1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
     
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

Vintage SVT effects loop?

Discussion in 'Amps and Cabs [BG]' started by boroman, Aug 23, 2012.


  1. boroman

    boroman

    Apr 27, 2009
    boro
    I had problems with keeping the volume down with my MTI SVT on almost all shows. Almost all the time "too loud on stage" and I was like on "1" on the scale...

    One time someone suggested inserting volume pot between preamp out and power amp-in, so this will behave like real master volume. A friend made my that passive box with volume control. Works great! But I have some questions: I dont understand how it works in the amps which have no master volume. So when you turn up the volume the preamp volume increases and power amp volume increases too at the same time? And amps with master volume are like volume1=increasing preamp volume, master.vol.=increasing power amp power? So what I'm really doing with the volume pot inserted between preamp and power amp is turning down the preamp, but the power amp with no change (dependent by the volume pot on the channel I am playing)?

    Just wondering
     
  2. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    A master volume is typically just a passive vol pot at the end of the preamp stage, before the power section. Putting a vol pot in the effects loop is the same thing.

    An amp with no master vol knob just doesn't have that attenuator after the preamp.

    In all cases the power section is on full blast at all times. The master is not regulating the power section directly, it is just regulating how strong of a signal goes into that power section.
     
  3. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    You're actually adjusting how much of your preamp signal you are allowing the power amp to see and therefore amplify.
     
  4. 96tbird

    96tbird PLEASE STAND BY Supporting Member

    Man this interests me. I have an old v4b. So to clarify the stock vol knob controls the pre level and the power section is 100% all the time?

    So I make a box with a pot (or get a vol pedal), plug it into the effects loop and viola: master volume mod without altering the chassis?

    So I can overdrive the pre with the stock vol and control output with the new master vol "pedal"?

    What value pot do I need? Oh boy, oh boy! (yeah I'm dumb about electronics!)
     
  5. boroman

    boroman

    Apr 27, 2009
    boro
    Is that really true? :eek:
    I cant overdrive channel in SVT even when it is on maximum "10" (and the effects loop volume turned down to listenable limit). Is that because of construction of these amps?

    Search ebay for "volume box". These are cheap. But you can build yourself. This is a passive box so it does not need power supply. And yes, this is the "mod" without drilling and modifying the amp.
     
  6. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    The way I said it is oversimplified and semantically arguable, but yes it's true. The amount of wattage a power amp puts out is directly proportional to the level of the signal you feed into it. The master knob does affect the overall volume, but the power section could put out its full rated wattage even with the master knob only 10% up for example, depending on how high the signal levels are before the master. So the power amp has the potential to be at full blast at all knob settings above zero.
    An all-tube amp like the SVT gets its overdrive from both the preamp and the power amp. You have to drive both of them hard to get the classic SVT distortion. That means not turning down the volume. High volume, loud. You can get half the sound at lower volume by driving the preamp extra hard and then turning down the master; you may need a boost or OD pedal to drive the preamp hard enough.
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    A vintage SVT power amp requires 0.25V to drive it to 300W output. That would be like your gas pedal to the floor. Less gas and the car drives slower. Less voltage and the amp puts out less wattage.

    The volume box is like a governor that limits the speed of the engine. It limits the pre-amp output voltage so the power amp can't be driven to full power. You can overdrive the pre-amp with the amp's volume control, then limit the output with the volume box.
     
  8. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    No, I'm sorry but that's off. An engine governor prevents a car from going over a certain speed (say, 60 mph); but the master volume does not and cannot prevent the power amp from going over a certain amount of wattage (say, 100 W). The power section absolutely CAN be driven to full power even if the master knob is down partway. All that's required is to send a stronger signal into the preamp.

    The volume knob is more like a car's brake pedal, in that it does reduce your speed, but it doesn't have a direct relationship to the specific speed. The speed you slow down to depends on how fast you are going at the time you press on the brake. With the brake pedal down 50% of its travel, you could still be going over 60 mph, if for example you had been careening downhill at 90 mph beforehand.
     
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Replace governor with "wife that keeps you in check by reminding you how fast you are driving".


    [​IMG]

     
  10. lburton2

    lburton2 Les Is More

    May 15, 2008
    Detroit, MI
    I've seen this happen with rigs i've played through in the past, but didn't quite understand why.. At least I know that's how it's meant to be
     
  11. This is quite possibly the easiest stompbox in the world to build. Only takes two jacks, a treble bypass cap, and a volume pot. If you really wanted to get technical you could add an indicator LED, battery, and stomp switch to pull it in/out of the effects loop. If you want to avoid the stomp switch you can just use a very large knob and control it with your foot- turn it all the way to the right to allow full signal to the power amp.

