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70s SVT Bench Test Results

Discussion in 'Amps and Cabs [BG]' started by Mark Reccord, Feb 3, 2017.

  1. Mark Reccord

    Mark Reccord Supporting Member

    So after some discussion in this thread about the frequency response of the SVT I ran a set of measurements on my mid-70s model last night. The impetus for this was that I had done a basic frequency response sweep a while back and in the thread referenced above, @JimmyM said that Bass Gear Magazine had published a suite of measurement on @tombowlus 's SVT a few years back. My recollection was that my measurement of the basic EQ flat frequency response did not match BGM's, but I wasn't 100% sure so I redid them last night. The tl;dr of it is that the pronounced bump between 100 and 400Hz they measured on Tom's doesn't show up on mine. There is a very slight bump on mine but it's around 0.6dB max, and I'd bet that it's because the bass pot isn't perfectly at flat even if the indicator shows it being at 12:00. Of course, now the trick is to figure out why they differ. In the interest of determining which amp is the outlier, I am going to get my hands on at least one more of them and run the same tests.

    A few notes on my methodology: I don't have pics because by the time I thought about it I had the test setup torn down. I am planning to check the frequency response of the power amp alone this evening so I will get some snaps then.

    I used ARTA, which is a free RTA/spectrum analyzer, to capture the data via my recording interface. I've used the software many times and have verified that it's reasonably accurate with my oscilloscope. The software/interface cannot handle speaker level signals so I used a pair of resistors to divide the amp's output voltage by 10 so that it wouldn't overload the analyzer. I also ended up having to transformer isolate the signal because there was a ground loop between the amp and the interface that resulted in a bunch of spurious noise. I used a very high quality 1:1 transformer for this (Lundahl LL1570 if anyone's interested) and it is essentially transparent for this test. Its frequency response is much wider than the amp's is and it can handle +16dBu. The amp would have to be producing 63V at its outputs to overload the transformer after the voltage divider. I connected my dummy load to the amp's speaker out (2x225W wirewound resistors in parallel for 4 Ohms) and tapped the voltage divider resistors off that. Measurements were taken at 10V output (approx 25W) but I did check the frequency response at power levels from about 1W up to 300W and there isn't much difference.

    So now the screen captures:

    Channel one with all EQ knobs at 12:00 and the ultra buttons both out. It's pretty flat with rolloffs beginning around 50Hz and 7k. There is a very slight bump in the low end range but it's only about 0.6dB.

    Channel One, EQ flat Ultra Lo in the + position, showing the deep mid cut it creates.

    Channel One, EQ flat, Ultra Lo in the - position.

    Channel One, Ultra Lo at 0, Ultra Hi +. This was pretty weird, that's a lot of boost. I had to roll the level back considerably to sensibly capture it.

    Channel 2 was quite different. Quite rolled off in the low end with the EQ flat and Ultra Lo off.

    Channel 2 with the Ultra Lo on.

    And finally channel 2 with Ultra Lo off and Ultra Hi on.

    I didn't manage to get captures all of the EQ settings but they look very much like the ones BGM did so I didn't think it was necessary.

    I also did an output test and found that I couldn't get much more than 250W out of it before THD went beyond 5%. I'm not sure whether this is something a little off inside the amp or if my spectrum analyzer is not very accurate. More on it later.
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Interesting Mark.

    The PEC tone stage modules used in many Ampeg amps are often out of spec. They are a network of carbon composition resistors and ceramic capacitors. Resistors are typically beyond +/-20% for example. If you redo the test on the preamp only, it would be interesting to perform a number of sweeps, dialing the tone controls to see how flat you can get it.

    More details on the tone module here: Technical - Amplifier | TalkBass.com.
    Tbone76 and bobyoung53 like this.
  3. Agreed, good post! Not bad for a guy from Newfie! :D

    You must know of Michael T Wall?
    Mark Reccord likes this.
  4. xj98jeep


    Jan 3, 2016
    Awesome, great info. I love science!
    Mark Reccord likes this.
  5. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    The svt I had measured close to flat as well.
    Mark Reccord likes this.
  6. ThisBass


    Aug 29, 2012
    I think that 250 watt at 5% THD is a pretty good number and actually more of "clean" watts I'd ever expect.
    Two antiparallel diodes feed to ground within the signal path at the input stage of the power amp input make it impossible to get "clean" 300 watt out of the amplifier.
    With 250 watt I'd expect more of THD than 5%.
    IME clipping "riddle" at the signal peak starts around 360 watt with 70's svt amplifiers.

