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Author Topic: HOW TO LOWER HV Supply  (Read 16858 times)
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flintstone mop
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« on: December 30, 2007, 06:47:55 PM »

I would like to lower an HV supply from 820vdc to around 600vdc. It's a Heathkit HP23A. I do not want to use a Variac. Regulation goes out the window.
A supply I built sags under full load from a 6146 final and 6550 mod pubes.
It's using a voltage doubler. I see some 100k resistors that prolly could be reduced to 47k and (lower voltage??) Shortcut engineering happening here

Fred
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Fred KC4MOP
flintstone mop
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« Reply #1 on: December 30, 2007, 07:55:48 PM »

I guess this post can float down the log.........It just hit me that if I throw a choke in the P.S. at the input to the filters instead of the capacitors that it would knock the HV down the 100 or so volts that I'm looking for. I forget what a choke input vs capacitive input does to HV.... there was a magic number that a good Ham friend of mine told me that would be the reduction in HV with a choke inpoot.
Have a Great New Year
Fred
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Fred KC4MOP
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« Reply #2 on: December 30, 2007, 08:42:00 PM »

Hi Fred,

As long as the choke is of sufficient inductance and the load on the supply is sufficient to keep the filter acting as a choke input then the magic number is about .9 times the transformer secondary.  However, as I recall doesn't the HP-23 series of supplies use a voltage doubler circuit?  Someone with more knowledge of this will have to chime in on conversion to choke input.  The other problem is that, depending upon what you are powering, you will need to draw a fairly significant idle current to prevent the choke input filter from acting like a capacitive input filter.

Rodger WQ9E
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Rodger WQ9E
flintstone mop
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« Reply #3 on: December 30, 2007, 09:25:35 PM »

You have a good point Roger about loading. I could juggle the values of the bleeder resistors.
You are correct on the voltage doubler diodes to get the 820vdc.
I'll have to stare at the scheezo some more

Fred
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Fred KC4MOP
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« Reply #4 on: December 30, 2007, 09:44:39 PM »

Fred,

I pulled my HP-23A manual and I don't see an easy way to get to 600 volts.  If you are not using the low voltage or bias outputs then you could probably series the 2 secondaries and end up with about 500 DC volts under load with a standard capacitor input filter (The two secondaries in series would add up to 407 volts and you could expect to get around 1.22 times this voltage as loaded output with reasonable sized capacitors).  Is 800 volts too high for the safety of components in your intended circuit?

Rodger
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Rodger WQ9E
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« Reply #5 on: December 30, 2007, 10:05:24 PM »

Why not use both power supplies one for the modulator and the other for the final
6146 will love 800 volts.  HP23 is not that big of a supply. You could switch to a full wave bridge but I don't think you will get quite 600 volts.
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« Reply #6 on: December 31, 2007, 10:35:35 AM »

What happens when you series up the secondary windings and throw a full wave bridge across it? Yup HP23 is no big iron.
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W3RSW
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Rick & "Roosevelt"


« Reply #7 on: December 31, 2007, 10:37:56 AM »

Yeah, the damn thing was designed for two 6146's SSB,  peak 'n hold for very short "burst" voice peaks via the filter caps. Can't take continuous current of the 6146 class C, plus the mod. quiescent current without sagging.  Add the mod. peaks and you have instant distortion, sheer mud.

I never understood why Citation, Heath, et. al. used voltage doublers. Guess that half the windings of slightly thicker guage wire was cheaper than a full center tapped winding of thinner wire, not to mention the "huge" cost of two more diodes in a bridge.  

I mean the "economy" supply could have gotten half voltage for the lower voltage stages using another, widely distributed and known design without resorting to a doubler.

Anyway the 6146 running class C will draw enough current by itself to act as a stiff bleeder if that's the way your lashing up the rig.

what the h' is going on with the site; I'm getting overwriting in this window.
Will send it a see what happens.
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RICK  *W3RSW*
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« Reply #8 on: December 31, 2007, 12:22:48 PM »

I don't think you can use choke input for a voltage doubler.  The fundamental design consists of two halfwave rectifiers, each with capacitive input, wired so that their outputs are functionally in series.  We once had a late 30's broadcast receiver with voltage doubler right off the power mains, to achieve approximately 250 plate volts.  The speaker field winding served as a choke, but that was a smoothing choke for the additional section of  filtering following the doubler circuit.

