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Ampeg SVT-3 Pro Tube Questions

Which Pre-Amp Tubes for SVT-3 Pro

  • Total voters
    86
F

f.c.geil

May 12, 2011
3,381
265
#1
I just bought this SVT-3 Pro with the stock (SovTek) tubes, and it sounded terrible (No volume, soft/furry at every gain level). I had a few tubes lying around, so threw them in, and it's better, but not fantastic (The bass is very soft/furry, and it's overly middy. If I try to bring up the bass or lower the mids, I lose all definition.). Right now, I have a mix of Sylvania and RCA (all vintage, ca 1959-1963) 12AU7 WA with one 12AX7. I realize this is pretty much the opposite of how it came from the factory, but that's what I had. I am ready to get better tubes, but need to know what y'all are using for that awesome Ampeg tone. I really like to be able to get good and clean, but want to have that classic overdriven Ampeg sound on tap when I turn up the gain. So, what do I go with?

If you have a preferred brand, that's good information to have, too, so please share
 
#3
While it's possible that you have a tube problem, please do not rule out the most probable cause: your mosfets need biasing. This is a known problem with the SVT-3pro. Once the mosfets are biased properly, you'll have way more headroom than you may effectively need. Do a search and you'll find a good thread on this topic.
 
#6
Mine has the torroidal transformer and had Electro Harmonix tubes in it when I got it. The bias was fine, and it has plenty of volume. The only problem I have is slow response, but from what I've heard, that is kind of how these are, and if I was doing old school R&B, it would be a go to amp for me.
 
#7
I'm not necessarily thinking I have a tube "problem." I'm thinking that the SovTek tubes are lousy, so I want to replace them with quality tubes. As long as I'm doing that, I may as well get the best sound I can from them.

I'll look at biasing the MOSFETs.
 
#8
B-string said:
don't get too hung up on the WA suffex. :)
Click to expand...

+ several...... The mil tubes are often not as good as the good consumer ones that came as original equipment back when tubes were made by real tube companies.

Mil tubes are reliable, generally low to middle range on techie tube specs, and often have NO particular selection for hum, microphonics, etc. and in some cases no particular noise specs.

So you can end up with a noisy or microphonic tube with a "reliably ordinary" gain, etc.

And, if you have a lot of 12AU7 and one 12AX7, that right there sounds like the main problem...... 12AU7 have lower plate resistance, translating in nearly every case to a low gain and possibly other issues when used where a 12AX7 should be used.
 
#12
Isn't it supposed to be a 12AX7 with a 12AU7 in front of it? At least, that's what it was when I got it (used). Unless I'm not understanding how it works, there's three tubes in the tone section, and two in the power section. Those two are one of each in stock form, with the 12AU7 closer to the front of the unit.
 
#14
tekdiver500ft said:
Isn't it supposed to be a 12AX7 with a 12AU7 in front of it? At least, that's what it was when I got it (used). Unless I'm not understanding how it works, there's three tubes in the tone section, and two in the power section. Those two are one of each in stock form, with the 12AU7 closer to the front of the unit.
Click to expand...

That should be correct... the 12AU7 goes in socket J12.

That is the second, or "output" tube....that drives the mosfets directly
 
#16
I've tried all different brands and types (vintage NOS vs. Modern) in my 3-Pros and really don't notice any difference at all. The next time I re-tube, I'll just go with all new Sovteks. I also would make sure you have the correct tubes in the correct positions, and not substitute tube types like you have mentioned above. If the sound is furry or fuzzy or not clear - it may not be the tubes. Your MOSFET bias may need to be adjusted. This is a simple adjustment that any qualified tech can make, and should only run you a bench fee.
 
#18
stiles72 said:
I've tried all different brands and types (vintage NOS vs. Modern) in my 3-Pros and really don't notice any difference at all.
Click to expand...

The most important tube is the V1 - the one nearest to the jack input. I think only there you can clearly hear the difference between the tubes. The other two preamp tubes are not that important. The pair near the vent - 12ax7 and 12au7 are responsible for the volume and clarity of your amp.

Sovteks WA are noticeably different than any other tubes I've tested. I didn't like them at all and I do not recommend them.
They work great in distorted / overdriven guitar amps tho!
 
#20
I don't know if you can safely put AT7 in place of AU7. I would not recommend that.

For the best result you should use good quality 12au7. It is a dual triode tube, both triodes should give equal result when testing using a special unit for measuring tubes. Cheapest 12au7 sometimes are not well "paired" inside.

This rule is not "transferable" for 12ax7 - their halves should not give the same result.
 

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