Announcement

Collapse
No announcement yet.

Been thinking about a simplified attenuator, wonder if anyone has tried it

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Been thinking about a simplified attenuator, wonder if anyone has tried it

    I have a few small OT's with 4 ohm outputs and quite a few 16 ohm speakers. I figured I'd connect a big 5 ohm 25 watt ohmite d25 series wirewound to the output TF's (and amp, of course!) and have taps for full voltage and a few steps, probably calculated to give 2 or 3 dB steps. A single pole multiposition switch would take care of the switching. The 5 ohm resistor would always be across the output. I would be using it with a low wattage push-pull amp in a house of worship (church) setting where low volume is always a big help. I have a few old worn out (or are they just broken in?) greenbacks that might be a nice match for this kind of thing at low volume. The amp won't care as long as there's some load- I could probably do just as well with a larger value resistor like a 7.5 ohm. I guess the question is how it would sound.

    Thoughts?

    I guess I could always buy some new speakers but like any cheapskate guitar amp builder it's fun to build with stuff you have laying around.

    Jamie

  • #2
    Taps on what? The wirewound resistor?

    I've used a 15 ohm sliding rheostat as an attenuator for years. I found it on Ebay for about $20. It seems to be made of iron wire and I guess the extra inductance helps to present a more speaker-like load.
    Click image for larger version

Name:	sliding-rheostat-753577.jpg
Views:	1
Size:	9.2 KB
ID:	831363
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

    Comment


    • #3
      Hey, that's pretty neat! Maybe I should look for one of those to play with.

      Yes, I was referring to a wirewound resistor with multiple clamp-on taps. It would be built into the amp and the switch would be mounted next to the volume and tone controls on the amp.

      Jamie

      Comment


      • #4
        Originally posted by imaradiostar View Post
        I have a few small OT's with 4 ohm outputs and quite a few 16 ohm speakers. I figured I'd connect a big 5 ohm 25 watt ohmite d25 series wirewound to the output TF's (and amp, of course!) and have taps for full voltage and a few steps, probably calculated to give 2 or 3 dB steps. A single pole multiposition switch would take care of the switching. The 5 ohm resistor would always be across the output. I would be using it with a low wattage push-pull amp in a house of worship (church) setting where low volume is always a big help. I have a few old worn out (or are they just broken in?) greenbacks that might be a nice match for this kind of thing at low volume. The amp won't care as long as there's some load- I could probably do just as well with a larger value resistor like a 7.5 ohm. I guess the question is how it would sound.

        Thoughts?

        I guess I could always buy some new speakers but like any cheapskate guitar amp builder it's fun to build with stuff you have laying around.

        Jamie
        You're going thru a lot of trouble for nothing. The amp will work just fine with 16 ohm load - just won't deliver full power, which it also wouldn't with a passive attenuator.

        Comment


        • #5
          Right...but then I wouldn't have an attenuation option which I like. I was just toying with the idea of a fun little tweak that's simpler than the traditional two resistor lpad idea that most attenuators are based on.

          Thanks for the tip on Aiken's article. I'll have to look for that. (whoops, wrong thread)

          JT
          Last edited by imaradiostar; 10-15-2013, 05:22 PM.

          Comment


          • #6
            Originally posted by radioguy View Post
            You're going thru a lot of trouble for nothing. The amp will work just fine with 16 ohm load - just won't deliver full power, which it also wouldn't with a passive attenuator.
            I wouldn't assume that not knowing the amp. That load gets translated through the OT, and a shift too far in either direction can be bad for the output section.

            OP: One thing you could do is make a little wiring network that would switch in series/parallel loads. If you've got a 16 ohm single speaker, you could switch in a 16 ohm load in parallel to the speaker, then bring it back to 16 ohms to match the OT tap by putting a 8 ohm load in series with the speaker. That would dump off some watts, so make sure to get resistors rated for such.

            Comment


            • #7
              I should clarify for all those watching the thread- the idea is to have a low-volume amp that's working hard. The 4 ohm outputs are convenient because the amp is more than happy to drive 5 or 7.5 ohm resistive load without damaging itself and the speaker just goes along for the ride. The amp in question would be a simple class AB or A design with no negative feedback, probably 6aq5 or EL84 based. The plate and screen voltages would be chosen for safe output into a wide variety of load impedances.

              Jamie

              Comment


              • #8
                Originally posted by Steve Conner View Post
                Taps on what? The wirewound resistor?

                I've used a 15 ohm sliding rheostat as an attenuator for years. I found it on Ebay for about $20. It seems to be made of iron wire and I guess the extra inductance helps to present a more speaker-like load.
                [ATTACH=CONFIG]25747[/ATTACH]
                Oh, an how does it sound, Steve? I've used a THD hotplate since about 2000 and I like it a lot. I've built resistive stepped attenuators for friends but never for myself and I think it's about time I tried one. The ones I've built have sounded good.

                Jamie

                Comment


                • #9
                  I'm not assuming anything. I have designed over a hundred amplifiers, many of them with tubes which always use output transformers, and I know exactly how they work. Increasing the load impedance on the output tubes merely reduces their plate current swing, which correspondingly reduces the power output P=I(sq)R. If you think something "bad" would happen, tell us what it is. Contrary to most popular belief, vacuum tube output stages are extremely tolerant of impedance mismatches. In fact, it is not that easy to really define what you mean by "impedance match". In this context it means nothing more or less than then the load Z at which maximum undistorted power out is developed.

                  Comment


                  • #10
                    Originally posted by radioguy View Post
                    I'm not assuming anything. I have designed over a hundred amplifiers, many of them with tubes which always use output transformers, and I know exactly how they work. Increasing the load impedance on the output tubes merely reduces their plate current swing, which correspondingly reduces the power output P=I(sq)R. If you think something "bad" would happen, tell us what it is. Contrary to most popular belief, vacuum tube output stages are extremely tolerant of impedance mismatches. In fact, it is not that easy to really define what you mean by "impedance match". In this context it means nothing more or less than then the load Z at which maximum undistorted power out is developed.
                    You're a new guy, why not take it down a notch and allow people to see that you're knowledgeable rather than blowing someone out of the water right away? We're a pretty friendly bunch here and with the exception of a few members try not to brag about what we have built or done. Those that are industry experts such as RG Keen have earned respect by being humble and helpful.

                    Furthermore, Trevorus is right. In general as plate impedance increases, screen current increases. For many designs and output tube types this leads to catastrophic screen failure. This a proven concept in many guitar designs and the effect is shown in RCA tube datasheets and if I recall correctly, discussed in the radiotron designer's handbook.

                    Jamie

                    Comment

                    Working...
                    X