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Increasing clean headroom in vintage SVT?

mattvon

mattvon

Jan 22, 2006
456
3
Minneapolis, MN
www.myspace.com
#1
Hi all, I've got a 1970 SVT (factory 6550s) loaded with GE KT88 tubes...it's the best sounding amp I've ever played, and I love it.

Here's my question--it breaks up just a little sooner than I'd like. Anyone know a easy way to increase clean headroom? I've thought about replacing the preamp tubes, but I'm not sure what would be a good swap for the Sylvania 12DW7s that are in there. Or would a power tube swap likely yield better results.

Anybody have any ideas? Given how great the amp sounds, I'm hoping to find a non-radical way to delay the onset of distortion...
 
#3
Someone will know more than me- thats 4 sure, & hopefully they'll chime in here, But KT88's IME should be fine for clean headroom. Most likely a change of pre or driver tubes will be the go! I've found Bob at Eurotubes.com to be extremely helpful in these matters. The JJ tubes he sells are superb IME/IMO also- email him, I think he'll have some helpful advice. In addition to the replies U get here!
 
#4
mattvon said:
Hi all, I've got a 1970 SVT (factory 6550s) loaded with GE KT88 tubes...it's the best sounding amp I've ever played, and I love it.
Click to expand...

GE NEVER made KT88's. Are you sure about the brand?

Here's my question--it breaks up just a little sooner than I'd like. Anyone know a easy way to increase clean headroom? I've thought about replacing the preamp tubes, but I'm not sure what would be a good swap for the Sylvania 12DW7s that are in there. Or would a power tube swap likely yield better results.
Click to expand...

Where is your volume knob when the amp breaks up? With most basses, 11 o'clock is about the limit before you're into overdrive. It's a VERY 'gainy' preamp. Also I'll echo the comment about active basses, some onboard preamps have very high output voltage.

Anybody have any ideas? Given how great the amp sounds, I'm hoping to find a non-radical way to delay the onset of distortion...
Click to expand...

You'll have to be way more specific.
 
#8
Thanks to all for the replies. All of my basses are passive.

To PBG's points:

While I'm sure on the manufacturer being GE, I'm wrong about the tubes--they're 6550s. Apologies--that's just what the previous owner told me. :hmm:

As for my volume knob position, I'm getting into distortion at about 10:00 or 10:30.

By non-radical I meant I'd like to avoid altering the circuit in any invasive manner. It's recently been recapped (again, according to the previous owner)...basically I was hoping to swap in different pre- or power-amp tubes to achieve my goal. If that's not likely to work, I'd be open to other methods. Hopefully that's way more specific enough for you.

I've considered other/another cabinet. Currently I'm playing Reverend Rumblefish->SVT->Berg NV610. I also have a Schroeder 410R/1212R I can put in play. The Schroeder is louder, but I prefer the sound of the Berg.
 
#11
I couldn't get away from the distortion either, which is what pushed me away from my SVT.

I loved it dirty - sometimes, but sometimes I like it nice and clean. :hiding:

It seems that the higher output basses were the ones that could give me more clean headroom. I used a passive jazz bass, and that thing would start distorting at lower volumes. My epi Tbird would go louder and cleaner for some reason.

Anyway, I love my Carvin B1500 and don't miss the SVT much aside from the collectability factor. :ninja:
 
#12
Yeah, volume isn't a problem! :bassist:

And I do have lots of pre-amp tubes around, I was just hoping for a bit of direction to save some time.

Another thought has occurred to me--the Rumblefish is REALLY hot for a passive bass...it might be worth trying a couple of other basses at gig levels to see what I think.

Oh, I should make clear--I'm not complaining about the SVT tone--in my opinion, there's none better...I'm just looking for a bit more headroom.

Keep those thoughts coming, and thanks!
 
#15
brothernewt said:
judging by your profile... you've got tons of preamp tubes around you could swap out to just see if that helps.
Click to expand...

+1. When I had an SVT, I upgraded all the tubes in the preamp, which was a mix of cheap Russian and Chinese tubes. At the time I put in two Electro Harmonix 12AX7EH’s, two JAN-Philips 5814A (12au7’s), and one JAN-Philips 12AT7WC. I personally found the new preamp tubes gave me more clean headroom. Looking back on it now, I would have probably substituted Tung Sols for the EH's, but it definitely made a difference. It didn't break up as soon as the old configuration.
 
#16
mattvon said:
While I'm sure on the manufacturer being GE, I'm wrong about the tubes--they're 6550s. Apologies--that's just what the previous owner told me. :hmm:

It's recently been recapped (again, according to the previous owner)...basically I was hoping to swap in different pre- or power-amp tubes to achieve my goal. If that's not likely to work, I'd be open to other methods. Hopefully that's way more specific enough for you.
Click to expand...

