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Screen Mode a pair of 4D32's




 
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Author Topic: Screen Mode a pair of 4D32's  (Read 2431 times)
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WA4WAX
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« on: April 17, 2022, 03:23:55 PM »

From way back when. 


https://worldradiohistory.com/Archive-Radio-News/40s/Radio-News-1949-06-R.pdf


......How to match for push-pull?
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« Reply #1 on: April 18, 2022, 01:32:54 AM »

On page 110..

Do you mean matching for RF circuit as shown (because no coil data is given)?

I pray someone correct me if I am wrong here:

For RF matching a push pull vs pi network circuit,
1. take the pi network reactance values for plate load for two parallelled tubes and multiply by 4 for the plate-to-plate value of the center tapped L and C reactances (double XL, double XC).

why?  
Because it is a tuned transformer with similar impedance ratio relationships as an audio transformer.
The push pull load will have the same signal voltage across half of it as the total across the single ended load
The push pull load will have half the signal current through each half of it as the total through single ended load
(same total number of tubes, same power).


The BVL-10 coil is a 10-meter plug-in coilset from Barker & Williamson. Also is the MVL-10 - so one for each band, with suitable two-section tuning caps. Maybe there are specs for their use online or hopefully will be added locally?

someone has some online here to look at:
https://antiqueradios.com/forums/viewtopic.php?f=15&t=228639

Or just go with two tubes in parallel and a pi network. Simpler bandswitching.

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« Reply #2 on: April 18, 2022, 02:51:26 AM »

couple of other good articles in that issue!

page 44 - 3-tube hi-fi compressor and expander circuit (as part of the project discussed)

Page 46 - Transient audio distortion reduction circuit for power amps - simple 25K pot and 2 Ohm resistor added  to overall negative feedback circuit smoothly varies the setting from voltage feedback to current feedback from the load connected to a 25W tube type power amp. I have used and very much appreciated this simple idea when driving single full-range speakers from 6" to 12". Current feedback 'forces' the voice coil to better follow the audio waveform regardless of speaker impedance variations over the frequency range. To me the current feedback improves the sound of a cheap speaker within its limits. YMMV.

Page 50 - Transmitter using Taylor Super-Modulation (Rufus P. Turner, one of my favorite authors). Note that this uses 807s in push pull as class C RF amps, and that two paralelled 807s is roughly one 4D32 from an impedance matching standpoint, and coil data is given so, think about the OP's question.

Page 56 - Speaker cabinet for the SWL and hi-fi AM folks - a fairly compact LF baffle for a small speaker. It's food for thought and not about little 4" desktop or reciever speakers. It's more about the slightly larger type that can sit atop the rack and angle down to the listener. They mention 60hz, so OK.
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« Reply #3 on: April 26, 2022, 06:08:03 AM »

The grid-leak must be 5k/4W with 20mA (bias - 100V) !
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« Reply #4 on: April 27, 2022, 10:27:35 PM »

From way back when.  


https://worldradiohistory.com/Archive-Radio-News/40s/Radio-News-1949-06-R.pdf


......How to match for push-pull?

This would be an awesome build if one paralleled the 4D32's linked to a pi net output network.

Instead of a shunt modulator, which is a rather power hungry way to modulate, I would use a 6EW7 using this cathode modulation circuit:

Phil - AC0OB

 

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« Reply #5 on: April 28, 2022, 09:40:52 AM »


Instead of a shunt modulator, which is a rather power hungry way to modulate, I would use a 6EW7 using this cathode modulation circuit:

Phil - AC0OB

 

Phil, Iím having trouble figuring out what the circuit you posted modulates. I think itís output (through the 51 ohm resistor) is audio with a variable dc offset. Given the amount of resistance around the tube (150k cathode R),  it doesnít seem like it can provide or sink much current.

So how does it work and what does it connect to?

Ed
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« Reply #6 on: April 29, 2022, 12:06:54 AM »


Instead of a shunt modulator, which is a rather power hungry way to modulate, I would use a 6EW7 using this cathode modulation circuit:

Phil - AC0OB

 

Phil, Iím having trouble figuring out what the circuit you posted modulates. I think itís output (through the 51 ohm resistor) is audio with a variable dc offset. Given the amount of resistance around the tube (150k cathode R),  it doesnít seem like it can provide or sink much current.

So how does it work and what does it connect to?

Ed

The first stage of the 6EW7 is a Class A RC coupled amplifier which amplifies the audio from the speech amp to give high peak-to-peak audio to the second stage, a cathode follower circuit with a high input impedance and a low output impedance. The 6EW7 is simply a higher current version of the 6DE7.