    Not sure if it's a good idea to evaluate svt power with THD numbers.
    For 1% THD I'd expect around 100..150 watt at best for svt tube amplifiers (for good reason).
    Last edited: Feb 3, 2017
  7. Mark Reccord

    Mark Reccord Supporting Member

    I agree that THD isn't a great metric when talking about SVT output power but I'm pretty sure I was able to bet a little more than 300W out of if at about 5% the last time I tested it (about 10 years ago). Not that there's much difference between 250 and 300W anyway. :D The power test was a bit of an afterthought but I figured I'd do it while I had the test stuff set up.
  8. ThisBass


    Aug 29, 2012
    I think with the svt amplifier THD numbers should be considered in a different way that does not directly apply to common rules of audio meaurements.
    But doesn't this apply to MI amplifiers in general?
    See the RH450 confusion

    addressed to the craftsman,
    audio and MI does not equal an electric tool
    Last edited: Feb 3, 2017
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    For the 70's SVT, they claim 300W RMS minimum continuous at less than 3% THD into 4 or 2 ohm load, 20 to 20K. Perhaps an exaggeration.

    The specs say 0.25V at the input of the power amp to generate 300W. On the vintage amp, they used anti-parallel 1N456 diodes; in the modern amps they use 1N3070. They are there to clip spikes.
  10. Mark Reccord

    Mark Reccord Supporting Member

    Couple of pics of my test setup. Had to put the amp on the floor because my bench is full of other stuff. Haha!

    I did run a couple of measurements this evening. I plugged the signal into the power amp in and measured the power amp's frequency response. Pretty much dead flat in the passband with rolloff beginning around 100Hz and about 7k. This is what I would expect as there is nothing in the power amp circuit that would otherwise change the frequency response. I think the low and high rolloffs mostly come from the OT (primary inductance, interwinding capacitance).

    And I also measured the preamp out. I fiddled with the EQ to get it as flat as possible, which incidentally was pretty much exactly noon for all knobs. The low frequency roll-off is the result of the output impedance of the preamp output interacting with the input impedance of my interface since it doesn't do that when only connected to the SVT's power amp.

    Roxbororob likes this.
  11. Mark Reccord

    Mark Reccord Supporting Member

    Hey, we're smarter than usually given credit for! Fish is good for your brains, right? :D

    Yeah, I've crossed paths with Michael Wall occasionally over the years.
    Roxbororob likes this.
  12. ThisBass


    Aug 29, 2012
    So the diodes see around 0.5 Volt peak when the amplifier is clipping at around 360 watt.
    Note that saturation of a diode is about 0.6..0.7 volt and diode characteristic goes exponentiell
  13. arai

    arai Banned

    Jul 16, 2007
    Interesting thread, thanks for sharing.
    I wonder why ampeg would say it puts out 300 watts at <3 % thd when actual watts are closer to an rh450's watts
  14. 12bass


    Jan 2, 2003
    Victoria, Canada
    Waiting for the inevitable headline comparing the two to appear on Breitbart....
    JJR58 likes this.
  15. ThisBass


    Aug 29, 2012
    0.25V rms * 1.41 = 0.35V peak
    I'd expect some THD around 5% with generic diodes anaway.
  16. ThisBass


    Aug 29, 2012
    cause average MI watts is more important than watts measured with a sine wave like to evaluate an electric motor.
    Mark Reccord likes this.
  17. Mark Reccord

    Mark Reccord Supporting Member

    Well, this particular one is 40 years old and hasn't had new power tubes in 10+ years so it might not be quite up to the original spec. :D
    Sartori likes this.
  18. Roxbororob

    Roxbororob Supporting Member

    Jun 8, 2015
    Nice work Mark.
    Im watching this thread (hope to learn something)...
  19. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Man, thanks for doing all that research, Mark! Very informative. I'm curious about why the difference, too. Ch 2 looks like how it sounds on my 69, though. Don't know why it measures like mine sounds and ch 1 doesn't, but I'll be interested in what you find out.
  20. Mark Reccord likes this.

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