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Screen modulation winding-bump up with a transformer or clamp tube?




 
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Author Topic: Screen modulation winding-bump up with a transformer or clamp tube?  (Read 524 times)
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KI4YAN
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« on: July 14, 2020, 03:54:40 AM »

Still working out how I can use one of my "Family 14" modulation transformers. I've got the parts I need to plate modulate a 4-65A tetrode, under the following conditions:

DC Plate voltage:      1300V
DC Screen Voltage:     250V
DC Grid Voltage:        -125V
DC Plate Current:      120mA
DC Screen Current:     35mA
DC Grid Current:         12mA
Peak RF drive Voltage: 225V
Peak A-F Screen Voltage for 100% Modulation: 175V
Driving power:            2.7W
Plate Input Power:      156W
Plate Output Power:    122W

Screen Dissipation:       9W
Grid Dissipation:          1.2W
Plate Dissipation:         34W

Modulating Impedance of the Plate: 10,833 Ohms
Modulating Impedance of the Screen: 7143 Ohms (Figured it just like the plate impedance...or do I need to calculate using the audio voltage listed?)

Modulation Transformer has windings of 10,000 ohms, with a second winding at 1000 ohms, meaning I'll have a 10,833 ohm winding and 1083 ohm secondary windings. Not quite the 7143 ohms I need...

Can I use this 1000 ohm winding to drive a step-up transformer to get up to the 7K screen impedance? Or if not a transformer (maybe too much phase shift?) drive the grid of a screen modulator tube, like a 6W6 or something? I'll have to have a screen clamp tube anyway for keying and safety, so maybe feed the screen clamp some audio for modulation?

Note that the peak screen grid audio voltage for full modulation is given in the datasheet as 175V on a 250V screen supply. If I should be using this voltage to calculate the modulating impedance of the screen, then it's 4861 ohms. Less, but still more than the 1000 ohms the mod transformer calls out.

Other options for this third winding include using it as an audio feedback winding, I'm thinking about using a long-tailed pair to do input and phase splitting for the modulator-this would allow negative feedback to be easily wrapped around the modulator for improved performance. Could go so far as use half the winding, if the whole winding is too much.
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WBear2GCR
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« Reply #1 on: July 14, 2020, 03:28:22 PM »


Not sure what you are doing.

You say "plate modulated 4-65".
So the screens are not modulated. Fixed supply.

Or are you using a pair of 4-65s as modulator tubes?
In which case, again, the screens certainly can be fixed voltage.
Standard tetrode operation.

IF you are putting signal on the screens? Then what are you doing
with the grids? And why not drive the grids?

The purpose of matching impedances is to transfer power, as long as
your voltage swing is adequate and you back it with enough power, getting
to the exact impedance ratio is not that important.

I think you are indicating a modulation transformer that has a 10:1 impedance
ratio?? Ummm... not likely to want to work, if this is the true ratio?

Got a picture of said iron?

Might work out better to cathode modulate that tube - not sure it's ever done
that way though... Hmmmm...

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« Reply #2 on: July 14, 2020, 09:28:13 PM »

Still working out how I can use one of my "Family 14" modulation transformers. I've got the parts I need to plate modulate a 4-65A tetrode, under the following conditions:

DC Plate voltage:      1300V
DC Screen Voltage:     250V
DC Grid Voltage:        -125V
DC Plate Current:      120mA
DC Screen Current:     35mA
DC Grid Current:         12mA
Peak RF drive Voltage: 225V
Peak A-F Screen Voltage for 100% Modulation: 175V
Driving power:            2.7W
Plate Input Power:      156W
Plate Output Power:    122W

Screen Dissipation:       9W
Grid Dissipation:          1.2W
Plate Dissipation:         34W

Modulating Impedance of the Plate: 10,833 Ohms
Modulating Impedance of the Screen: 7143 Ohms (Figured it just like the plate impedance...or do I need to calculate using the audio voltage listed?)

Modulation Transformer has windings of 10,000 ohms, with a second winding at 1000 ohms, meaning I'll have a 10,833 ohm winding and 1083 ohm secondary windings. Not quite the 7143 ohms I need...

