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Powerbrake after all....

8/22/2000 11:09 AM
Chris	Powerbrake after all.... Some of you may remember a thread a little while ago concerning my wish to tame the volume of my JMP 2204 50 watt head. I was about to order some Mazda 6V6's as advised by certain knowledgeable and helpful ampagers, when I was passing a Music store in London..... There it was in the window for Ł70 ($80-90) s/h. A Marshall Powerbrake. What could I do? Honest guys, I couldn't help myself! Guess what ? It sounds AWESOME. The key seems to be moderation: DO NOT max all volume controls and expect anything other than mush, regardless of attenuation level. DO NOT set attenuation so high as to be only tickling the speakers. I get the fattest, most organic power stage OD with the following settings. Preamp 6-7, MV 8-9, Powerbrake -6 to 9dB. This gives me TONE at loud rehersal/ gig volume. The key, I think, to getting these things to work is to drive the speakers at a respectable level. Oh joy......!! Chris

8/22/2000 11:09 AM

Chris

Powerbrake after all....

Some of you may remember a thread a little while ago concerning my wish to tame the volume of my JMP 2204 50 watt head. I was about to order some Mazda 6V6's as advised by certain knowledgeable and helpful ampagers, when I was passing a Music store in London..... There it was in the window for Ł70 ($80-90) s/h. A Marshall Powerbrake. What could I do? Honest guys, I couldn't help myself! Guess what ? It sounds AWESOME. The key seems to be moderation: DO NOT max all volume controls and expect anything other than mush, regardless of attenuation level. DO NOT set attenuation so high as to be only tickling the speakers. I get the fattest, most organic power stage OD with the following settings. Preamp 6-7, MV 8-9, Powerbrake -6 to 9dB.
This gives me TONE at loud rehersal/ gig volume. The key, I think, to getting these things to work is to drive the speakers at a respectable level. Oh joy......!!

Chris

8/22/2000 4:14 PM
Rebel420	I personally think that most people are overlooking one MAJOR factor when they say a powerbrake/hotplate DRASTICALLY changes the tone (ok 2 factors). 1) Spoeakers, speakers react differently being driven to different levels. 2) and most importantly, the human hearing is MUCH different, much more sensitive to minor tonal differences at low to moderate levels than it is at higher levels, and at the pain threshold that most marshalls can push us all to

8/22/2000 4:14 PM

Rebel420

I personally think that most people are overlooking one MAJOR factor when they say a powerbrake/hotplate DRASTICALLY changes the tone (ok 2 factors).
1) Spoeakers, speakers react differently being driven to different levels.
2) and most importantly, the human hearing is MUCH different, much more sensitive to minor tonal differences at low to moderate levels than it is at higher levels, and at the pain threshold that most marshalls can push us all to

8/22/2000 7:27 PM
Dave Charneski	Right On! About 4-5 years ago, I spent a year in tonal heaven with a Power Brake and Tone-Master. What you say about moderation is true. However, on the Tone-Master Drive channel, I maxed the Master Volume and slowly increased the gain just until I had the sustain and fullness I wanted. I also mismatched speaker impedances to increase saturation and reduce the need for higher attenuation levels. Most of the time, I got away with -6dB. Yes, the PB colored the tone, but it still sounded MUCH better than getting only preamp distortion. The only reason I abandoned that rig was that I got tired of hauling a head, cab and another combo for loud clean tones. I did try a Hotplate, but it sounded lousy with mismatched impedances and, with matched impedances, didn't sound as fat and "good" as the PB and mismatched impedances. Plus, at full power (i.e. matched impedances) through the Hotplate, the tubes got way too hot. But, no matter what, you gotta' drive those speakers to a certain level in order to get good tone. I also just spent a year and a half with a Guytron and the same thing applies - gotta' work those speakers. Dave

8/22/2000 7:27 PM

Dave Charneski

Right On!

About 4-5 years ago, I spent a year in tonal
heaven with a Power Brake and Tone-Master. What
you say about moderation is true. However, on
the Tone-Master Drive channel, I maxed the Master
Volume and slowly increased the gain just until I
had the sustain and fullness I wanted. I *also*
mismatched speaker impedances to increase saturation
and reduce the need for higher attenuation levels.
Most of the time, I got away with -6dB. Yes, the
PB colored the tone, but it still sounded MUCH
better than getting only preamp distortion.

The only reason I abandoned that rig was that I got
tired of hauling a head, cab and another combo for
loud clean tones. I *did* try a Hotplate, but it
sounded lousy with mismatched impedances and, with
matched impedances, didn't sound as fat and "good"
as the PB and mismatched impedances. Plus, at full
power (i.e. matched impedances) through the
Hotplate, the tubes got way too hot. But, no matter
what, you gotta' drive those speakers to a certain
level in order to get good tone. I also just spent
a year and a half with a Guytron and the same thing
applies - gotta' work those speakers.

Dave

8/23/2000 10:01 AM
Chris	Thanks Dave. I find that I can get (semi!) clean sounds with my present rig, just by switching the humbucker on my guitar to parralel. Cuts the vol. just enough to clean things up, but retains crispness and definition. Cheers Chris

8/23/2000 1:43 PM
Dave Charneski	The problem I had... ...with trying to get clean out of the attenuated amp is that it always sounded kinda' "hollow" to me. With the two channels in the TM, I could definitely get a clean sound, but it was drastically different than the unattenuated version. That may be quite different with different amps though. The Hotplate colored the clean tones less, but suffered from the problems I already mentioned. Dave

8/23/2000 1:43 PM

Dave Charneski

The problem I had...

