The AM Forum

Boring stuff for most of you but not for this guy!

I kludged together the B+ supply on the bench.  The transformer is new to me so testing it at least unloaded is in order.  I am using a string of caps for ~47uF.  I have a 4-20H swinging choke in the CT lead to ground.  Full-wave rectifier diode strings.

Unload voltage is ~2200VDC.  The 100k 100W gets quite hot even dissipating 50W!

The next step is to add the step-start circuit.  I have a programmable relay and 10 ohm 50W resistor for that.

Cheers

Rich

Rich,

Looks like fun lab work! 

One thought, you might want to pick up a piece of teflon or plexi to put under the board when doing tests like this one.  I am not sure what your bench surface is made from but typical counter top materials don't necessarily make good HV insulation and that could get interesting.

The resistor will get hot, 50 watts is around 170 BTU per hour so it makes a fair little heater.

0nly crazy fools think HV is a bore

Consider a fuse in the primary just large enough for testing and no more.

Honestly, that picture scares me.

I have a healthy respect for HV. I got across an 6js6 in my Tempo One while adjusting it when I was a JN. That left a mark. I thought from that point on, I was going to be super-careful. About 10 years later, I got across an 850V Capacitor bank in a studio strobe power pack. That knocked me down. I guess I wasn't careful enough.

Now, I'm still nervous when working around HV, especially considering the project I have coming , and as far as I'm concerned, that's a good thing! :o

By the way Rich, nice meter and probe. I have a similar Fluke setup. I just cannot believe how accurate that probe is, or how accurate and responsive Fluke stuff is in general. They make you pay for it though. ;D

Whilst I was still in the Navy, I was making the final service call on what was to be my final cruise since the ship was going in for overhaul. I opened up one of our PPI's. I forgot to shut down the HV breaker and to -5KV off of the cathode circuit of the CRT. That one still hurts to this day!  :P

Rich

Nice test setup,  put a square of ceramic tile under your load resistor to keep it from burning the table top.   50 watts through a 100w resistor will get it quite hot.  Get a small fan and blow some air on it.

I use all 225-300 watt Ohmites for testing power supplies.  I see you were testing with the lower voltage taps on your xfmr.  You need to load the xfmr down with about 300-400 ma to see what your loaded voltage will be.  You should be somewhere around 1500 volts.  I think you're using a UTC choke, looks like a S-38 IIRC.  That choke should work OK in the positive lead with only 1500-2000 volts.

To build more reliability into the bleeders you should use two 100w resistors.  Two 50K 100watt Ohmites wired in series,  each will run cooler and last longer. 

As part of your metering circuits you can measure the voltage at the mid terminal between the two bleeders.  You should measure half of your full plate voltage.  The purpose of this is to be able to tell that the bleeders are not open, thus leaving the supply with no bleeder current (which is not a good thing).

Fred

PS  Don't get electroplated working on the HV supply.  Be careful as you work, it's easy to make a mistake.

I do similar up to around 3500V but the variac and meter are a lot further away from the hot stuff.

For the 7500V xfmr test I used the garage floor with a sheet of very dry plywood. The diodes and bank of resistors were mounted with their brackets on another piece of vertical, L angle bracketed, plywood with a large floor fan blowing on them. The Variac and Fluke meter were as far away as the HV probe allowed. I ran that with a 51uF @ 10KV 125# Sangamo oil filled for an hour at a wimpy 4000W load after giving it a 10 minute shakedown run.  The transformer didnt get much above room temperature.

Considering how that transformer and PS was run in a RF heater with a 14KW Pd water cooled triode my setup was far more conservative :o I was also right next to the garage 240V breaker box but the test ran fine while I read a book.

    I test all my HV stuff by feeding the primary of the XFMR
from the secondary of a rugged filament XFMR.  five volts
or 6.3 in will "tell the tail" so to speak. If the numbers work
out perfectly then all ya gotta worry 'bout is insulation
breakdown.  "Duck and cover"  "Switch to saftey"

73

/Dan

The old buzzard HV plate meters in my HF-300 rig use 10 ma movements instead of the usual 1 ma.  The multiplying resistors are a string of about half dozen large wirewounds; look like at least 25 watts each, all mounted in a factory made perforated metal cage.  Those multipliers would make a good safety bleeder on their own.  When I go to standby, I can see the plate voltage bleed down slowly.  By watching those meters, I know the HV has bled off.

In my modulator supply I use a pair of 50K 200 watt wirewounds in parallel for bleeder, and in the rf final PS I use a single 30K 200 watt wirewound. With choke input and 28 mfd, I get good static and dynamic regulation, and I don't use step-start.

