DR. Z Airbrake owners unite!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by retrobob, Dec 16, 2007.

  1. retrobob

    retrobob Member

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    If your an original owner of a DR.Z Airbrake power attenuator,
    you will notice in the instruction manuel it states:

    "When using the Airbrake with vintage amps, such as an AC30, set amp to the 8 ohm tap with a 16 ohm load. This will keep the transformer running cooler.
    Same with a 50w Marshall. Set amp to 8 ohms with a 16 ohm cab."

    First of all, why will this mismatch keep the transformer running cooler?
    And second, Wouldn't this be better for any amp, new or old? :?:
     
  2. conundrum

    conundrum Member

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    I believe the Airbrake is 8ohms in itself, and you usually want to set your amp for the impedance of the atten. regardless of the speaker cab. It's not really a mismatch, just a round about way of saying it.
     
  3. retrobob

    retrobob Member

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    DR Z doesn't specify the unit as any particular ohm rating that I know of. But you can use it with any amp, just as long as you match the impedence from the amp to the cab no matter what ohm it is, per instructions. (Except for the vintage amp instructions.)

    Its a nice unit! But I would feel better having some solid answers.
    Especially since Vox & Marshall amps have a reputation of not tolerating
    mismatched impedences in general. But its OK with an Airbrake in between?
    Just want to make sure my vintage amps are happy!
     
  4. ChickenLover

    ChickenLover Member

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    The Z Airbrake has a resistance and does not 'match' anything. Yes, the manual states that you can use it with a 4, 8, or 16 ohm amp but you could also use an 8 ohm cabinet with a 4, 8, or 16 ohm amp.

    At no attenuation it's just a straight shot to the cabinet...so with a 16 ohm cab your amp will see a 16 ohm load.
    At one click down it puts a 24 ohm resistor in parallel with the cab...so with a 16 ohm cab your amp will see 16 ohms in parallel with 24 ohms which is close to 8 ohms.
    As you increase the attenuation it just adds more and more resistance in series with the speaker cab while still maintaining the 24 ohm resistor to ground...so your impedance goes up as you attenuate more.

    As you can see...not only does it not 'match' anything...the load your amp sees is different for each setting. But it's not that far off...I've never had a problem with mine and it sounds good as long as I don't go too far.
     
  5. retrobob

    retrobob Member

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    Thanks CL,
    Now wer'e getting somewhere!
    So with that explanation, I can see where mismatching the load in the first place will give you a closer to normal matched load as you increase the attenuation. :BEER
     
  6. reaiken

    reaiken Member

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    [​IMG]


    Here's what the impedance of an Airbrake-type attenuator looks like for a 4, 8, and 16 ohm cabinet, plotted as a function of the attenuation setting (assuming you have the series resistance taps properly calibrated for the nominal 8-ohm design).

    Note that the 16 ohm speaker is the upper plot (black), the 8 ohm is the middle plot (blue), and the 4 ohm speaker is the lower plot (yellow).

    As you can see, the impedance varies quite a bit. For example, at -3dB attenuation, the impedance is 11 ohms for a 16 ohm cab, 8 ohms for an 8 ohm cab, and around 6 ohms for a 4 ohm cab. At -12dB, the impedance is 15 ohms for a 16 ohm cab, 14 ohms for an 8 ohm cab, and 13 ohms for a 4 ohm cab. The higher you go, the more this type of attenuator approaches a 24 ohm load, although I think the number of steps is limited in the Airbrake design to a maximum of around 16 ohms if you used the full resistance of the series arm.

    Also note that the attenuation steps are only accurate for an 8 ohm speaker load. If the load is 16 ohms, the attenuation steps are 1/2, i.e. at -12dB you are really only -6dB down, and at 4 ohms they are twice, i.e. at -12dB you are really -24dB down.

    Not very "universal" after all, huh?. ;) They do sound pretty good, though.

