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tube guys: svt chop up Q

Discussion in 'Amps and Cabs [BG]' started by staindbass, May 15, 2011.


  1. staindbass

    staindbass

    Jun 9, 2008
    hi, not really chopping it up, i just want to try a vintage blueline preamp through crown modern amps. on the schematic, (thanks dr tube !) pins 1&4 are the output, it looks like i only need 6.3v to the power the filimants on 7&8 , but on pin #5 i cannot tell what it needs. my only guess it is plate voltage, i could measure with a multimeter, but i dont know if its ac or dc. am i missing anything else to make this work ? thanks, johnny a.
     
  2. staindbass

    staindbass

    Jun 9, 2008
  3. If you don't know whether the B+ voltage is AC or DC I'm not sure you should be attempting this.

    Yes you will need 6.3V AC at a couple of amps for the heater supply. The B+ should be in the range of 300V DC at about 100mA and very well smoothed. That will get the pre-amp section working.

    There's a gotcha, (isn't there always?) and that is the output impedance from the 6C4. It's way too high to properly drive a SS power amp. The 100n output cap will need to be much larger. I would suggest driving an output transformer - I'm sure Jensen will have a suitable one - with a primary impedance of 100KΩ and a secondary impedance of 600Ω. That will take care of the impedance mismatch. The next question is whether the output will have sufficient voltage to drive a power amp to full output.
     
  4. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    Are you saying that you want a preamplifier out signal from the head to drive your power amps? Why not just pick up an SVP-CL preamp instead? To do what I think you want to do with that head, I would install a quarter inch jack and pot to control the level and then tap the signal at the appropriate spot. Everything is already powered asuming you are starting with a working head so you don't need additional transformers. If you are wanting to pull the preamp out of it, that is a whole different ball of wax and you will need to build a custom power supply for it, which I wouldn't recommend. I may be misunderstanding your intent though. You could also use something like the Countryman 85 to steal a signal from the speaker out, which would also capture the tone of the power section as well, which is where a lot of the magic is. You would need to keep the head hooked to a cab though. Suhr/CAA also makes a box that will do this. If you go this route, stick to a cab as the load as the attenuators/load boxes really change the response.
     
  5. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    The .1uF is setting the low frequency attenuation so I don't see any reason to make it larger. That won't help drive the power amp, it will just change the tone. The cathode follower has a low output impedance so I think that is the best bet for driving the power amp - works fine with a 12ax7 anyway, but not sure how the 6C4 would compare... but there should be plenty of gain available in that circuit. I would take out the 100K and replace with a 1 or 2.2M so you aren't dumping all that signal level to ground, put a 1 Meg pot right before that stage in place of the 1 Meg resistor, and give that a whirl. Actually, I wouldn't do any of this to a vintage SVT, but hey... :)
     
  6. Spencer!

    Spencer!

    Jun 25, 2006
    Seattle
    Owner, Pike Amplification & 3Leaf Audio
    If you just want to try it, why not take the signal from the preamp out jack on the back?
     
  7. It will cut the low frequencies way too much when working into the 10-20KΩ input impedance of a SS power amp. You must have considered this in your own pre-amp design.

    Per your last sentence (not quoted) I wouldn't do this either.
     
  8. IntrepidCellist

    IntrepidCellist

    Sep 10, 2009
    Manhattan
    It would be cheaper, simpler, and safer to get an SVP-CL or one of JGR's preamps.
     
  9. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    I did indeed and I have not found it to be an issue. If there is any problem with driving the poweramp, you will see it with a loss of high end, not low end. There are so many rolloff points in a preamp that a single one, unless set to the extreme, will not make a huge difference. It all works out to a cumulative sum which is where it can get tricky getting everything to work in a complimentary fashion. IMO/IME, a lot of preamp designs, both for guitar and bass, suffer from having too large a value of coupling caps/cathode bypass caps which leads to mud. Full frequency response (both low and high) in a preamp is not necessarily a good thing, though the highs are more of an issue in high gain designs since they contribute to buzzy tone and oscillations. You can't really generalize since every preamp is different, and this is where some of the art of voicing an amp comes into play. All that said, it is a point to consider, and may likely take some trial and error to get correct.
     
  10. staindbass

    staindbass

    Jun 9, 2008
    thanks bassman paul, all good info. actually the signal would go thru an avalon 737sp before hitting the crowns, with my luck the mismatch will stilll apply... the preamp has line, mic and instrument inputs, i will check to see the different impedances. whats the impedance of the preamp after the 6c4, 100k ohms? johnny cash, the circuit in the svp-cl is not the same is it? i havent seen all of them but most of the new ones use 12ax7s and 12au7s. good but not the same. yes i wanted to use it as a separate preamp, without toting the 80lb powersupply around. (4 flights up the stairs to the rehearsal space)nothing would be hacked, its together with connectors, no wiring to unsolder, and can be put back in the amp when im done trying it. i guess like spencer suggested i could try the preamp output if the powersupply is too difficult. i wouldent want to mod anything inside, just use it as it is. the avalon is great, but the blueline has some character that the reissues and avalon dont have. thanks for the advice. johnny a.
     
