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Tube amp damping specs

Discussion in 'Amps and Cabs [BG]' started by K Dubbs, Jan 19, 2006.

  1. K Dubbs

    K Dubbs Just graduated from OSU, Go Bucks!

    Mar 16, 2002
    Toledo, Ohio
    I've never seen any manufacturer quote damping factor specs for their all tube heads/power amps. Then again I've never seen a spec quoted for slew rate either? Does anybody happen to have these figures for any of the following?

    Ampeg SVT's
    Yorkville YBA200
    Mesa 400+
    Fender Bassman 300 Pro
    Eden VT300B
  2. chucko58


    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    I believe the damping factor for most tube amps would be pretty low. I'd be surprised if you could get a DF of 10. The series resistance of the output transformer, combined with less global feedback compared to solid state, are the reasons.

    I wish I could throw some real numbers at you, but I can't.

    Fortunately DF is mostly a marketing spec of little consequence. No one ever gets the full DF of their solid state amp anyway because of an unavoidable little inconvenience known as "speaker cable".
  3. BbbyBld


    Oct 13, 2005
    Meridian, MS
    Peavey has found the opposite to be true for guitar and bass amplifiers! Especially on our Transtube amplifiers and the Classic 400.
  4. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Tube amp damping factors are lower than solid state. Some really primitive amps have no negative feedback. But any of the more powerful heads (e.g., the ones on your list) have negative feedback. Their damping factors are not going to be huge, but still respectable, and probably comparable to one another.

    I remember a DF of around 25 or 30 for my dad's Heathkit tube hi-fi amp.
  5. K Dubbs

    K Dubbs Just graduated from OSU, Go Bucks!

    Mar 16, 2002
    Toledo, Ohio
    I suppose I should have noted that while I am primarily a solid state power kind of guy, I do know a fair amount about tube circuitry. I should also have noted my intentions for the question, as doing such a thing would help you folks answer me.

    Things I've got in mind when I ask the initial question:

    1. Amps with damping factor below 20 (or 10 or 50 depending who you ask) are prone to modification of the input signal at the output stage. While with tube amps this usually sounds great and is described as a "rich" or "smooth" sound, the amp's ability to control the voice coils with muscle usually suffers in these amps.

    2. For this reason, most cabs-to-be-paired-with-tube-amps are sealed cabs to preserve better cone control as compared to ported designs with variable acoustic loading support.

    3. Any sort of electrical resistance after the output transformer of a tube amp murders the damping factor and results again in lesser cone control ability.

    4. If speakers are wired in series with the output transformer, one of them will receive a differently damped power waveform. Thus all made-for-tube-amp-cabs are wired all woofers in parallel to avoid this problem.

    5. As the impedance load drops (from 8 ohms to 2 ohms lets say) in SS power amps, the damping factor deminishes and THD increases. I'm not sure if this applies to tube amps with output transformer taps or not. Anybody know for sure?

    So what I'm getting at is the following: Since tube amps are already notoriously low on the damping factor (and probably slew rate as well), and most of the time people actually enjoy the "smooth" or "rich" character of the modified output signal, are there any sonic disadvantages to multi-woofer-in-series-wired cabinets due to the drop in DF as "seen" by the less damped speaker compared to all-parallel-wired-speakers cabinets?

    Put shorter...when an amp already has significantly depressed cone control ability, does kicking it down yet a little further produce audibly/electrically noticable effects for the less damped speaker?
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The significance of damping factor is one of those things that audiophiles debate ad nauseum even though it really doesn't matter. It's in their nature. Don't go there, lest you be tempted to buy kilobuck cables and cryo-treated everything.
  7. K Dubbs

    K Dubbs Just graduated from OSU, Go Bucks!

    Mar 16, 2002
    Toledo, Ohio

    Well, damping or any other buzzword aside, Bill, have you found in your experience any detrimental effects of using series wired speakers with transformered outputs/tube amp outputs?
  8. ANY amp, regardless of its (usually) mythical damping factor, is affected by what it's hooked up to. Ever seen an EAW, BSS, or Meyer multidrive processor? Guess what they're compenstating for other than EQ and phase even with supposedly flat PA amps? For that matter have you ever seen voice coil wire?

    It's the electrical loading, not the acoustical loading. Ported cabs have LOTS of weird peaks and valleys in impedance and if they peak too high with tube amps, the electrical feedback is audible in the cab's sound, usually for the detriment.

    Yep, and again, that applies to ANY amp. One could even argue that given the difference in stability this can induce, you'll hear it MORE with a solid state amp.

    Ditto, my above comments. Physics applies to all amps, no matter what kind of output devices they use.

    This has to do with current capability. Transistors get less linear under heat stress from excess current. Since tube amps are constant power for a tapped range, going lower slightly drops power since more OT windings are required and power is lost as heat. If the range is not tapped, the amp simply puts out less power and distorts quicker due to a drop in plate voltage.

    See, that's the funny part. You see people saying over and over that tube amps have a lower damping factor and slew rate- but they don't explain why or how or even by how much. You know why? It doesn't matter. There are a host of other factors that affect tone MUCH more significantly than damping and slewrate, like the speaker. It's a non-issue.
  9. K Dubbs

    K Dubbs Just graduated from OSU, Go Bucks!

