Ampeg SVP-PRO overdrive / tube issues... HELP!

pablomigraine · #1 11-08-2010, 12:50 PM

So... I have a mix of Groove Tubes and Sovtek tubes in my SVP-Pro preamp.... and although the resultant tone is nice and buttery... the overdrive just isn't doing it for me. In fact, the problem is I cant really seem to get any overdrive!

One of the features I always loved about this pre (and the SVT-II-Pro) is the "Drive" control. However, turning Drive all the way up right now does little more and than add some upper midrange harmonic content (which is more a function of that circuit's equalization behavior than actual overdrive). Increasing input Gain with the Drive control anywhere past noon gives a little sweeter sound.. but just it seems like its gonna start to break up real sweet... BAM!! It takes a nosedive straight into the most unpleasant preamp-tube fuzz and static you've ever heard. Awful. NO breakup whatsoever... not even that nice clangy midrange response you're supposed to be getting when using Drive. Just totally saturated buzz. When increasing gain with Drive below noon, or even off, has the same effect, just with less midrange. The resulting response is either dead clean or static/fuzz mess.

Now, time was when I knew what each of the tube positions was responsible for, and time was when you could count on getting good results by putting a nice quiet GT 12AX7 in #5 and a sweet hi-gain EHX 12AX7 in #1 but now I just don't know...

I really want to avoid the expense of re-tubing the whole preamp... as I already went thru a bundle finding a quiet enough tube for #5 to silence the hiss this preamp is notorious for. What positions should I change out, and what tubes should I buy? I'm looking for a little natural compression and nice beefy overdrive that's real sweet in the mids for purposes of cutting through.

Help me out TB!!

pablomigraine · #2 11-08-2010, 03:11 PM

Anyone?

..........Bueller?

GMSweet · #3 11-08-2010, 04:59 PM

I'm far from an expert here, but I'll try to point you in the right direction.

What do you have in V4?

Post #60 in this thread goes over the different tubes and their functions. There should be a 12AU7 in that socket. If it was replaced with a 12AX7 the gain factors may be off. My SVP-PRO can get all the way to nasty with the drive cranked to the right. Don't be afraid to crank the gain as well if you need more. Your peak light should light up like a Christmas tree.

I hope this helps!

Matt

pablomigraine · #4 11-08-2010, 06:49 PM

Quote:

Originally Posted by GMSweet

I'm far from an expert here, but I'll try to point you in the right direction.

What do you have in V4?

Post #60 in this thread goes over the different tubes and their functions. There should be a 12AU7 in that socket. If it was replaced with a 12AX7 the gain factors may be off. My SVP-PRO can get all the way to nasty with the drive cranked to the right. Don't be afraid to crank the gain as well if you need more. Your peak light should light up like a Christmas tree.

I hope this helps!

Matt

Yeah the 12AU7 is in the right spot. It just goes from clean straight to totally awful blown out static fuzz with nothing in between.
..

12bass · #5 11-08-2010, 07:11 PM

Sounds like there's a circuit problem somewhere. Do you have a copy of the schematic?

Depending on the mix of gain/drive, mine goes through the continuum from relatively clean to slight edge to full-on tube fuzz. As the drive knob is turned further clockwise it shelves off some upper treble "fizz" to smooth out the overdriven tone.

GMSweet · #6 11-09-2010, 05:47 AM

I'm hoping someone more knowledgeable than I will be able to step in, but here's my thoughts...

1) I would pull the 12AU7 and look at the pins to see if there is any corrosion going on. Perhaps a visual inspection will show somethings else as well. If everything "looks fine", re-seat it and test.

2) If you still have some of the tubes from when you were trying out various tubes, you could try a 12AX7 in V4. My understanding is that by doing this you really shouldn't get any distortion because of the gain factor difference between a 12AU7 and the 12AX7. If you are still getting "blown out static fuzz" when turning up the drive control, then I believe 12bass is on the right track with it being something else in the circuit. If the problem goes away, you may be able to just replace the 12AU7.

3) Amp tech time unless you are handy with schematics and circuits. I'm of no help there.

Hopefully it's just the 12AU7 that's bad.

Good luck,

Matt

pablomigraine · #7 11-09-2010, 07:10 AM

Quote:

Originally Posted by GMSweet

I'm hoping someone more knowledgeable than I will be able to step in, but here's my thoughts...

1) I would pull the 12AU7 and look at the pins to see if there is any corrosion going on. Perhaps a visual inspection will show somethings else as well. If everything "looks fine", re-seat it and test.

