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Author Topic: Converting AC to DC filiments  (Read 26603 times)
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ke7trp
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« on: April 28, 2012, 03:39:40 PM »

I would like to convert my Speach amp from AC to DC filiments. I have found several circiuts on the web.


One way is to use a simple hum balance pot.    Term 1 goes to one side of fil trans secondary. Term 2 goes to ground and terminal 3 goes to the other side of the fil trans secondary.  A 100 ohm pot is suggested.  You can dial the hum away.



The other system is to use DC offset.  You make a simple voltage divider off the B+ to get around 4 volts DC to the center terminal of the 100 ohm pot.  Then you can adjust the hum to zero.

I guess also you could use a Diode and a cap to make a DC supply for the filiments but this is not suggested in my cases. I guess that is because the voltage will be changed.

What circiuts have you used?  Any luck?



* 5F1humFree.gif (5.81 KB, 576x529 - viewed 486 times.)

* 568humAdjust.gif (2.33 KB, 504x150 - viewed 456 times.)
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« Reply #1 on: April 28, 2012, 03:52:03 PM »

Neither one of those adaptations is a DC filament operation.

They work fine, however, as an out-of-phase additive to get the hum nulled out.  This method is a lot easier to do than working up a filtered low-impedance DC source.

Cathode tubes are easier to run on DC than filaments, as there will be no tendency to pull more electrons from one side of the filament, causing a shortened life.

Lots of audio equipment uses out-of-phase global feedback to reduce noise as well as hum artifacts.

Vintage radios that used field coil speakers sometimes had a secondary winding there to lower the hum level right at the speaker.

73DG
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ke7trp
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« Reply #2 on: April 28, 2012, 03:54:31 PM »

Yes.. Of course a 100 Ohm pot is not producing DC.   Roll Eyes

So your suggesting the hum balance pot and or some feedback?

C
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WD5JKO
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« Reply #3 on: April 28, 2012, 04:03:30 PM »



Clark,

  It would be helpful to know the tube types used in the low level stages of the T368. Also are the filaments grounded to chassis on one side? Is the chassis steel or aluminum? Are there any audio transformers in the low level audio chain that are near a power transformer or a filter choke? If so are these transformers encased in iron , aluminum, or open construction.

  I am asking because the solution to your issue depends a lot on your unique situation.

Oh, is the input audio return tied directly to the chassis, or is it tied to some sort of audio common that might have an impedance (like 10 ohms) to chassis ground?

Jim
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« Reply #4 on: April 28, 2012, 04:26:48 PM »

I do recommend the 'electronic' method of limiting hum rather than the brute force track.  

A simple thing to try is flipping the polarity of the primary on the power transformer feeding that stage.  It may very well be causing an 'additive' hum, and trying that may get the 'subtractive' reduction you are looking for.

73DG
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« Reply #5 on: April 28, 2012, 05:36:58 PM »

Dennis. I will perform this test today.  Thanks for the tip! 

Here are the screen shots of the 60, 120 ect on the SDR radio receiver.  Both transmitters where tested into the same dummy load. The SDR was hooked to the antenna switch nearby. 


Keep in mind that I am zoomed in to the lower portion of the audio spectrum and that I have used the peak slider to make these visable as large peaks.  Use the DB scale at the right to compare noise floor to PIP at 60 and 120.

Icom 756 Pro has 20 DB or so on the 60 cycle and 35 to 40 on the 120.
The T368 has about 55 to 60DB down for 60 Cycle and 55 to 60DB on 120.

Nobody has ever mentioned hum on the Icom ever.   Can someone please let me know:

1. Did I read the plots correctly?
2. Are these figures well below what is considered a clean signal?
3. At what level can signals like this be heard by the casual listener?
4. Am I waisting my time trying to further reduce the 60 and 120 on the T368?
5 Should I tell the freaks that send flex radio plots to my inbox of my transmitters they should jump off a bridge?

Jim.  The Preamp tubes are 12au7s  Yes they are by sealed potted Crosley/saratoga transformers. I think Copper or aluminum cases.  I will look over the schematic again for the wiring question. The T3 preamp is known for all kinds of hum.

