Converting AC to DC filiments

[The Slab Bacon]

<snip> "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!" <snip>

Hmmmmm............ Sounds like they had that problem all along It must have some magnetic coupling to something else. Is there a fil transformer close by??

Many older designs used structured frequency response to eliminate the trash that made it's way into the audio. This was a very common thing that rears it's ugly head and bites you in the ass when you try to produce HI-FI audio. You end up opening up the floodgates to the garbage. Most of the all stock ones sound like the operator has a clothespin on their nose.

Unfortunately, I am not familiar with the innards of a T-3, but I have heard some that sound real good. I wonder if they have eliminated the hum by not using the low-level stages and feeding them with line level audio further up the line. If not, the soup can testing may be the next step. (Or find someone that has a good sounding one and ask them what they did)

[KA2DZT]

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!

I would get rid of any iron core chokes in the low level audio stages.  Just by-passing it while it's still connected in the circuit probably would have made the hum louder.  You need to cut the whole filter out and place it over on the table someplace.

Rotating means just that, rotating the core to reduce hum pick up.  Minimum pickup occurs when the fields are at right angles to each other, in theory.

Fred

[ke7trp]

Ok.


So Next step here is to get a Sat when I am bored, then modify the amp. I need to remove the Filters from the chassis, Braid the filiment wires and install Hum balance Pot.  Then back in and test again I guess.

I wish I had another speech amp to install and then I could just gut this one and install the Circiut out of the 1950s handbook and start Fresh.

C

[ke7trp]

While thumbing through the manual,  I noticed I overlooked TWO Can capacitors.  C17 and C18 on the modulator deck.  One is 1,100 UF and one is 400UF.  Voltage is not given. 

These caps are hooked to the center tap of the filiment transformer for the Bias supply.  It looks like it might be a source of bias for the carbon mic?

Can someone please help me and look at the following schematic for C17 and C18.  They are 60 years old, I am guessing they are shot. They are Typical twist lock malory can capacitors, I cant read the voltage rating on the caps.

I have caps here near those values in 63 volt rating.  Do the values need to be exact?  It looks like some kind of half wave supply.  If those are dead, Could this contribute to the hum? I cant imagine they are good after 60 years! 

C

[Opcom]

C17 and C18 are very important. The current waveform through R19 would be a "fullwave rectified" sinewave, like raw DC, without them. If they are bad, the carbon microphone will receive its DC current with a generous helping of hum.

As for the bias for filaments, I have always read the filament should be positive to prevent leakage current to the cathode. It has something to do with the way the parasitic heater-cathode "diode" works. I'm not sure I ever saw a detailed scientific explanation until now, never looked for it just believed. It's an ancient practice. The link mentions the RCA RC-26. The attachment is the paper from 1936.
http://www.diyaudio.com/forums/tubes-valves/187134-elevated-heater-voltages-40v.html

As for those RF chokes, maybe they are only needed if there's a problem. If your set did not have them, why add them? As for their metal, a ferrite core choke might be better than an iron core one for rejecting hum but that is only an opinion. It's a question for a transformer expert, not me!

[ke7trp]

So C17 and C18 are only for bias supply for carbon mic?  The carbon mic works great with the same low level hum that the rest of the rig has.  Only, You sound like you are being called to the principals office 


What voltage would you suggest as replacements?  Its confusing because it shows them hooked to the hv supply. They must be low like 50 or less.  They did not make HV caps in those values anywhere near that size in 1951.

As for the chokes..  The Basic and first version had the chokes.  Then, this A model removed the chokes.  The B and all other versions had them installed.  Its like they pulled them out and then put them right back on the next model.

I ordered some. I might put them in. Why not? 

I am very confident that the low level hum is the Filter/filiments in the 12au7. If I remove the 12au7 the hum is gone.

How about a I sacrafice an old 12au7 and snip its Fil pins OFF>  Then plug it back in with no fil power as a test.

C

[KA2DZT]

The voltage rating of those two caps would be low.  Your 63 volt caps would be fine.  I think you should check the two 8ufd caps that are connected to the two 6hy choke.  They would be high voltage caps (450v).