    Mar 16, 2002
    Toledo, Ohio
    So Psycho,

    You'd say that there's an audible difference between parallel wired speakers and series wired speakers with both SS amps equally to tube amps?

    I seem to remember you saying something once about the importance of keeping speakers wired in parallel. I don't remember (if it was indeed you who made the comment) with reference to what you were making the comment.
  10. Here's why its important: tube amps can have multiple taps for different secondary impedances; solid state can too, but that requires an output transformer, and you're right back into all the "problems" with slew rate and damping because it's the output transformer that is the limiting factor for damping in most high powered tube bass amps.

    Simply put, you can easly check the difference with most tube amps, but with s/s, there is no way to get around the current difference of differing value loads. It's apples and oranges.
  11. mksolid


    Jan 4, 2005
    So would you guys mind mentioning which cabs are these "made for tube amp cabs"?
  12. There are no cabs specifically made for tube amps, but often sealed cabs sound better with tubes.
  13. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I would consider it quite unlikely. Assuming that both speakers are identical, and the amp is designed to drive the resulting load impedance.

    Probably the most important factor governing the sound of mainstream amps is the frequency response of the preamp, which is often anything but flat. Second is distortion. Damping factor and distortion are related, as they are both driven by the open loop transfer function. The damping factor has to be quite poor in order to have an effect, as the largest resistance in series with the speaker is the voice coil itself.
  14. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    One possible explanation is the response curve of tube preamps, some of which are quite simplistic. The classic Fender 3-knob tone stack has a "scooped mids" curve, emphasizing the bass. Thus it might sound better driving a speaker with less bass response, such as a sealed box. In other words, the preamp and speaker might simply be EQ'ing one another.

    Naturally this is just a hypothesis, and I don't know how other amp brands implement their tone controls.

  15. Dirty little connection secret: series and parallel both disappate the same amount of power in equally resistive circuits being fed equal amounts of power, but the series connected speakers will have voltage distributed equally while the parallel will have equal current distribution. Sometimes, it makes an audible difference, sometimes not.
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Not so long as the final load was correct for the transformer tap being used. This question has been addressed in the Journal of the AES on more than one occasion and the concensus has always been that the ultra high damping factors of some SS amps are of no particular benefit, and that the low damping factors often seen with tube amps are of no particular detriment. That being the case, using measures with speakers with the intent of altering the damping factor is also of no particular benefit. Don't get me wrong, damping factors do influence response, but on the list of things one must be concerned with it's much closer to the bottom than the top.
  17. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Just curious, but if damping factor is not an issue, what do you guys think is the controlling variable between amps using similar tube configurations, but which exhibit very different cone control and/or low frequency performance out of the same cab?

    Also, FWIW, the Eden VT-300A/B has a footswitchable damping factor control. Here's what the manual has to say:

    In the low setting you get normal vaccum tube damping response. That is an open round soft low end that works well with 4 string Basses and traditional tones. In the hi position you get a tighter more defined sound suitable for slapping and more modern playing styles.

    I really like this control, and I generally preferred the sound in the hi position. The low end definitely tighted up. With the low setting, it was more round and "dubb-like." To my ears, the difference is quite audible with my NV215 or with a vintage SVT 8x10. Of course, this control is only switchable through the horrible footswitch (which causes enough problems with sound cutting out and pops, etc. that I don't use it). So, did Eden get it wrong, or is this control really changing something else (EQ?).
  18. Damping factor certainly becomes a sonic factor when it gets very low (ie below about 5 or 10). The lower the damping factor (ie higher output impedance), the more impedance variances in the load can affect the output voltage of the amp. Amps with very high output impedances tend to have trouble driving reactive loads. I wouldn't classify any of the big tube bass amps as having very low damping factors as they are all Class AB with some amount of feedback. Single-ended 300B amps, on the the other hand have output impedances of a few Ohms and tend to have trouble with low end (hence our use of solid state amps for that purpose :D).

    As for slew rate, I think that any sensibly designed audio amplifier reacts plenty fast enough for audio band signals. Our single-ended 300Bs with no NFB reproduce 20kHz square waves as accurately as the Brystons we use for low end do. Tubes are too slow for switching purposes, but most switching situations are well above the audio band (100kHz+). So are BJTs for that matter.
  19. K Dubbs

    K Dubbs Just graduated from OSU, Go Bucks!

    Mar 16, 2002
    Toledo, Ohio

    Hey Tom,

    This is something I was curious about as well. If DF has as little effect as these knowledgable folks are saying, what does the eden switch do to the signal to alter the output so audibly? It must alter something else than DF. The newish yorkville yba200 has a similar control that I think they call "resonance."? Mark, Psycho, Bill, Fdeck you guys have any ideas on these?
  20. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    There are (at least) two possible ways to change the damping factor. First is simply to insert a resistor in series with the speaker. This would simply change the frequency response. Second is to alter the open loop gain of the power amp. In the latter approach, damping factor is intertwined with frequency response and distortion.

    I think the controlling variables are frequency response and distortion.
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