Harley Benton - PA100 Power Attenuator [schematic]

[Manfred]

Since like you say, the error occurred suddenly and you could assume the following.
The connection contact of the resistor winding was bad or loosened which resulted in a higher contact resistance of a few ohms at the shown position.
The power caused by the voltage drop and the current at this contact resistance could have led to overheating at this point.
If the schematic is correct then the L-pad is loaded with maximum half input power.
What exactly happened is hard to say, maybe the L-pad was faulty from the beginning.

BurnedLocation.jpg (33.72 KiB) Viewed 5947 times

[spoontex]

I think that I will try to replace the LPAD, the rest of components seems to be ok. Aware don't turn the amp at max. And when I have some money I buy another one, like the Webber Mass or something like that.

Or definitely I will install the master volume and play with this and the attenuator. But, I don't like this idea. Drilling a new hole into the chassis and modify the amp... I don't know.

Thanks a lot.

[phatt]

I think that I will try to replace the LPAD, the rest of components seems to be ok. Aware don't turn the amp at max. And when I have some money I buy another one, like the Webber Mass or something like that.

Or definitely I will install the master volume and play with this and the attenuator. But, I don't like this idea. Drilling a new hole into the chassis and modify the amp... I don't know.

Thanks a lot.

hi Spoon,
Even if you replace the Lpad it will eventually happen again *Especially* if the Amp is run flat out with the Attenuator dialed down low.
You need to back off the signal that runs through the Lpad. If it interests you you can modify the unit with some DIY by adding a low value R in series with the main resistor and then join the Lpad at that join. (See schematic below, I've pointed to the burn out shown in the picture you posted )

This trick will dramatically reduce the energy that passes through the Lpad. The only down side is you can't dial back up to full volume.
But then that is what the bypass switch is for anyway.

As Mr *BMX* has already mentioned at end of first page, these Lpads are designed for 2/3way crossover use so when they say 100Watts that is referring to *whole system wattage* so you need to de-rate that figure by at least half if you want to use it for full bandwidth attenuation.
Bass freq has up to ~10times more energy than higher freq so Lpads used in a crossover can indeed handle 100 system Watts because the bass is rolled off by 6~12dB in the crossover.

The reason they fail;
The outer winding of Lpads is 10 Ohms of thick wire, while the inner wire is much finer wire and measures 40 Ohms. so when turned down to
very low maybe 80% of the energy is passing through the last few turns of the fine wire.
Which on my Lpad is about 2 Ohms and maybe 4 turns of fine wire.
So with a pair of 6L6 pushing full power your signal might be peaking over 80Watts.

[bmxguitarsbmx]

I got that explanation from you Phatt! It's in the Jet city Jettennuator thread. Makes a lot of sense. In fact, it may be 10x power/Octave to reach the same dB level. Like the "Bass player should buy a 1000Watt head to keep up with the guitarists 100watt head" rule.

[j_flanders]

It's a Harley Benton Power Attenuator. In this web has a schmeatic:

https://blog.brixit.nl/blog/2016/2/harl ... -teardown/

I measured all the amp inputs:

4Ohm:
lpad min: 8.3R
lpad max: 16.6R

8Ohm:
lpad min: 12.3R
lpad max: 20.6R

16Ohm:
lpad min: 16.3R
lpad max: 24.8R

I found that the switched jack from 16R are always open. I replace the jack, and I have this values now:

4Ohm:
lpad min: 4.2R
lpad max: 5.7R

8Ohm:
lpad min: 8.5R
lpad max: 9.8R

16Ohm:
lpad min:16.3R
lpad max: 24.4R

But anyway, this don't attenuate nothing.

I have the 'updated' version of this attenuator, the one with the cooling fan which is powered by the audio/AC signal.
I remember getting the same readings when I measured them right after I got the unit. I found it kinda odd.
Is that a design flaw, or intentional? Should the speaker impedance be put in parallel to those values to get correct results?

I'm using this attenuator with a 5watt Fender Champion600. I guess the attenuator won't burn out but I'm more concerned about the load on the amp. 16r is 4 times the requested 4Ohm speaker load.

[phatt]

I'm not a qualified teck but Here is what I have personally experienced and tested with good results, FWIW;
As long as the Amps output has a low value load then It's unlikely you will blow the amp.
Exact values of resistance is not the issue.
Remember that when driving a resistor your amps output is no longer driving a Z.
When driving a Z (impedance) the resistance alters quite a lot over the Audio bandwidth.