That is exactly what I needed to know. Given the inaccurarcy of the tube type from the previous owner's description, it could be likely the amp never was recapped. Ever blown the fuse? If you can post a pic of the amp with its grille removed I can tell you if it has been recapped for sure.

It's also possible that the bias is set wrong and you're not getting full power. I'd check that first since it's easy to verify non-invasively.

Subbing different type preamp tubes won't help you, and will in fact decrease your headroom. A 12AX7 in place of a 12DW7 in a circuit designed for the DW will distort much sooner and decrease the drive to the power section, so you'll be getting more distortion with LESS clean volume.

As for my volume knob position, I'm getting into distortion at about 10:00 or 10:30.
Click to expand...

For a high output bass like your Reverend, that sounds normal. Do you dime your bass's master volume?
 
#17
Psycho Bass Guy said:
That is exactly what I needed to know. Given the inaccurarcy of the tube type from the previous owner's description, it could be likely the amp never was recapped. Ever blown the fuse? If you can post a pic of the amp with its grille removed I can tell you if it has been recapped for sure.

It's also possible that the bias is set wrong and you're not getting full power. I'd check that first since it's easy to verify non-invasively.

Subbing different type preamp tubes won't help you, and will in fact decrease your headroom. A 12AX7 in place of a 12DW7 in a circuit designed for the DW will distort much sooner and decrease the drive to the power section, so you'll be getting more distortion with LESS clean volume.



For a high output bass like your Reverend, that sounds normal. Do you dime your bass's master volume?
Click to expand...
PBG, you're a tube amp machine! It's so odd you mentioned the fuse popping--when I first received the amp, I plugged it in and played and blew the fuse on the back within the first 20 minutes of playing. I went to Radio Shack and bought a correct replacement and it's been fine ever since (about a month of daily, medium-volume playing)

I will take the back cover off and shoot some pics over the weekend. Thanks so much for offering to help me ID the caps.

I usually play with my Rumblefish volume nearly pegged, but with a little room for my (very occasional) lead while playing live.
 
#18
mattvon said:
PBG, you're a tube amp machine! It's so odd you mentioned the fuse popping--when I first received the amp, I plugged it in and played and blew the fuse on the back within the first 20 minutes of playing. I went to Radio Shack and bought a correct replacement and it's been fine ever since (about a month of daily, medium-volume playing)
Click to expand...

Blowing it once might mean something or it may have just have been worn out. In an amp with bad filter caps, the power supply will draw more current out of the wall to try and keep the output devices energized. It will distort much sooner than normal and have less volume, BUT it will also blow the fuse very often.

I will take the back cover off and shoot some pics over the weekend. Thanks so much for offering to help me ID the caps.
Click to expand...

You don't have to take the back cover off. Just pop out the front grille, which is held on with plastic friction fasteners. The main supply filter caps will be sitting over to the right next to the power transformer when viewed from the front. If they're metallic looking and not colored plastic coated, chances are they're the original caps.

I usually play with my Rumblefish volume nearly pegged, but with a little room for my (very occasional) lead while playing live.
Click to expand...

Try running your amp louder and your bass quieter, but I don't think that's the answer.
 
#19
to determine whether you are distorting the preamp or
power section, you could turn the volume down & use the
level control on the graphic eq (i beleive this is post preamp)
as a boost, to see if the power section is distorting at
medium volumes.
also you can bypass the preamp section altogether. just
plug your bass in to your cheap $hit mixer, then out to
the 'power amp in' jack on the back. this will allow you to
see if your power amp is what it should be. just make
sure you get a clean strong signal from the mixer first or
you will be wasting your time.
 
#20
I would suggest to check the bias at first. Its deadsimple and costs just a minute.
If this doesnt improve take out the power tubes and let them check from an experienced tech. He puts it in a tube tester and can tell you immediately if they are worn out or not.

GE are great tubes and very expensive these days, but also this tubes dont last forever.

If the tubes are ok - then bring the amp to the tech.

If he checks the filtercaps, Phase inverter balance and everything else and the amp is fine and still has too little headroom for you, then there is just one thing I can think of.

As sbd mentioned before - antiparallel diodes in the poweramp are the magic ingredient of the trademark svt tone. These can be removed by your tech really easily. The sound then is cleaner, but also lacks of the typicall svt grind.
If the power tubes start to clip, then it can sound really harsh. These diodes can be replaced back very easy.

Me personal - I would never do that modification as I love the sound of my SVT.

If your amp is fully serviced and works as it should and you still need more clean headroom, than look for a cabinet with a better efficiency or in the end:
look for another amp, that gives you more clean headroom.
 

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