Marshall Power Brake PB100 - need help please

[Thoma]

Just received a Marshall Power Brake PB-100 from 1993.
Its case looks good and a first test with relatively low power shows it works as expected.

If you own a PB100, would you please do me a favor and take a measurement of the DC resistance over its input, please?
Please measure for both the 8Ω and the 16Ω setting.

Thank you very much in advance!

 

[mickeydg5]

This may be interesting. Hopefully someone out there with a PB100 responds.
May I ask why?
The 16ohm Input should be slightly more resistance if measurable by most common meters.

 

[JohnH]

It's a good check, just to be sure. I don't have a PB, but looking at the schematics online, it runs using a one-sided transformer (an auto-former) for managing all inputs, attenuation and outputs. That's all fine but it means that the DCR readings that you measure aren't related to the nominal ohms that it's designed for. Hopefully it gives the right ohms at audio frequencies (around 400-500 hZ), but thats much trickier to test. At dc resistance, I'm guessing the measurement will be significantly lower.

The comparison to another PB is a good idea, and I agree with @mickeydg5 .

 

[Thoma]

Exactly!



As I was quite surprised by the values, I had a look at the schematics as well.
And only after that I connected it to the Amp.
Better safe than sorry.
However, I am looking forward to getting some measurements from other folks.

 

[JohnH]

One more suggestion, if anyone is able to make these mesurements: For consistency, best if the PB is completely unplugged, no speaker.

 

[Thoma]

... PB is completely unplugged, no speaker.

You're spot on of course.
I meant to imply that in my original post.

 

[playloud]

With mine, I get 1.2 ohms on the 16 ohm setting, and fluctuation between 1.1 and 1.2 on the 8 ohm setting (my meter, a Fluke 83V, clearly doesn't have adequate precision!)

The caveat is that mine is modded, with a 33R resistor bypassing the 1mH inductor (but don't think that should affect the DC readings?)

Haven't had any issues yet! I agree with what @JohnH says about DC readings differing significantly from impedance at relevant frequencies.

 

[mickeydg5]

Well that is how inductors work but he was only asking about DCR at the INPUT.
As I said the two impedance options would be close with the 16 being slightly more.

Now, @Thoma , you give us your DCR measurement values please.

@playloud , a Fluke 83V is pretty decent.

 

[Purgasound]

If you're measuring a transformer winding the results don't necessarily mean much. I'm getting 1 ohm with the front control set to max. It will lower the reading as the control is lowered.

The purpose of the impedance switch is to match the impedance for the amplifier. It's not going to measure like a speaker coil.

 

[mickeydg5]

I figured it would be around 1 ohm which is why I made the " if measurable by most common meters" comment.

By the way you guys, a way to be more accurate is to touch your leads together and read the measurement, then subtract that from the circuit measurement, unless your meter has a zero-out function (like mine ).

 

[myersbw]

To me, it seems you're wanting to see if that PB100 is giving you the proper load at the desired input impedance. (At least that would be ideal, yet not exacting per the device. So, how about this. You could just wire a cable...or a box with 2 jacks and a cable. Series two jacks (thus isolating ground connections)....feed one jack to the PB100 (set for 8 ohm)....feed the other jack that's in series to a known good 8 ohm speaker...and that cable attached is fed to the amp (set at 16 ohm). Or, wire it however is convenient so to make the PB100 and a standalone speaker (cab, etc)...in series, not parallel, be the total amp load.

Feed a fixed sine wave into the amp and measure the Vrms across each of the series jacks or connections. If they were matched (not likely), each Vrms measurement taken across the speaker and PB100 input should be the same or close to it. That would be the ideal as it would prove the PB100 is indeed providing the ideal match when used alone.

Anyways, I think you get the drift. You're just comparing the PB100 input measurement to the constant (good 8 ohm speaker or resistor). If it's close to balanced...it's a win. If not, compare the ratio of the two and see how far the PB100 is off. We're just doing a practical comparison measuring the PB100 loading with respect to a known good speaker or resistor loading. (i.e. comparing a voltage divider measurement.)

Watcha think? If you don't have a signal generator, a phone and the right app & adapter can work in a pinch to create a sine wave generator of which a true rms meter will want to see for an easier measure. Or, use a scope if you have it.

Cheers!

 

[Thoma]

With mine, I get 1.2 ohms on the 16 ohm setting, and fluctuation between 1.1 and 1.2 on the 8 ohm setting

Very good!
That's exactly what I get with mine as well, measured with an HP Digital Meter.

Thank you very much for checking, @playloud !

I just wanted to confirm that mine is OK in that respect.
I've got a Tom Scholz Power Soak as well.
And as it uses Resistors, it shows corresponding DC resistance and is more in the ballpark for 8Ω/16Ω.

 

[Thoma]

I'm getting 1 ohm with the front control set to max.

Thank you very much for sharing your measurements, @Purgasound !

 

[CrystalBearer]

One more suggestion, if anyone is able to make these mesurements: For consistency, best if the PB is completely unplugged, no speaker.

I do have a '94 powerbrake PB100 with me which has been officially retired from service at least 2 decades ago.
How exactly am I going to take a measurement at 8 ohms and 16 ohms respectively in a completely unplugged no speaker manner ? Any connection between the power brake and the amp ?
Is there any indication from the ohm readings which I can tell the power brake is still working ?
Say, a lower 7 ohms /15 ohms or no readings at all ?


Update :

2.3 ohms @ max same readings with 8 ohms and 16 ohms
1.3 ohms @ before noon same readings with 8 ohms and 16 ohms
1.2 ohms @ near min same readings wih 8 ohms and 16 ohms
no readings @ min same with 8 ohms and 16 ohms
Only using 1 speaker cable plugged into 1 output @ 8 ohms

 

[Harlequin tusk]

I have a power brake, how do I measure this? With a speaker cable plugged in?

 

[Harlequin tusk]

I get 1.9 @ 16 ohms and 1.8@ 8 ohms with dial on Max

 

[Thoma]

Update :

Thanks for your update!
Values look correct.
Funny: My PB100 is sitting on on my 6100LE right now.
Great minds think alike.

 

[Thoma]

I get 1.9 @ 16 ohms and 1.8@ 8 ohms with dial on Max

Thank you very much!