SIB Varidrive  [traced]

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SIB Varidrive  [traced]

Postby Smoky Black » 01 Mar 2013, 00:44

Thought I'd share this one before I start modding it! Kind of a FET boost into a lower voltage Vox TB style couple stages followed by a Butler Tube Driver tonestack.

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http://www.theinside.net/misc/Varidrive/SIB%20Varidrive%20Schematic.pdf

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Bernardduur (04 Mar 2013, 08:52), ik6gpy (01 Mar 2013, 21:58), imaradiostar (07 Mar 2013, 05:05), loylo (02 Mar 2013, 09:27), Lucifer (01 Mar 2013, 23:39), Manfred (02 Sep 2015, 20:32), modman (19 Mar 2013, 13:53), oh7hhi (01 Mar 2013, 13:45), rocklander (01 Mar 2013, 02:09)
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Re: SIB Varidrive

Postby Bill_Mountain » 01 Mar 2013, 21:40

It looks pretty cool.

The 470k & 470k voltage divider before the second tube stage looks backwards.
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Re: SIB Varidrive

Postby Smoky Black » 01 Mar 2013, 22:33

Bill_Mountain wrote:It looks pretty cool.

The 470k & 470k voltage divider before the second tube stage looks backwards.


Hey, Bill.

I double checked and that's the way it is!
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Re: SIB Varidrive

Postby imaradiostar » 07 Mar 2013, 05:04

I traced one of these a while back. I found a few errors on your schematic, it should be easy enough to verify if I'm right by desoldering and measuring or looking closely at the pictures. I guess it's also possible that I'm wrong or they're different, but I don't think I am!

-treble pot is a 500k linear with a 47k resistor between the bass end and the wiper
-treble connects to the output pot via a 220k resistor
-output pot is a 100k audio with a 220pf cap across it
-gain is a 1 Meg audio pot
-bass is a 100k audio pot
-mid is a 10k linear pot
-4n7 cap connects between 22k "slope" resistor and ground, not on the mid wiper side

I built a clone using the above values and it sounds like a Varidrive should. It's one of my favorite gain boxes of all time, for bass and guitar.

As a mod I would like to recommend using a higher gain Jfet, adding some grid stoppers, and using the treble boost network from a dual rect. I used a smaller (calculated) source cap for more shelving response. It sounds, per friends that should know, mostly like a dual rect in that mode. I believe I used a 2n5457 and a pot as a source resistor to get the source voltage at about half of the available supply voltage. I'll get specifics of my mods if anyone cares to try them. I was planning to put them on a footswitch so I could have both modes available.

I noticed a 100pf cap in your pictures, on the underside of the board. What is it connected to?

Again, thanks for posting.

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Re: SIB Varidrive

Postby imaradiostar » 07 Mar 2013, 05:05

Looks like the 100pf is on the gain control. Makes sense.

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Re: SIB Varidrive

Postby imaradiostar » 07 Mar 2013, 05:11

One other thing I forgot to mention- the power transformer is a dual 120 volt primary dual 6.3 volt secondary transformer. One of the primaries is used as a secondary and the leads have been cut off of the other secondary.

In other words:

-120v in to one of the primaries
-6.3v out to tube filament
-other 120v primary ISN'T connected to wall power- instead it connects to a bridge rect and provides the B+ for the circuit. You'll note in the pictures that the bridge rect is on the primary side of the PT and connects to the caps on the opposite side via a single trace seen through the board.

Sorry to keep posting like this!

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Re: SIB Varidrive

Postby imaradiostar » 07 Mar 2013, 05:42

Here's a picture of my clone/prototype. It's powered by a nixie supply from ebay instead of a bulky EI lam transformer. It was wired up to the input jack at first and I couldn't get it to be quiet- it injected all kinds of noise into the circuit and anything else on the power supply because it's a high frequency switcher. I moved the nixie power input to the output of the 7806 regulator that was running the tube filament and now it's quiet and works great with my Visual Sound 1 spot. It draws less than 400ma, which is a lot, but not so much for a tube pedal. I settled on a 6112 tube because it's tiny and sounds great.

I can share my layout...it's not great but it works if you're using an eyelet board and don't mind wires all over. Like I said, it's a first prototype. I'm sure the guys on this site will clean it up and make it amazing in no time!

The "dual rect" mode switches a 680k/.002uF cap in series with the gain pot and puts a .47uF cap across the source of the 2n5457 Jfet. I ended up with a 4.7k source resistor and a 50k trimpot to set the drain voltage.

