airbrake?

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Guy77
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Re: airbrake?

Post by Guy77 »

RJ Guitars wrote: Thu Aug 17, 2017 5:46 am I have a few of these being field tested and so far nothing but good news. Mine are all in smaller amps than you are talking about here... 35 to 45 watts. In my most recent implementation I have added a bypass switch so that you can send 100% of the power to a speaker if that is your preference.
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Hi , I was looking at the various designs of attenuators listed in this post (thanks to everyone here for the great contribution ) and it appears that the one posted by RJ (RJ power broker) is one of the least expensive to build. I have 3 mouths to feed so always on a budget .
Hi RJ the question I had regarding your design is if it could get down to bedroom levels in the way that Ken Fishers original design does, or close to it? I have built Ken's design but have recently lent it out and looking to build another one.

Thanks everyone
Sorry for double post.
Cheers

Guy.
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RJ Guitars
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Re: airbrake?

Post by RJ Guitars »

Guy, I suppose the fair answer is no, not really. You can drop it down to a very low level, but my design sorta stops before you get to that ultra low level that most wives, kids, and neighbors really appreciate. You are probably familiar with the non-linear characteristics of audio sound and know that a few watts is still really loud to our ears. My design only drops the power to about 20% at it's lowest level... which can still be fairly loud. Sadly, in my life I usually can't play loud and most of my distortion tends to come from some sort of pedal. I do like the way the attenuator allows me to use more of my primary volume control and get a little response out of my front panel tone controls.
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Guy77
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Re: airbrake?

Post by Guy77 »

Thanks for the quick response RJ! I was just about to add something to my post and that was would it be safe to replace the 25 ohm rheostat in your 8 ohm version with a 40 ohm or higher one. Would this take the volume down more?
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RJ Guitars
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Re: airbrake?

Post by RJ Guitars »

Guy, I'll need to look at it a bit more closely, but if I just add that 40 Ohm value into the spreadsheet I used to calculate things it goes in the direction you like... at this point I don't see a down side to it.
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Guy77
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Re: airbrake?

Post by Guy77 »

Thanks again RJ!
I see your graph shows the power coming down nicely from over 20 % with the 25 ohm rheostat to close to 15 % with the 40 ohm version. This could take the 50 watt John Mayer clone I am building down to about 7 watts.. So much to build and so little time to improve my guitar playing. :D
Sorry for interrupting your thread rockgod212, Hope your build works out nicely.

Cheers
Guy
rockgod212
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Re: airbrake?

Post by rockgod212 »

not a problem, I like to read all info.
rockgod212
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Re: airbrake?

Post by rockgod212 »

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rockgod212
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Re: airbrake?

Post by rockgod212 »

curious about using the middle lug of the big shunt resistor (12.5ohm) for bedroom setting would that make a better impedance match when in bedroom setting? seen people do it that way, not sure why though?
rockgod212
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Re: airbrake?

Post by rockgod212 »

my build has a gray hill 3 position switch, 25 ohm/100 watt shunt with adjustable center lug, 25ohm/100watt (ohmite) rheostat, 150ohm/50watt (clarostat) rheostat, dpdt switch for the .1/400volt cap, jacks/wire, a nice vented big box for ventilation.

just done testing this thing:
1st position- bypassed- 16ohm cab/16ohm amp load
3rd position- rheostat controls attenuation- 16ohm cab/8ohm amp load
3rd position- bedroom setting- 16ohm cab/ 16 ohm amp load

now I jumped a wire in bedroom mode to the center lug of shunt resistor (12.6ohm) brought the amp load down to 9.9ohm with 16 ohm cab, now if I use the center lug of shunt resistor would that alter the wattage it could take? just curious.

where to set the amp load now is the big question?

getting anxious to try this creation out.
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romberg
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Re: airbrake?

