Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mostro on February 03, 2015, 06:41:28 am

Title: Resistive Atten: Some questions - dissipated power, mods
Post by: Mostro on February 03, 2015, 06:41:28 am

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:

(http://i68.photobucket.com/albums/i5/Morocotuco/Weber-Atten_zps2zfwzlva.jpg)
[IMG:1000:667]http://i68.photobucket.com/albums/i5/Morocotuco/Weber-Atten_zps2zfwzlva.jpg (http://s68.photobucket.com/user/Morocotuco/media/Weber-Atten_zps2zfwzlva.jpg.html)


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:

(http://i68.photobucket.com/albums/i5/Morocotuco/Resistors_zpsct3703v3.jpg)
[IMG:800:574]http://i68.photobucket.com/albums/i5/Morocotuco/Resistors_zpsct3703v3.jpg (http://s68.photobucket.com/user/Morocotuco/media/Resistors_zpsct3703v3.jpg.html)


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.

Title: Re: Resistive Atten: Some questions - dissipated power, mods
Post by: 2deaf on February 03, 2015, 11:26:31 am

Mouser has 50W and 100W resistors.  Try 284-HS100-8.0F  to get started.

You might want to put the amp into standby before flicking any switches.

Title: Re: Resistive Atten: Some questions - dissipated power, mods
Post by: Mostro on February 03, 2015, 01:32:02 pm

Quote from: 2deaf on February 03, 2015, 11:26:31 am

Mouser has 50W and 100W resistors.  Try 284-HS100-8.0F  to get started.

You might want to put the amp into standby before flicking any switches.

Thanks for the answer 2deaf but I live in Argentina and mail order is not an option now for me. Regarding putting the amp on standby, yes, it´s a good idea, but you know how we tend to forget those things in the heat of the moment... that´s why I asked the first question. The idea is to make it foolproof, sort of.