If I pull the two preamp tubes, the Hum goes down to a point where I cant even hear it here.  But there is still a blip on the plot.



* t368.jpg (176.08 KB, 768x1280 - viewed 531 times.)

* icom 756 pro.jpg (185.43 KB, 768x1280 - viewed 471 times.)
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Steve - K4HX
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« Reply #6 on: April 28, 2012, 07:20:51 PM »

Quote
4. Am I waisting my time trying to further reduce the 60 and 120 on the T368?

Anything more than 40 dB down isn't worth worrying about unless you just want to see what can be done.
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« Reply #7 on: April 28, 2012, 07:40:13 PM »

Some cathode type tubes hum because of a high-resistance path, heater to cathode leakage. It's obviously less annoying in their level stages than lower.

Heater to cathode leakage due to the inherent diode, and or hum from the heaters, can sometimes be reduced by applying a DC bias to the heater circuit so it is 25-50V above ground. Hi-fi amps frequently do this. Some take the voltage conveniently from the top of the output stage's cathode bias resistor.

Some audio guys run directly heated tubes from DC rather than the old way of using a transformer center tap for the GND.

About the DC polarity on a filament eating one side of the filament coating faster, isn't one solution to periodically reverse the polarity? Just have a latching relay that swaps it each time the gear is turned on.

Some modernistic folks will run their directly heated tubes from a high frequency AC source, 100KHz or some frequency that is not audible. That takes yet another special power supply.
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« Reply #8 on: April 28, 2012, 08:00:01 PM »

Another thing I forgot to mention is that grounding the CT of the filament supply essentially makes the hum frequency half, or 30Hz on a 60 Hz supply reference to ground.

That low a freq is easily not a player in most uses, as audibly reproducing it is not easy.

73DG
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k4kyv
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« Reply #9 on: April 28, 2012, 11:01:40 PM »

I run the filaments on my old mic preamp, and the VFO tube, off the same DC supply.  I once had a neat little hermetically sealed regulated DC supply that put out 6.3 volts at about 3 amps, but it crapped out.  I replaced it with the 12 VDC supply that used to run my 2m CB rig (which I haven't used in years), with a resistor in series to drop the voltage down to 6.3.

The VFO filament was  more for the regulation than for the DC, since even a volt or so change in  line voltage caused my VFO to shift enough to move the frequency 100~ or more on 40m, which was a problem when I tried to work 40m CW. The single-ended 12AX7 mic preamp had loads of hum with a.c. on the filament, but it went away when I ran it off DC.

The push-pull pre-amp and balanced output D-104 combination I have been using lately works just fine running the preamp tubes on a.c.
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« Reply #10 on: April 29, 2012, 01:07:35 AM »

I run my 12AY7 mic amp filament off a 21 volt DC supply.  Voltage on the filament runs from about 21V down to about 15V.  The remaining 15 volts is dropped through a resistor to ground.  So the filament is about 15-20 volts DC above ground which also helps to reduce hum. The same 21v supply is used for the 12VDC relays using dropping resistors.

The filaments for the VFO tubes run off a regulated 6.2 volt DC supply.  This eliminated the minor drift due to slight line voltage changes.

Fred
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« Reply #11 on: April 29, 2012, 08:28:56 AM »

Clark, any chance you can post the T-368 speech amp schematic?

Here is a great quote from Bruce W2XR from 2008 (HumApache thread):

"And do all of the audio grounds return to a single common point very close to the audio input connector? I'm just wondering about the possibility of a ground loop here. A 60 hz hum can frequently spell a ground loop. When looking at the hum pattern on a scope, if it is not a clean 60 hz sine wave, this is a good indication of a ground loop. If you have multiple grounds within the audio section of this rig, you are asking for trouble. All of the grounds must return to a single common ground point, using the popular star-ground topology; you can't take the expedient path of using the grounding lugs on a tube socket to ground the components for that particular stage."

Many good points brought up on this thread including how DC filaments don't always fix the problem; topic ended though without resolution:
http://amfone.net/Amforum/index.php?topic=13900.0

Looking back, I bet the hum was magnetically coupled to the open frame audio interstage transformer, that is if there is one on the Apache..