Always mount wirewound power resistors in a vertical position so that the hollow interior serves as a chimney to keep air flowing and cool the resistor by convection.

Just tested a 40 kv beam supply at work last week. It can deliver 2.5 amps continuous.

That pic still scares me.

In a prior life I worked on 100-500KV supplies at up to 200ma. Its a long gone company called High Voltage Engineering in Burlington MA. Trying to build a million volt accelerator for universities killed them when the feds cut the funding.

I 'member them.  I worked at Raytheon Bedford and paid occasional calls to our Spencer lab in Burlington

VERY scary, fear is a good thing. 

Fred,
   If the ground side bleeder opened then you would see full voltage.  If the high side bleeder opened then you would see no voltage which would be dangerous as you would have no bleeder and an indication of zero volts!

Rich

Don

I think the heat radiates to the greater surface area which is the outside surface.  This is not to say that the inner surface doesn't get hot but I think the outside surface gets hotter.

I have often thought about what you say but many wirewound resistors are mounted vertically with a screw and fiber washers covering the openings on both ends.  So, I'm not sure how important it is to have the ends open at all.  Another thought,  since heat rises, I wonder how much of the heat from the bottom end of the resistor rises along the vertical surface and causes the upper end of the resistor to become progressively hotter than the bottom.

It's possible that mounting the resistors horizontally may in fact be better since both ends will get a equal amount of cooler air moving around the surface.

Just some thoughts

Fred

Rich,

You're exactly correct.  The point is that if either bleeder opens you have no bleeder current.  The extra metering circuit is just to serve as an indicator of the bleeder circuit working or not working.  The point you're missing is that it would be an extra meter circuit in addition to your usual plate voltage meter.

This can be done with a separate meter (if you like lots of meters) or with a switch and one meter.  You would need two sets of meter multiplier resistors, both sets being the same resistance.  One is connected to the plate supply line and the other is connected to the midpoint of the two bleeder resistors.  If the bleeder meter is reading half the full plate voltage, the bleeders are not open.

Just an extra bell and whistle if you like bells and whistles.  It is just a way of always knowing that the bleeders are working.  It's up to the builder to determine how important that is to go through the extra work and parts to add that feature into their transmitter.

Fred

You could use the two series resistor bleeder, but it seems to me that just adds one more component that can fail as opposed to one larger bleeder.

Even better would be two bleeders in parallel each with twice the resistance you are targeting.

Then if one fails, you still have a bleeder.

Another thing about volt meters is that the string of meter dropping resistors is also a redundant bleeder resistor.

It might take a while, but the PS caps would be blead to zero by the meter circuit.

I have a question because I am figuring out bleeders for my p.s. and I have read that around 200K to 500K resistors are recommended because lower values result in too much current in the bleeders that could be used for the real p.s. load.  I see values here of 25K or 50K which seem low (but bleed down the caps fast) but if you have say a 2500 v. p.s. and 50K bleeder you are continually loosing 125 w. and 50 ma. to the bleeder (if I did the figuring correctly).   I was going to series two 200K fifty watt w.w. resistors for a 4.5 KV 1A supply with around 36 uF.  This is a cap input filter supply, no choke.

Rob

Dam, all this talk of bleeders in power supplies got me looking at my RCA BTA1 and noticed that there are no bleeders and other than the tubes the only DC path to ground is the four meg resistors in series with the plate voltage meter. The question is do I need one considering that the PA tubes and the modulators have their filaments on all the time and the plate contactor is only engaged when the transmitter is keyed? Wonder if I should hang a couple 100 K 100 watt resistors in series across the output of the plate supply in series? The 600 volt B+ supply has a voltage divider (R15/R16) that will serve to drag down that supply but have to wonder about the HV. I am not as concerned about stored charges when everything is switched off because one of the first things was to clean up and repair all the interlocks and HV shorting switches on the transmitter.
KA3EKH

Thats why BC rigs were equipped with a chicken stick ;D

Its a lot better to series a bunch of lower value wirewounds than use one high value. The high value use finer wire and are more prone to opening, anything over about 75K is problematical as anyone who has purchased or tested hamfest treasures has found out.

If using choke input and one oil filled cap figure on the bleeder taking 10% of transformer rated current. If its a string of modern electrolytics in a capacitor only filter a 100K 5W MOX across each will run cool and be sufficient; some use 3W to save a few pennies.

Carl

Fred,
    I didn't realize you were talking about a second meter, that makes sense.

I have been wondering if I could use a simple neon lamp as a HV indicator.  The question is would a current limiting resistor, sized for ~2KV, still light the lamp at say 500V?  I would put one on the front of the chassis and one on the back.