Can I use this 1000 ohm winding to drive a step-up transformer to get up to the 7K screen impedance? Or if not a transformer (maybe too much phase shift?) drive the grid of a screen modulator tube, like a 6W6 or something? I'll have to have a screen clamp tube anyway for keying and safety, so maybe feed the screen clamp some audio for modulation?

Note that the peak screen grid audio voltage for full modulation is given in the datasheet as 175V on a 250V screen supply. If I should be using this voltage to calculate the modulating impedance of the screen, then it's 4861 ohms. Less, but still more than the 1000 ohms the mod transformer calls out.

Other options for this third winding include using it as an audio feedback winding, I'm thinking about using a long-tailed pair to do input and phase splitting for the modulator-this would allow negative feedback to be easily wrapped around the modulator for improved performance. Could go so far as use half the winding, if the whole winding is too much.

Yep, would be nice to see a schematic so we can tell what the heck you're trying to accomplish.


Phil
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KI4YAN
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« Reply #3 on: July 14, 2020, 10:57:18 PM »

I am plate modulating a 4-65A. I have a transformer that has a center-tapped 8400 ohm primary winding, a 10000 ohm secondary winding with 9K and 8K taps, and a tertiary winding that is 1000 ohms center-tapped.

Here's a shot of the winding information on the transformer:



I'm trying to determine if that 1000 ohm winding will have enough voltage swing to modulate the screen voltage, as is needed for full plate modulation of a tetrode.

If I CAN'T get enough voltage swing out of this winding, I'll try to use it as an audio feedback winding for the modulator. Not trying to do both at the same time, though.
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KI4YAN
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« Reply #4 on: July 15, 2020, 03:34:37 AM »

Here's a rough-in schematic for what I'm wanting to do. I just don't know if that third transformer winding will swing enough voltage.



Hope that's enough to see what I'm planning. Next step is to drive the transformer primary with 10 volts AC or so, and measure the other windings.
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WBear2GCR
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« Reply #5 on: July 15, 2020, 02:37:03 PM »


Figure the turns ratio from the impedance ratios.

Then divide the Primary voltage swing by the turns ratio of the Primary to Secondary
(the lower one in this case) and that is the swing you will get from the lower Z secondary.

Remember in your schematic the AC screen voltage is swinging around the 300vdc "bias"
voltage ur putting on the screens through the xfmr.

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KI4YAN
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« Reply #6 on: July 15, 2020, 04:32:52 PM »

Turns ratios:

Primary winding to RF Plate winding: 1:1.095 when configured on the 10K tap, 1:1.035 on the 9K tap, and 1:0.976 on the 8K tap

Primary Winding to Screen Winding: 2.898:1

So if I run Class AB1 807's at 600V on the plates, 20mA idle cathode current, and -32V grid bias, the primary voltage swing from one plate will be about 550 volts. That would put the total end-to-end (full 8400 ohm winding) at 1100v primary voltage swing.

So if the primary winding sees 1100v end to end, then I divide by 2.898 to get the voltage swing on the 1000 ohm winding that I want to use for the screen modulation. That gives 379 volts swing, over the whole winding, or 189volts if I only use half the winding.

Even if I'm wrong and I only count the 550V swing of one modulator tube, that's still 189 volts over the whole winding.
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KI4YAN
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« Reply #7 on: July 15, 2020, 04:41:36 PM »

Armed with that information, I figure I should actually check my numbers to see if the 807's will make enough power.

Push-pull 807's with 600V on the plates, 300V on the screens, -32V grid bias, 550V plate voltage swing at full audio drive.

Transformer using the 10K tap with the single 4-65A, which presents a 10,833 ohm modulation impedance.

That back-calculates about 9.099K plate-to-plate that the 807s will see-so that makes power output about 66W before taking into account transformer losses. If I figure 15% losses, that's 56W audio power delivered. Not quite enough.

If I bump it up to 700V on the 807's plates, with the same grid bias, that's 76W delivered power.