    Randall Aiken
     
  7. GAT

    GAT Gold Supporting Member

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    So Randall, if I have a Komet Airbrake using a 16 ohm cab, should I set the amp's impedance to 8 ohms? This would be at 2-3 clicks down. Thanks!
     
  8. ChickenLover

    ChickenLover Member

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    Randall, does the fact that the Airbrake is purely resistive make it less likely that there will be problems when running the amp into a higher Z-than-expected load? That is...when setting the amp to 8 ohms but then plugging into the Airbrake (set for a lot of attenuation) and then into a 16 ohm cab, the amp sees a greater than 8 ohm load which is usually an undesirable thing. Does it being purely resistive lessen the chance of amp/tube injury?
     
  9. mooreamps

    mooreamps Senior Member

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    I'm not sure I would believe his claims. In my mind, the only way to reduce the stress on a power tube is to reduce the amount of plate current which flows through it, using an authetic gm Power Brake.

    -g
     
  10. retrobob

    retrobob Member

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    Inquiring minds want to know!
     
  11. phsyconoodler

    phsyconoodler Member

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    As far as I know a greater than 8 ohm resistance into an amp set to 8 ohms is not an issue.It's when the reflected ohms impedance becomes a lot lower,closer to 2 ohms that makes the OT go runaway and fry.
    The DR Z is basically two L-Pads in a box.One has a stepped setting and the other is simply a 100 watt L-Pad.I'm going to open one up and see what's inside.
    Lets see,two 25.00 L-Pads and a box = 300.00.Hmmm.....
     
  12. ChickenLover

    ChickenLover Member

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    psychonoodler,
    My understanding (or lack thereof, lol) was that running into a lower than expected load stressed the tubes (which of course could then fail and hurt the OT) but that running into a higher than expected load stressed the OT due to the higher flyback voltages (possible internal arcing or something). But I would guess that a purely resistive load would not have those flyback voltages...so therefore OK. Any of this sound wrong?
     
  13. markh

    markh Member

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    The "flyback" voltage is a byproduct of a *way* too high output impedance, i.e. open circuit. I don't think +/- 100% of nominal will give the tubes or the OT a heart attack.

    --mark
     
  14. benttop

    benttop Member

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    This chart gives me a brain cramp. :)

    On the left (vertical scale) it says "Input impedance" but that must really be the output impedance of the amp, because the Airbrake doesn't really have anything but one jack to get the signal in. So OK, fine, that's the amp's output impedance. I think..

    Then we have the horizontal scale, labeled in increments of one. One what? Db? Just below that we have the statement that it's Attenuation (still no unit of measurement though) and the explanation that it's for an 8 ohm cab, but 16 ohms is divided in half, while 4 ohms is multiplied by two. The explanation helps in that I can see it means to divide or multiply those numbers, whatever they may be.

    But whoops, just below that, we have another legend, that identifies the three lines with the colors as being associated with 16 ohm, 8 ohm or 4 ohm speakers. Huh. Didn't we just establish that the chart is for an 8 ohm cab? Isn't the speaker in the cab? GAAA! This is where the brain cramp comes into play! :) Doesn't that legend really mean to associate the colored lines with the amp's output impedance? I hope so, or I just plain can't read this chart at all!

    All that said, nice graphic! :)
     
  15. reaiken

    reaiken Member

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    No, that is the input impedance of the Airbrake (note the title of the graph: "Airbrake attenuator actual input impedance"). This is the load the amp will "see" if connected to the Airbrake with one of the three speaker impedances shown by the three colored lines.

    Yes, attenuation is usually measured in dB, particularly when talking about power attenuators.

    That's correct. Since the design of that attenuator is only accurate (in terms of attenuation steps) for an 8 ohm load, you have to multiply the attenuation value on the x-axis of the curve by 2 if you have a 4 ohm load, or by 1/2 if you have a 16 ohm speaker attached. Basically, what it says is that if you set the attenuator up where it has -4dB attenuation with an 8 ohm cab at one click on the switch, it will have -2dB with a 16 ohm cab, or -8dB with a 4 ohm cab.