I like the tube substitution of a 5814 (Low noise and microphonic rated 12AU7). Might help depending on the cause of the hum.

Once in my QRO Central Electronics 20A I had magnetically coupled hum from an added filter choke to one of the audio interstage transformers. I experimented with a hum neutralizing circuit consisting of a resistor (about 22 ohms 2W) off one side of the 6.3 VCT filament winding to chassis ground. The magnitude of the induced ground current, phase, and location to chassis were all important. I could make the hum far worse, and considerably better with experimentation. If the hum had been pure 60hz and sinusoidal, the technique would have worked with a deeper null. Instead I had to replace that filter choke with another that was encased in a steel can. Hum gone!  



Jim
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« Reply #12 on: April 29, 2012, 11:03:13 AM »

Another thing I forgot to mention is that grounding the CT of the filament supply essentially makes the hum frequency half, or 30Hz on a 60 Hz supply reference to ground.

That low a freq is easily not a player in most uses, as audibly reproducing it is not easy.

73DG

How does that work? It's push pull -so 120Hz seems more likely
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« Reply #13 on: April 29, 2012, 01:12:10 PM »

The famous Collins PTO and its clones drift substantially with even a minuscule change in filament voltage.  The regulated DC on the osc. tube cured my VFO (a highly modified T-368 master oscillator/multiplier unit). I had the exact same problem with the 75A-4.  I use an updated version of the Sola constant voltage transformer to run the whole receiver.  It is relatively small, about 3 X 4 X  5 inches, the buzz is just barely perceptible and it runs cool. It was designed to run a PC, according to the vendor who sold it to me.

One advantage of a FET or other solid state vfo.  No tube filament voltage to regulate or filament hum to suppress.
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« Reply #14 on: April 29, 2012, 01:54:29 PM »

I got an old Sola from my employer on eBay.  That thing made so much noise and threw off so much heat I hated using it and eventually gave it away.
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« Reply #15 on: April 29, 2012, 02:28:21 PM »

I ran some more tests this morning.  Hooking up the Green wire on the power to the rig lowered the 60 hum by 8 DB.  I have the green wire from the wall socket and I have an seperate station ground system.  Hooking up the station ground does nothing. But when I hooked up the green wire, The hum did Drop a bit. I guess I will leave that on.

I think I might try swaping the HOT and Neutral line on the plug next. 

The remaining hum can be varied by changing the modulator BIAS control on the front panel.  If I bias the tubes off, The hum is gone.  If I turn the current up to 150 to 200 MA, Then it sure does get noticable.   

I think I will have to go into the preamp to get anything lower now.  I am considering bypassing the first stage.  Running TWO 12au7s like that back to back is a tremdous amount of gain.  I think its simply to much for anyones sake. 

Keep in mind that the rig came with a 12aT7 which then fed a 12au7.   If I run this way, the hum is super strong.  Removing the 12AT7 and inserting a 12au7 reduced the hum way way down. 

I will get a schematic up here shortly!

C
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« Reply #16 on: April 29, 2012, 04:21:16 PM »

Here are the schematics Patrick. 

I have the "A" model Speach AMP.  This was a one time run.  The reverted back to the basic design and used that through until the F which was changed again I understand.

I have included TWO  schematics.  One of A and one of the other design.

If you notice that on the "other" SA's they used two 2.5 chokes on the input.  My Speach amp does not have those chokes.  Only resistors.

Also.  On the Filiment supply,  There are two .01 caps but only on the First and second preamp tube.

C


* A model SA.jpg (826.27 KB, 1952x3264 - viewed 489 times.)

* FIL power for SA.jpg (903.11 KB, 1952x3264 - viewed 476 times.)

* later model SA.jpg (846.56 KB, 1952x3264 - viewed 445 times.)
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« Reply #17 on: April 29, 2012, 11:27:58 PM »

I got an old Sola from my employer on eBay.  That thing made so much noise and threw off so much heat I hated using it and eventually gave it away.

Some of the AC regulators are noisy and they are all inefficient when not under a decent load. When the load is near full, the buzz is less and the waveform is nice (nicer..) The advantage is low cost and simplicity.