If you dig up some old radio schematics you will find that some had a hard wired 30~50 Ohm resistor across the speaker output terminals. This was a safety backup in case the speaker or connection went open while driving a signal. Open circuit for Valve amps is often fatal if it's driving a large signal at the time. A dead short is not an issue for Valve output while the "inverse" is usually instant death for SS circuits without current limit circuitry.

As long as your amps load is lowish, ie only around 3~4 times the output Z it will be ok.

If you want to research this further then the UA (Ultimate Attenuator) is a good example of how one can exploit this little understood conundrum of Resistive loading of the power stage of Valve Amps.
OK though it's not exactly the normal Attenuator being discussed here it gives clues as to what needs to happen to extract the best possible outcome (Tone wise).
The UA uses a 30-Ohm load resistor and then simply taps off the signal and re-amplifies it via it's own in built SS Amp. It might seem sac-religious to use a SS output for a Valve rig but it works well.

How it works;
The important bit to understand 30-Ohms is the **AVERAGE** resistance over the audio bandwidth.

Loading an 8-Ohm output power stage of a Valve Amp with an 8-Ohm resistor just turns you valve rig into a very expensive/heavy cheap dirt pedal, just kills the dynamics of the Valve output but the moment you up the R then it allows the power stage to breathe.

Why I know this is because I did something very similar to the UA system about 10 years back to my small Valve rig and it has never failed,,Often running for hours at full volume. The second amp was just a big 120Watt SS head driving a quad box at the time.

If you want to research this when taken to an even higher level then google *Guytron GT100*
It's 2 Amps in one head, Amp 1/ drives 2 EL84's in pp,, Amp one drives a resistive load (most likely 30ish Ohms) then Amp2/ is a full blown 100W section, 4 EL34's.
This gives incredible control over absolute singing OD at any level you desire,,probably one of the best systems around.

Now back to Lpads;
No matter what you use to attenuate there will be loss.
Whether you use a resistive, inductive or other hi teck load you will likely interfere with the back EMF that the speaker returns directly to the output but in my experience you can get some very good results.
This EMF and speaker breakup stuff comes under the *law of diminishing returns* lol. You end up spending a fortune only to find you have only improved the outcome by a smidegin.
Use my schematic as example, you can use a resistor that is higher than the rated Z out of the amp then add an extra couple of Ohms at the bottom of the load R then tap off into the Lpad and then drive the speaker. Try a few things out and hear for yourself, maybe start with 10R and 2R and tap the junction with the Lpad. Work up from there until you find a sweet spot just stay under ~30R. My setup uses 24-Ohm on an 8 R output.

Now the **CAVEAT** please note that when Amps fail while driving attenuators or load boxes the Attenuator often gets the blame.
Advice ***NEVER*** drive any Valve Amp at full tilt into a load, a speaker or a banana.
Unless you know Full well the Amp has been Load tested.

There would be a lot of Amps around that would not pass a full load test (and that's some Big names as well), so don't fool yourself just because it looks tough with checker plate on the front.
I've lost count of the Marshall's I've had on my bench with voltages way above what the power valves can handle.
Hope it helps folks out there, Phil.

[j_flanders]

Thanks for the reply phatt. Great post, highly informative! Love the load test smiley.
I'll check out some of the references (UA, Guytron).
Bottom line is that Mr. Benton did well making the resistive load higher and that I certainly shouldn't change that 16Ohm switching jack to get closer to 4 Ohm on the 4 Ohm amp input.

[j_flanders]

Here's what I traced when I got the unit which is slightly different from the schematic posted above. I didn't trace the line out or the mic out btw.

[okgb]

Looks weird having a bridge rectifier as part of the speaker circuit

[j_flanders]

Looks weird having a bridge rectifier as part of the speaker circuit

Here's another project using a similar setup:

https://arduino.openexpanse.com/en/ardu ... cle/id/330

I measured the AC voltage at the speaker output of my amplifier and it seemed that it could get as high as 50 volts. The 50 Ohm resistor is intended to reduce the input voltage to something the DC voltage regulator can handle as input (up to 40 volts). The 39-volt Zener diode, together with the resistor, is intended to protect the voltage regulator should the voltage after the bridge rectifier exceed 39 volts. The 2000 uf capacitor is not so much for filtering the DC from the bridge rectifier but to allow the fan to run for a brief period after the speaker input signal disappears to provide additional cooling.

I think Marshall power break uses something similar:
http://schematicheaven.net/marshallamps/powerbrake.pdf

[phatt]

Maybe recheck your drawing of the HB Attenuator, my guess is that the NEG speaker output is also at Common/Ground to input, you can easy check with a meter to see if it is Common.

