The AM Forum

Hello all,

I have a very nice UTC filament transformer rated 5V@30A, I believe the part number is H20. This is a beautiful transformer that I would like to use in my next project. However, it has no center tap on the secondary. How would I use this transformer to power the filaments of, say, a pair of 250TH's? A simple hand drawn schematic would be nice.

Thanks, Ron W8ACR

250TH's filaments are 5V at 10.5 A. What's wrong with just putting the filaments in parallel?

Ron,

    I presume your looking for a way to use that transformer and be able to return the cathode current to ground without getting an imbalance, or 60 hz hum on the 250 TH cathode. Two options come to mind:

1.) Make a centertap from two resistors in series where each end is across the power transformer, and the junction of the two is the tube cathode return. These resistors need to be pretty small in ohms to not add a cathode bias, like two 10 ohm 2W resistors.

2.)  Use another transformer (much smaller) like a Radio Shack 12.6 VCT @ 1 A. Use the 12.6 volt winding just like the resistor. Here the DC resistance of the winding will be pretty low, so there should not be much added cathode bias like with the resistors.

Jim
WD5JKO

Hi Pete,

I thought there had to be a ground connection to complete the DC circuit. I understood this to be the purpose of the usual center tap. In other words, if one lifts the center tap the circuit is broken. The only other option that I could think of would be to ground one side of the filament transformer and likewise ground one of the tube filament connections.

Or - more likely -  is my assumption way off base?   ???

Ron

Yes Jim, that is what I was referring to. I understand your instructions OK. That sounds simple enough.

Thanks, Ron

I've used a 12.6 V center-tapped small filament transformer wired in parallel across the main transformer.  Then use the CT of small filament transformer as a "phantom" center tap.  Works great.

I am confused now.  I thought you needed CT for changing bias (if cathode driven) between cutoff and conduction.

Also, what is DC path to ground here about.  Isn't fil. current AC?

Rob,
       You still need a DC path to ground (or B-) for a complete circuit. No path to ground for the cathode, no current can flow through the tube. In a circuit using a directly heated cathode tube, it is usually done at the filament transfoma center tap to balance the AC offset and eliminate any hum problems from an AC powered filament.

Applying the biass to the filament center tap us usually done in a grounded-grid amplifier because it is the simplest way to do it.. If you want to apply the biass directly to the grid of a GG amp, you have to keep the grid at RF ground, while applying the biass, thus complicating things a slight bit and requiring a few more parts. Applying it to the filament CT is just the easiest and simplest way to do it.

That is what I was going to suggest.

Pete

The filament transformer sounds great, but if  you are planning to use a cathode resistor anyway to provide protective bias to the tube, they you might as well use the two resistor method.

In that case, each resistor would need to be twice the nominal cathode resistance, since as far as plate/cathode current is concerned, the resistors are in parallel.

The two-resistor (or midtapped resistor) method has been in use since the advent of broadcast receivers using directly heated a.c. triodes dating back to the 1920s (226, 171 and 245 tubes).  The beauty of the circuit is that the resistors are effectively in parallel for cathode current, thus keeping voltage drop to a minimum, while they are in series across the low-voltage filament supply, thus pulling minimum current from the filament transformer.  The midtapped choke idea (or auxiliary midtapped transformer winding) offers the best of both worlds, although perhaps more expensive and takes up more space. Of course, the best idea of all is to have an accurately tapped main transformer winding, but you have to make use of what you have on hand.

If this a linear amplifer circuit in Grounded Grid configuration, the filament voltage is almost always AC while the cathode bias is DC provided by a zener brought in to the CT of the tranformer or the midpoint of the resistor circuit.

The bottom line, first, tell us what the project is??

Frank thanks for the explanation--I get it now.

  I have a Gonset GSB-201 linear amplifier with four G-G 572B tubes in parallel. This amp is good for 200W AM 80-15m, and a little less on 10m due to what appears to be high tank Q losses. Anyway I noticed from the beginning that on AM I had about 5% 60 Hz AM modulation that had a pure sine wave shape on the scope monitor. Looking at the schematic I notice that the filament transformer for the 572B's had no centertap. I was shocked saying WT_ at what appeared to be a glaring design short coming on this amplifier.

