regulated screen supply for 4cx5000

[KL7OF]

Looking for a circuit to provide a regulated 1200 volts to the screen of a cathode driven 4cx5000......The screen current is under 30 ma in operation...  The screen voltage is taken from a dropping resistor off the plate supply..

[K1JJ]

Looking for a circuit to provide a regulated 1200 volts to the screen of a cathode driven 4cx5000......The screen current is under 30 ma in operation...  The screen voltage is taken from a dropping resistor off the plate supply..

Hi Steve -

I assume this is for a linear amplifier...   It's generally not a good idea to use the HV supply as a source, especially if you are running serious HV.   It's rarely done in commercial use. I would build a separate adjustable regulated 1200V supply. There are circuits around the web for the regulator.   The final plate loading has a big effect on the screen current, thus the need for an adjustable screen supply to get proper screen current after a loading adjustment..  Use a screen current trip circuit in case the HV drops out and leaves the screen on alone.

T

[KL7OF]

This is an FRT-39 (GPT-10K) Transmitter.....The 4cx5000 final stage is linear.  TMC designed it with a dropping resistor for the screen supply....They had a regulator circuit using resistors and pass transistors...When I got this transmitter , that circuit had been removed and a subchassis with 8 VR150 tubes was substituted to provide the 1200 V for the screen...This has worked well ..I am just wondering if there is another way to do this ..It seems those pass transistors are unobtainium and the VR 150 tubes don't last forever......so Im looking for alternatives...I saw a circuit using a 4-400 as a regulator....(no thanks)  and another circuit using a rare Hi current Zener diode..(no thanks again)  Can Zeners be combined to provide Hi current hi voltage regulation???

[K1JJ]

Yep, it can be done - and I have done it before, but I always went back to a separate screen supply for simplicity and control.  Solid state devices never seemed to last when used in these HV circuits.

The last circuit I made used a string of 200V 50W zeners taken off of a 2KV supply.  Even though they were bypassed well, switching them in and out popped them over time.

Screen regulation has always been a challenge with high power tetrodes, thus the reason I switched to triodes. (8877, etc)  (Plus triodes are usually cleaner unless NFB is used with the tetrodes)

Using a 4-400A or similar (or 813) tube regulator may be a good idea if you want a bullet-proof solution.

There's a few guys who build these rigs commercially on this BB -  and maybe they will suggest a time-tested solution...

T

[WD5JKO]

Steve,

  IF you wanted to pull off the High B+, look at what Central Electronics did with the 600L amplifier. They use an 812A triode as a series pass tube. This to regulate the screen of an 813 tube, and pull from a ~ 1800v Plate supply. They also use a 6AQ5 regulator, and a +300v reference from two VR tubes. The concept seems simple, but implementing might require different tubes to keep things within ratings.

http://ce-multiphase.com/600l_schematic.pdf

Jim
Wd5JKO

[KL7OF]

this is how TMC did it....The 1200V line at the upper left is the screen supply for the 4 X 5...
 

   I have the original TMC regulator board.and I see that the transistors used (2N3792) are not unobtainium....http://www.newark.com/multicomp/2n3792/bipolar-transistor-pnp-80v-to/dp/01H1374?mckv=sY6h9qvsL_dc|pcrid|117072058768|plid||kword|2n3792|match|p&CMP=KNC-GUSA-GEN-SKU-MDC-NF&DM_PersistentCookieCreated=true&gclid=COm7nrH3utECFdm3wAod0PsL7g

 Makes me wonder why it was replaced with VR150s...

[DMOD]

Jim W5JKO gave a meaningful alternative for a regulator.

However, I would use a 572B in lieu of the 812 since that tube has higher gain and would make a more sensitive regulator.

Why the use of PNP transistors with 80V Vec specs? Must be an interesting and complex regulator schematic.

Phil - AC0OB

[KD6VXI]

Steve,

I've had boards made up for the 8170/8171 screen bias.

