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Ampeg SVT test - 69'er vs 78'er SVT

Discussion in 'Amps and Cabs [BG]' started by jvdb, Oct 16, 2009.


  1. jvdb

    jvdb

    Jul 26, 2006
    I had two sets put up. An SVT Blue Line with a straight back from '69 or '70 (etched lines) and my 78 SVT with my 78 towel bar.

    The Blue line stack. Well conserved, recently retubed (not sure about the caps) and the straight back.

    VS

    My 78 stack with towel bar. Also recently retubed and recapped.


    First I tried my 78'er. Sounded OK. Then switched over to the Blue Line stack. This one sounded FANTASTIC! Tighter, louder, warmer, rounder. So much better. So at first I thought I was getting sick... was this my SVT I loved so much. I was thinking of selling mine and buying the Blue Line.

    Thank God my guitar tech came along and adjusted the bias. Now the two heads had a comparable bias. WOW, world of difference! Now my head sounded even better than the Blue Line. As loud, as warm but tighter!

    After that I could start the 8x10 A/B test. The straight back IS the better sounding cab. Louder, rounder, punchier. Everything the 78'er has but better.

    A week after that, I played a show with a guy who normally uses an 06 Ampeg 8x10. This time he used my 78'er. He was blown away. So much tighter, clearer and punchier. Hahaha!


    I'm now thinking of buying that straight back. Test drive the three 8x10's (I also have a 73'er which I couldn't test, a 78'er and the straight back) and sell one of them. The 'worst' sounding that is.

    Conclusion: cabs make a difference. A lot!

    But what struck me the most was the whole bias thing. Mine was biased at 75 and the Blue Line at 60. Gonna do some more testing with this.
     
  2. you were cooking your valves running them that hot!
     
  3. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    i don't think so. the SVT's spec is to run each side at 72mV so +3mV isn't going to make much difference. and I definitely wouldn't run them at 60mV, as it'll have alot less headroom.
     
  4. metron

    metron

    Sep 12, 2003
    Denver
    For those with SVT bias experience - how reliable/accurate are the pots and test points on the front of the chassis? I have heard that the best way to bias is to use a scope but if the test points are equal at 72 mV then is that good enough? I have a multimeter but no scope at home.
     
  5. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    the system that is built into the amp has ALWAYS worked perfect for me. i have a scope, frequency generator, distortion meters etc, but have never needed them to set the bias on an SVT. just a good DVM is all that is required. I use a Wavetech or a Fluke. however, in order to set the output driver balance (phase inverter), you should use a freq generator and a dummy load with a DVM.
     
  6. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Hey Jaap, did the blue line have 6146B's or was it retrofitted for 6550's?
     
  7. jvdb

    jvdb

    Jul 26, 2006
    @Jimmy: The Blue Line is fixed to run on 6550's.

    @ John: So I'm not frying/freezing my tubes at 60?

    And less headroom? The thing is twice as loud with the bias set at 60. And the tone is 10 times better. Warmer, rounder, more pressure.... way better!
     
  8. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    OK, then I'm not upset your head sounds better anymore ;)
     
  9. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    I always set mine to pull the factory spec's bias, which is 72mV per side. 60mV per side would be running them a bit too cool (bias voltage too high) and would yield less headroom.
     
  10. jvdb

    jvdb

    Jul 26, 2006
    Thanks... like I said... gotta do some more testing. Used it last night and tonight and it sounded awesome.
     

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