Pin 9 of the second stage and the 51 ohm resistor feeds composite audio to the grids of the 4D32 Pin 2. It will source 65 mA easily. Current sinking is mostly provided by the screen grid current with help from Rk2.

RVD1 and RPR sets the screen grid voltage of the 4D32 to about 150V quiescent.

I use this same circuit in the T-150 and others have used it to screen modulate higher powered tetrodes. I have annotated it for the 4D32's.

This circuit allows you to control the output power of the transmitter because it controls the voltage on the second stage's control grid, which in turn controls the voltage on the screen's of the Finals.

This is just a suggestion as an alternative to the 'shunt' modulator in the original article and substitutes one tube for two.

* 6EW7 Modulator for Power Tetrodes.pdf (42.73 KB - downloaded 69 times.)
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« Reply #7 on: April 29, 2022, 06:19:56 PM »

Here is how I envision the circuitry for the RF deck:



Phil - AC0OB

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« Reply #8 on: April 29, 2022, 06:43:20 PM »

No Phil !
For 2 x 4D32 (100Wa), under B+ = 680V and Vg2 = + 150V, Ip must be only 0,22A.
The carrier will be 50WHF and the Pi net-work must be revised for Q = 10 !
Best 73
John
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« Reply #9 on: April 29, 2022, 08:57:21 PM »

No Phil !
For 2 x 4D32 (100Wa), under B+ = 680V and Vg2 = + 150V, Ip must be only 0,22A.
The carrier will be 50WHF and the Pi net-work must be revised for Q = 10 !
Best 73
John

Greetings from across the pond Jean-Pierre.

The Q in the article was not specified so in my Weigand Pi-net equations, which are programmed in MatLab with the Terman corrections, I used a Q = 12. If the OP wants a different Q of 10, which he did not specify as well, then the Pi-Net values will differ by about 15%.

A Q of 12 is normally thought of as median Q for Amateur Radio Pi-net circuits.

500mA@680V is about the peak current required for the plate voltage rail.

Interpolating from the curves for 680V Vp, the current per plate is 227 mA.

The power supply for a dual 4D32 is designed for a peak system current of 600mA@680V since peak plate current is 454mA + 65mA for the modulator = ~ 520 mA.

The PS requires a 405VA transformer.

If one examines the many Vintage circuits for Amateur Radio transmitter Finals, you will notice the circuits were not designed using ClassC plate modulated and or screen modulated transmitters@100% modulation, rather, the manufacturers used the "RF Power Amplifier and Oscillator" curves and specs.  

For this transmitter, I used 680Volts Vp which is 70 volts below the typical operating specs for "RF Power Amplifier and Oscillator" curves and specs.

As a final note, the Viking I with the 4D32 Final had a typical plate supply voltage of from 660V to 710V.

If you would like to present a suggested circuit as well, I am sure all would be interested.


Phil


 

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« Reply #10 on: April 30, 2022, 05:51:13 AM »

OK Phil.

The power supply must be designed for a stable quiescent current of 250mA (under +600V) from the PA (2 x 4D32), the peak current being provided by the filter capacitors.
I don't know how to publish a diagram on the site, but here are the characteristics for G2 modulation of 2 x 4D32 :

- HV = + 600V
- Ip  = 250mA (steady)
- VG1 = - 100V
- IG1 = 20mA
- VG2 = + 150V
- Carrier = 50W (steady)
- Wa = 100W (maxi)
- Pi-Network for Q = 12 at 3700kHz for Zant = 50 ohms (from QST 11/1955)
- ZHF = 500 x 600/2X250mA = 600 ohms
- L = 2,75ĶH
- CPT = 850pF
- CPL = 2850pF

You can see my AM BGV-2014 (500WHF = 2 x 813) on french site ARACCMA.
My AM BGV-2018 is a G2 modulation of 813 under 1500V (with clipper).

Best 73
Jean-Pierre
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« Reply #11 on: April 30, 2022, 05:07:44 PM »

I have plenty of 4D32's......and a nice Kenyon T-655.  Thoughts?
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« Reply #12 on: April 30, 2022, 07:28:03 PM »

I have plenty of 4D32's......and a nice Kenyon T-655.  Thoughts?

I think it would be a fun build.

If I wasn't already working on a Valiant I think I might start cutting sheet metal myself.


Phil - AC0OB

* 4D32 Screen Grid Modulated Class C Parallel 4D32s.pdf (118.56 KB - downloaded 71 times.)
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« Reply #13 on: May 01, 2022, 04:28:44 AM »

All my frames are laser cut. Cheap !
Look at...


* BGV-2018 (2).JPG (1614.77 KB, 3264x2448 - viewed 73 times.)

