Use choke or ?

[k7mdo]

Anotherr question for the power supply under construction is what circuit to use for screen voltage adjusment?  I have attached one that I lifted from some old military gear and it purports to have "ADJ" but of course without building it it is hard to say how much.  The voltage reference tube is a 5651 which I have one of... that is always good....  the power transformer will likely produce 375 VDC for screens and I would like to be able to adj down to 200 VDC for some applications.  The circuit pictured is used at 250 VDC setting per the diagram.  The data I find on the 5651 is 85 volts which is a little odd to my understanding.

Does anyone see any issue with using the attached circuit... or is there a more sensible route?  I do want to stay with tubes.

Tom

[N2DTS]

There is a good circuit for adjustable regulated voltage using tubes in the 15th edition of the Bill Orr handbook
(black and yellow) on page 711.

It uses a 6as7, a 6sh7 and a vr150.

I used it for the screens of my 4cx250b modulator deck I built 35 years ago.
Its still working with no crap outs.

Another way to do it is a string of VR tubes with an adjustable dropping resistor, plug in the combo of tubes that gives you the voltage you need.
There are 75, 90, 105 and 150 volt VR tubes...

For an adjustable supply with less regulation, a small variac supply works well and is very easy to do.
For RF decks, a screen current overload relay is a good idea.

I do a variac supply, a screen overload relay, and a series dropping resistor that drops about 30%of the screen voltage for AM rigs. Its fool proof, and adjustable for the lowest distortion and almost any tube you chose to plug in.
 

[Opcom]

The 0A2-0D3 are voltage regulator tubes using 5 to 40 mA depending on type, and keep within 4 to 8 volts of their rating depending on type.

The 5651 is an 87V voltage reference tube and keeps to 3V within its 1.5 to 3.5mA rating. Have a look at the H/P 400 VTVM for another use of it. It's an example of something that keeps about 1% precision. The 5651A is rated 85.5V.

The 992 is similar to the 5651 in using low current, but uses 0.4 to 2mA and regulates to within 8V.

For a 250V screen, the regulation can use any of them quite nicely.
For an oscillator, maybe the 5651 would be useful. Of course the design and the other parts of the regulator and of the oscillator must also be of fine quality, to take advantage of it.

If subbing, remember to change the circuit to accommodate the regulator or reference tube's proper current.

I like the optical spectra of gas tube regulators.

[k7mdo]

In laying out the chassis for the power supply I see that I have the opportunity to add a -50 VDC supply for grid bias? the winding is on the filament transformer I will use and would be easy to add to the projected supply.

I wonder if negative 50 VDC is just too unlikely to be needed to bother wiring it into the supply?

Tom

 

[WA6PBJ]

Likely this will not be read as this topic is old, but for my 2 cents: capacitors today are very much larger (higher capacity) than back in the 40's for example. SO you get far better regulation and filtering today. You don't need a choke input because SS rectifiers today (again not available in the early days) can tolerate high surge currents (at start up, etc) where vac tubes can't not (flash over, etc). So the choke which presents an 'open' at startup saves these old vacs from high surge...thus the use/need of a choke. Not needed today. You can get same or better regulation using large cap input today vs even a choke input with large caps. The choke is relatively redundant. Interestingly, another benefit of doing C input (no choke) is 1.4 X Vrms of the transformer. Get .9 using a choke. No wonder big amps (albeit SSB) do so well with smaller HV transformers and all in a desktop box. But let me say this: I LOVE THAT MERCURY VAPOR look!!! One more thing: IF your rig is vintage and uses these vac rectifiers, and you are planning some sort of mod and staying these vacs, you best consider doing the choke/leaving it in place. You will 'enjoy' having to determine whether the choke really regulates like your mod requires. Its not just a one-size-fits-all. Choke inductance vs ripple vs load swing ==>need to do the math to see whether the choke will work. I personally try to avoid mods on vac tube supplies for this reason.