    The treble bypass cap is important- when you turn down the pot you will lose treble, just like when you roll off your volume knob on your bass.
     
  12. 96tbird

    96tbird PLEASE STAND BY Supporting Member

    Like a 500k pot? 250k?
     
  13. spankdaplank

    spankdaplank

    Jan 19, 2003
    Philippines
    Back in the days when I was playing full time(was it really 35 years ago!??) I had the same problem with my '72 SVT. Playing in the small clubs with the volume on less than three resulted in the tone and "bounce " being sucked out. Without thinking about it too hard , I borrowed the guitarist's passive DeArmond volume pedal and inserted it between the pre out and power in jacks. I have no idea of the ohm value of the pot inside. It worked really well, as I remember. There was still some loss of tone from not driving the power section as hard, but my basic signature sound was restored.
    An added benefit in using the volume pedal was, well, I could adjust my volume on the fly while more or less keeping the tone the same.
     
  14. BogeyBass

    BogeyBass

    Sep 14, 2010
    I dont think the pre out and ext amp in work like a common effects loop. they are tied to the same place and dont actually break the internal preamp like a effect loop does.

    if it does you would use a 500k or 1meg pot.

    the guitar v4 you could overdrive the pre a little, it only overdrives one of the tube stages and it really didnt sound very sweet. very squishy and wierd sounding. dont get me wrong sounded great for bass because you basically turned it all the way up and that distortion was good. but the preamp overdrive was bad. tried preamp distortion for guitar and bass, guitar needed a pedal for sure. amp is clean as heck for guitar
    and again even with guitar the preamp overdrive just sounded wierd and squishy.

    otherwise with the op passive volume control can work, no different than using the bass volume control, so you can get tone sucking using the passive pedal. this would be just going straight into the amp input. so somebody might be able to recommend a active pedal, like a simple EQ or something that has volume control. this way you could dim the volume going in without causing tone losses. basically a active volume control.

    otherwise master volume yes just controls the signal level going to the power section just like the channel volume would.
    But master volume comes later in the chain. So you can turn the channel volume, or on some amps gain knob. To overdrive the preamp. Basically sending to much signal into the preamp to get distortion. Of course all this extra gain needed to get distortion. Would cause the power section to go full blast as well. So the master volume is later in the chain to now turn down the signal to control the poweramp volume, but still retain the wanted distortion from overdriven preamp stages before the master.
     
  15. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Why not just leave it at 1? That's what I do when I use one. Sounds pretty awesome to me.
     
  16. 96tbird

    96tbird PLEASE STAND BY Supporting Member

    You mean 1 o'clock?
     
  17. anderbass

    anderbass

    Dec 20, 2005
    Phoenix. Az.
    I've been gigging my sansamp VT pedal into my 72-SVT and early SVT-II non pro, master-volume heads for quite awhile now. I know it might seem like sacrilege but this freakin little pedal truly gives me that "cranked-SVT" tone and feel at any volume.

    Using this pedal, I have absolutely no need or desire for separate pre & master volume knobs. Before I used the pedal, I did and was constantly trying to play my SVT's too loud trying to achieve that sweet spot.

    Before I had the VT-pedal, I added a SVT-CL to my arsenal for a while in hopes of it solving my lower-volume gig woes, but It never gave me that magic mojo I get from my older master volume SVT's so I sold it off.

    At outdoor/large-venue gigs where I can crank my SVT's volume knob up to the 11 o'clock range I dont miss my VT-pedal at all, at lower volume gigs I get that same tone and most importantly FEEL by simply adding my VT-pedal...

    No chit, that's one amazing little pedal for solving the "SVT at lower volume" dilemma.
     
  18. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I thought he was talking about volume at 1 out of 10. 1 o'clock would mean serious cranking.
     
  19. 96tbird

    96tbird PLEASE STAND BY Supporting Member

    I thought so, I was "JimmyM drives his v4b at 1 o'clock? That's gonna give the world a headache an' 'splode some drivers!!
     
  20. boroman

    boroman

    Apr 27, 2009
    boro
    The main problem is that my pot behaves like it has no sound when turned from 0-0,5 on the scale, and then from 0,5-1 it like logarithimic x2 ;) so hard to dial good volume because even a slightest change on the pot is dramatic in volume (happens sometimes when you play loud and the pot is shaking because of high volume and its turning down).

    Yesterday I recorded with the volume box into the effects loop and (as suggested here) a pedal booster (Zvex Super Hard On). AWESOME.

    My volume box does noit have any treble bypass cap. I haven't heard any treble loss even at attenuating a lot.

    BTW, Is this a good idea trying to put the booster on the effects loop to distort power stage a little?
     

Share This Page