* BGV-2018 (10).JPG (1569.89 KB, 3264x2448 - viewed 69 times.)

* BGV-2018 (11).JPG (1623.65 KB, 3264x2448 - viewed 77 times.)
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« Reply #14 on: May 04, 2022, 04:39:57 PM »

...a nice Kenyon T-655.  Thoughts?

You need at least 550 mA so the Kenyon won't work.

Phil - AC0OB
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« Reply #15 on: May 04, 2022, 09:23:12 PM »

...a nice Kenyon T-655.  Thoughts?

You need at least 550 mA so the Kenyon won't work.

Phil - AC0OB


That is right- see datasheet attachment:

Also, these old transformers were mostly designed for choke input filters. If you put a Cap input filter on it, the current peaks will cause a lot of heating, so you may not get even the 250mA specified, from it.

You might consider a 400vA Antek 500vac transformer instead.
$72 plus shipping and they can handle C-input. Put the 400mA secondaries in parallel and use a bridge. Should get 650volts DC.

https://www.antekinc.com/as-4t500-400va-500v-transformer/





* EFF5C853-1617-476F-9AE9-8A1A49337385.jpeg (385.46 KB, 1460x843 - viewed 65 times.)

* E87AD204-A224-4842-B1D1-E9C0A5D30DCB.jpeg (352.44 KB, 2723x1797 - viewed 59 times.)
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« Reply #16 on: May 05, 2022, 09:08:14 PM »

...a nice Kenyon T-655.  Thoughts?

You need at least 550 mA so the Kenyon won't work.

Phil - AC0OB


That is right- see datasheet attachment:

Also, these old transformers were mostly designed for choke input filters. If you put a Cap input filter on it, the current peaks will cause a lot of heating, so you may not get even the 250mA specified, from it.

You might consider a 400vA Antek 500vac transformer instead.
$72 plus shipping and they can handle C-input. Put the 400mA secondaries in parallel and use a bridge. Should get 650volts DC.

https://www.antekinc.com/as-4t500-400va-500v-transformer/

I specified the

https://www.antekinc.com/as-4t475-400va-475v-transformer/

sheet 2 because it had a bit more current reserve.


* 4D32 Screen Grid Modulated Class C Parallel 4D32s.pdf (118.56 KB - downloaded 41 times.)
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« Reply #17 on: May 06, 2022, 12:58:01 AM »

...a nice Kenyon T-655.  Thoughts?

You need at least 550 mA so the Kenyon won't work.

Phil - AC0OB


That is right- see datasheet attachment:

Also, these old transformers were mostly designed for choke input filters. If you put a Cap input filter on it, the current peaks will cause a lot of heating, so you may not get even the 250mA specified, from it.

You might consider a 400vA Antek 500vac transformer instead.
$72 plus shipping and they can handle C-input. Put the 400mA secondaries in parallel and use a bridge. Should get 650volts DC.

https://www.antekinc.com/as-4t500-400va-500v-transformer/

I specified the

https://www.antekinc.com/as-4t475-400va-475v-transformer/

sheet 2 because it had a bit more current reserve.



Either will work as long as secondaries are hooked in parallel and connected to a bridge. I chose the 500vac model because it will be closer to the 675vdc on your schematic.
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« Reply #18 on: May 07, 2022, 03:58:42 PM »

How about a UTC S-44? I have one of those transformers.  It was liberated from an old mass spectrometer.
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« Reply #19 on: May 08, 2022, 03:03:05 PM »

...a nice Kenyon T-655.  Thoughts?

You need at least 550 mA so the Kenyon won't work.

Phil - AC0OB


That is right- see datasheet attachment:

Also, these old transformers were mostly designed for choke input filters. If you put a Cap input filter on it, the current peaks will cause a lot of heating, so you may not get even the 250mA specified, from it.

You might consider a 400vA Antek 500vac transformer instead.
$72 plus shipping and they can handle C-input. Put the 400mA secondaries in parallel and use a bridge. Should get 650volts DC.

https://www.antekinc.com/as-4t500-400va-500v-transformer/

I specified the

https://www.antekinc.com/as-4t475-400va-475v-transformer/

sheet 2 because it had a bit more current reserve.



Either will work as long as secondaries are hooked in parallel and connected to a bridge. I chose the 500vac model because it will be closer to the 675vdc on your schematic.

Yes and thank you. I was willing to give up a few volts for a tad more current  Cheesy

Phil
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« Reply #20 on: May 08, 2022, 11:36:29 PM »

How about a UTC S-44? I have one of those transformers.  It was liberated from an old mass spectrometer.
a

Questionable. According to the UTC catalog it is meant for a choke input filter, and the current spec is on the ragged edge.

Phil
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