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Thread: Attenuators between a tube amp and the guitar speaker: some measurements and theory

  1. #106
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    I don't think reactive loads are bad per se otherwise a lot more people would have complained over the years which is not the case. Also bigger companies wouldn't have jumped on that train either. It's more like what mhuss said earlier:

    I've often wondered if the bad rep attenuators get for blowing up output stages isn't due to the fact that most people wouldn't run their guitar amps on "11" if they had to deal with the full volume. Except for maybe arenas, it is sort of self-limiting. With an attenuator, one is much more likely to run the amp way hotter then normal. And particularly with the tubes being made today...

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  2. #107
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    Fully dimed through an attenuator, I have seen the screens actually sag & then melt/ short out.
    Pentode screens are delicate (more so than beam tretrode screens) and heat up much faster than the plates. Screen dissipation increases strongly when the plates are driven into saturation. Screen dissipation is further increased with a more horizontal loadline, i.e. high load impedance. This is the reason why I warn against doubling speaker impedance.

    If power tubes actually die early when using a power attenuator, there are only 2 possible explanations. Either the tubes would have died just as well when using speakers at prolonged periods of tube saturation. Or the attenuator impedance does not match the speaker impedance over the whole frequency range.

    I have seen and measured the frequency responses of several "reactive" power loads/attenuators that showed way too high impedances around the resonance and above 1kHz. The reason was that they didn't provide appropriate damping for the inductors used. This could be an explanation for screen failures.

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  3. #108
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    Yes, as shown in the first post in this discussion, the magnitude of the impedance of a speaker could appear to imply that it is nearly perfectly inductive at higher frequencies. But a measurement of the full impedance shows that this is an illusion because there is a large resistive component that also is also rising in magnitude with frequency. Maybe some people designing attenuators do not understand the difference.

    For the other possible explanation, I am not convinced that guitar amps are as fragile as that implies. The screen resistors are there for a reason. In a guitar amp, tube life might be nasty, brutish and short, but they do reach adulthood. My ears tell me that much of the hard rock recorded over the years used output stage distortion, often very heavy. Of course, this was encouraged by the lack of a master volume control on how many popular Fender models?

    I do not think they were replacing the tubes every couple of hours.

    Quote Originally Posted by Helmholtz View Post
    Pentode screens are delicate (more so than beam tretrode screens) and heat up much faster than the plates. Screen dissipation increases strongly when the plates are driven into saturation. Screen dissipation is further increased with a more horizontal loadline, i.e. high load impedance. This is the reason why I warn against doubling speaker impedance.

    If power tubes actually die early when using a power attenuator, there are only 2 possible explanations. Either the tubes would have died just as well when using speakers at prolonged periods of tube saturation. Or the attenuator impedance does not match the speaker impedance over the whole frequency range.

    I have seen and measured the frequency responses of several "reactive" power loads/attenuators that showed way too high impedances around the resonance and above 1kHz. The reason was that they didn't provide appropriate damping for the inductors used. This could be an explanation for screen failures.

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  4. #109
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    I don't think anyone meant anything as extreme as tube replacement 'every couple of hours'.
    How would you be with 'significantly more often than without the attenuator'?

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  5. #110
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    EDIT: I forgot to say that the restive divider used in the Russian circuit can be replaced with a transformer. This does a better job of copying a reduced voltage from the load to the speaker. What you want is a "bridging" transformer, one that is considerably higher than 8 ohms looking into it.
    That's the principle of my Marshall PB 100. I uses a speaker emulating circuit connected to a multi-tapped autotransformer for power attenuation.

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  6. #111
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    Could someone explain how to calculate the voltage and current distribution in the modeled circuit? I'm not sure how to specify power handling requirements where the network has reactive/storage devices and resistance/dissipation components.
    Here are some simulations of current/voltage distribution(s) for an emulation of a 8 Ohm 4x12 Marshall cab. Input voltage 37Vrms (171W apparent power), corresponding to a clipping 100W head, kind of worst case:

    1) At bass resonance (116Hz)
    SEres.pdf
    2) At 400Hz
    SE400.pdf
    3) At 5kHZ
    SE5k.pdf

    Resistor power is easily calculated from the voltage across them.