Recall that a capacitive input filter raises the DC power supply voltage (under light load) to 1.414 X rms output of the a.c. voltage source.  Choke input yields 0.9 X rms a.c. voltage.  So changing from choke to capacitive input filter should raise the DC output voltage to 1.57 X choke input filter voltage.

To maintain choke input, the maximum load resistance in ohms must be 1000 X the inductance in henries.  For example, if you use a 5-25 hy swinging choke, the bleeder resistor should be no more than 25000 ohms.  Of course, if there is a static plate current, such as on a slopbucket leenyar or class AB or class B modulator, the static plate current supplies some of the bleeder load and a higher  resistance bleeder may work.

The above figures are for full wave rectification.  It is different for half wave rectification. I have so seldom used half wave rectification that I would have to look up the numbers for that configuration.

It's always a good idea to use capacitors with voltage rating at least the capacitive filter voltage.  That way, if you lose the load or the bleeder opens up, you don't run the risk of blowing the filter caps from over voltage, although in my experience oil capacitors can stand considerable over voltage before they crap out, but I wouldn't run the risk, since rectifier, transformer and other crapouts may occur if the filter cap shorts out.
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Don, K4KYV                                       AMI#5
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flintstone mop
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« Reply #9 on: December 31, 2007, 02:20:32 PM »

It looks like I might go with a dual supply on this transmitter. It's my Elmac. 6146 final and I like the sound of the 6550's modulators, but the RF outpoot drops to 28 watts when the 6550's are in ckt.
The supply I built was in the ER mag. A quickie supply designed for the Elmac. It really looks like a copy of the Heath. The B+ goes down to 525vdc with the 6550's and the 28 watts out.
With the 5881's the RF output is 35 watts and the B+ is 540
With all modd tubes removed the RF is 40 watts and the B+ is 585vdc.
I do not have a brand new 6146 to compare. I have a fairly good one and another that puts out less RF.
I'm not shooting for 50 watts out. I want at least 40 for tuning the linear for that magical peak power then I reduce the RF out by a solid state screen control device on the 6146 for 12-15 watts driving the linear.
If someone has an idea that I am missing on this technical merry go round, please let me know. It's getting might complicated for such a simple transmitter. I have read in ER where folks modify their Ranger with KT88's and are not complaining about P.S. problems, unless it's humming right along with a good 600vdc. I'll look into the some of the suggestions mentioned above.
Thanks as usual.
Fred
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Fred KC4MOP
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« Reply #10 on: December 31, 2007, 03:30:50 PM »

Hi Fred,

Which issue of ER has the supply schematic?  If the transformer is up to the job then you should be able to decrease the voltage drop under load by using somewhat larger filter capacitors.  I assume you are using solid state rectifiers, if not that is a quick way to gain some volts.

Rodger WQ9E

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Rodger WQ9E
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« Reply #11 on: December 31, 2007, 03:46:52 PM »

If you place a zener diode in series with the b+ it will provide a constant voltage drop. You may need to use more than one to get the voltage and power rating you need. I've done this many times and it works well.

Darrell, WA5VGO
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flintstone mop
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« Reply #12 on: December 31, 2007, 06:39:31 PM »

I thought this was going to be easy, BUT I need to know if there is an ENGINE to help me find ER articles??? I went to the ER website and nothing there.
I'll email Ray. It was a neat P.S. designed for the Elmac AF67 or 68 and possibly for the receiver as well.
Fred
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Fred KC4MOP
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« Reply #13 on: December 31, 2007, 06:58:52 PM »

Hi Fred,

I found it, it is Feb. 2005 P.36.  There is a search engine for ER at http://home.wi.rr.com/n9oo/ersearch.html

Wow, a line operated voltage doubler...  OK  you could try increasing the value of the two 100 uf 450V caps to a higher value (say 160 or 220 uf) which will give you a bit more voltage under load.  No load you should currently be seeing up to around 490 volts now and increasing the filter caps shouldn't push it past this but should give you a little more voltage under load.

I have to say even though this one is designed for safety I am not a big fan of direct line generated voltages. 