Based loosely upon AG6K design.

I've given the production to a friend.   I can supply info via email if you want.

These have been used to provide screen voltage to a 4cx15k.


I also had HV rectumfrier / filter C boards made up at the time.   Nobody seemed interested,  so I shelved the ideas.

--Shane
KD6VXI


Edited to add pics front and rear of regulator board.

Rich's schematic is here,  which he states is based upon the datasheet.   http://www.somis.org/D-a-08.GIF

[KD6VXI]

You'll notice the air gaps.   Had problems at extreme high screen voltage.   Air gap insulation stopped all that nonsense.

--Shane
KD6VXI

[KF7WWW]

I too would be interested in one of those boards..

[W3GMS]

Steve,

Not sure what the HV suppy potential is, but for something simple that I have often used, I would develop a reference voltage with a bunch of HV Zener diodes in series to get to your 1200V point.  I would use that reference voltage to establish the gate voltage on a FET used as a series pass.  I would use a resistor divider to divide down the plate voltage to a level that would minimize the voltage across the FET series pass transistor.  That would reduce the dissipation greatly that the FET had to dissipate while delivering the 30MA of load current.  With 500V across the FET, that works out to be 15W of power that the FET has to dissipate.   The input divider network would have to dissipate some power, but if its not in that network, its in the pass element(s)!  I use some steering diodes around the FET for protection based on how the voltages appear at turn on and turn off. 

Joe-GMS

[KD6VXI]

I too would be interested in one of those boards..

I've sent my friend a text asking for pricing on the bare boards.   I'll post when I find out.

--Shane
KD6VXI

[Opcom]

On page 56 of this document is the accurate and rugged vacuum tube HV regulator to go with that. Tube is cheap and you already have the air. It shows 2 pass tubes but you need just one for 30mA.

http://69.36.164.11/tube/man/STCx.pdf

[w4bfs]

On page 56 of this document is the accurate and rugged vacuum tube HV regulator to go with that. Tube is cheap and you already have the air. It shows 2 pass tubes but you need just one for 30mA.

http://69.36.164.11/tube/man/STCx.pdf

this works well .... first saw it done this way in 1969 ... the 4cx250 was dunked in an oil bath for cooling ...worked fine, lasted a long time

[KD6VXI]

My response was lackluster.   He went back to school  and has sold the prior inventory.

Anyone know of a fab house?   I have artwork,  and if there is enough interest,  I'll have more built.

Am toying with a dual regulator.   Screen and grid.  Maybe up to 1.8 kv in on the screen side,  - 4 to - 500 on the grid, regulated down a few volts?

--Shane
KD6VXI

[Donnie SWL]

A few more....

[Opcom]

The 572 may require an inconvenient grid current of 20-30mA, depending on voltage across it and the peak load current. The dissipation in the driver and its plate resistor may be higher in order to supply it requiring a larger driver tube. It would be very interesting to see a regulator circuit with a zero bias tube. If going with convection cooled regulator tubes, an 813 might do well and could be easier to have than an 812. 812s are going for only $60-70. 813s same but Chinese are about half price. Just sayin.. consider spares and the 'next guy' who might inherit it.

[KD6VXI]

Semi Update.

I am unable to get more of the screen supply boards that I posted a pic of due to a myriad of circumstances.

BUT,  I found a workaround.  I bought a CNC to mill,  etch and drill my own.

If all goes well,  I'll be able to make some more of these in the next month or so.  The CNC says it should be here in a couple weeks.

--Shane
KD6VXI 

[DMOD]

Looking for a circuit to provide a regulated 1200 volts to the screen of a cathode driven 4cx5000......The screen current is under 30 ma in operation...  The screen voltage is taken from a dropping resistor off the plate supply..

Regulate it with audio and drive the SG with 4-65A's.

This tube makes an awesome Screen Modulated AM transmitter.


Phil