    The chosen frequencies do not necessarily represent worst case for specific components. E.g., the 12.5mH inductor current increases below the bass resonance to around 5A.
    In real guitar signals, power is distributed over a frequency band with a probability maximum at middle frequencies, so some de-rating of component loads at the frequency extremes is feasible. But distortion increases high frequency energy content.

    Current distribution will be different with different component values.

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    Last edited by Helmholtz; 12-30-2018 at 04:41 PM.
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  7. #112
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    Quote Originally Posted by Helmholtz View Post
    Here are some simulations of current/voltage distribution(s) for an emulation of a 8 Ohm 4x12 Marshall cab. Input voltage 37Vrms (171W apparent power), corresponding to a clipping 100W head, kind of worst case:

    1) At bass resonance (116Hz)
    SEres.pdf
    2) At 400Hz
    SE400.pdf
    3) At 5kHZ
    SE5k.pdf

    Resistor power is easily calculated from the voltage across them.

    The chosen frequencies do not necessarily represent worst case for specific components. E.g., the 12.5mH inductor current increases below the bass resonance to around 5A.
    In real guitar signals, power is distributed over a frequency band with a probability maximum at middle frequencies, so some de-rating of component loads at the frequency extremes is feasible. But distortion increases high frequency energy content.

    Current distribution will be different with different component values.
    That's kick-ass, man. That was super helpful to see some visual examples like that.
    Made it clearer to think of it in terms of Kirchhoff's laws.
    I will say, that at first glace I was kind of surprised that the inductance presents that much of a difference at 400Hz vs 116Hz.

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    What is the effect of a resistive load versus a speaker load on the amplifier waveform?

    The top panel shows the waveform with resistive load, the bottom with the speaker load. (The voltage levels are very different because the sample ports on the two loads have very different levels of attenuation.) The resistive load is 7.8 ohms; the speaker load is that described earlier.

    The amp uses EL84s, grid biased for about 70% dissipation limit. You can tell this is hard overdrive because you can see that one tube shuts off before the other turns on as a result of charging of the coupling capacitors. The asymmetry might be a result of differences in the drive, output tubes, or both; I have not looked into this.

    The frequency is 300 Hz; thus the fundamental is at the minimum point in the speaker impedance load where the impedance is real. The bass resonance is not involved in this test; the differences are a result of the damped inductor that has its major effects above 300 Hz. The effect of the speaker load is to "round" the waveform, reducing the content of high harmonics while distorting the waveform on a time scale that introduces more lower harmonics.

    Click image for larger version. 

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  9. #114
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    Thanks for this impressive piece of evidence, Mike!

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    Last edited by Helmholtz; 01-03-2019 at 03:22 PM.
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    Quote Originally Posted by Helmholtz View Post
    Thanks for this impressive piece of evidence, Mike!
    Thanks. One goals of this work was to look at how a speaker affects a tube amp waveform without going deaf or getting murdered.

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  11. #116
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    Quote Originally Posted by Mike Sulzer View Post
    What is the effect of a resistive load versus a speaker load on the amplifier waveform?

    The top panel shows the waveform with resistive load, the bottom with the speaker load. (The voltage levels are very different because the sample ports on the two loads have very different levels of attenuation.) The resistive load is 7.8 ohms; the speaker load is that described earlier.

    The amp uses EL84s, grid biased for about 70% dissipation limit. You can tell this is hard overdrive because you can see that one tube shuts off before the other turns on as a result of charging of the coupling capacitors. The asymmetry might be a result of differences in the drive, output tubes, or both; I have not looked into this.

    The frequency is 300 Hz; thus the fundamental is at the minimum point in the speaker impedance load where the impedance is real. The bass resonance is not involved in this test; the differences are a result of the damped inductor that has its major effects above 300 Hz. The effect of the speaker load is to "round" the waveform, reducing the content of high harmonics while distorting the waveform on a time scale that introduces more lower harmonics.