I took another look at the schematic.  I don't like the fact that the two 8ufd caps are grounded back to the center tap on the filament xfmr.  Probably should have been grounded to the neg return or chassis ground if one is used.  Anyway, the two 8ufd caps are grounded through the high capacitance of the 1100ufd cap.  So if that cap is open you have no filtering on the mod bias supply.

I think you did mention that the bias voltage affected the hum.

Have to take another look, not sure about my comment above.

Took a third look.  The two 8ufd caps are grounded through the 100 ohm resistor.  That doesn't make much sense since there will always be some ripple left unfiltered.  The 8fd caps should be grounded at the ground point of the 100ohm resistor.

Fred

[ke7trp]

Fred, Thank you very much.  I had read that section of the schematic half a dozen times last week scratching my head.  WHy did they use a 100 ohm?  This makes no sense to me. 

1. I have some 1000UF at 63volt caps here. Will that be sufficient for the replacement of the 1100?  I can test them all and find the high one in the batch.

2.  I have some 250UF at 50 volts caps here.  I will use two for the replacement of the 400 uf can. I will find an equal match.

3.  I will move the ground point to the other side of the 100 ohm or chassis ground for both 8UF caps.  I have already replaced the caps with 22 UF 500 volt electroylitics.  This change is simple and will take moments.

Sound good?

[KA2DZT]

Moving that ground point may or may not make any difference, give it a try.  The 1000ufd cap should be OK, not much difference between 1000ufd and 1100ufd. a few 250ufd will work fine for the 400ufd.  The voltage drop across the 100ohm resistor is for (I think) the carbon mike.  

That whole power supply is nothing more than an ordinary FW power supply.  The difference is that they use negative lead filtering.  Usually the ground point would be at the CT of the HV winding. This would make the whole supply voltage positive with respect to ground.  The most negative point is the HV winding CT.  The most positive point is the CT of the filament xfmr.  Even if there were no ground point you would still read the full output voltage of the supply with a volt meter placed from the HV CT (neg) and filament xfmr CT(pos) (400-500v).

There is a voltage divider across the whole supply (3500ohm, I think is was 350ohm and the 100ohm resistor).  Now add the ground point somewhere along the voltage dividers.  In this case it is at the junction of the 100ohm and 350ohm resistors.  So you can see that with respect to chassis ground the HV CT is still more negative than the chassis ground. Hence you have a negative bias supply with respect to chassis ground.  From the chassis ground point,  the filament xfmr CT is still more positive than chassis ground. Hence a positive voltage for the carbon mike bias.  A carbon mike needs some current flowing through it to work.  This positive low voltage also has its own CRC filter circuit (1100ufd, 100ohm, 400ufd.

Because the 1100ufd cap is across only a 100ohm resistor with nearly 4000ohms in the whole voltage divider, you can see the voltage division is about 40 to 1.  With a total voltage from the supply being a max of about 500 volts you can see that there is probably no more than about 15 volts across the 1100ufd cap.

Hope some of this helps you to understand your power supply circuit.

Fred

[The Slab Bacon]

Interesting circuit........... the 2 8uF caps are the inpoot filterz for the whole shebang but..... The positive side of that supply goes to ground through a voltage divider circuit, to provide a low B+ for other things. The 1000 and 400 are fairly low voltage caps. If you are going to buy them, I'd go for 160v caps just to have a little cushion factor. If bad, they could cause a hum issue. Be sure to replace the 8uF caps as well as they are filters for the modulator biass.

I have seen similar circuits in old 30s broadcast receivers, Philco was famous for them, except they put it in the negative lead (seondary center tap) to ground side to obtain some biass for the outpoot tubes. Also check all of the associated resistors to make sure they haven't gone high.

[The Slab Bacon]

Many older designs used structured frequency response to eliminate the trash that made it's way into the audio. This was a very common thing that rears it's ugly head and bites you in the ass when you try to produce HI-FI audio. You end up opening up the floodgates to the garbage. Most of the all stock ones sound like the operator has a clothespin on their nose.

