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Driver Transformer




 
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W9ZSL
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« on: January 10, 2019, 01:03:42 PM »

I have a Stancor A-4762 Poly-Pedance Driver Transformer which has a split primary for a pair of tubes. Is it possible to use it single-ended by shorting out the 2 red B+ center-tap wires and then run one of the remaining (brown) wires to the tube and the other to B+ or would I be better off only using 1/2 of the primary? The schematic calls for a 3:1 primary to 1/2 secondary ratio. The transformer provides provision for 2.6:1, 3:1, 3.2:1. 3.4:1. 4:1. 4.5:1 and 5:1 ratios.
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KA2DZT
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« Reply #1 on: January 10, 2019, 01:25:03 PM »

Try using the whole primary otherwise your step-down turns ratios will be 1/2 of the ratings.
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W9ZSL
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« Reply #2 on: January 10, 2019, 01:27:23 PM »

I've been trying to attach the instruction sheet. Let's try this. It worked! It's a bit hard to read (old). The modulator is the same as the one discussed with Bob Mayben a few weeks ago. I figured the whole primary
was the way to go. I've included the original diagram. The 811A's are on the same chassis, I'm eliminating the feedback circuit and substituting a 6L6 for the 6V6.

* Driver Instructions.pdf (138.04 KB - downloaded 51 times.)
* Driver 2.pdf (95.5 KB - downloaded 47 times.)
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KA2DZT
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« Reply #3 on: January 10, 2019, 01:33:45 PM »

I've been trying to attach the instruction sheet. Let's try this. It worked! It's a bit hard to read (old). The modulator is the same as the one discussed with Bob Mayben a few weeks ago. I figured the whole secondary was the way to go.

The whole primary
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W9ZSL
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« Reply #4 on: January 10, 2019, 01:36:47 PM »

Right. PRIMARY. Typo...or brain fart.
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WBear2GCR
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« Reply #5 on: January 10, 2019, 03:18:41 PM »

YMMV.

I'd not use this transformer "single ended".
If that is what we're talking about?

The reason is that the core is not designed for any significant unbalanced DC.

IF you want to use it single ended, then the way to do that is "parafeed" which is a type
of modified Heising. So, a choke to the plate of the tube, and then a cap to the transformer.
The choice of choke and cap does effect the LF frequency response. Clearly the rolloff, but
also it may produce a bass/midbass "bump" due to resonance developed with the two "L"s and the C.

So, it is a good idea to sweep it...

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W9ZSL
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« Reply #6 on: January 10, 2019, 07:49:39 PM »

Well, fortunately I haven't gotten far enough along to have wired it in yet. I have a plan "B" using a pair of 6V6 tubes. Anyone else have a comment? I hate the thoughts of punching two extra holes in the chassis.
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Patrick J. / KD5OEI
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« Reply #7 on: January 10, 2019, 11:49:25 PM »

It might be as well to punch two octal holes in the chassis on the driver side, and use a snap-in cap for one if it turns out it is not needed. Just plan for it in case you want to do tweaking with push pull later, the hole will be there already.

My only electronic suggestion is that your driver be able to make at least 2-3x the required wattage cleanly, for the sake of fidelity and good regulation over the half cycle to those class B grids. A push pull pair of drivers may be more linear, use two 6L6s in class A @ 250V?  The transformer seems to have the split winding advantage.
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WZ1M
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« Reply #8 on: January 11, 2019, 01:14:25 AM »

I have a good driver transformer out of a Johnson Ranger, if any one is interested.
Regards,
Gary
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W9ZSL
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« Reply #9 on: January 11, 2019, 11:13:07 AM »

My second choice was a pair of 6V6 tubes. I have enough parts for either one. The 6V6 unit will put out 12 watts. There is enough room on the chassis for the extra tube/s so I think I'll go that route. Of course, I can always slip in a pair of 6L6's. Transformer is good for 15 watts.
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Patrick J. / KD5OEI
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« Reply #10 on: January 11, 2019, 04:54:36 PM »

Exactly! Bet it's going to sound great! I'm a fan of 6V6 as well. Two in the SX-28 sound mighty fine.
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W9ZSL
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« Reply #11 on: January 11, 2019, 05:19:58 PM »

I just picked up the front panel. We have a fabricator in town who cut the meter hole and did a textured black powder coat job on it for $21. Looks great. I'll be posting pix as I go along. The mod iron is ART-13 and I left plenty of room on the chassis for something larger.
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WBear2GCR
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« Reply #12 on: January 12, 2019, 10:18:42 AM »

A pair of 6V6 is more than sufficient to drive 811a into B2.
(we're driving 811s? I hope)

Actually a pair of 6W4 is suitable as well.