    Click image for larger version. 

Name:	ResistiveVsSpeakerLoad.png 
Views:	53 
Size:	341.6 KB 
ID:	51755
    Nice traces! Im wondering how the bias excursion caused in part by the capacitor coupling effects the output waveform under heavy drive vs a directly coupled drive? My amp is dc coupled EL84s, so I could adjust the bias to 70% (HT 320V) and compare waveforms once I construct the reactive load. Actually, I use a 1X12 eminence red fang i can scope. Ill just use ear protection. The resolution on my digital scope is total shit, so i may end up using the old 465 instead.
    The resolution on your images looks great though. What are you using?

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    Quote Originally Posted by Mike Sulzer View Post
    Thanks. One goals of this work was to look at how a speaker affects a tube amp waveform without going deaf or getting murdered.
    Two questions:

    - In an earlier post you mentioned "high" voltage spikes at the power tube plates. Did these only show at the primaries?

    - Does your amp employ global NFB?

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  13. #118
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    Quote Originally Posted by SoulFetish View Post
    Nice traces! Im wondering how the bias excursion caused in part by the capacitor coupling effects the output waveform under heavy drive vs a directly coupled drive? My amp is dc coupled EL84s, so I could adjust the bias to 70% (HT 320V) and compare waveforms once I construct the reactive load. Actually, I use a 1X12 eminence red fang i can scope. Ill just use ear protection. The resolution on my digital scope is total shit, so i may end up using the old 465 instead.
    The resolution on your images looks great though. What are you using?
    Thanks. With dc drive you should see less rounding since you can get higher peak current, but you do not know for sure until you try it. I have some concern for the screens of the EL84s, but with 320 volts I hope you are OK.

    For signal sampling I am using an Apogee Element 24. It feeds the scope from Electroacoustics Toolbox. When I measure impedance, I use the Element 24 and my own software.

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  14. #119
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    Quote Originally Posted by Mike Sulzer View Post
    Thanks. With dc drive you should see less rounding since you can get higher peak current, but you do not know for sure until you try it. I have some concern for the screens of the EL84s, but with 320 volts I hope you are OK.
    The screens are right around 300V, and have 1k/5W screen resistors. Plus there is shared cathode resistance adding degeneration for some compression effect. Im not worried, but im putting it through abusive testing to make sure.

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  15. #120
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    I'm resurrecting this thread and project because I think it's a good one, and I was hoping to find out if anyone has made any progress on building/completing/testing this and hear what kind of results they got. It's been on the back burner for me for a while, but I have some time and a little extra cash to start sourcing parts. One of the challenges has been finding the balance of cost and size efficiency to use film caps for the ≈390F capacitance.
    I found some really good deals today on 100-150F film caps, with voltage ratings from 100-500V and was going back and forth over +/- $4 (man, I must be painfully cheap.) Anyway, I was working in my shop on another project and I found a stock of 50F/250V electrocubes caps I got a while ago totally forgot about. There were like 12-15 of them in the box, so I decided to make a capacitor bank using 8 in parallel.
    Click image for larger version. 

Name:	400F_Capacitor_Bank.jpg 
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    Best part is I get to hold onto that $4.
    I was going to buy a pair of Jantzen 1031 0.47mH 18 AWG Air Core Inductor for Lh0 & Lh1 and wanted to double check that these are appropriate.
    I know that Mike used the secondaries of a filament transformer for the large inductor (Lb) in the prototype. But if I was sourcing out a designated filter choke for the project, can you guys offer up some suggestions to get me in the right direction? I was thinking 4-5A rating (maybe 6?). The whole unit is probably going to have to be large anyway to accommodate the caps. Go big or go home, I say.
    The last question for Mike, Helmholtz, Nickb (and others) is what kind of ideas where you thinking about for the secondary solid state drivers?

    for those who may be interested, here are some great prices on film caps for the capacitance needed:
    130F-500V
    110F-450V
    120F-250V
    100F-450V
    100F-100V
    100F-63V

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    what kind of ideas where you thinking about for the secondary solid state drivers?
    In principle you could use any linear, low output impedance (power) amp. Even an integrated amplifier with tone controls. Just make sure to attenuate the dummy load signal to a level that suits the slave amp's input sensitivity using a voltage divider of total resistance between 1k and 5k. If you attenuate to line level, you may insert line level effect units between dummy load and slave amp, e.g. reverb, delay, chorus, equalizer...or even go quasi stereo with one dry and one wet signal into a dual channel amp connected to guitar speakers.

