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svt-vr preamp tube question

B

bass bart

Sep 14, 2005
65
1
38
Genk Limburg Belgium
www.myspace.com
#1
First off, if your from europe and in the market for an svt-vr, you might want to check this out:
Invalid Link Removed

As far as I know, that's pretty cheap for a new svt-vr in europe... (the price is in €)

Now what I wanted to ask.
In the preamp of an svt-vr, there are 3 x 12AX7 and 2 x 12AU7
If one would change the 2 x 12AU7 to 12AX7, that would overdrive the amp more at lower volumes?
And the driver tubes are 1 x 12AX7 and 2 x 12AU7
What would changing those 12AU7's to 12AX7's do?

Thank you!
Bart
 
#2
Great link for the music store in Europe, its nice to see the prices and selection. I can't tell you about the tubes and changing them. My experience with the Vr was with its loud fan and 2nd channel static. Have you ever played a VR? you should try to get to one if you havn't. I bought one brand new and the sound was good but the noise of the fan and some 2nd channel static was too much for me to put up with.
Good luck ,,,,,,,:cool:
 
#3
I've already ordered one, it should arrive the end of this week.

I haven't played a vr yet, but those sell second hand here in belgium for 1300 to 1500€ so if I don't like it, I can just sell it for the same price I bought it for :)
but I don't think that will be the case, I have played a few cl's as backline and really like the sound, and now stumbled upon the vr for that price, and from reading reviews, it is said it sounds a bit better than a cl
 
#4
Actually, there's 5 pre tubes and 3 driver tubes (1-12ax7 and 2-12au7), not counting the 6- 6550's.....

Generally, you would want a aggressive 12ax7''s, but with that, depending on what tubes you get can introduce a little noise.... Au7's tend to be more cleaner than ax7's.

I could be wrong....


I've been told that Electro harmonix makes a decent hi-gain 12ax7 (EH12AX7).
 
#5
I would leave the types in their own place, but you can play around with high gain 12AX7 and/or different brands. I found this line-up from another thread:

V1=12AX7
V2=12AU7
V3=12AX7
V4=12AU7
V5=12AX7

And my based on some research and help from others here at TB in regards to my SVT-HD, this is how it goes (not 100% for the VR though, so if anyone knows better, please correct)

V1+V3+V4: Channel 1
V1: Channel 1 preamp + the bass and treble switched boost and cuts
V3: Channel 1 EQ (bass + treble pots)
V4: semiparametric mids

V2: Channel 2 preamp and EQ

V5: Cathode follower, drives power amp + external amp

The two input channels get combined at V5.

So I'd try one channel at a time. Say if you want channel 1 to breakup, put higher gain 12AX7s in spots V1, V2, V3 - Some higher gain tubes are Tung-Sol 12AX7, and from what I understand some RFT ECC83 (12AX7). Maybe try just that change in V1 and see how it goes.

You can also play with the Phase Inverter tube. If it currently has a 12AX7, try a 5751.

Finally, at the cost of some volume, you can get early distortion by lowering the voltage a little on the BIAS.
 
#7
Yes, that is my understanding too. I guess the result out of the speakers is distortion, so either Pre or Power, it's happening.

I would honestly just play around with the tubes in V1, V4, and Phase Inverter though. I just don't know what the long term effects are of lowering the voltage through the BIAS, but I'm guessing it's safe as that's how the Hybrid amps (like the SVT-3Pro) get distortion... by lowering the tube voltage.
 
#9
Do you need slapped? Leave your poor amp alone... The 12AU7 invertor setup is there to send the correct half of the phase to the correct triplet of final tubes. Jacking bias around... I mean its your amp, melt it if you want. But document your mods so the tech can figure out what happened to the amp to cause the damage. BTW the 3PRO doesn't jack the bias, they do it by lowering the plate voltage on the driver tube. Yes this in effect makes the tube think it has its bias jacked UP... you clip a tube by raising bias, not lowering it. Lowering it runs it further towords class A, generating massive heat.

If you want to change the tone of the amp, change out V1 this is called "tube rolling". You will be able to substantually change the charcter of the amp without breaking it.

A nice clean Sovtek will sanitize and red Groove Tube will add nasties. You want more overdrive at the same volume, add a peddle infront of the amp. Depending on the peddle and the settings on that peddle you will completely change the amp's character.

BOB
 
#10
Well, I wasn't thinking about changing the bias :)

I have this epiphone valve jr and did some preamp tube swapping in it, so I was was thinking if it would be possible with the svt, and if yes, which tube could be changed.

Thanks for your reply Bob, I'm not going to touch the phase inverter, just try different tubes in V1, I really wouldn't want to damage the amp...

and tearalong, obviously the germans can't convert pounds to kg :)
it's actually (according to the ampeg site) 85 pounds, or 31.7kg.
 
