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In Reply to: RE: Where from that hum?? posted by vinnie2 on September 11, 2016 at 14:13:25
Somebody ought to outlaw hum. Man what a pain in the patootie! I need a check on my methods here to make sure I have got this right. Trying to start at the top so i eliminate the hum from my preamp before I try getting rid of it on the amp.
To measure it on the preamp I turned it on but had no inputs on. I then measured across the out put rca cable and got about 10-12 mvac. Is that the correct way to measure it> I also reran the psu on ducan amps to make sure I had that right. Using the schem above I came up with about 2 mvac of ripple. If some one who uses psud2 a lot could check that I would appreciate it.
Follow Ups:
On the ripple, I get 3.5mv AC peak to peak. 3.5 / 2 = 1.75 mv peak X .707 = 1.237mvac rms.I have questions.
1. Where did you get a 10Hy choke with 30 ohms DCR?
2. The sim shows an output voltage of 285vdc into a load resistance of 5k ohms. That would be 57ma of current. Does your preamp circuit draw 57ma. of current?
3. Is there a volume control before the preamp tube(s) and was it turned all the way down when you made your measurement?
BTW The ripple on the audio output of the preamp will not be at the same level as the ripple coming out of the power supply.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/13/16 09/13/16 09/13/16 09/13/16
Opps, just checked the data sheet and that should be 155 ohms for the 10H and 105 for the 5H.
The 5k ohms is the bleeder resistor that is actual on the psu just as drawn. I still haven't figured out how to calculate the actual load; more delving into the instructions needed I guess. The volume control was all the way off. and I shorted the input rca just to be sure.
The ripple out of the audio out of the preamp has been amplified by the preamp right? What should i b e shooting for there, less than 5mvac like an amp or less?
I think you meant 1.237 mvac, not vac right? If that is the case that is close enough as I was just eyeballing it from the graph.
mvac, yes. Post corrected.
I'm not sure what to shoot for using a DHT in a preamp. With IDHT in a preamp, I over build the outboard power supply and outboard DC heater supply and get no hum at the output of the preamp.
But that is going to be different with a 26. Or maybe not if you did a really good DC filament supply.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
mt
Yes, that is what I found on the 845 amp. This morning I was measuring the total ripple including the preamp and found I had more ripple on the preamp than I thought I had.
You have many possible sources of hum.
1. B+ supply in preamp. 120Hz
2. Filament supply in preamp. 60Hz if it's AC, 120Hz if it's DC.
3. B+ supply in power amp. 120Hz
4. Filament supplies for input and driver stages. 60Hz if they're AC, 120Hz if they're DC
5. Filament supply for the 845. 60Hz if it's AC, 120Hz if it's DC
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
5. Filament supply for the 845. 60Hz if it's AC
That's not correct, at least not in tubes with large filament structures like the 211 and 845. Those tubes also generate considerable harmonic energy at 120 Hz. I first discovered this when I built a circuit attempting to null the 60 Hz in a 211 amp. It worked, but there was still too much 120 Hz energy. I eventually discarded the whole idea. It seemed futile to try to duplicate the entire harmonic signature being created by the filament for purposes of nulling. That was when I moved to DC, and later to ultrasonic heaters.
--------------------------
Buy Chinese. Bury freedom.
"Those tubes also generate considerable harmonic energy at 120 Hz."
You are right. But he WILL be able to null the 60Hz. There's not much he can do about the 120Hz harmonic. I think he already went with DC for the 845 so it's moot.
I just thought if he had a list of all the possible sources of hum it might help him track some of them down.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What set up did you use for dc?
I tried several, including a simple bridge/filter, a bridge/filter/regulator (linear), and a switcher. Switchers have a potential advantage insofar as their high frequency step-down transformers isolate the DC from common-mode noise on the AC line. It was my experience in every case, however, that DC heating of the 211 sounded dry and lifeless compared to AC, regardless of how the DC was derived. That was the impetus behind my search for a practical (i.e. inexpensive) method of ultrasonic heating.
--------------------------
Buy Chinese. Bury freedom.
That's been my experience too, although the 845 doesn't seem to lose as much. Did you ever get a finished version of the ultrasonic heater? If so did you or could you you post the design?
Vinnie, I never did finalize a sine wave version, but I'll tackle that eventually. Meanwhile, the square wave doesn't seem to create any audible issues. If you search the forum and read up on the work that's been done in this area, I think you'll have enough to duplicate my efforts. One thing to remember about these supplies is that everything ahead of the 12V output (step down) transformer is attached in one way or another to the AC mains. If you're inclined to do a lot of experimentation, a small isolation transformer would be a great investment.
--------------------------
Buy Chinese. Bury freedom.
Thanks TK, I will see what I can find.
Triode_Kingdom did all the leg work on this using a simple 12v supply off Ebay.
You can no longer find the exact supply but one similar. I did this last year using TK methods and it worked perfect!
Triode_Kingdom Supply Link
--------------------This is the content and basic instructions--------------------
The first thing you have to do is pop the top off the box so you can get at the circuitry. Removing the PC board is a simple matter, once the box is open. Next, ferret out the connections at the four rectifier diodes and determine which leads represent the + and - output. You will then need to add a capacitor across those points, approximately 450uF / 200VDC.
Now, the next time you plug in the supply, it won't work. You'll discover that a small resistor (0.1 or maybe 1 ohm) located at the edge of the board has opened. That occurs because this is a fusible resistor, and the surge created by charging the new capacitor is too much. Replace the resistor with a 5 ohm, 5-10 watt part.
The next issue to deal with is the output frequency. 25 KHz is too low for audio work. The board contains a small toroidal transformer, and that needs to be modified. Remove the transformer from the board and you'll see it has two separate windings, about five or six turns each. Cut one end of each winding loose and unwrap all but one turn of each. Resolder the loose end of each winding. The next time you fire up the supply, it will output a 12V squarewave at approximately 60-65 KHz.
The last thing you'll need to do to use the supply is to add a dropping resistor at its output. The supply outputs 12V, so for a 211 drawing 3.25A, you'll need approximately 0.6 ohms. I use two 0.33 ohm resistors, one in each leg of the filament. I should add that I much prefer this approach to any sort of constant voltage regulation, or to modifying the turns ratio of the power supply's output transformer (not the same transformer mentioned previously). At the very least, the dropping resistors help to mediate the inrush current when the tube is cold. I mount the resistors directly to the filament posts of the 211 socket.
To use the supply, you'll need to mount the PS circuit board in a larger case. It won't fit in the original plastic container with the addition of the larger resistor and the filter cap. Make sure the PC board is well insulated from anything metal. The input circuit (oscillator transistors, diodes, filter cap, and toroid) are electrically connected to the AC line. In fact, only the two output leads (driven by the conventional output transformer) are isolated.
The Ebay link is the supply I used, just search for "12v halogen supply
This morning I measured the ac on just the 10 volts of dc for the 845 filament with the preamp disconnected from the 845 amp. It was only 2 mvac. This tells me I am ok with the dc filter I am using on the 845. However, when I hooked the preamp back up it jumped up 19 mvac. I think my problem has been the preamp all along, so I am going to concentrate on getting that down below 5 mvac and see what that does for me.
What is it that shows up as ripple at the speaker terminals that you measure? That is only AC isn't it?
Edits: 09/13/16
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