  11. staindbass

    staindbass

    Jun 9, 2008
    input impedance : mic 850/2500 transformer balanced, line 20k ohms and instrument 1 meg ohm.
     
  12. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    I have this rig; it is very nice but does not sound like an SVT either DI or through a power amp; had the same experiences with a 3 Pro.

    A Countryman can take the speaker output from a SS amp and convert to line level; can you do this with a tube amp?
    Never tried but think this might be worth a quick try if it doesn't hurt the amp.

    Have also used a 300 watt ballast resistor on the output of a V4B and then took the preamp to a DI; this sounded great; never tried this with an SVT though
     
  13. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    hey johnny! jerrold tiers, who used to be chief engineer for ampeg in the slm days, says the svp-cl is based on the original svt preamp, and to my ears, it does sound an awful lot like it, but i think two things will be deal breakers for you. first, it's got separate gain and master. and second, it's a single channel pre, so if you're wanting to chop your blue line to use both channels together as you talked about before, you're out of luck.

    if i were you, rather than chop your beast, maybe i'd talk to a custom builder about getting an exact duplicate of the two svt preamps in a box. it will likely cost out the wazoo, but what good is playing in a band like staind if you can't spend way too much money on custom gear made to your exacting specs? ;)

    btw, 4 flights of stairs? holy cow!
     
  14. Just for the sack of sharing information, the 6C4 is the same a half a 12AU7.
     
  15. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    wow, you'd think they wouldn't all be microphonic in that case.
     
  16. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Hey Jimmy;
    Do I remember that you took the speaker out of an SVT through a Countryman?

    Back the the topic; I just can't get my SVP-CL to sound like a mic'd SVT in the studio.
    Not that you'd expect that to be the case but me thinks that this is what the OP is looking for especially if an overdriven tone is the goal.
    John K's clips did seem to show this to be the budget pre that kicks through a console.
     
  17. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    i did indeed, jim, but that involves the power section, which johnny is trying to avoid.

    also, considering that most of the time, he goes for super clean sounds, i don't know that his goal is overdrive here. could be wrong about that, but just going by what he's done in the past.

    ordinarily, i'd suggest something like the jule amps monique, but considering his last thread about the svt, i don't think anything's going to satisfy him except the exact two pres in his blue line.
     
  18. staindbass

    staindbass

    Jun 9, 2008
    i agree jimmy, but half the fun is doing it myself lol. if i get the other preamps mentioned i will always wonder what a "real" one would sound like. soundmen before have told me "no matter how highend the gear is onstage it doesnt fully translate through the front of house and no one will probably notice" and i reply , yes im aware of that. its purely for personal enjoyment onstage". but like i said its not really chopping it, it will remain unharmed. hi jim c, no we use a di right from the bass, then a di after the avalon with slight eq and compression. a mic on the 18 and horn. i think the 6c4 might be more microphinic due to just one plate. the 12ax7's 2 plates may damp each others vibration. the 6c4's i have i looked at seem to have tilted micas, the micas are not held level or flat but on the 12ax7 they will hold everything level because of the 4 point suspension. the 6c4 micas do not touch the glass evenly all the way around the glass. maybe thats why. i have never seen that jonny k clip, i like a touch of grit sometimes but mostly clean.
     
  19. elgranluis

    elgranluis

    Feb 14, 2003
    El paso, TX
    Im with jimmy here. Id go for the custom stuff or getting some sort of technician to work out the schematic (voltages, loads, etc) for you (since you want to go DIY).

    As jimmy says, there is no way around the single channel that the cl has. You can easily get rid of the master gain pot, no problems so basically you would need a different starting point.
     
  20. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    never really studied the looks of the 6c4 before, but it appears you are right on all counts, johnny. the jan philips 6c4wa looks more rugged, but apparently they have the same problems as well. oh well, not a biggie to me...mine still sounds great, but jerrold says that's why they ended up getting rid of it after the skunkworks heads.

    anyway, good luck with it...it sounds like a lot of fun. and you're right...as meticulous as you are, it'll always be a "what if?" scenario if you don't use that exact preamp. and also, all those soundmen who say "you won't notice it on a gig?" f them! let's all play behringer amps if that's the case!
     

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