long time since I heard an SVT, but I recall one time in a club that had a lot of balloons as decorations.  Every time the bass player would really get on it, balloons would start to pop.  Serious power with the 2 8x10s.  At the time I remember thinking--finally, bass as it should sound.

[nateflanigan]

Greg, PM sent.  Turns out my friend is thinking of selling his 2000s, unfortunately I don't think I can afford it  Hopefully I can convince him to put some money into it and get it tuned up so I can at least hear it at it's best.

Pullshocks, I had a similar experience.  I was probably 18 or 19, and saw a hardcore band from SF play in a church basement, the bass player had an SVT.  It was the first time I could really hear (and feel) the bass at one of those shows.  I've wanted one ever since.  I haven't played that kind of music in a very long time, but I recently put together a heavy/punk/what have you band, so naturally I thought I gotta get an SVT.
 

[SoundmasterG]

A 2000S is another amp that benefits from the original style cabinet intended for it. Great amps though! Where an SVT is punchy and sort of indistinct with lots of mids, a 2000S is very precise and articulate when clean, and when overdriven...well think of mid 70's/early 80's Rush bass tone.....nice and fat and aggressive. You can't hide with a Sunn amp as much as you can with an SVT.

I'll respond to your PM.

Greg

[Jack1962]

Simple answer to your tone problem , the mixture of preamp tubes is your tone problem , a lot of people like to mix and match preamp tubes and sure you can come up with a good mix , but IMHO that is your tone problem , install a set of the same make and model and I would almost bet , that your tone will GREATLY improve .

[nateflanigan]

it's been a bit, it took me some time to order a dummy load, get my oscilloscope back from a friend who borrowed it etc etc.
So here's whats going on...

- Power amp checks out a-ok

- Pre-amp, not so much.  Running test tones in, 1k behaves ok but the lower the frequency the worse the output.  By the time I'm down to 40 hz the amp's output will barely reach 5v without distorting. 

To be clear, running a .25v tone at any frequency directly into the power amp, produces a nice clean sine wave measuring 30v across the dummy load.  Regardless of which pre-amp channel I feed the tone into, I get the same poor response.  This makes the 6c4 and surrounding circuitry suspect.  So, I ordered a replacement, no improvement.  I pulled out the chassis, checked voltages, they're high, but not crazy high.  It looks like R45, has been replaced (one of the resistors off of the cathode of the 6c4) schematic says 47k.  I pulled the resistor, it measures 62k.  For all the world it looks like the color codes read 470 ohm.  Sooooooooooo, next steps?  Replace the 47k resistor, look for other suspicious repairs, continuity checks, all that sort of stuff. 

I'd like to try looking at the signal before the 6c4 tube.  Should I do that before or after the 100k mixing resistors? 

Thanks!

[nateflanigan]

Things seem good.  I replaced that questionable resistor, now I'm able to amplify from 1k down to 40hz to ~30v across the dummy load and get a pretty clean looking sine wave on my scope.

I don't have a bass cab (or neighbors) at home that could take the SVT cranked up to full volume to do an actual listening test so I'll have to wait til I can get it back to my practice space.