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KT88 / 6550 : I need my amp to have a heftier low end - what tubes?

Discussion in 'Amps and Cabs [BG]' started by KF2B, Jan 22, 2017.


  1. KF2B

    KF2B

    Jan 28, 2013
    Finland
    Greetings,

    So this is the situation - I´ve been using a guitar tube head (THD Bivalve) to power my bass cab (Aguilar DB 285JC) for low volume noodling. The sound is awesome otherwise but I' d like it to have more oompf! in the low end. According to the manufacturer (I asked THD) the 6550´s should be the best bet for bass. The amp arrived with el34´s which I switched for a pair of Genalex Gold Lion KT88's. I´ve been using them with great results with the amp on low voltage setting (from the manual):

    "HI V/LO V Switch: This switch determines the plate voltage of the power tube. At the Hi V setting, depending on which power tube you use, approximately 450 to 470 volts are applied to the plates (a tube that draws more current will have a lower plate voltage). The amp has twice as much power at this setting than it does at the Lo V setting and results in a sound that is punchier, more aggressive and more “in your face.” At the Lo V setting, depending on which power tube you use, approximately 300 to 320 volts are applied to the plates. This setting will give you a sweet, warm and smoother sound with a slower attack."

    So... that is what the manual says. I prefer the sound with the low voltage though as the high voltage appears too "glassy". Now my question is would the 6550's have more heft on the low end compared to the KT88`s?

    If, which tubes would you recommend?

    Thanks!

    edit: edited a typo: el84 > el34
     
    Last edited: Jan 22, 2017
  2. First I've ever heard of different variations of the same tube having different frequency responses in the same circuit.
    It will be interesting to hear about how this works from those much wiser than me.
     
  3. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Low end limit comes from hwo well matched the output transformer is to the valves, and its physical size. If its one of those amps you can throw any valves in, the OT probably isn't very right for any of them.
     
  4. Just spitballin here...

    Is it correct to assume that if the operating impedance of tubes in the original amp design have been optimally matched with the transformer that using a different tube that presents a different operating impedance will result in a less than optimal transfer of energy?

    And does the overall impedance mismatch result in an apparent difference in the frequency response of the circuit?

    And if that is the case, is it fair to say, that the apparent increase in low end is had in exchange for a less efficient amp?

    And then can we say the tube itself does not have a lower frequency response, but rather it is the tube's less than optimal operating parameters in the circuit that affects low end either to enhance or degrade it? Thus there is a trade off between power and frequency response?

    And then, as you mentioned, if the output transformer isn't capable of passing the lower frequency, then all you've done is make the amp work harder for no additional low end at all?

    Does any if that make sense?
    Thanks
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    First of all, I don't see how you can fit a KT88 or 6550 in place of an EL84 unless you are a most excellent magician. I am assuming you mean EL34???

    Low frequency response is a function of many aspects of the entire amp's design, especially the preamp, which may intentionally be voiced for guitar.

    Regarding the oft-mentioned output transformer size, in fact size may or may not be an indication of low frequency performance because there are core materials that support higher linear flux density and thus require a smaller core to achieve greater low frequency performance.

    Changing to different output tubes can change the tone and feel of the amp, not so much low frequency performance, but due more to the different transfer curves, where on the load lines the tube operate, and the differences in output impedance that is reflected to the load.

    The KT88 and to a lesser degree the 6560 may not operate all that well at the lower voltage and deliver less power than you might expect. They may also not bias correctly if the bias network is not designed to accommodate those tubes operating at the lower voltages. Additionally, its important to pay attention to heater current draw, different output tubes have very different requirements.

    If the amp was originally designed for EL34's, paying attention to voltage limitations, you might find more suitable performance by switching to 6L6 class tubes (with the necessary bias change). The output impdance of this tube more closely matches the plate load that the output transformer was designed to provide to the EL34 type tube.
     
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The power supply design also plays a roll in low end performance for bass. Some guitar amps are designed to have an earlier onset of distortion. Distorted guitar can be desirable.

    The lower notes of a bass can tax the power supply more. The plate voltage sags. Change the plate voltage and the bias requirements change., All this can cause more distortion and affect the performance of the amp.

    Can a guitar amp with the right speaker cab sound good with a bass throuh it? Yes.
    Does this combination always work? No.
    Does a pushed guitar amp perform the same with a guitar or bass plugged in? Usually not.
     