My personal suggestion is to add a bit of resistive "load" to the secondary.
This is for two reasons:
- the transformer will likely look "at" being lightly loaded more favorably than being unloaded
- the grids will look like "unloaded" until they draw current - then the poor hapless tubes
will be called upon to produce current.
- the tube's load line requires a plate load, the primary does NOT automatically provide that.

The idea is to create less of a jump between the no-current to current conditions.

Again YMMV.

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W9ZSL
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« Reply #13 on: January 12, 2019, 02:17:12 PM »

Yes, I'm using 811A's with the secondary CT to ground. For now, I won't be running over 1250 Volts on their plates. The particular 6V6 driver is kind of "all purpose" in that it can be used to plate modulate a small transmitter in the 20 watt range, or it can screen or cathode modulate higher-power rigs. The instructions also state that a driver transformer can be substituted for the modulation transformer.  The plate-to-plate impedance of the 6V6 tubes will be around 12,500 ohms.  With a ratio of 3:1 to 1/2 secondary, that would turn out to be in the neighborhood of 4,160 ohms / 811A grid if I'm figuring that correctly.  So what resistance would you suggest on the sec? Of course, here's the rub. The original plan was to use a single driver tube like Bob Mayben (6L6) since this is the same schematic. However, my transformer is for push-pull. Keep in mind, I can change the ratio big-time. All along, I planned on using one of the two driver systems so making a change is no problem.

* IMG_20190110_0003.pdf (138.04 KB - downloaded 19 times.)

* 6V6StancorA.jpg (208.1 KB, 1280x1360 - viewed 73 times.)
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KK4YY
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« Reply #14 on: January 12, 2019, 04:29:49 PM »

I *think* that the stated ratio, 3:1 to 1/2 secondary is the turns ratio, not the impedance ratio. If so, that's a turns ratio of 1.5:1 primary to full secondary, which would be a 2.25:1 impedance ratio from primary to full secondary. So, to reflect back an impedance of 12500, a resistive load of 5555 ohms across the full secondary would do it. That said, I don't think you want to load it down that much. Something like 10k - 20k across the full secondary (grid-to-grid) would be where I'd start. There's no crime in trying different values and seeing how it goes.

Burning resistor smoke never hurt anyone - cough cough.

Don
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W9ZSL
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« Reply #15 on: January 12, 2019, 05:02:41 PM »

I've always had a problem with matching.  I've seen plenty of information about matching the modulator to an RF final, but little on inter-stage iron. All I know is the original, single-ended driver transformer was a UTC S-8 which was rated at only 5 watts and the instructions called for the 3:1 primary to 1/2 secondary ratio.  Yeah...turns ratio.
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WBear2GCR
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« Reply #16 on: January 15, 2019, 08:47:25 AM »


My view on this is fairly simple, since ur driving grids, and most of the time there is no current drawn,
the entire problem boils down to voltage swing. The grids need to develop sufficient voltage swing
to drive them fully. In the case of the 811a, it's nominally a "zero bias" class B tube, so everything is
driving the grids positive, but there is not much current draw until the tube is pushed hard, which of
course it will be to make 100% or better modulation.

That means that the ratio can be empirically determined by testing in the circuit.

The PP driver tubes want to be in Class A, in that they are biased on heavily, thereby making the load
line mostly impervious to the added current drawn by the grids.

I need to revise my earlier statement, since there are zero bias tubes and unless there is bias applied
because the B+ is on the high side of the 811s rating (not a bad idea) the tube will be in the positive grid
region the whole time. So, the driver tubes will be seeing current draw, and there won't be a transition
point between no grid current and grid current - a smoother load line, which is good. I'd still likely want
a light resistive load on the secondary, this is based on my observations of driver transformer performance
in audio amps (same thing as a modulator more or less).

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W9ZSL
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« Reply #17 on: January 15, 2019, 04:45:35 PM »

The 6V6's are operated AB1.
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Patrick J. / KD5OEI
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« Reply #18 on: January 15, 2019, 05:37:40 PM »

A pair of 6V6 is more than sufficient to drive 811a into B2.
(we're driving 811s? I hope)

Actually a pair of 6W4 is suitable as well.



6W6?
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