Just a return note on this one. When I did the Invader 2000 that was one of the biggest problems I had to deal with. Once I opened it up for some bass response,
everything started showing up in the audio. The factory "eliminated" the problem by narrow banding the living hell out of the audio. It was quite a can of worms to deal with...........

[ke7trp]

Fred, Frank.
Thanks alot for helping me out!   I understand how this works now.  I was scratching my head on why they would have that 100 ohm resistor there for the ground.  I want to keep the carbon mic working and make this rig look stock.

I will yank the deck out again after work and install the 1000uf/63volts caps under the chassis and maybe I can find a high reading 250 to replace the 400.  Otherwise, I can series two 1000s and find a combo thats near 400.  I ordered some 100 UF 450 caps and received 1000s/63. The seller on ebay told me to keep the 1000s and sent me 450s.  I got 20 of them so why not use em!

I will also check those resistors frank.  Maybe one or more are shot.

The rig is working great now guys.  The hum level is low and the rig is running great. But as mentioned.. If you remove that audio filter and add in the higher value cathode bypass caps or more fidelity, You OPEN the flood gates for TRASH and the sharks smell blood and come swimming in for MHZ's away 

All of this is just for fun and to keep me busy and out of trouble.

THanks again!

C

[KA2DZT]

The problem with the circuit is the 8ufd caps are grounded in the wrong place.  They should be grounded at the chassis ground, the junction of the 100ohm and 350ohm resistor.  Once they're grounded there you can see that now the 1100ufd cap has the correct polarity(two filter caps in series) to complete the 8ufd cap filtering back to the PS positive which is the filament xfmr CT.  The way the circuit was built the positive side of the 8ufd caps is connected to the positive side of the 1100 cap.  The 8ufd filtering is completed to chassis ground through the 100ohm resistor.  This will always leave some level of ripple on the mod bias supply.

The voltage across the 1100ufd and 400ufd caps is low, probably not more than 15 volts.  You don't need 160volt caps.

[The Slab Bacon]

The problem with the circuit is the 8ufd caps are grounded in the wrong place.  They should be grounded at the chassis ground, the junction of the 100ohm and 350ohm resistor.  Once they're grounded there you can see that now the 1100ufd cap has the correct polarity(two filter caps in series) to complete the 8ufd cap filtering back to the PS positive which is the filament xfmr CT.  The way the circuit was built the positive side of the 8ufd caps is connected to the positive side of the 1100 cap.  The 8ufd filtering is completed to chassis ground through the 100ohm resistor.  This will always leave some level of ripple on the mod bias supply.

How do you figger, Phred? ? ? ? The 8uF caps are across the total of the rectumfryer outpoot. They are connected from the filament of the rectumfryer to the center tap of the secondary winding. They are filtering the whole high voltage outpoot of the power supply. Just the positive end after the filters is held up slightly above ground (the 100 ohm resistor) to allow to tap off a small of + to operate other things. That whole voltage divider is also in effect the bleeder stack as well.

They used that same bleeder / voltage divider in old broadcast radios for many years to tap off some biass for the audio outpoot tubes. They just put it in the negative side instead of the positive side. The negative side was usually insulated from ground with a cardboard tube and a phenolic mounting wafer and tied to the center tap of the power transformer secondary. Which was then grounded through a resistor. The biass pick-off point was the junction of the secondary CT and negative side of the filter can.
I have been zapped many times when that resistor opened up with time. If you were to directly ground the 8uF caps it DEFINATELY create a hum. I have seen this with my own eyes when I misconnected a replacement cap in an old broadcast set.

[KA2DZT]

Frankie,  I disagree.  The way the circuit is designed there will be some level of ripple that can be measured from the chassis ground to the CT of the filament xfmr.  The thing you're overlooking is that the bias for the mod works from chassis ground.

Build a power supply and put a resistor in series with the ground lead that runs to the neg terminal of the filter caps.  Let me know how much ripple you can measure at the neg terminal of the filter caps and ground.