Rich

The bleeder not only discharges the caps, but maintains a minimum load on the power supply to maintain regulation.  Broadcast transmitters typically use a common HV power supply and run hours at time, if not 24/7, so the RF power amplifier maintains a constant load, acting as a low resistance bleeder, typically 4K or 5K.  That way, the HV to the modulator is well regulated. A 100K or higher bleeder is satisfactory if a safety bleeder is used at all.

If the power supply is to run only a class-B modulator, or in a ham transmitter, the final will also be used on CW, or if the power supply is for a slopbucket  leenyar, the load will be intermittent, so a lower resistance bleeder is necessary to keep the plate voltage from soaring at minimum load.  To maintain critical inductance, so that a choke input supply doesn't try to act like a capacitor input supply, the bleeder resistance should be no more than 1000 times the number of henries of inductannce at minimum load.  If a 10 henry smoothing choke is used, the bleeder needs to be no higher than 10K! That's why most supplies use a swinging choke.  With a 5-25 henry choke, the bleeder should be no higher than 25K.  A better choice for swinging choke would be 8-40 henries, allowing a 40k bleeder.  Sometimes other components such as the meter multiplier or the static plate current on the amplifier add some minimum load, so the bleeder may often be satisfactory if the resistance is a few K higher than 1000X the nominal choke inductance.  Also, a smoothing choke will actually show a little extra inductance at minimum load even though it is not designed to be a swinging choke.

The low resistance bleeder does use up power, generate heat and reduce the maximum current available from the supply to the real load, but that is the trade-off for good regulation.  Poor regulation can result in distortion, spurious emission products and even damage/failure of power supply components, modulation transformers and reactors, bypass  caps and tubes if the high voltage soars to unsafe levels under light loads.

Don, thanks for the plain spoken information; I don't think I would find anything that concise in a handbook.  Carl--I never considered a high value w.w. opening up so I'm glad i asked. 

Looks like I'd better dig around for some 5 or 10K 50 to 100 watt w.w. jobs to series together.

Rob

Following that logic Don a 25K bleeder across a 2500V PS will be dissipating 250W. I dont believe Ive ever heard of such a low value used with a 5-25H swinger.

Both the ARRL and West Coast HB's use the 10% of full load current rule which Ive followed for over 50 years without losing 866's, etc. So then a 2500V @ 400ma KW input amp would use a 62.5K bleeder which would dissipate 100W. A typical bleeder was/is 50K or 75K and a pair of parallel 100K/100W were often used. Knowing now the problem with those high value wirewounds Id use 4 50K or 75K in series parallel.

Carl

But what did the plate voltage  soar to at minimum load?  Even if nothing blows up, with poor regulation the CW waveform will be horrendous and the output from the linear or modulator will be excessively distorted.

In addition, the output capacitor must be sufficient, in order to maintain good dynamic regulation.  Just because the plate voltage meter stays still  doesn't mean that the instantaneous voltage isn't jumping all over the place with a varying load.

If you are running a plate modulated AM rig with common power supply for PA and modulator, this won't be a problem because the PA will maintain sufficient minimum load on the supply.

Rich,

Not sure about using a neon lamp as a HV indicator.  If you are referring to using it as a indicator for the bleeder resistors it wouldn't work.  When you use two bleeders in series, two things can happen, depending on which bleeder opens you can have either no voltage or full voltage at the midpoint connection of bleeders.  I'm not sure how a neon lamp would react to having twice the voltage on it if the ground side bleeder opened.  Using a meter is better as it can read half the HV and would still have enough scale left to read the full HV.

Keep in mind, you really do not need any indicator for the condition of the bleeder resistors.  The whole idea only works if you use two bleeders in series.  If you use only one bleeder resistor or two in parallel you can't use this scheme at all.

Using a neon lamp to monitor your HV in general is not a good idea, you really need a meter to be able to measure the voltage.

Fred

As long as the 10% rule is met and the filter components follow the published info its a non issue. There will always be some sag.

There is no evidence to show that a high value bleeder has any bad effect on SSB.




Thats been covered to death on here.





Which is right out of the same HB chapter as the 10% rule.

It seems to me a lot of these issues could be addressed by including a volt meter in your power supply.

IF you get into the habit of checking the B+ voltage when you turn on the power supply and you notice the voltage is much higher than usual, then you can figure it's possible the bleeder has blown. It's also possible the choke has shorted turning the filter into cap input.

If the output voltage under load is lower than usual, that would  perhaps indicate one of your recitfiers is out and you are running with 1/2 wave rectification.

Also, when you power down, get into the habit of watching the voltage go down.

If it goes down slower than usual, that is also a sign the bleeder is shot.

And, of course, double check the volt meter before you open the supply up to work on it.

Plus, having lots of meters looks cool.