[W2PFY]

Tom said

But how would I know if that 30H choke was used as Heising reactor or a power supply choke? I seem to remember there is a difference, though I've used any rated choke I've had available in the past for Heising reactors

Just a couple points that I have observed. One thing is that chokes designed for heising have a polarity. I learned that when Tim gifted me a choke for the Westinghouse mod section. I forgot which lead was to go to B+ but I would think it would be the one closest to the core? The other difference between the two types of chokes is that the heising usually have a higher breakdown voltage rating than a PS choke. Most heising chokes for high power almost always have spark gaps on them but I have seen a few regular chokes so equipped. The other thing I have heard but not seen is that a heising chokes core may have a gap in the laminations?

[Opcom]

purpose-designed heising or not, a large choke like 30H can make HV transients when the current varies abruptly. The transients amplitude can be close to the same as the HV supply voltage.  I think it's OK for AM linears and less OK for SSB or CW.

Break-in situations can be complicated with large value chokes. The resonant filter might be a better choice if that's to be done.

the L value for a resonant choke has upper limits because too much reactance at 120Hz will limit the resonant current and the setup acts more like a non-resonant setup.

[n1zpy]

Thanks, Tom, glad my post was helpful.  I know of no way to definitively determine whether an old transformer was designed for choke or capacitor input, other than looking at spec sheets and application notes from the manufacturer.  But measuring the DC resistance of the secondary, and knowing the voltage and current the transformer will produce, will give a good indication of how much heating you might expect from capacitor-only filtering.  

I was thinking about what you said regarding all your power supplies used just capacitors for filters.  It reminds me of my first strapping linear that I built while in the Air Force in 1968,  It employed a pair of Eimac 4-400A tubes, B&W 800 plate choke, and an Illumitronics PI-195-2 Plate inductor.   The band switch that tapped this came from a surplus BC-375 Tuning Unit.  Never had one crap-out with this amp, and it still has the original tubes.  It still produces legal limit power out.

Looking back, I probably was very lucky that the power transformer never failed.  It was a UTC S-48, intended for full-wave center tapped rectifier configuration, with tube rectifiers and either choke or cap input.  I used the transformer in a full-wave bridge configuration, using the entire secondary.  Input was rated at 115 volts.   I did not use an automated step-start, but I had a 120 volt, 10 amp GR Variac in the primary circuit.  I also had a tune/operate switch that put a 120 volt, 250 watt photoflood bulb in series with the transformer primary in the tune position.  I never turned it on at full voltage without it in tune position.  

The rectifiers were large devices like used in the HV radar power supplies in the F4 Phantom, they looked like large cartridge fuses.  Those seemed to last forever if not subjected to large start-up surges.  The capacitor bank consisted of four each GE Pyranol 14uf at 2KV, configured in series parallel with equalizing resistors, for an overall rating of 14uf at 4KV.  Not considering safety margins as a teen, I ran it up to 4KV on a regular basis.  The caps still live too!

 That UTC transformer works as well today as when it was new in the 50s.  Used with due caution, these transformers will work FB without the choke, but I still prefer choke input when the parts are available.

I opted to not use a choke in the high voltage power supply I built for a homebrew amp that uses a pair of 4-400 tubes.  Today I picked up a socket and a 250 watt bulb to add a softstart/inrush protection similar to the way described above.

[w7fox]

I am also using a capacitor input filter in my homebrew 813 amp.  The transformer has an 1800 volt, 800 ma secondary.  Remember that there is no free lunch, putting a big capacitor in as the filter will get you near 1.4 times rms voltage, but you will only get about half the current rating.  In my case it runs at 2500 volts and 400 ma which is what the 813s call for.  This used to puzzle me about voltage doublers too, it seemed like you were getting something for nothing until I found the right design information.  You only get about 1/4 the secondary current rating out of a voltage doubler!  That is why I like choke input filters, the secondary current rating is what you get...at least from a full wave bridge that is.  You get more than the secondary rating from a full wave center tapped secondary.  One warning though, on very old transformers, the ratings displayed on the transformer case may actually be DC voltage and current, the manufacturer assuming you are using mercury vapor rectifiers, choke input filter, and grounded centertap.  I ran into this while testing a transformer of unknown origin, and the ac voltages I measured didn't match those marked on the case until I ran through the equasions for choke input. (0.9 x rms).  Enjoy the adventure.