  Thinking hastily, I forgot about the center tapped transformer idea as mentioned above. I found a 50 ohm 25W rheostat in the junkbox, and went with that as the dual resistor method also mentioned above. So with this I get 25 ohms each side, or 12.5 ohms to ground for the cathode current. Since E=I*R, at 600ma * 12.5 ohms we get 7.5V drop (cathode bias). This certainly reduced peak power output of the amplifier, and reduced the gain a little. Then thinking about it, isn't this a form of negative feedback? The hum is gone, and the AMP is very linear now; almost as good as an 8877 based amplifier.

Jim
WD5JKO

The series'd 10 Ohm resistors across the 3-500Z filaments make a nice CT and this works very well for the DC return.

The Gonstipated amp that I re-did for Derb had the same issue. It picked up a very noticable hum. I was a bit disdraught to see that the factory had grounded one side of the non center-tapped filament supply.

I did the 2 resistor fake center tap mod to it and all was fine afterwards. IIRC, I used a pair of 10 ohm 10w dogbones to do it.

The Clipperton L and similar 572B/811 amps from Dentron were the same. With more C the hum is way down on SSB/CW but may be noticable on AM.

Here are a couple of the techniques that folks have described in the thread. The caps would be required for for a conventional grounded filament amplifier. 

Why couldn't you use 2 diodes?
Put the anodes of each diode to either side of the filament and tie the cathodes together and apply the bias there.
I have seen this done on an sb-200.

W9BHI

When reading this topic from top to bottom, I had the same idea....
I've never used two diodes, but have seen a few schematics using this configuration.
The cathodes on each end of the fillament transformer, and the anodes joining at B-

Martin, PA4WM

Or zener diode. A great way to make bias for the tube. I tried it once on an 833A and the bias stayed steady at 90 volts no matter what the plate voltage was. It was a class B lineeeeeeer.

Wouldn't that result in ripple at 120Hz on the filament?

That is what you are trying to get away from with the two resistors.

Dave

Exactly, HUMMMMM - take an old linear amplifier like a Gonset GSB-201 with a 6V winding that had one side grounded. By lifting that side of the transformer and the four 811 fils off ground and making an artificial CT the, center can be properly grounded for nice HUM Free AM. Timmy turned me onto this idea.  Mike WU2D

Well just to satisfy my morbid curiosity I took my old SB-220 out of it's cabinet and flipped it over.
I disconnected the center tap of the filament transformer from the relay contacts.
I took two 3 amp 1000vpiv diodes and connected their anodes to the filament leads before the choke and connected the cathodes together and connected that to the relay contacts.
I fired it up and it loaded up just fine.
It had 4 milliamps less plate current than before because of the diode drop.
On air test showed NO hum in the signal.

W9BHI

Sorry guys, I was still stuck on the grounded transformer with no center tap but I did not quote it so it looked like I was reacting to the diode comment. Even looks that way to me. Here is the schematic of The GSB-201 Linear. Notice how the filament transformer has no center tap and that one side is grounded. That is the situation which can cause hum. Since we are talking linears - Grounded Grid - I redrew the schematics and added the diodes.

Mike WU2D

It is difficult for me to understand why the two-diode approach will not cause hum.
Non-linear circuits are difficult to analyze but...

1. The two-diode circuit will steer the plate current through one diode or the other as the filament transformer's voltage swings between positive and negative. I.e. if the filament voltage is across the back-to-back diodes, and if the filament voltage is not zero, then only one diode can be conducting. This behavior is similar to that of a mixer.

2. When the plate current is passing through the left-side diode, then a portion of the right-to-left AC voltage across the filament will add to the bias voltage. When the plate current is passing through the right-side diode, then a portion of the left-to-right voltage across the filament will add to the bias voltage.

3. Since the switching between the diodes is caused by (and therefore, synchronized with) the AC filament voltage... the result will be that the voltage added to the bias will be a rectified (approximately) 60 Hz signal. Thus it will contain harmonics of 120Hz (assuming everything is symmetrical.. e.g. the diodes are matched).

Stu

The two diode circuit is basically a full wave rectifier -- without a filter.

You will end up with a 120 Hz pulse train -- with each successive pulse on opposite ends of the filament.

But, if we are talking 5 volt tubes like 3-500z's then the pulse train is only 5 x 1.41 volts in amplitude and perhaps you won't hear it as it's only a small percentage of the 3KV plate voltage.