Compare the cost of a Sola mains regulator to one of those variac-controlled buck boost units, it's 5 to 1 or more. Comparing it to a GE Inductrol the difference is even more extreme.

here, there are one or two Sola and similar-type filament transformers on the shelves, as well as a few little 100-200VA ones some OM has put on a chassis with a plug and socket. I am saving them for projects. They are small and I don't think the annoyances will be noticed. Those kind of small Sola regulators are pretty innocuous.

My low voltage rack has two 24V/25A Sola DC supplies in parallel for working on military gear, and a 48V/5A one. I am very happy with them. I do not mind the buzz or the inefficiency but they don't run all the time. $20 each - It was far cheaper than buying active-device regulated supplies.

I'm just saying don't knock them all, and understand they have to be well loaded to avoid the annoyances. One thing they do well is eat large AC voltage excursions that would otherwise blow up the served equipment.
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« Reply #18 on: April 29, 2012, 11:49:55 PM »

Here are the schematics Patrick. 

I have the "A" model Speach AMP.  This was a one time run.  The reverted back to the basic design and used that through until the F which was changed again I understand.

I have included TWO  schematics.  One of A and one of the other design.

If you notice that on the "other" SA's they used two 2.5 chokes on the input.  My Speach amp does not have those chokes.  Only resistors.

Also.  On the Filiment supply,  There are two .01 caps but only on the First and second preamp tube.

C

I don't see anything there that ought to make a difference. The RF chokes should not contribute to hum unless they are ina  field and have a low-Z connection at the input end.

But you mentioned 4V as a bias on the filaments. That is interesting because I have always been told at least 25V and as much as 50V. If the hum is in any way from a H-K emission or the like, 4V is not enough to resist 6.3V.

Have you tried operating the unit, and then removing and restoring heater power from the suspected tube? An A-B test, As in, setting it up where you can disconnect it completely (both leads), even with a DPST switch. The hum should either do nothing or go away, or, anyway something has to happen.. It's an experiment, maybe a pain to do but it should give an answer as to whether the hum is due to filament power or something else, on a per-tube basis.

At one time I had a couple of those "test extension sockets", a 7 pin and a 9 pin, that I'd opened and wired with a DPST switch for exactly that purpose, because customers would complain of hum in their amplifiers and about half the time it was the tube. But a tube checker would not catch it.

The pic is of a MX-1258/U extension set.


* P2060101.JPG (82.53 KB, 640x480 - viewed 432 times.)
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« Reply #19 on: April 30, 2012, 02:38:58 AM »

I looked at those schematics you posted.  Not sure what they are but, I would get rid of those filters with the iron core chokes.  I once built a filter in one of my HB rigs.  It used a iron core choke.  It picked up a lot of hum from other xfmrs on the chassis.  I tried everything to get rid of the hum pick up.  Finally I had to remove the choke and filter.

You also mention something about the bias voltage affecting the hum level.  Make sure your bias supply is well filtered.

Fred
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« Reply #20 on: April 30, 2012, 09:46:04 AM »

Like Jim said, is it magnetic coupling? ? ? ? Some years back I had a situation that drove me absolutely NUTZ! ! ! ! ! I was using my DX-60, and then my little 5w piss weaker to excite my Junkston T-bolt. I had a terrible hum problem that was driving me crazy. First I tried DCing the filaments in the VFO - no help. Then I DCed the fils in the DX-60 - still no help. Then I chased it down looking for anything that might be causing a ground loop - STILL NO HELP!

I was about to reach for the BFH when the revelation came. I had been seeing the hum on the scope, but I hadn't actually listened to myself, so I put on the 'phones and starting listening to myself. Then the revelation came.........

I forgot to hit the plate switch on the T-bolt and there was no hum! ! !  As soon as I hit the plate switch the hum was there. Hmmmm............ Then I noticed that if I moved my chair, the hum would change  Huh  Huh

It ended up being the Dynamic mic that I was using at the time was picking up the magnetic field from the T-Bolt plate transformer! I had been had! ! ! ! !  