  7. Come on youse guys, leave me something to say!! :(

    Ah! It was a rainy day in Pizzavile and a foggy day here in Toronto so dismal and dreary is the outlook. :(
     
    Fishheadjoe, Robert B, JimmyM and 3 others like this.
  8. KF2B

    KF2B

    Jan 28, 2013
    Finland
    Thanks guys! @agedhorse - my bad, meant el34´s, correct. The THD Bivalve is a a odd one in that it accepts pretty much any tube you can throw at it and in any combination:
    [​IMG]
    It might be a futile task trying to get a stronger lower end of it if it has been voiced for a different purpose than bass ampm which it is not. The sound is excellent though with the pair of (THD suggested) 12AT7´s and the KT88´s. They did say 6550´s though, but I put the KT88´s instead - might try the 6550´s though...

    There is no difference re the lower end whether I play it on low / high voltage. The sound on the high voltage is more strained for a lack of a better word but not gutsier. The low voltage makes things smoother, as in a good way. The low end is there, yes, but it lacks the necessary muscle to get that oomphf thing going on. It is a shame as otherwise it´s superb.
     
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    There might not be enough available current to take advantage of the additional voltage in the high position. Could also be that the tube plate impedance is not well matched to the transformer's primary impedance via reflected load, or a host of other possibilities.

    If it was me, I would try 6L6's before either the KT88 or 6560
     
    Rip Van Dan likes this.
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    When all else fails, talk about the weather. At least there isn't much snow.
     
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Perhaps a different cab would resolve the issue. I doubt that a tube change will give you the dramatic increase in low end that you are looking for.
     
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Ok, I see which amp you are talking about... IMO not a great choice for bass as it's a "class A" single ended cathode biased design that's voiced (IMO) very well for guitar. This is kind of like putting heavier strings on a guitar and expecting it to tune up well for bass... it's not designed from the start for that kind of application. It's no surprise that the KT88's gave you those results, I don't think any of the other options will work for you either. Just my experience.
     
    blindrabbit, Rip Van Dan and KF2B like this.
  13. That’s the result of global Warming. 2016 was the hottest year on record as was 2015 before it. The wild weather that we are experiencing is sadly a by product of that.

    OK I’ve got to blow my node now! :(
     
  14. micguy

    micguy

    May 17, 2011
    If I were you, I'd hold up on spending money on tubes. Most guitar amps have capacitors in the circuit that roll off low end. No, I can't tell you which ones do that, but unless you understand what's limiting the low end, you probably won't get anywhere, except a bunch poorer changing tubes. The output transformer also may or may not pass the lowest octave of a bass - the amount of iron you need to pass the low e on a bass is a lot more than passing the low e on a guitar.
     
    KF2B likes this.
  15. ficelles

    ficelles SUSPENDED

    Feb 28, 2010
    Devon, England
    I have a THD Univalve I have modified for bass. It shares the pre-amp circuit with the Bivalve in most respects, and the stock circuit has no less than three low-pass filters dumping low frequencies to ground with a cutoff frequency of around 100Hz. I think it's -12dB per filter. I have modified mine by changing two of the filter caps to lower the cutoff frequency to around 30Hz, and at the same time put a mod in to reduce hum in the transformer section. This is why your Bivalve is weak in the low end, so I'm afraid changing tubes won't help... but on the upside my Univalve now has huge low end and sounds awesome through a bass cab with a 6550 in the power slot.

    There are schematics online and the filters are fairly obvious, and f = 1/2piRC gets the cap value for the cutoff you want. Btw obviously only do these mods yourself if you are happy working inside an amp that can give you a huge shock from stored charges even when unplugged!
     
    Last edited: Jan 22, 2017
    AstroSonic and KF2B like this.
  16. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Another challenge with single ended designs is that the transformer core must support more (average) DC current than a push pull design, another reason to limit the low frequencies and improve the core saturation margin.

    There's a lot of things about amplifier design that lays between the lines. The details are what will get you every time.
     
    dawind99, KF2B and spankdaplank like this.
  17. ficelles

    ficelles SUSPENDED

    Feb 28, 2010
    Devon, England
    Indeed and it's probably worth saying that the mods I have done aren't sanctioned by THD and could result in a wrecked amp...
     
    KF2B likes this.
  18. corinpills

    corinpills

    Nov 19, 2000
    Boston, MA
    Non technical answer, based on my personal experience only, I find KT88s to be a bit girthier in the bass than 6550s.
     
  19. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    In one of these amps?
     
    Old Garage-Bander likes this.
  20. Coke Kane

    Coke Kane Supporting Member

    May 16, 2010
    San Gabriel, CA
    I experimented with most 6550s when I had a SVT. JJ tubes have the most bass.
     

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