#11
i've done ALOT of tube rolling in my VR's preamp. i would not recommend swapping in an AX for an AU (never did that) as, even though they have the same pinout, are very different tubes for different applications (have different gain and current). to achieve what i liked best, i ended up with a nos telefunken 12AX7 in V1, a nos RCA cleartop 12AU7 in V2, an amperex 12AX7 in V3, a nos GE 5814A (triple mica) in V4 and a nos RCA 7025 in V5. with this tube compliment, it sounds indistiguishable from my '69 SVT head. the head has plenty of gain, and if you want more drive/grind/distortion at low levels, i'd just recommend using a VT pedal in front of the amp.
 
#12
^ and that is how it is done!

I ended up with a GE 5751 5-star in V1 on my HD. It went from sounding like a Junior Bacon Cheeseburger with Fries to a Double-Whopper with Extra Cheese, Large Fries and a Chocolate Milkshake by just that one tube change.

I also found V3 in my case to respond well by using a Raytheon 5751 Windmill getter in that spot. Seemed to give a tiny bit of reverb-ness to the sound, as well as making the changes when switching the Mids selector switch a little more pronounced.

I also have a medley of tubes in mine. It's fun to tube roll. Just did my SVT-3PRO this weekend too. I was surprised how much the tubes actually changed the character on that amp as well.

That said, I'm a believer in Tube rolling. Most changes are subtle, some not even noticeable, but some are pretty clear a change has been made.

Forget the Voltage change suggestion. It’s just something I noticed when I was BIASing my amp. I misread the Voltmeter and ended up with very low voltage. Sounded very grindy and fuzzy, almost as if I had a distortion pedal in front of it, but I did lose volume and overall punch/power, still pretty loud though.
 
#13
I have a sort of related question - I recently picked up a 74 SVT and the pre-amp came loaded with 12AX7s in place of the 12dw7s (and the 12dw7s scratched out on the tube chart with 12ax7 written over them). I put fresh JJ 12dw7s in the head since it had a bit of hum and I was hoping it was noisy preamp tubes, but the hum hasn't really gone away. I've got a set of new power tubes on the way as well, since I have no knowledge of the age of the current ones and there is a bit of distortion on some notes, so I'm hoping that may clear some of it up.

To go to my point, I was surfing the web and came across someone talking about modding an SVT to use 12ax7s instead of 12dw7s - most people said just swap them but someone mentioned changing out a few components in the pre-amp to better suit the 12ax7s. The amp is going to see a tech in a few weeks anyways, but has anyone (looking at you JohnK ;)) seen this type of thing done? Would there be any harm to putting back 12dw7s if it has indeed been optimized for 12ax7s?
 
#15
i've read in some sources where you can just install 12AX7's where the 12DW7's go, but i also read about a modification that replaces a couple of resistors in order to substitute 12AX7's and make them run in the same manner as 12DW7's.

i believe that i have it here somewhere, so i'll see if i can locate it.
 
#16
here it is:

SVT12AX7mod.jpg


if the previous tech used that method, all you have to do is check resistors #R25, #R7 and #R6. if they are not the stock values and have been changed, change them back to:
Stock SVT:
R25= 1.5K
R6=47K
R7=100K
 
#17
johnk_10 said:
if the previous tech used that method, all you have to do is check resistors #R25, #R7 and #R6. if they are not the stock values and have been changed, change them back to:
Stock SVT:
R25= 1.5K
R6=47K
R7=100K
Click to expand...

Amazing. Thank you. Do you think these are easy to access for a quick check (ie save myself a trip if they haven't been changed) or should I not waste my time trying and just bring it in? I'm assuming they're under the faceplate but I haven't looked into how to go about pulling that out...
 
#19
I think I have a question similar to this. I was going to start a new thread but maybe not. Anyway, I play mainly rock, and I will be getting a VR shortly, but I am a "TEWB-NEWB". Since the VR doesnt have a separate gain knob, and I want to be able to get that hot grindy, gritty rock tone without having to deafen everyone, so what tubes should I use? Higher gain ones? I don't want to buy the CL because everyone says the VR feels more "open".


Sorry for jacking your thread!
 
#20
coreyfyfe said:
Amazing. Thank you. Do you think these are easy to access for a quick check (ie save myself a trip if they haven't been changed) or should I not waste my time trying and just bring it in? I'm assuming they're under the faceplate but I haven't looked into how to go about pulling that out...
Click to expand...

they are easily accessible in the top preamp chassis. all you have to do is remove the top chassis, and it has a aluminum cover plate so you can get to both sides of the pcb. it's a super simple job with a solder-sucker and a few resistors, since everything is numbered.
 

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