    After I have finished my Marshall PowerBrake attenuator project, I will experiment with my old and trusted Dynaco Stereo 120, which has become part of my lab equipment.


    BTW, if you make the capacitor(s) switchable you can emulate the bass response of different speaker/cabinet types. Closed cabinets raise the bass resonance frequency (e.g. vintage Marshall 4x12: fres=120Hz). This would require a cap value of 150 to 180 with a 12mH choke.

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  17. #122
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    Quote Originally Posted by Helmholtz View Post
    BTW, if you make the capacitor(s) switchable you can emulate the bass response of different speaker/cabinet types. Closed cabinets raise the bass resonance frequency (e.g. vintage Marshall 4x12: fres=120Hz). This would require a cap value of 150 to 180 with a 12mH choke.
    Interesting idea, i had thought about the need for an cab/speaker emulator circuit for a possible direct out. But, I like the idea of having Variations in the reflected load. Too bad they didn’t make small variable auto transformers, you could probably use something like that as a variable inductor to dial in a pretty broad range of LC resonance.

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    I was going to buy a pair of Jantzen 1031 0.47mH 18 AWG Air Core Inductor for Lh0 & Lh1 and wanted to double check that these are appropriate.
    But if I was sourcing out a designated filter choke for the project, can you guys offer up some suggestions to get me in the right direction? I was thinking 4-5A rating (maybe 6?).
    I like to insure myself so in the reactive loads I've built so far I was using 1,2-1,3mm thick copper wire (~16AWG) air core inductors. Same for the low freq inductor. For caps my choice was two 100uF/350V motor start caps in parallel (this way they can fit into an 1U rack enclosure). All this is for a load that can handle cranked 100 Watts.
    Considering the LF inductor choices you can use a Hammond 159ZJ 10mH/5A choke or go for this one for example:
    https://www.parts-express.com/erse-s...-coil--266-946
    For those in Europe at Banzai they have the full Mundorf list of caps and inductors.
    You can also check Ebay and Aliexpress for 200uf 250-450V CD60 motor start caps (for a 8 Ohm load).
    Also I'm running 2 or 3 power resistors in parallel on a massive heatsink fan cooled (Marshall Powerbrake fan circuit) to make sure that if one them fails the amp will have some load left (although with a massive fan cooled heatsink that's very unlikely to happen).

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  19. #124
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    I was also have looking at a 15AWG .47mH jantzen 1518 as well. For a few bucks more, I’ll go with those.

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  20. #125
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    Quote Originally Posted by SoulFetish View Post
    Interesting idea, i had thought about the need for an cab/speaker emulator circuit for a possible direct out. But, I like the idea of having Variations in the reflected load. Too bad they didn’t make small variable auto transformers, you could probably use something like that as a variable inductor to dial in a pretty broad range of LC resonance.
    Not sure if we mean the same with "emulate". I meant emulate the bass impedance of different speakers/cabinets. The attenuated and amplified dummy load signal needs to be connected to a guitat speaker.
    A direct out for full range reproduction would require additional filtering to emulate the frequency response of a guitar speaker.

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  21. #126
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    Quote Originally Posted by Helmholtz View Post
    Not sure if we mean the same with "emulate". I meant emulate the bass impedance of different speakers/cabinets. The attenuated and amplified dummy load signal needs to be connected to a guitat speaker.
    A direct out for full range reproduction would require additional filtering to emulate the frequency response of a guitar speaker.
    Don’t sweat it. I knew what you meant.

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