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 Post subject: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 1:11 am 
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Joined: Aug Tue 04, 2009 2:15 pm
Posts: 393
Location: Elmira, NY
I bought this at an estate sale. No schematic.
It appears to be a high power plate Modulator with integral power supply.
I tried tracing out a wiring diagram but someone else has been inside so I don’t have a complete circuit.
It has 8 vacuum tubes:
(1) 0B2
(1) 5U4
(2) 866
(2) 2A3
(2) VT-4C

Large HV Plate Xfmf
Huge plate Choke
Huge Filament Xfmr
Large 940 V, Ctr tp Xfmr
Large 10 Hy Choke
Small 3:1 interstage Xfmr
2nd larger 3:1 interstage Xfmr
110/140 Xfmr

The HV Mercury Vapor supply is just that with some filtering. I believe it provides the B+ for the plates of the VT-4C’s, and then connected to the secondary of the modulation xfmr the onto a external transmitter final.
What exists is an interstage Xfmr feeding two 2A3’s, their output feeds the VT-4C’s, and the VT-4C’s at the modulation Xfmr. I am unclear how/where the center tap of the first interstage Xfmr is connected. I’m sure the 110/140 V. Xfmr and 0B3 are connected some how. This is where I need help as these connections were cut.
I believe the 940V ctr tp (470V supply) is for the 2A3 B+..
For Modulator duty; do the primary and secondary of the modulation Xfmr connect to the same high ‘high’ voltage?
I tried to draw/trace out what is known the best I could. As I stated some of the wiring is missing. Can anyone Identify this circuit and fill in the blanks?


Last edited by Y2KEDDIE on Aug Sun 11, 2019 4:32 pm, edited 1 time in total.

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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 3:27 am 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 73
Location: Texas, U.S.A.
...
Typical circuits, different tube part numbers but essential circuit functions are the same. The class B tubes that drive the modulation transformer may or may not utilize the same B+ as the modulated RF amplifier. The 6B4's or 2A3's shown are connected to the primary of T1 in the 500 watt class B modulator schematic. The 810's correspond to the VT-4's (but electrical ratings are different).
Cathode bias is developed for the 2A3's or 6B4's across a resistor connected between ground and the center tap of the filament transformer.
Attachment:
Speech_Amp_Driver_Radio_Handbook_1942_p296.jpg
Speech_Amp_Driver_Radio_Handbook_1942_p296.jpg [ 170.8 KiB | Viewed 349 times ]

Attachment:
Typical_driver_circuit.jpg
Typical_driver_circuit.jpg [ 149.87 KiB | Viewed 349 times ]
Attachment:
500Watt_mod_Radio_Handbook_15th_ed.jpg
500Watt_mod_Radio_Handbook_15th_ed.jpg [ 119.3 KiB | Viewed 349 times ]

References:
W. I. Orr, The Radio Handbook, 15th ed., 1959, pp. 647 - 661
ARRL, The Radio Amateur's Handbook, 18th ed., 1940, pp. 191 - 201

Eric LaGess
WB5HDF


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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 4:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9005
Location: Beaver Falls, PA. USA
A class B modulator, with a driver stage; a typical setup for plate-modulation of the RF final amplifier. Your tube list doesn't include any low level audio "speech amplifier" tubes, so they must have been in a separate unit. It won't be hard to trace the circuit out. See where the plate connections of the 211 tubes go; that will show if the modulation transformer is part of the unit.

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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 4:31 pm 
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Joined: Aug Tue 04, 2009 2:15 pm
Posts: 393
Location: Elmira, NY
Thank you, this is what I was looking for. I have 20th edition Radio handbook, and different editions of ARRL Handbooks.

My error; I have. 2A3’s driving VT-4C’s, not 211A’s.

The center tap of secondary feeding VT-4C’S is connected to a 0B2 pin 2, and pin 5 grounded. I haven’t seen Bias applied here in other circuits, maybe this is an error.

My circuit ,as it is, has 2 interstage transformers and one modulation transformer:
Interstage, 2A3’s, interstage, VT-4C’s, and then 275 watt modulation transformer.
No resistors or capacitors, except in Bias supply and power supply

The ‘H’Hv connects to center tap on primary, and one side of secondary of the modulation transformer.