A little re-arranging of things totally eliminated the problem! And then I eliminated the T-bolt entirely just becasue it was a worthless POS......... Grin  Grin

I wonder if a T-3 could be picking up the plate transformer field with one of it's own interstage transformers, or something similar.
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« Reply #21 on: April 30, 2012, 10:13:42 AM »

But you mentioned 4V as a bias on the filaments. That is interesting because I have always been told at least 25V and as much as 50V. If the hum is in any way from a H-K emission or the like, 4V is not enough to resist 6.3V.

I have heard of using DC bias on the filament before, but never tried it. Of course, that would work only with indirectly heated cathode tubes.

Should the DC bias be positive, or negative, or does it make any difference?

For reducing hum I have had worthwhile results using an isolation transformer, so that both sides of the mains voltage are floating from ground and the equipment is isolated from the mains ground (green wire).  This may eliminate otherwise baffling ground loops.  Sometimes it helps to use an isolation transformer with a mid-tapped secondary with the tap grounded so that each side of the line is balanced, 60 volts above ground. Treat the 120v a.c. line like a balanced audio line. Try both the mains safety ground and a separate ground rod as your grounding point (for safety's sake do this only when using an isolation transformer), and see if it makes any difference.

If the iso transformer secondary is not mid-tapped, you can simulate one by bridging a mid-tapped choke across the output. The CT winding of a transformer, such as the HV secondary of a small power transformer or the primary of a push-pull audio output transformer, with all  the other windings left unused, will serve as an excellent choke for this purpose. Another possibility is a small variac.  Set the output voltage to mid-range, as measured with a voltmeter, and ground the variable tap. With the variac, you may be able to make a small adjustment to the mid-tap setting and null for minimum hum. Again, do this only in conjunction with an isolation transformer; otherwise you will probably blow a fuse or burn out the variac.
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« Reply #22 on: April 30, 2012, 10:44:54 AM »

A DC bias on directly heated cathode tubes is done all of the time in grounded grid amplifiers. It actually works out about the same as applying the bias to the grid.  Also in an ART-13 (which uses DC on all of the fils) uses the connecting points of it's series-parallel connected filaments to pick off the bias for the 811 modders. And maybe even the 813 (dont remember for sure).

Applying a bias to indirectly heated tubes, in theory should make no difference as the filament is "supposed" to be insulated from the cathode.

but if there is some heater to cathode leakage (which isn't supposed to be there) it could well make a difference. Capacitive and maybe even inductive coupling of the fils to cathodes could, however induce a hum from the AC potential on the fils from small amounts of stray AC that end up "leaking" through and showing up on the cathodes of high gain stages.
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« Reply #23 on: April 30, 2012, 01:29:44 PM »

1. So every one agrees the circiut with the 2.5 choke used on the previous and later speach amps are NOT needed?  I was thinking of adding them.

2. What about the 01 bypass caps on the Filiments at the first tube?

3.  Would you try the hum balance pot on this circiut?

4.  DZT,  I bypassed the High pass filter and the HUM is very loud. That filter has 50DB supression at 200hz and below. I think it said 60 or more at the 60/120hz point.  I cant bypass it. If i do, The hum is super super loud.  I do have the LOW pass filter bypassed.  It chops the Freq off at 2500 and up.  The goal is to reduce the hum and then bypass that filter.

One thing to NOTE is that printed ON THE FILTER is instructions to "Rotate" the transformer to reduce hum pickup!  Do they mean to flip the leads???  Or do they mean to ROTATE the Iron in the mount???  I found that to be odd!


Frank.  I have had the same kind of things going on here at time to time!   I found the problem was my Laptop power supply.  When I unplugged it, the station worked normaly again.  Lots of odd things can happen!



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« Reply #24 on: April 30, 2012, 01:58:03 PM »

One thing to NOTE is that printed ON THE FILTER is instructions to "Rotate" the transformer to reduce hum pickup!  Do they mean to flip the leads???  Or do they mean to ROTATE the Iron in the mount???  I found that to be odd!

  They mean to rotate the iron core! Must be one of those transformers with a single centered mounting thread and a jam nut. If so, this might be the "Bingo" you were looking for. A Campbell's soup can (if magnetic) over it might also lessen the hum.

Jim
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