Also the 10uF on the Bridge Rect is upside down.
Regards the suspect resistance on 4 Ohm you may find that the switch in the plastic 16-Ohm socket is not making contact when the plug is removed, it's a common issue with those type of sockets.

BTW, the bridge (BR1) in the Marshall schematic is drawn wrong, rotate it 90' to left for correct orientation.
For a small Amp I doubt you would need the added complexity of adding a fan but if it's there then hey use it.
Phil.

[freefrog]

Just a contribution to state that I've got a PA100 with suspect readings apparently due to a botched inner wiring...

All was fine with the 4 Ohm and 8 Ohm inputs... but when I've tested the 16 Ohm input, DCR reading were very high and always changing.

There was a cold solder on this female jack plug.

Once this issue corrected, the 16 Ohm input worked but the other inputs exhibited high DCR readings: around 8 Ohm for the 4 Ohm input, around 12 Ohm for the 8 Ohm one.

So I've connected to ground the unused lug of the switching jack, on the 16 Ohm input... and it worked, since the DCR readings are now those expected from 4/8/16 Ohm impedance inputs.

Also: the line output didn't work. The resistor between rheostat and pot had a leg touching the housing of the pot...

I hope this explanation to be helpful for PA100 users: before to use it, check the inner wiring of this thing, people!

If I had connected my VOX to the 16 Ohm input without checking it with a DMM, my amp would have seen a load of +/- 150 Ohm... :-/

[j_flanders]

Maybe recheck your drawing of the HB Attenuator, my guess is that the NEG speaker output is also at Common/Ground to input, you can easy check with a meter to see if it is Common.

Also the 10uF on the Bridge Rect is upside down.

Thanks. Both correct.
I've added a corrected version of the schematic below.

Just a contribution to state that I've got a PA100 with suspect readings apparently due to a botched inner wiring...

Thanks for your contribution. Your first one after signing up 11 years ago?!

Also: the line output didn't work. The resistor between rheostat and pot had a leg touching the housing of the pot...

Yes, I see how easily this can happen:


Your post made me open mine up again and do all the readings again.
I think I made some mistakes when I first did them (connecting the speaker cab to the inputs at the back of the attn instead of the output.)
Anyway, so this is what I read as far as DC-resistance goes:

Speakers by themselves:
4 Ohm speaker: 4,3 Ohm
8 Ohm speaker: 8,7 Ohm
16 Ohm speaker: 14,3 Ohm

Attenuator without a speaker attached:
4 Ohm input :
-with level knob at 5 o'clock (max position, minimum attenuation) : 5,4 Ohm
-with level knob at 7 o'clock (min position, maximum attenuation) : 4,5 Ohm

8 Ohm input :
-with level knob at 5 o'clock (max position, minimum attenuation) : 9,3 Ohm
-with level knob at 7 o'clock (min position, maximum attenuation) : 8,5 Ohm

16 Ohm input :
-with level knob at 5 o'clock (max position, minimum attenuation) : 0,5 Ohm
-with level knob at 7 o'clock (min position, maximum attenuation) : 0,5 Ohm

Attenuator with a speaker attached:
4 Ohm input :
-with level knob at 5 o'clock (max position, minimum attenuation) : 3,5 Ohm
-with level knob at 7 o'clock (min position, maximum attenuation) : 3 Ohm

8 Ohm input :
-with level knob at 5 o'clock (max position, minimum attenuation) : 8.7 Ohm
-with level knob at 7 o'clock (min position, maximum attenuation) : 8,1 Ohm

16 Ohm input :
-with level knob at 5 o'clock (max position, minimum attenuation) : 15,5 Ohm
-with level knob at 7 o'clock (min position, maximum attenuation) : 15 Ohm

And the corrected schematic:

[freefrog]

Thanks for your contribution. Your first one after signing up 11 years ago?!

Yep, sorry for that. I was and am still member (or moderator) on other forums, was and am still busy and...life hasn't been quiet here these last 11 years.

Thx for your new DCR measurements anyway: if memory serves me right, they are almost exactly what I've measured myself in the same conditions.

[javableak]

Interesting conversation about the HB Pa 100, I just received one, I opened it, and the first question I ask myself is, is it possible to increase its power, I mean, if now it only supports 100w, ok. How to modify it so that it supports more power? For example: if I connect a 200 or 300w amplifier, increasing the dissipating resistances it has.

Would it be possible?