Fred

[The Slab Bacon]

Frankie,  I disagree.  The way the circuit is designed there will be some level of ripple that can be measured from the chassis ground to the CT of the filament xfmr.  The thing you're overlooking is that the bias for the mod works from chassis ground.

Build a power supply and put a resistor in series with the ground lead that runs to the neg terminal of the filter caps.  Let me know how much ripple you can measure at the neg terminal of the filter caps and ground.

Fred

Phred,
          It shouldn't matter. The filter is between the positive output lead and the negative output lead (2 chokes and 2 8uF caps) All of the filtering for the HV side is between + and - period! A voltage divider between + and - why would you want to put the filtering between the divider and ground? ? that would induce a hum in the outpoot. Take a very close look at the skizmatic that Clark posted. It is right by design. Exactly the way that I would do it if I needed to build it. That basic circuit but flipped over to the negative side has been around longer than either of us.

Take a real close look at the schematic and put your glasses on and scratch your head a few times. I think you'll see what is going on. I do value your opinion and respect your knowledge, but............... You are dead wrong here. It should make no difference.

The 400 and 1000uF caps are put there because that small voltage shows up to the low level AF stages where even a very small percentage of ripple applied to the grids would be highly amplified as it passes through the audio chain. Basically a "just for good luck" deal.

[KA2DZT]

I'm going to run a few experiments later tonight to see what exactly is going on with that type of circuit.  I'll let you know what I find.

Fred

[ke7trp]

After work i pulled the deck out. Cap c17 was open.  C18 measured 500uf. Super leaky. I installed a new 1200uf and two a 500uf.  The ground runs from a very large resistor. Must be 100 watt. It measured ok. Going to fire it up in a few moments.

[KA2DZT]

Frankie,

I did a quick experiment.  I have a power supply that has it negative and positive terminals isolated from chassis ground, it's a floating supply.  The supply is well filtered.  I measured the ripple with a DVM set for AC.  The supply had nearly no ripple with the meter leads connected to the neg and pos terminals.  Just what I would expect.

I then try another AC ripple measurement.  This time I connected one meter lead to chassis ground and the other to one of the PS terminals.  I was reading the full AC ripple with this measurement, just what I thought would happen.  So, even though the PS is well filtered, when measured between chassis ground and either neg or pos terminal the full AC ripple is there.  If I grounded the unused terminal to chassis ground the ripple completely disappeared.

So what have we learned.  A well filtered supply that is floating, will still have full AC ripple between chassis ground and the supply terminals.  When one of the supply terminals is grounded the ripple disappears.

In the xmtr supply that we have been commenting on, the PS is floating except for it is grounded through the 100ohm resistor.   So with a 100ohms in series with the pos point to the chassis ground you end up with some level of ripple as measure from chassis ground and the pos point (filament xfmr CT).  The amount of ripple is somewhere between no ripple (zero resistance to chassis ground) and full AC ripple (open to chassis ground).

This residual amount of ripple appears at the pos point and also at the neg point (bias for the mods).  So even though the supply is well filtered, the whole bias supply has a ripple when measured from chassis ground.

This can be eliminated by grounding the 8ufd caps to the chassis ground point instead of the CT of the filament xfmr.  With this, you then need the 1100ufd cap to completed the filter back to the fil xfmr CT.

This is my 3 cents on the subject.
Maybe we can get some more opinions.

Fred

[ke7trp]

Ok.  I got the deck back in.  The hum level is now so low I think it would never be noticed.  Its only 20DB over the noise floor on the scope.  I can hear a little hum using headphones and listening to the monitor rxer.  It might not be 60. It might be 120. 

The hum does not vary with MOD Bias now.

The hum does vary with the Mic gain a little. I think that is expected because of the filiment hum.

I wondering about grounding that center tap..  Should I try it and what circiut change needs to be made?

C

[Opcom]

Just to answer on cap voltage - the current rating of the transformer there is 220mA, therefore the most current that could pass through the 100 Ohm resistor in the resistor stack R19 R22 R23 is 220mA, and that would make 22V. Since it is possibly a rectified sine and would have a peak value, multiply by 1.414, and the minimum cap volts might be 31VDC.

the resistor R19 should dissipate no more than 220mA * 31V, or 7W.