...with trying to get clean out of the attenuated
amp is that it always sounded kinda' "hollow" to
me. With the two channels in the TM, I could
definitely get a clean sound, but it was drastically
different than the unattenuated version. That
may be quite different with different amps though.
The Hotplate colored the clean tones less, but
suffered from the problems I already mentioned.

Dave

8/23/2000 1:08 AM
Steve A.	Re: Powerbrake after all.... Chris: As much as we all gripe about the Marshall Power Brake, living in a condo with thin walls I find it indispensable for testing out my amp projects... You can crank your amp up to full power to check for weird oscillations or whatever without having the cops shut you down. The key seems to be moderation... Exactly! As one person pointed out, setting the attentuation down one click is like having a 50W Marshall instead of a 100W. So each click is -3dB? Looking at the knob you don't want to set it much lower than 2 o'clock... I had been using my Power Brake as a load box for biasing tubes, etc., but it was pointed out that the dc resistance of the PB is around 1.8 ohms regardless of the switch settings. So I picked up a 6 ohm/50W resistor at a surplus shop to use when I just need a load... --Thanks! Steve Ahola

8/23/2000 1:08 AM

Steve A.

Re: Powerbrake after all....

Chris:

    As much as we all gripe about the Marshall Power Brake, living in a condo with thin walls I find it indispensable for testing out my amp projects... You can crank your amp up to full power to check for weird oscillations or whatever without having the cops shut you down.

The key seems to be moderation...

    Exactly! As one person pointed out, setting the attentuation down one click is like having a 50W Marshall instead of a 100W. So each click is -3dB? Looking at the knob you don't want to set it much lower than 2 o'clock...

    I had been using my Power Brake as a load box for biasing tubes, etc., but it was pointed out that the dc resistance of the PB is around 1.8 ohms regardless of the switch settings. So I picked up a 6 ohm/50W resistor at a surplus shop to use when I just need a load...

--Thanks!

Steve Ahola

8/23/2000 2:00 AM
Randall Aiken	Steve, regarding the 1.8 ohms resistance - this is the DC resistance, and is completely irrelevant. What counts is the AC impedance, which follows the typical speaker/inductive load impedance curve, i.e. peak at resonance, trough matching the specified impedance at 400Hz, then a rising impedance following that. The reason the DCR is so low is because of the tapped-inductor in parallel with the load. It has a low DCR, because of the high power rating, but by the time you get to the frequencies of interest, the AC impedance is correct. They are perfectly safe to use as a load, unless you are testing solid-state amps below 50Hz or so! Reactive loads are good for testing amplifiers using global negative feedback, because they can sometimes cause a marginally stable amp to oscillate, when it won't into a purely resistive load. It is always a good idea to test into a reactive load while looking at the output on the scope and adjusting all the knobs to make sure there is no setting that can cause oscillation. If the amp does oscillate, it may need less negative feedback, or phase compensation of the loop. Having said that, you want to use a purely resistive load when testing amps for power output, because the power measurement is only accurate into a purely resistive load. If you are using a reactive load, the actual output power is equal to the voltage multiplied by the current, multiplied by the cosine of the phase angle between them. Highly inductive or capacitive loads can fool you into thinking you have a higher output power than you really do, because, even though the voltage is higher, the current is not in phase with it, so the real power output cannot be accurately determined by measuring the voltage, squaring it, and dividing by the specified impedance, you must multiply by the cosine of the phase angle. Randall Aiken

8/23/2000 2:00 AM

Randall Aiken

Steve, regarding the 1.8 ohms resistance - this is the DC resistance, and is completely irrelevant. What counts is the AC impedance, which follows the typical speaker/inductive load impedance curve, i.e. peak at resonance, trough matching the specified impedance at 400Hz, then a rising impedance following that.

The reason the DCR is so low is because of the tapped-inductor in parallel with the load. It has a low DCR, because of the high power rating, but by the time you get to the frequencies of interest, the AC impedance is correct.

They are perfectly safe to use as a load, unless you are testing solid-state amps below 50Hz or so!

Reactive loads are good for testing amplifiers using global negative feedback, because they can sometimes cause a marginally stable amp to oscillate, when it won't into a purely resistive load. It is always a good idea to test into a reactive load while looking at the output on the scope and adjusting all the knobs to make sure there is no setting that can cause oscillation. If the amp does oscillate, it may need less negative feedback, or phase compensation of the loop.

Having said that, you want to use a purely resistive load when testing amps for power output, because the power measurement is only accurate into a purely resistive load. If you are using a reactive load, the actual output power is equal to the voltage multiplied by the current, multiplied by the cosine of the phase angle between them.

Highly inductive or capacitive loads can fool you into thinking you have a higher output power than you really do, because, even though the voltage is higher, the current is not in phase with it, so the real power output cannot be accurately determined by measuring the voltage, squaring it, and dividing by the specified impedance, you must multiply by the cosine of the phase angle.

Randall Aiken

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