[soulsonic]

Interesting conversation about the HB Pa 100, I just received one, I opened it, and the first question I ask myself is, is it possible to increase its power, I mean, if now it only supports 100w, ok. How to modify it so that it supports more power? For example: if I connect a 200 or 300w amplifier, increasing the dissipating resistances it has.

Would it be possible?

A good starting point could be to add a high powered fixed resistor L-Pad before the rest of the attenuator. But to do that correctly, you have to design it for one impedance. But, you can calculate it to drop the power heading into the attenuator significantly, to bring it down to a range it can handle.
I have done a similar thing when I tried making an attenuator with one of those same "100w" variable L-Pads many years ago, and discovered it can't even hardly take 40w without smoking. I forget the values I used, but it was a simple thing. You can find L-Pad calculators online; you just put in the impedance and how much you want to attenuate and it gives you the resistor values.

[javableak]

Interesting conversation about the HB Pa 100, I just received one, I opened it, and the first question I ask myself is, is it possible to increase its power, I mean, if now it only supports 100w, ok. How to modify it so that it supports more power? For example: if I connect a 200 or 300w amplifier, increasing the dissipating resistances it has.

Would it be possible?

A good starting point could be to add a high powered fixed resistor L-Pad before the rest of the attenuator. But to do that correctly, you have to design it for one impedance. But, you can calculate it to drop the power heading into the attenuator significantly, to bring it down to a range it can handle.
I have done a similar thing when I tried making an attenuator with one of those same "100w" variable L-Pads many years ago, and discovered it can't even hardly take 40w without smoking. I forget the values I used, but it was a simple thing. You can find L-Pad calculators online; you just put in the impedance and how much you want to attenuate and it gives you the resistor values.

Thanks "soulsonic", the problem is that my knowledge of electronics is somewhat limited, that's why I'm here, to try to learn something. I would really appreciate some guidance diagram on how to increase the attenuation power of the PA100, I know how to read electronic diagrams, anyway, I will try to understand the L-Pad

[soulsonic]

Interesting conversation about the HB Pa 100, I just received one, I opened it, and the first question I ask myself is, is it possible to increase its power, I mean, if now it only supports 100w, ok. How to modify it so that it supports more power? For example: if I connect a 200 or 300w amplifier, increasing the dissipating resistances it has.

Would it be possible?

A good starting point could be to add a high powered fixed resistor L-Pad before the rest of the attenuator. But to do that correctly, you have to design it for one impedance. But, you can calculate it to drop the power heading into the attenuator significantly, to bring it down to a range it can handle.
I have done a similar thing when I tried making an attenuator with one of those same "100w" variable L-Pads many years ago, and discovered it can't even hardly take 40w without smoking. I forget the values I used, but it was a simple thing. You can find L-Pad calculators online; you just put in the impedance and how much you want to attenuate and it gives you the resistor values.

Thanks "soulsonic", the problem is that my knowledge of electronics is somewhat limited, that's why I'm here, to try to learn something. I would really appreciate some guidance diagram on how to increase the attenuation power of the PA100, I know how to read electronic diagrams, anyway, I will try to understand the L-Pad

It's one of the simplist circuits possible.
Try the calculator here: https://www.diyaudioandvideo.com/Calcul ... adCircuit/
For example, if you choose "8db" attenuation with 300w of power going in, you will see in the result that the "speaker" - in the case the attenuator - would only be getting 47.55 watts. You find that it gives odd values for the two resistors, and this can be a little tricky to match, but I found that you don't have to be precise and using close whole values will often give acceptable performance. The calculator shows the circuit diagram too, which is only two resistors, and says how much power each resistor will be dissipating, so you can choose one the correct size.

[javableak]

Something that I don't like at all in the PA100 and what I don't like is that the volume for the line output is the same as that of the attenuator, that is, the attenuator is at 1 and the line volume potentiometer is at 10. .. listen with very little volume connecting it to a sound card. I mean, I can't play in my room with the amplifier at maximum power, the attenuator at 0 or 1, because I won't hear anything through the line output!! horror!!

[soulsonic]

Something that I don't like at all in the PA100 and what I don't like is that the volume for the line output is the same as that of the attenuator, that is, the attenuator is at 1 and the line volume potentiometer is at 10. .. listen with very little volume connecting it to a sound card. I mean, I can't play in my room with the amplifier at maximum power, the attenuator at 0 or 1, because I won't hear anything through the line output!! horror!!

You can change where the line out connects to the signal. In the schematic, it shows the line out being connected after the wiper of the L-Pad. Move this connection to the input lug of the L-Pad (or to the input jack, might be even better) and then you can turn the attenuator down low without killing the line out signal.