In "normal" or "Varidrive" mode the 680k/.002uF network is shorted and the .47uF cap is lifted, lowering the gain and flattening response.

It's kinda bright in rect mode- I'd like to add some capacitance to ground before the first grid but haven't gotten around to it. I don't wanna kill the tone in the other mode. As it is now it works really well with the guitar's volume knob and cleans up beautifully.

This is a great pedal- It can do anything from smooth pretty EJ style leads to Incubus or Opeth style crunch depending on settings. I use it with a Boss OC-2 occasionally for 311 covers- very cool. :D

I know some engineers that swear by this pedal set for light grit into a DI for bass tones. I haven't done it recently but I recall it being very cool.

Anyway, I'm happy to share what I've learned. Go solder some stuff!

Jamie
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Re: SIB Varidrive

Postby imaradiostar » 07 Mar 2013, 05:57

Just noticed I got the 470k thing wrong on my prototype, I'll have to fix it and get back to you about the tonal difference. It'll double the signal getting to the second triode as well as making the clipping of the second triode much sharper/crunchier.

It could also explain the excess top end on my prototype- 470k is one heck of a grid stopper and should cause a pretty substantial treble rolloff. It'll be down -3dB around 3k on a 12ax7! It won't be quite as bad on the 6112 with its lower gain and slightly lower input capacitance, but I'm sure it'll sound better with the 470k in the right place.

Must fix.

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Re: SIB Varidrive

Postby Smoky Black » 08 Mar 2013, 16:39

Yes, thanks for adding those clarifications. I took the pots out the other day and noticed that I it was 100k output put and some of your other corrections.

Honestly, this pedal was not doing it for me, sounded too much like a pedal and not like an amp. I really attribute it to the weird tonestack. I tried a few different mods. I tried doing the second tube gainstage as half a phase inverter, bootstrapped and all, but that didn't sound very good. I decided to bias each stage a little colder to get earlier clipping and less output so I could remove more voltage dividers. I used a high plate dumble setup, 220k plate/3.3k cathode into 150k plate/2.2k cathode. I've got a 39k grid resistor on the first and a 180k on the second, 68k grid to ground on the second instead of 470k. I removed the .022/470k network before the first tube stage and the .1/270k/270k after the second stage, and the .001 snubber on the 220k. At this point there was a lot of gain and it became unstable, so I put a 200pf cap from plate to cathode on the first tube stage to get some local feedback.

I redid the tonestack to more of a marshall type stack, 56k slope, 500 pf treble cap, .022 bass and mid caps, 250k/25k/500k treble/mid/bass pots. Now it's pretty amplike and it's very easily controlled by the guitar volume. I think the jfet stage might be a bit bright. What do you suggest? I was thinking of replacing it with a 6AV6, actually.

I'm experiencing a bit of brightness/lack of bass, too. When I get a chance I may listen to just the sound coming off the FET. My initial thought is to increase the 4.7mf cap on it to something much bigger. I already increased the coupling cap to .1.
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Re: SIB Varidrive

Postby devastator » 10 Mar 2013, 13:14

Big thanks for the schematic and informations !

looks really nice circuit , we should also add a Mosfet buffer at the end to drive long cable too. The tonestack seems to be a Fender one with a bigger treble cap (1n). It must cut off a lot of mids , right ?

I'd like to know the way they did their Echodrive blue and red. I think it's nothing nothing new but should be interesting to see .

I should try those subminiature tube one day.

I'm experiencing a bit of brightness/lack of bass, too. When I get a chance I may listen to just the sound coming off the FET. My initial thought is to increase the 4.7mf cap on it to something much bigger. I already increased the coupling cap to .1.


my experience is that tonestack as we know them give their own charactere to the sound. They are filters so they modify what's coming into. You can remove it and try to filter the sound with simple RC filter in the signal path. Works well ,but with less variations on the sound, if you like "raw" tube sound.
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Re: SIB Varidrive

Postby Smoky Black » 10 Mar 2013, 22:49

Ok, I think the source of the harsh top end was the biasing of the fet. I dropped the 10k on the drain to a resistor that was around 7.9k. That helped dial out the harshness a lot. I also upped some of the couplers even more. I'm thinking those xicon MPP caps might have something to do with the treble quality which I didn't like. They're mostly gone now with all the mods I've done. Whereas before, I still preferred my DIY vero OCD (thanks FSB!) to the stock varidrive, now the varidrive is sounding extremely bad ass.
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Re: SIB Varidrive

Postby imaradiostar » 14 Mar 2013, 07:13

It's funny how different people like different things. My cousin is a studio engineer in Nashville and he said there were 4 Varidrives at a session he did last week- his and the ones brought by the artists. He's sold quite a few people on buying Varidrives after they used his at a session.

