#1
I just read through this whole article and it seems like a great idea to make my own attenuator for my amp.

http://www.guitarplayer.com/article/power-attenuator-project/930

I am however concerned that with this attenuator's specs being rated at 100w, and my amp (6505) being at 120w, i may encounter some issues.

i have been told that tube amps can output up to 50% more than their rated wattage when distorted, and with my amp being the way it is, it is always VERY distorted
so i worked out my maximum output from my amp to be 180w. now i want to crank it up further than my usual level of 2 on the post gain, to about 4.5 there. taking that figure as a percentage of the pots rotation (it seems logical to me) at 45%, i worked out 180*0.45=81. now 81 i believe to be my total output wattage, and 81w is less than the 100w of the attenuator, so i guess i am safe? i also worked out that with my post gain at 5.5 i get 100w of output, using the same idea used earlier.

am i doing the right maths or am i totally missing stuff?

is it safe to build and use a 100w attenuatore if i run my 6505 at 4.5 PG?

if not, for building an 8R attenuator, would changing R1 & R3 to 4R 200w and R2 & R4 to 8R 100w work? i guess that i would get <200w then, and with my amp putting out (by my total guesswork...) 180w, i could run it full out? i would only run it to 5 maximum, but on principle and i doing okay here?

also, in the article, when it says about the 4/8/16R versions, is that for the amp's output, the cabinet, or both.

i hope i havent missed anything out and made myself look like a total plank!!

Cheers guys
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#2
Unfortunately your hypotheses are not correct. Power output from amps does not have a linear relationship with the volume (or post, or any other) knob. Luckily, Peavey is rubbish at rating their amps so you're not going to get 180 out of your amp, and with your settings you're probably not at 100 watts.

HOWEVER

Attenuator ratings ought to be used very, very conservatively. The amount of heat being dissipated, and the potential for destroying the attenuator and your amp if it fails, are very serious. So you would not want to run a 100 watt amp with a 100 watt attenuator, and I would go further and say that personally I would not even run a 100 watt amp on a 150 watt attenuator. A decent and common rule of thumb is to get an attenuator with twice as much handling as your amp can put out.

Luckily, using two or four resistors, or just bigger ones, is a fine option as long as you're being very careful about your heat dissipation. I can't stress enough how important it is to get proper heatsinking and venting. If you screw up, and a solder joint heats up and comes loose, your amp is going to all of a sudden be running on an open load at very high output. It will melt your output transformer, and possibly a bunch of other stuff.

Long story short, get a big metal box with great venting (and a fan would be a good idea, too) and over-spec the crap out of your resistors. The more heat you can dissipate with bigger resistors and good heatsinking, the safer your attenuator will be.
#3
cool okay, thanks for that. i am probably going to put it in a 2U rack mounted chassis. the resistors will be stuck (with TIM) to a huge heatsink. vents and fans galore too. that should keep me safe

so, seeing as a 100w attenuator doesnt seem to be a great idea, am i correct with the whole thing at the bottom of my initial post? doubling the wattage, but keeping the values of the resistors at 4R and 8R accordingly?

cheers
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#4
Yes, with the caveat that it might be cheaper to get two resistors of twice the value and use them in parallel instead. But that works.

To answer one of your other questions, the attenuator's impedance is what the amp is looking for. The cab's impedance after the attenuator doesn't matter enough to worry about. So if your amp is set at 8 ohms, use an 8 ohm attenuator, and plug in any 2/4/8/16 whatever ohm cab you like.
#6
I have read somewhere that LPads can suck your tone like my nan does eggs, when you get to high attenuation levels. is this true? is there any way by which i can combine the concepts, so like 2 switches and a dial? so i could achieve 1) OFF, 2) -6db, 3) LPad level -db 4) -6db AND LPad attenuation level?

if i was to implement the LPad circuit in place of the -12db switch (in the design in my first post) my amp would still be seeing the same load it wants, but with a really broad range of attenuated levels?

legit?

cheers
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#7
The thing you posted is an L-pad. It just has a switch and resistor instead of a rheostat. It will sound about the same. Any cheap attenuator design like this is going to sound like crap except on the lightest attenuation settings. Even really good attenuators have that problem.
#8
how far would you say i could go before i hit the "my tone has turned to crud" stage?
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#9
Depends on your amp and its settings, but of the attenuators I've used, most of them were noticeable almost immediately and total crap anywhere below 9dB.

Attenuators get a lot of hype because on paper they should let you turn the power section all the way up and get whisper quiet volumes out of the speakers. In practice, I'd say 95% of the time you're better off with a good master volume setup than an attenuator.

If you need a loadbox for recording, sure. If you're playing your original Plexi at 8 and need it just a little quieter for that bar gig, sure. For a 6505, which has a super clean power amp? Hell no. Totally not worth it. It's going to sound worse than just turning the post gain down.
#10
alright, so i am probably better just keeping the money for a choke and new tubes then. brilliant, thanks for the advice Roc8995!!
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