    No, you're missing the entire point of the graph. The chart is not for an 8 ohm cab only, it is for a 4, 8, and 16 ohm cab, as shown by the three colored lines, one for each cabinet impedance. The x-axis attenuation values, however, are for an 8 ohm cab (as indicated by the legend "Attenuation (8 ohm cab)", followed by the note "16 ohm = 1/2, 4 ohm = 2x" to indicate that the x-axis attenuation values must be multiplied by 1/2 or 2 to get the correct attenuation at those settings for 16 or 4 ohm cabinets.

    The graph is designed to show the load impedance the amp will see at different levels of attenuation when connected to one of three cabinet impedances: 4, 8, or 16 ohms.

    The legend at the bottom is to tell you that the attenuation steps with this kind of attenuator will vary by a factor of 2 or 1/2 if you are not using an 8 ohm cabinet.

    What this means is that the amp will "see" a wildly varying load, depending on the speaker load impedance connected and the attenuation setting. That load can vary from as little as 4 ohms to as much as 16 ohms for the attenuations shown on the graph, but it can go as high as around 24 ohms for higher levels of attenuation ("bedroom" volumes). If you wanted to correctly match the amp to the load, you'd have to set the output impedance of the amp to different settings, depending on the load connected and the attenuation setting.

    To summarize, if you want to know what load impedance your amp sees at say, -8dB, for instance, you would just follow the line up from the -8 point on the x-axis until you got to the colored line that represents your cabinet impedance.

    Example: if you were using a 4 ohm cabinet, you would follow the line up from the -8dB point until you reached the yellow line (as indicated in the legend at the bottom), and you would see that the input impedance of the airbrake is around 9 ohms. If you had your amp set to 4 ohms (which you normally would for a 4 ohm cabinet), you would have a 1:2 mismatch of impedance.

    If you wanted to know what range of impedances the amp would see over all clicks of attenuation, you would start at the leftmost x-axis point, where at 0 dB attenuation, the amp would see a 4 ohm load (perfect impedance match), and then look over at the right side of the graph, where at -12dB, the amp would see a 13 ohm load (3.25:1 impedance mismatch).

    If, instead you had a 16 ohm cabinet, and were set to, say -2dB, you would follow the -2dB line up until you reached the black line, where you would find that your amp would see around a 10 ohm load. If you had your amp's output impedance selector set to 16 ohms, it would then see a 1.6:1 mismatch of impedance.

    Why, thank you. :)

    Randall Aiken
     
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  16. reaiken

    reaiken Member

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    Resistive loads are indeed safer than inductive loads for higher impedance loads, because inductive loads can rise much higher than the nominal impedance at the resonant point or at the upper frequencies. A resistive load at a 2:1 mismatch will remain mismatched at 2:1 over all frequencies of interest, while a 2:1 nominal mismatch with a reactive load will vary from 2:1 up to possibly 12:1 or even 24:1 at the resonant point. Higher than normal impedances are usually responsible for transformer, tube, or tube socket arcing failures.

    In addition, resistive loads are much less likely to cause instability problems, like parasitic oscillations, because they don't have the phase shift associated with reactive loads. A purely resistive load will not have the same level of overshoot/ringing on square wave edges, either, so it won't have the higher-voltage "spikes" on these edges that can cause arc-over.

    Typically, it is not the just the higher resistance load that causes problems. A 2x, 3x, or even 10x resistive load is not really a problem for the amp, because the tube voltage is limited by the plate voltage - the output can only swing to a max of twice the plate voltage on either side of the output transformer. For example, if you had a 500V supply, the max you could ever get is 1000v on each plate, or 2000V p-p across the entire output transformer primary. The transformer is usually designed to handle this. In reality, the output tube cannot drive to 0V at full saturation, especially in class A1/AB1/B1, and the transformer winding resistance also limits the upper range, so you wouldn't even get the full 1000V swing on each plate, more like 950V or so. If you drive the amp to full clip, you'd never get more than around 1900Vp-p across the transformer primary, no matter whether you had it connected to the nominal 8 ohm load or a 32 ohm load.