...Both op points are definitely going to put the 807's in ICAS territory.
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K8DI
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« Reply #8 on: July 15, 2020, 04:43:42 PM »

Turns ratios:


Another way to look at this is compare only the two secondaries, forget for the moment whether your mod stage can "get there". They are 10K and 1K, that's 10:1 Z, so 3.16:1 turns/voltage ratios, with
1300v on the plate. So (tetrode cutoff voltages aside) we want 1300v swing on the plate line, up to 2600 and down to zero. Dividing by 3.16, that is 411 volts swinging the 300v screen around -- up to 711v, down to -111v.  I'm pretty sure the negative screen is going to make the amp pretty non-linear/distorted as the modulation goes up. Using half the secondary for a 206v swing is probably a better starting point, and you still may have to do some finagling to get it linear -- some kind of voltage divider deal or maybe a higher standing screen voltage so it cant go as low under modulation, I wouldn't know without experimenting a bit...

Ed
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KI4YAN
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« Reply #9 on: July 15, 2020, 06:18:29 PM »

Well, the screen wants to see 175V of swing, on top of 250V standing voltage. Using half the winding gets 206V going by the 10:1 ratio between the two secondary windings, that's down to 44V on the screen, when the plate is near 0 volts, and up to 456V when the plate is near 2600V. That does seem better, at least the screen isn't going negative. I need to edit my rough-in sketch to reflect that.

Seems like I don't need to drive the 807's that hard to fully modulate the RF final-maybe.

According to my 1951 copy of "The Care and Feeding of Power Tetrodes", "In Plate Modulation it is necessary to introduce not only amplitude modulation of the plate voltage, but to develop about 70% amplitude modulation of the screen voltage as well, for 100% carrier modulation." Also, later in the same section: "Where Modulation voltage appears on an electrode of a tube, the r-f bypass capacitor of this electrode should be kept to about 0.002mmfd or less in order to avoid bypassing high modulation frequencies."

So that's how the datasheet came up with a screen grid voltage swing of 175V for a DC supply of 250V. 175V is 70% of 250. Using half the 1K winding on my transformer, and a DC screen supply of 250V, AND using the 10K full winding for the plate, I'll get about 82% modulation of the screen.

So, using the full secondary for the plate, I'll get 82% modulation on the screen. Using the 9K tap, things get further from the desired values in every respect-so using the full winding it is.

How can I reduce the amount of modulation to the screen, if it turns out 82% is too much?
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K8DI
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« Reply #10 on: July 16, 2020, 09:14:12 AM »



So that's how the datasheet came up with a screen grid voltage swing of 175V for a DC supply of 250V. 175V is 70% of 250. Using half the 1K winding on my transformer, and a DC screen supply of 250V, AND using the 10K full winding for the plate, I'll get about 82% modulation of the screen.

So, using the full secondary for the plate, I'll get 82% modulation on the screen. Using the 9K tap, things get further from the desired values in every respect-so using the full winding it is.

How can I reduce the amount of modulation to the screen, if it turns out 82% is too much?

Simple but a physically large/harder to find solution -- Heising styled reactor ...  see image. Choose the resistor ratio to load the winding to 1K, keeping in mind the parallel resistance of the screen itself. Choose the resistor ratio to also give the desired swing.

You could cut out the choke and cap, but then you'll have to also deal with the screen current divided between the resistor and the secondary, where the resistor provides no modulation and dissipates power. Either should work, but from an experimenting perspective, I'd prefer the R1-R2 pair be a 1.5-2K 10w wirewound pot I can just crank around until the modulation is cleanest....

Ed



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KI4YAN
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« Reply #11 on: July 16, 2020, 02:05:41 PM »

I'll make sure to leave room on the chassis for that. Reading through Care and Feeding of Power Tetrodes, they do a complete walkthrough of setting up a 4-65A in a plate modulated configuration, and go on to use a screen grid voltage swing of 85% of the screen voltage so I probably will be ok with using just half the winding and getting 82%. If I end up not using the extra mod transformer winding then I can always use the extra space for a screen choke too.

I think it is time to start fleshing out and detailing the schematics now!
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