Post by romberg »

rockgod212 wrote: Fri Aug 25, 2017 2:40 am
where to set the amp load now is the big question?
I would plug a DMM into the jack where you are going to connect an amp. Have your cabinet plugged into the attenuator. Then measure the resistance for each setting of your switch. For those switch positions that have an active rheostat, measure the minimum and maximum resistances you get by adjusting it. Then try setting the amp to the impedance which is closest to your measurements.
rockgod212
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Re: airbrake?

Post by rockgod212 »

ok I just took some test readings, heres what I have..........
16ohm cab tested
bypass mode 13.3 in/ 13.3 out

25ohm/100watt rheostat- measured the ohms of rheostat in various positions to get readings in a stepped configuration like the big variable resistor would have been.
0 ohm-8.9 (lowest setting)
4ohm- 10.3
16ohm- 13.7
20ohm- 13.7 (probably the highest setting, as I will never get this low on the dial)

150ohm rheostat/50watt- same as above, measured resistance in stepped configuration with full shunt
0ohm- 15.2
18ohm- 17
40ohm- 18.6
74ohm- 19.7
103ohm- 20.8
134ohm- 21.3
159ohm- 24.4

so my conclusion is this- looks like in,
mode 1/ true bypass mode- 16ohm load/16ohm cab
mode 2/ rheo1- 8ohm load/ 16ohm cab
mode 3/ rheo2 BR- 16ohm load/ 16ohm cab

oh what to do now?

should I set the amp (marshall 50watt plexi) to 8ohm load and run a 16 ohm cab and forget about it, or do I need to worry about anything (impedance issues) when switching between modes?
rockgod212
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Re: airbrake?

Post by rockgod212 »

observation of my mode 2, looks like if I added 1 of the 8ohm/50watt power resistors I have off of the wiper of rheo1 back to cab outputs, I could get that section to a 16ohm load for a 16ohm cab, I might go ahead and try it and see what readings I get. any thoughts?
rockgod212
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Re: airbrake?

Post by rockgod212 »

rockgod212 wrote: Fri Aug 25, 2017 10:39 am observation of my mode 2, looks like if I added 1 of the 8ohm/50watt power resistors I have off of the wiper of rheo1 back to cab outputs, I could get that section to a 16ohm load for a 16ohm cab, I might go ahead and try it and see what readings I get. any thoughts?
25ohm/100watt rheostat- measured the ohms of rheostat in various positions to get readings in a stepped configuration like the big variable resistor would have been. readings without the added 8ohm resistor.
0 ohm-8.9 (lowest setting)
4ohm- 10.3
16ohm- 13.7
20ohm- 13.7 (probably the highest setting, as I will never get this low on the dial)

readings with the added 8ohm resistor
0- 11.6 ohm
4- 12.6 ohm
11-14.2 ohm
16-14.9 ohm
20-15.5 ohm

the readings with the added 8ohm resistor are closer to a 16 ohm load, are these readings close enough to run a 16ohm load?

anyone have any thoughts on this, just trying to avoid having to switch impedance between the 3 modes.
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RJ Guitars
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Re: airbrake?

Post by RJ Guitars »

In my experience, if the amp is looking for 8 ohms and sees something less then I get a little more nervous than I would if it was looking for 8 ohms and it actually saw something slightly greater. There are statistics and logic that back that up but generally speaking your output transformer health is at an increasing risk as the load impedance approaches zero ohms. If you believe in Gerald Weber, he gives a rule of thumb that anything within 100% of the expected load your not going to blow your output tranny... of course he is not responsible to buy you a new one so take that for what it cost you. YMMV rj
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rockgod212
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Re: airbrake?

Post by rockgod212 »

I think I have those backwards 0 ohm is full up volume and 20ohm is volume way below, I just measured the rheostat itself to find approximate ohm' age positions with the rheostat itself instead of variable lugs on a resistor. the ohm positions on the rheostat were made out of circuit, so there is always a load when using the rheostat, I made it more complicated than it needed to be.
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