If you don't mind my asking, what kind of amp and guitar are you using it with?

There are a lot of ways to go with the Varidrive mods. At one point I was kinda over it so I modded the tone stack to marshall values and played with some stuff. I later decided that wasn't working and put it back to stock. I realized then that the strength of the crazy tone circuit values is versatility. I could plug that pedal into any amp with guitar or bass and get some really great sounds happening with a bit of tweaking.

I'm still torn on my "high gain" varidrive mods. It cleans up really nicely but it's a little over the top. With only two stages distorting it doesn't quite have that Dual Rect crunch and it loses a little of the thick almost big muff quality it had stock at higher gain settings. I have to experiment some more when I get the time. I might try adding some more caps to kill the highs or possibly another big grid stopper like the 2nd stage in a Dual Rectifier preamp- once again, this could make for a big treble rolloff.

I do know, from building some amps with two stages distorting like the varidrive, that you can mod this thing to do a lot of cool sounds. As for "amp likeness"- I don't know what to make of statements like that. I feel like it's a very subjective thing- more widely varied than amps are in the first place since the amp becomes a part of the sound of the pedal. I guess that's a discussion for another thread. I guess clipping diodes sound like a JCM900, so a lot of pedals can be "amp like" without even having tubes.

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Re: SIB Varidrive

Postby Smoky Black » 14 Mar 2013, 18:28

I've been using the Varidrive with a DIY JTM45 type amp with an early seventies 1960B loaded with Blackback G12M's and a DIY AC30 type amp (top boost and EF86 channels) with a 2x12 loaded with Celestion Golds. Been playin' strats with it!

My expectation of the varidrive was that I'd get all the distortion at low levels from the pedal. I think the intent of the varidrive may be to build a better tube screamer, and I'm just not a tube screamer type. I think the varidrive has those thick, opaque tube screamer like mids, whereas I'm looking for transparent tube-amp like mids. It really seems to be that tone stack. I feel the same way about the tube driver; it's got the opamp and that weird tonestack which just make it "pedal-like."

I did do a couple more mods, decreased the slope resistor to 30k from 56k which kind of brought it back a bit towards the stock voicing, bringing in more midrange. Upped the grid resistor on the first tube stage to 70k from 33k, to get rid of some blattyness which I attributed to a bit of blocking distortion. I find the marshall stack to be more flexible, at least with my amps. With the stock tonestack, the mid and treble seemed to almost do the same thing. With the 25k mid pot from the marshall stack, as you increase it, you're really moving towards flat, and the treble and bass pots become somewhat less effective, which I like.

I'll see if I can post some clips, it's definitely closer to what I'm looking for. I'm going to remove the appr 100k resistor into the volume pot and put a voltage divider after the pot to see if I can get a more usable volume range. It's a little hot now on the output level.
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Re: SIB Varidrive

Postby Smoky Black » 19 Mar 2013, 04:28

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Re: SIB Varidrive

Postby snofla1900 » 19 Mar 2013, 16:14

Sounds very good to me ! What strat did you use ( pickups ) ?
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Re: SIB Varidrive

Postby Smoky Black » 19 Mar 2013, 17:19

snofla1900 wrote:Sounds very good to me ! What strat did you use ( pickups ) ?


Thanks, man! Tons of slop in there but, whatever.

It was a parts strat, all parts sro62 neck, the laminated rosewood style, and an old alder body from the eighties that a guy from San francisco made in his basement. The bridge pickup is a Suhr FL standard with a Fralin base plate, the middle is a Suhr FL standard, and the neck is an early 90's Van Zandt I found on EBay that he wound for a particular guy's 64 strat.

DIY AC30 top boost, celestion gold, bit of timeline dbucket delay in there, too.

Final mods on the varidrive were removing the 100k or so resistor into the volume pot and putting a 470k/56k voltage divider after the volume pot along with a .002mf + 39k across the 56k to cut the top.
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Re: SIB Varidrive

Postby milamter » 11 Jul 2019, 21:10

Hi, I have this schematic, which capacitor goes on the drive pot ? any advise on building this is much appreciated :) and there is a triangle symbol on the tube that does not appear on the schematic, where should it be connected to?? i a bit scared about the power supply, I don't want to fry the tube or myself. thanks !!
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