2) If you still have some of the tubes from when you were trying out various tubes, you could try a 12AX7 in V4. My understanding is that by doing this you really shouldn't get any distortion because of the gain factor difference between a 12AU7 and the 12AX7. If you are still getting "blown out static fuzz" when turning up the drive control, then I believe 12bass is on the right track with it being something else in the circuit. If the problem goes away, you may be able to just replace the 12AU7.

3) Amp tech time unless you are handy with schematics and circuits. I'm of no help there.

Hopefully it's just the 12AU7 that's bad.

Good luck,

Matt

Well, we're certain that the problem is just the wrong tubes. When I say blown out static fuzz.... its TUBE static fuzz.... just not a very pleasant tube overdrive. Some tubes break up very nicely and some tubes just dont break up at all until they completely overload.... We're certain it isnt a problem with the circuit.

I was hoping to get some suggestions as to a good place to start insofar as what tubes I should start trying and in what positions....

Thanks!!

staindbass · #8 11-09-2010, 07:39 AM

i think you would find an rca blackplate would sound nicer than the gt's. if he had a 12ax7 in the 12au7 spot, he should have too much gain. he noticed less. sometimes half of a tube dies, (dual triode) and the other half works fine. the gain just goes low suddenly. do you have a tube tester or one you can use? johnny a. /staind

pablomigraine · #9 11-09-2010, 07:57 AM

Quote:

Originally Posted by staindbass

i think you would find an rca blackplate would sound nicer than the gt's. if he had a 12ax7 in the 12au7 spot, he should have too much gain. he noticed less. sometimes half of a tube dies, (dual triode) and the other half works fine. the gain just goes low suddenly. do you have a tube tester or one you can use? johnny a. /staind

Correct. I dont have a tester. I think I'm just going to be a big pain in the ass to my local shop, line up about a dozen tubes and just spend the day sorting this out. Anyone have suggestions where I should start? EHX for the input gain? Maybe a 12AT7 for drive? What brands?

12bass · #10 11-09-2010, 08:25 AM

The Tung-Sol 12AX7 sounds pretty nice and clear, IMO, at least for input gain (V1). My SVP has a really good sounding stock tube in V4... think it is maybe Philips 5814A, but don't want to open it up at the moment to confirm.

metalinthenight · #11 11-09-2010, 08:38 AM

I don't know anything about tubes or how to fix your problem, because I have never had to re-tube mine. Here is a thread with a schematic for the SVP-PRO:

Ampeg SVP-Pro Schematic Tutorial

My SVP-PRO has these tubes in it (definitely not whisper quiet

):

GT (Sov) 12ax7 in V1,
Ruby 12ax7 in V2 and 3,
Electro-Harmonix in V4

V5 is supposed to be a 12ax7, but the previous owner installed a NOS RCA 12at7 to help lower the noise floor feeding into the power amp. I'm not sure how much this helped, but I would hate to hear how noisy this pre amp would be with the 12ax7 if it is actually worse

.

With this configuration I can get nice gain/drive easily without tube-y fuzz/static (unless both the gain and drive are cranked).

EDIT: Here are the links to the SVP-PRO schematic:

Link to Main Board Schematic PDF
Link to Tube Board Schematic PDF

12bass · #12 11-09-2010, 08:43 AM

Although there have been concerns about excess noise with this preamp, I don't find my early-ish SVP overly noisy. As for V5, I've found that using a lower gain tube (12AU7/AT7/AY7) lowers the output level. Not sure if affects the overall noise much, as most of the gain is probably before that.

metalinthenight · #13 11-09-2010, 08:48 AM

Quote:

Originally Posted by 12bass

Although there have been concerns about excess noise with this preamp, I don't find my early-ish SVP overly noisy. As for V5, I've found that using a lower gain tube (12AU7/AT7/AY7) lowers the output level. Not sure if affects the overall noise much, as most of the gain is probably before that.

There is just a low hiss or hum that is always present when I am not playing (even when my board is muted). It isn't noticeable once I start playing, which is why I've kept the amp, but it is rather irksome when I really stop and think about it.

pablomigraine · #14 11-10-2010, 07:07 AM

So it would seem that that the V4 (12AU7) is used for the "Drive" control. Anyone have any experience using a 12AT7 or higher gain tube such as a 12AX7 in V4?

12bass · #15 11-17-2010, 10:14 AM

Ampeg SVP-Pro Schematic Tutorial

Old thread with a lot of good SVP info.

aledeville · #16 11-17-2010, 10:58 AM

IMO GrooveTubes and Sovtek are not very musical tubes.
Try a JJ, in my Fender TBP-1 they made a huge difference.
http://www.amplifiedparts.com/products/tubes/12AX7-S-JJ