There is a separate ‘H’H V supply for VT-4C’s, HV supply for 2A3’s, and Bias supply

Thanks for your contribution.


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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 4:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9005
Location: Beaver Falls, PA. USA
The VT-4C is the military version of the 211, which was widely available in the surplus market after WWII. A pair in class B will require a negative 80 to 100 volts grid bias.

It sounds like you have about half of a classic AM ham transmitter. Did you get the rest of the rig?

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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 5:14 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 73
Location: Texas, U.S.A.
...
The VT-4's do need grid bias. The OB2 will provide about 108 V across it. It would probably be wired into a bias supply circuit with the 5U4, similar but possibly simpler, than the bias circuit shown in the above 500 watt/810 diagram.
Attachment:
211_Class_B_audio.jpg
211_Class_B_audio.jpg [ 129.74 KiB | Viewed 311 times ]


OB2:
Attachment:
OB2.jpg
OB2.jpg [ 91.47 KiB | Viewed 311 times ]


...


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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Sun 11, 2019 8:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1602
Location: Canyon Lake, TX
Here is the GE data sheet for the VT-4C. Hope this helps.


Attachments:
VT-4C-1.jpg
VT-4C-1.jpg [ 452.01 KiB | Viewed 299 times ]
VT-4C-2.jpg
VT-4C-2.jpg [ 495.75 KiB | Viewed 299 times ]

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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Mon 12, 2019 3:04 am 
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Joined: Aug Tue 04, 2009 2:15 pm
Posts: 393
Location: Elmira, NY
I bought it along with a couple Swan 350’s, and an Invader 2000.
Our ham club in high school (1960’s), had the Invader 2000 and I remember the separate power supply with the mercury vapor tubes. I thought this was a homemade version of that supply. When I took it apart I found a 275 watt modulation transformer and the interstage audio transformers, along with a big HV transformer with MV Tubes.The Invader 2000 was just a cw, and sideband transmitter. My chassis looks home made and a bit rough. None the less I’m guessing the previous owner decided to build a power supply and modulator to use with the Invader 2000 in a high level AM mode.This chassis only has 8 tube sockets. The 1st interstage transformer primary was wired to a terminal strip. He must of had an external audio amp.
It appears to have all the voltage available that the Invader 2000 needs.
The wiring was pretty chopped up, but I think I’m on the right path. I stripped the chassis and I am about to wire brush it before I rewire.
It had a small 140V transformer with a selenium rectifier, couple resistors, caps, connected to the 0B3. I will redo that with a silicon diode.
It looked so simple , I felt I might be missing something....other than the preamp.
I’m hoping I can use it as it appears to be intended.


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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Tue 13, 2019 2:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9005
Location: Beaver Falls, PA. USA
Do you know what the input impedance of the input transformer is? If it is 500-600 ohms, a small PA amp would drive it with no problems. The amp will probably need an attenuator pad on its output to keep from overdriving the 2A3 stage. Good luck with the project!

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 Post subject: Re: High voltage supply / Modulato
PostPosted: Aug Wed 14, 2019 5:39 am 
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Joined: Aug Tue 04, 2009 2:15 pm
Posts: 393
Location: Elmira, NY
The 1st input stage transformer looks the most modern of all the parts. It is roughly 1-1/2 “ square, and it is plainly marked: ratio 1:3 red lead B+, blue plate, G,G, and ct. no other markings.
The grid wires were attached to the 2A3 grids, the 2A3 plates went to a thoradson 1:3, and then to the VT-4C’c, and then to the Modulation Transformer.
Alas! After stripping the chassis, wire brushing, I started reassembling. I accidentally dropped one of the Vt-4C sockets. It broke in 4 pcs. Ii took it apart tonight and glued the porcelain with JB Weld. I hope it takes and I can operate it until I
can find a replacement. It might be difficult finding a replacement. Hopefully I can find one at the next hamfest.
That’s what happens when I stay up late and try to build things, I get tired and careless.


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