Late 70's Fender SF - Pull boost feature

Discussion in 'Amp Tech Center' started by Stax1, Mar 29, 2020.

  1. Stax1

    Stax1 Tele-Meister

    Age:
    41
    Posts:
    134
    Joined:
    Apr 24, 2019
    Location:
    Over here
    Hi guys, thought I'd post this question up for the learned bunch that may have some technical knowledge of the late 70's Fender SF amps that have the super fancy 'pull boost' master volume.

    I have a late 70's Pro Reverb that has this and while I can see why lots of folks dont like it, I can find some pretty unique tones out of it. To me it sounds a lot like its trying to create a mid frequency boost, for cutting solos through a mix etc..

    Anyway, can anyone who has some technical understanding explain what the feature is doing in lay terms?

    Following - could this feature be modded to be more useful to more folks? ie, mod it to boost all frequencies? or be a treble boost?

    Also - what would removing it from the circuit do? Nothing..?

    Maybe the pull feature could be modded to remove the tone stack?
     
  2. lil scotty

    lil scotty Tele-Meister

    Posts:
    347
    Joined:
    Jun 29, 2016
    Location:
    Phoenix, AZ
    You will get some good answers shortly, I'm sure. I do believe that removing it from the circuit will do nothing. My understanding is that when knob is not pulled, it's just like it's non push/pull counterparts.
     
    Stax1 likes this.
  3. Dacious

    Dacious Poster Extraordinaire

    Posts:
    8,356
    Joined:
    Mar 16, 2003
    Location:
    Godzone
    The pull boost diverts the reverb drive signal and mixes it back into the dry as increased gain. One problem is the driven tone off the 12AT7 is put through a small value cap to stop bass flub. The distorted signal is a bit bright and thin and fizzy. The other problem is there's no way of altering the mix level independent of volume.

    Pulling the circuit might give a bit more channel gain in theory. It might do nothing. You could use the switch and jack for something else like a midboost.

    boost.gif
     
    Last edited: Mar 29, 2020
    Fretting out likes this.
  4. Stax1

    Stax1 Tele-Meister

    Age:
    41
    Posts:
    134
    Joined:
    Apr 24, 2019
    Location:
    Over here
    Ah, that would explain why it sounds a bit 'nasal' at some settings. it also explains (I think) why it lessens the amount of reverb in the tone - eg, pull the boost and the sound becomes a little louder overall, but the amount of reverb in the mix is the same is it was before pulling the switch, which has the effect of sounding like there's less.

    So if we were to change the value of that cap that it goes through (or remove it), would that make it sound fuller?
     
  5. Dacious

    Dacious Poster Extraordinaire

    Posts:
    8,356
    Joined:
    Mar 16, 2003
    Location:
    Godzone
    I had three amps with this feature - 70 watt Pro, 40 watt Vibrolux Reverb and Princeton Reverb, and it sounded like doodoo with all.

    It actually cuts reverb off, what you're hearing is the clean from the the vibrato channel injected with the same signal through tube gain stages. There's no mix or depth control to it. It's like early attempts by makers to add drive. Just not right sounds and not adjustable enough.
     
  6. Stax1

    Stax1 Tele-Meister

    Age:
    41
    Posts:
    134
    Joined:
    Apr 24, 2019
    Location:
    Over here
    Yeah its not a drive that inspires a lot.. My amp is a 70w Pro as well. I've been wanting to get it into a rehearsal so I can test the pull boost at volume and see if it helps it cut through a band mix or if it just comes out sounding anemic...

    I can get good tones using it, but it means adjusting other things B/M/T and channel volume. Its just a bit weird..
     
  7. tubelectron

    tubelectron Tele-Afflicted

    Posts:
    1,153
    Joined:
    Nov 25, 2010
    Location:
    France
    Yes - look at the tone-stack of the Boogie MKIIA below :

    [​IMG]

    It is a good volume-mid boost, and if you want to boost the treble also , remove the 5nF cap in // with the 22K. You can also increase or minor that 22K depending on the effect you want, as you can also play with the 5nF cap value...

    I have done this simple trick to several of my amps with success (SF DR, SF PR). I find it more usable on stage than the original "through reverb" boost circuit there was on these amps.

    -tbln
     
  8. Marcusin

    Marcusin TDPRI Member

    Age:
    57
    Posts:
    15
    Joined:
    Feb 28, 2020
    Location:
    Biloxi Ms
     
  9. Marcusin

    Marcusin TDPRI Member

    Age:
    57
    Posts:
    15
    Joined:
    Feb 28, 2020
    Location:
    Biloxi Ms
    Ive been doing this from when I was 17 the amp on the vid was my bandmates from the 80's im now 58 on my SF 's I put a active loop in using the tremolo circuit's 12Ax7 this plug uses the reverb circuit so the reverb knob is now overdrive, the old reverb tank if shaken sounds like a garbage truck to me, that's why I get rid of that tremolo circuit too in place of a good loop, I heard many experts, Tech's , guru's say bad things about The Pull Boost Master Volume Fender amp's But if you make that plug which if you dont like it pull it out you will find that your silver SF is a world class amp. now they need a closed back cab. vol 10 treb 4 mid 5 bass 3 reverb 10 master vol pulled and no more than 4 brite sw off plug in tremolo ch 1 make some real music THAT THE SF TRUTH ! 120k ohm 1/2 watt resistor ...
     
    Last edited: Apr 1, 2020
  10. Telecasterless

    Telecasterless Friend of Leo's

    Posts:
    2,252
    Joined:
    Jan 29, 2011
    Location:
    los angeles
    Well, it is April 1st after all.
     
  11. Stax1

    Stax1 Tele-Meister

    Age:
    41
    Posts:
    134
    Joined:
    Apr 24, 2019
    Location:
    Over here
    So was that just a late 70's amp with that reverb circuit jumped..? Sounded amazingly dirty for a Fender.. Then again, I've not really cranked mine up that much... Too f'n loud!
     
  12. SoK66

    SoK66 Tele-Afflicted

    Posts:
    1,325
    Joined:
    Oct 2, 2007
    Location:
    Four Corners USA
    The pull boost thing can be safely ignored by not activating it, as noted. A couple things, the volume pots are linear, like a real mid-late 50s Bassman and most other large tweeds (the schematics are wrong), which means you're going to get a lot more volume, sooner. Also, going from memory, they continue something Fender started in '76 IIRC, lowering the cathode resistor value and eliminating the bypass cap on the reverb driver. This drives the gain higher on the tube. It stresses the reverb transformer and eventually they'll fail, giving a horrid, arcing and unmusical distortion on lower notes. When I've seen this on ones where I eliminate the pull boost I revert the reverb driver circuit to the earlier BF/SF spec.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.