Resistive Atten: Some questions (NOT Airbrake)

[Mostro]

Hi guys.

I´m going to make my first resistive attenuator, for taming my AC30 inspired amp. Since it´s my first, I´m gonna make a simple one, to test the concept, if I like it in use later I might make an Airbrake.

So, I came across this one:

[IMG667]http://i68.photobucket.com/albums/i5/Morocotuco/Weber-Atten_zps2zfwzlva.jpg[/img]

It was published in Guitar Player´s web, an article by Gerald Weber. I´ll make the 8 ohm one, so I need 4R 100W and 8R 50W R´s.

Now, the questions:

1) If I use typical toggle switches for the DPDT´s (not the small blue ones, but the big power ones), those are non shorting ones, so when I switch from one position to the other, momentarily there will be no load to the output transformer, right? Will that be harmful for the amp? Any mod I can make to avoid that?

2) Clearly this schem is one L-pad module repeated twice. As it is I can get -6 dB or -12 dB with both modules engaged. The question is, can I add a third module to get -18dB?

3) Now the most important question. I can´t get 50W and 100W resistors, so I´m going to "make" them with lower wattage resistors. I came up with this:

[IMG:800:574]http://i68.photobucket.com/albums/i5/Morocotuco/Resistors_zpsct3703v3.jpg[/img]

Hopefully they are correct. The question I have is about the power dissipated in each resistor. I believe that in series resistors, the power dissipation adds, and in parallell resistors it´s shared, right? If so, taking the first composite R as an example, the two series 8R 15W are equal to a 4R 30W resistor, and the two parallell pairs woud take 60 W together, right?

Well, I hope I´m clear in my questions. don´t want to burn a resistor and have an open circuit that could damage my amp.

Any help apprecciated.

[vibratoking]

If so, taking the first composite R as an example, the two series 8R 15W are equal to a 4R 30W resistor

Two in series equals 16 Ohms 30W

[Mostro]

Vibratoking, yeah, made a mistake there, confused myself while typing. What do you think about the power dissipation question?

[vibratoking]

I believe that in series resistors, the power dissipation adds, and in parallell resistors it´s shared, right?

The resistance adds in series. The power dissipation is correct in both drawings you made. But be careful. If you change any of those resistor values then the actual power dissipation would also change. You have to use P=VI=V^2/R=I^2R.

[statorvane]

1) If I use typical toggle switches for the DPDT´s (not the small blue ones, but the big power ones), those are non shorting ones, so when I switch from one position to the other, momentarily there will be no load to the output transformer, right? Will that be harmful for the amp? Any mod I can make to avoid that?

I would simply throw the switch while the amplifier power is secured. Then restore power to the amp. Just avoiding something that could go wrong here.

[Mostro]

1) If I use typical toggle switches for the DPDT´s (not the small blue ones, but the big power ones), those are non shorting ones, so when I switch from one position to the other, momentarily there will be no load to the output transformer, right? Will that be harmful for the amp? Any mod I can make to avoid that?

I would simply throw the switch while the amplifier power is secured. Then restore power to the amp. Just avoiding something that could go wrong here.

Yeah statorvane, the logical thing would be to put it in standby when switching, but you know, we tend to forget those details when we are playing... the idea is to make it foolproof, kind of. That´s why the question.

[LOUDthud]

2) Clearly this schem is one L-pad module repeated twice. As it is I can get -6 dB or -12 dB with both modules engaged. The question is, can I add a third module to get -18dB?

It would be better to have one L-pad -6db and the other -12dB. Then by engauging both switches you get -18dB attenuation.