    The problem comes in because transformers are not ideal, and they have leakage inductances and things that can cause the output waveform to not be a pure, flat, square wave. It will have "ringing" or overshoots at the edges, because of the fast transition of the edge, and this overshoot can be very high, depending on the transformer design. Here is where the load impedance characteristics come into play. A reactive load, and especially a mismatched one, can cause this overshoot to be very large, and the spikes can exceed the voltage breakdown ratings of the output transformer or tube socket insulation. The same will happen with a resistive load, but not to as great an extent, but there will still be higher voltage spikes if you run into an impedance that is 2x or 3x the nominal value than you would see if the amp was loaded with the correct impedance.

    Here is an example of the voltage from plate-to-plate across the output transformer primary of a hard-driven amp into a correctly matched 16 ohm resistive load, with a plate supply voltage of 500V. Note the output transformer has to withstand a bit less than 2000V p-p:

    [​IMG]

    And here is that same amp being driven into a 16 ohm reactive load. Note the output transformer now has to withstand around 3000V p-p, because of the ringing and overshoot:

    [​IMG]



    Randall Aiken
     
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  17. Dr. Tweedbucket

    Dr. Tweedbucket Deluxe model available !!!11

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    As far as stressing your amp, it seems having the amp set at 16 ohms and using a 16 ohm cab on the first few clicks of the Airbrake would be the worst case, since the amp would be seeing around 10 or 11 ohms?
    Wouldn't it be easier on the amp then with a 16 ohm cab to set the amp impedence to 8 ohms if you were only going to attenuate around -1 to -5 dB or so? This get's a little confusing with non conventional settings.

    I was always told a slight mismatch was ok if the cab ohms setting was less than the cab, but not the other way around.
     
  18. ChickenLover

    ChickenLover Member

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    I was away on Christmas vaca and missed this...thanks a lot Randall, very informative as usual.
     
  19. ruger9

    ruger9 Supporting Member

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    Here's a related question:

    The Weber Mini-Masses use a speaker motor as the resisitve element. Weber site: "The MiniMASS employs an actual speaker motor for realistic interaction between the attenuator and the output circuit of the amplifier."

    Does this mean the Weber is a REACTIVE load as opposed to a resistive one?

    AND... if so... can a reactive load cause bias-shift distortion? I ask because I have been having a problem for awhile now when attenuating ALL my amps with a Weber. On powerful low notes, I get a sound which, after much online research, appears to be bias-shift distortion. The way I run my amps/attenuators is this: I crank the MV full (essentially turning it into a Non-MV amp), then use the regular volume (or "pre") as a gain control, using the attenuator as the MV. This allows me to get the power tubes into the equation alot more.

    But I can't live with this "boinging"/"angry bee" sound I get on certain lower notes...especially the E & A strings, low on the neck. I'm wondering if a different attenuator might work better. I've had friends on forums using the same amp I am (but a different atenuator...an Airbrake) set their amp exactly like mine, at the same volume levels, and they don't have this mysterious sound. Could it be that speaker motor in the Weber? Would a purely resistive load avoid this?
     
  20. soulsonic

    soulsonic Member

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    The MiniMASS is a crude reactive load. It has the inductive properties of a speaker, but it doesn't match exactly the sort of load a speaker gives because even though it is a speaker motor, it isn't moving and interacting the same way an actual speaker does. The actual attenuation is done with a Rheostat and it has additional resistors in parallel for impedance "matching" when you change the impedance setting. As you turn the volume down on the MASS, the rheostat pans between the speaker cab and the internal motor. The ugly distortion you are experiencing is likely due to the fact that the MASS presents a very nonlinear load and the amp probably doesn't like it. There is also the possibility that the amp's impedance is mismatched by the MASS.
     

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