The 8uF caps passing through the 100 Ohm resistor to GND won't have significant ripple added to them from R19 if R19 is bypassed by the 1100uF cap.

You could move the bias supply GND to the fil. CT and move the carbon mike power to the top of R19, but there is no benefit I can see doing that. It could have a negative consequence of coupling the mike power to the bias current.


Snipping the heater pins from a tube won't allow an a/b test for H-K leakage because the heater won't be hot and the tube won't work.

[ke7trp]

The 100 ohm is not a 100 watt. I meant 10 watt. Its one of those green flat resistors that mounts with two studs.

I think I will move on to the filiments in the SA.  All the tubes are grounded on one side of the tube socket and then one wire snakes around inside the chassis.  Kinda like a johnson rig.

I am considering making some twisted pair wires and run them to the fils for each of those preamp tubes.  I almost bet that solves the SA hum and will allow me to get rid of that Hi pass filter.  This thing is pretty tiny sounding with nothing below 300hz.

If that does not work. I am going to try the 100 ohm hum balance pot.
C

[KA2DZT]

The only thing I would try is moving the pos wire of the two 8ufd caps from the fil xfmr CT to the ground point of the 100ohm resistor.  The reason I suggest this if you look at the polarity of the 1100ufd cap you'll see that the pos side is also connected to xfmr CT.  You have two caps with their pos sides connected together.

If the 8ufd cap's pos side was at the ground point it will be connected to the neg side of the 1100ufd cap.  This is the correct way to have them connected.  You end up with complete filtering of the bias supply with respect to chassis ground.  This is what you want.  It eliminates any ripple issues with the 100ohm resistor.  The 1100ufd cap completes the filtering back to the fil xfmr CT. 

This is the same thing as when you stack filter caps in a HV supply.  The 8ufd caps in series with the 1100ufd cap has no reduction in the amount of capacitance.

Fred

[The Slab Bacon]

Clark,
        I wouldn't ground the center tap of the rectifier fil winding. You then wouldn't have a place to pick off the mic bias (and whatever else may get it's power from there). Why re-invent the wheel if you dont have to.

Phred,
         "in theory" if the power supply is "floating" there should be NO potential, AC or DC between either leg and chassis ground, short, sweet and simple.
But............ You always seem to get a small amount of capacitive coupling between the primary and secondary windings. Henceforth the 1000uF cap is basically a short at AC against the small amount of capacitance between windings.

Going back to old broadcast receivers that use that circuit in the negative side, they usually have one filter cap between B+ and the "floating" B- lead, then another one across the whole shebang. The one across the rectifier outpoot is the basic ripple filter, and the one across the whole shebang to ground acts both as another filter, and bypasses out any AC riding on the outpoot from capacitive coupling in the transformer.

I would also think that a mil-spec transformer like you have on a T-3 would have that calculated in and held to a close tollerance. (Spec writers are always looking for something to make themselves look necessary) Sounds like now, all Clark has to do is adjust the rotation of that one interstage core and he is good to go. 

[k4kyv]

I have rotated power transformers, and "played  chess" with them to find the optimum spot on the chassis, and optimum orientation, to produce minimum hum pickup. The same thing works with the resonant coils in a multi-stage transmitter. Once the coil in the first or last stage is located, the coil in the following or previous stage is temporarily resonated with small variable cap and moved around and rotated until an rf sensor indicates minimum coupling.  Then the first two stages are fired up and the next coil down the line is treated the same way.  With multiple stages and multiple tuned circuits, each subsequent coil may have stray coupling to more than one previous coil, so the location and orientation may not be where you would think.

This technique is especially useful in open breadboard construction where each stage is unshielded from the rest.

With an audio amplifier, a transformer may be picking up hum from multiple sources as well, even from a nearby piece of equipment.  I once had that problem when my speech amp was located on the table adjacent to the receiver.  Moving it some distance away significantly reduced the hum.