Rodger WQ9E
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Rodger WQ9E
flintstone mop
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« Reply #14 on: January 04, 2008, 05:04:18 PM »

OK Gang
I have had good success lowering the HV to unloiaded 650 vdc by the suggestion of throwing the 12 volt winding of the HP 23 in series with the primary.
PROBLEM.........The NEW HV supply cannot just sit there on all the time on the plates of the 6146 final. The tube started to glow red and was eventually damaged UNkeyed with just 600vdc on its plate. Soooo, I'm out of a 6146 that died. I have a WANT ad.......hi.
What happened? The screen vdc was off, it was just 600VDC on the plate?Huh
Anyway, the fix will be to have the new HV supply going through a relay and ALL B+ voltages will be off in standby.
BTW the RF outpoot is 40 watts with a pair of 6550's modulators WOW!!!! When the sunspots arrive I can operate the AF67 barefoot and enjoy the big boost in the audio now.
Thanks for the help and edumacation here.

PHRED MOPRADIO
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Fred KC4MOP
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« Reply #15 on: January 04, 2008, 11:50:30 PM »

Why did you leave the power on if it was glowing red?
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #16 on: January 05, 2008, 09:28:06 AM »

Fred:

"the screen voltage was off"

Was the 6146's screen grounded or floating?

Grounded (directly or through a resistance of a few thousand ohms) is ok for reducing the 6146's plate current to a safe value

Floating is not okay. If the screen floats... the tube will essentially (for this purpose) ignore the screen, and work like a triode.

Stu 
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
flintstone mop
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« Reply #17 on: January 05, 2008, 10:24:53 AM »

It seems like various tubes have different characteristics. I have operated certain transmitters before where the HV B+ was on the plates all the time. Turning ON the screen, the tube 4CX1500B, became active and provided 1500 Watts

I guess in this case (6146) the SCREEN was floating

Frank, By the time discovered the glowing tube it was too late. I was keying and un-keying the the TX and was wondering why the HV was dropping....then I looked up.
BTW I waited a while for the tube to "recover" and it's still making RF power. So, tubes can take a lot of abuse..........up to a point. I know the story with some of the ceramic pubes.

There's few small wrinkles left and then I'll put it all back together and install a little fan to circulate some air into the Elmac.
Fred
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Fred KC4MOP
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« Reply #18 on: January 05, 2008, 10:35:27 AM »

Sounds like all you need now is a resistor on the screen pin to ground.
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #19 on: January 05, 2008, 11:00:38 AM »

Frank

"Sounds like all you need now is a resistor on the screen pin to ground."

Grounding the screen (preferably removing the screen voltage source at the same time) is a good way to put the 6146 into "standby" mode.

However, placing a resistor from screen to ground will not work in transmit mode.

Adding a clamp tube (or a solid state equivalent using something like a comparitor driving a power bipolar transistor or FET) would work.

If the voltage on the grid (or a tap along the resistive path from grid to ground) rose above a set negative value (the other input to the comparitor), the comparitor's output would turn on the FET and drop the screen voltage to essentially zero.

Stu
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
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« Reply #20 on: January 05, 2008, 11:10:30 AM »

Or just a pole on a relay to do the same thing.
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flintstone mop
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« Reply #21 on: January 05, 2008, 08:43:41 PM »

All problems are resolved with the new B+ supply. This additional supply will also be switched using another pole on the existing relay used for the Elmac. I thought (and forgot about a cutoff voltage on the grids to stop current flow throught the pube) that I could leave HV on all the time.
My brain feels real good right now.

Fred
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Fred KC4MOP
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« Reply #22 on: January 06, 2008, 08:33:01 PM »

A guy told me about the "4X8" power supply. It uses a better scheme for the voltage doubling, may or may not work in your case.

A choke might be do-able between the final (double voltage) filter capacitor and the output of the dual full wave doublers. well it's worth a look.

The choke would go between the junction of D9-D10 and C4 ? C4 receives two full-voltage charges per cycle and is only a filter, not involved in the doubling, so it would seem a good place for a choke.

The explanation of it is here:
http://www.bunkerofdoom.com/lit/4x8/index.html



* 4x8_power_supply1.gif (7.32 KB, 925x367 - viewed 742 times.)
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