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Old 26th December 2009, 04:54 AM   #121
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Thanks Tubelab,
Cool stuff, thank you for sharing your understanding and experience. I do not understand all that I know about the interaction between/among the parts of the SET amplifier and the loudspeaker output device. Thanks to Thiele and Small we know that speakers and their boxes have Q. Amplifiers have Zo. I have not seen anyone tie the amplifier and speaker together in the concept of the total Q of the system. What I have seen is the operating assumption that amplifier has a Zo of 0 ohms and the speaker including box/baffle has a Total Q of 0.71. SET’s do not have a Zo of zero.
Bass is good, flabby is not.
The Millett interface and the dedicated PC will get a workout at my house.
DT
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Old 26th December 2009, 11:56 AM   #122
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Quote:
Originally Posted by tubelab.com View Post
My experience as an RF engineer has offered me another solution. I use an RF spectrum analyzer to poke around inside every one of my tube amp designs. The little 5842 that I use in the Tubelab SE can easilly sing away at 400 MHz. Just touching the grid pins with the scope probe kills the oscillation, but it is plainly visible with the spectrum analyzer's input placed in the vicinity of the tube, without physical contact. It would be beyond the expectations of a casual amp builder to do this, but many of these oscillations can be detected with a radio or analog TV placed in close proximity to the amp.
I have a couple of RF Sniffers, one was made with the flying head of a hard disk drive (I haven't been able to find that one for a while), another is some wire-wrap around a ferrite (like an a.m. antenna). I have seen one made with an Analog Devices detector/log converter. If you have a spectrum analyzer (I have an HP3577) you can poke around in the amplifier and see what's really going on. It's interesting to look at the switching noise on semiconductor diodes -- and see how these are attenuated.
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Old 26th December 2009, 07:06 PM   #123
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I have a couple of RF Sniffers, one was made with the flying head of a hard disk drive
I have a couple of them too. The one I use most is simply a piece of coax cable with a BNC on one end and the other end has the braid stripped back to expose about 2 inches of the center conductor. The end is covered with heat shrink to avoid fireworks when poking around in live equipment. The other has a coil with a ferrite core attached to the business end. I have no idea what the coil came out of.

I have an old HP141T mainframe with the 0 to 1.25 GHz RF plug in. I made it out of the remains of two millitary scrap units that I got cheap back in the 1970's when these units sold for about $3500 used. They can often be found for $200 to $500 now. The analog storage CRT on these units are rare so make sure that you verify functionality before buying a used unit. Don't expect the storage function to work too good, though. It sucked even when these things were brand new.

It is amusing to see just how much RF garbage comes spewing out of some common electronics. The worst offender is the PC. And guess what powers that sound card. My old PC audio analysis setup always had a few spikes in the 18 to 25 KHz region. These came from the CRT based monitor. I always had to turn the monitor off when saving plots for posting to my web site.

Quote:
Cool stuff, thank you for sharing your understanding and experience.
I have a bunch of material of this sort that will be posted to my web site when I finish rebuilding it all.

Quote:
I do not understand all that I know about the interaction between/among the parts of the SET amplifier and the loudspeaker output device
.

That particular interface is a very complex one. The most often overlooked phenomena are the fact that the speaker drivers are also microphone devices, and they have a DC resistance. The drivers can each produce a voltage in response to air pressure on their cones, and in response to mechanical vibration from the cabinet and other drivers inside the cabinet. Many users spend tons of time and money trying to achieve a very high damping factor (low output impedance) and often look for speaker wire with an extremely low resistance. It is important to remember that this is all effectively in series with the DC resistance of the drivers, which can be several ohms. A low output impedance is however important to prevent any EMF (unwanted microphone voltage) generated in the speaker system from making it back into the amplifier.
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Old 27th December 2009, 12:45 AM   #124
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Pete Millett,
CQ CQ are you monitoring Pete?
I have a question about SW4, the Grayhill 2p 6t rotary switch. I have the Millett interface powered up. SW4 looks to be a quality part selection. It comes with two small pins that insert into the switch body to limit the number of active poles. I want to only take this thing apart once to set the pins.
This is where I start making assumptions. The switch will have 4 active positions. The first, left most position will have “mV” in the lower right of the display. The next three positions as the switch is turned clockwise are the 2V,20V & 200V positions these 3 positions will all have “V” in the lower right of the display. Am I on the correct page?
BTW: The enclosure and part pieces including display all fit together well. Good job.
DT
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Old 27th December 2009, 02:48 AM   #125
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Originally Posted by DualTriode View Post
It comes with two small pins that insert into the switch body to limit the number of active poles. I want to only take this thing apart once to set the pins. This is where I start making assumptions. The switch will have 4 active positions.
Oh yeah, I didn't really mention that, did I?

Yes, the switch stops should be set for 4 positions. Position 1-4 are used, so you can follow the instructions that came with the switch as to how to put those little tiny pins in.

If I remember right, you make sure the flat on the shaft is opposite the number "2" on the back. Then you insert pins just to the left of number "1", and to the right of "4", as viewed from the shaft end. Then you stick the shiny label thing on the front of the switch body to hold the pins in.

I always make sure the pins are right by checking the switch rotation before sticking the label on.

Be careful, those tiny pins are hard to hold on to, and they disappear from the face of the earth if you drop them! I have sucessfully substituted a short piece of piano wire or other steel wire...

Pete
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Old 27th December 2009, 06:43 AM   #126
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Thank You Pete,
Facing the shaft end of the switch and with the flat of the shaft opposite the 2:00 o-clock position I placed the pins in the holes at 12:00 and 4:00 o-clock. Things seem to work OK. If you want to take a look at the Grayhill instructions look here http://www.grayhill.com/web1/images/.../Rotary_71.pdf
DT
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Old 28th December 2009, 12:51 AM   #127
TheGimp is offline TheGimp  United States
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When I got my switch it had no instructions with it. Recognizing that the pins and label were important, I put them in a 'safe place'.

I'll have to see what size piano wire I have.
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Old 28th December 2009, 01:33 AM   #128
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Quote:
I put them in a 'safe place'.
I put some nice guitar pickups in a safe place, haven't seen them in over a year.

Quote:
I'll have to see what size piano wire I have.
If I remember these switches, the pins from a dead Magnoval tube (.050) may fit.
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Old 28th December 2009, 03:09 AM   #129
TheGimp is offline TheGimp  United States
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Thanks. I'll dig through my dead tube assortment and see what will fit.
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Old 28th December 2009, 03:47 AM   #130
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Hello,
For temporary use I tested the switch with pieces of wire that were clipped from one of the watt resistors. That wire is most likely to soft to last but is near the correct size.
My guess at the size is closer to 0.010 inches perhaps a red hair more. Tubelab in your world a piece clipped from a guitar string may work.
Hint, look in the Dgikey box.
DT
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