6550 tubes for SVT et al.

Discussion in 'Amps and Cabs [BG]' started by Decedo, Apr 29, 2012.

  1. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Washington, Utah
    John K Custom Basses
    +1. the HP8903A is what i have (and IMO, it's a great unit) but for quick power measurements i also use my compu-bias digital meter with it's wattage probe. the compubias will only measure it with zero distortion (it calculates automatically), but it's reasonably priced and super great for setting the bias in tube amps (it's main purpose).
  2. Well that is what my tech tells me. I did not ask him at what % distortion or anything. It was using SED tubes and to be honest, he may have said 225, 235 or 255. This was last year and I was less concerned with the exact amount of watts that I got vs what it sounds like (it sounded way better with SED than with EHX tubes). So I didn't put a lot of importance on remembering the precise conditons and all.
    But it was certainly in the 200 club.
    I remember being annoyed because he said that he could have done a better job of biasing the tubes if the sextet had been matched. Apparently Tube Depot didnt't do any matching at all on this sextet. He said a 5 year old could have done a better job matching the tubes by the pretty boxes.
    Hey even if its only 175 watts it utterly destroys everything else on stage if I want it to. But I allow them all to continue to live. For now. :bassist:
  3. timv


    Jun 7, 2000
    Chandler, AZ
    Cool. The times I tested all of mine, I never got a chance to listen to the differences of 6L6 to 6550's so didn't hear the change.
    I also had really inefficient cabs back then, which I figured out later, so didn't play any of them for very long because I thought they were too quiet compared to the 400+ I had @ the time.
    I'm going through a D-180 right now and will do all of my tests again.
  4. well with 6L6 it puts out closer to 180 watts. That is a different tube with different volts, current draw and output.
    You can yank out a set of 6L6GC and drop in some 6550 but it will sound better (and give you a few more watts IIRC) if it's been biased right.
    I actually had a long talk with my amp tech one day(he lives a couple blocks from me and I see him all the time). I asked him about the possibility of getting the 6550's in teh 400 to put out as many watts as an SVT. He said it was theoretically possible, the constraining factor is the power supply, not the tubes or the output transformer.
    Though from what he tells me it would require a custom power supply transformer and stuffing a few more caps in there in addition to upgrading the bridge rectifier and some other components. So long story short, just buy another bass 400 if you want to be louder!
    Which I do plan on doing after I pickup a couple other amps. ;)
  5. I recently had my 70's SVT serviced and the tech said at 400 Hz Sine

    Clean: 41 volts into 5 ohms = 336.2 watts
    Dimed: 54.1 volts into 5 ohms = 585.4 watts