Need help getting new Homebrew rig going

[KL7OF]

The rig is push pull 100th's in the final..... modulated by 813s triode connected.  I am using a Viking 1 as an RF driver and a homebrew push pull 6L6 speech amp....The High voltage power supply is 4500V center tap, 1700VA transformer, K2AW diodes.5-15 hy swinging choke input. 11 hy smoothing choke. each choke filtered with a 4MF 6kv oil cap.  This supply operates both decks..Bias for the 100ths is supplied by a separate supply. -100V
Specs key down no modulation:  Plate Voltage 2000.  Plate current 250 ma, Grid drive 100ma . Modulator tubes idle at 50ma.
    I key the rig  and it puts out 400+ watts of carrier then I turn the mic gain up until the modulator current is hitting 250ma on peaks.(It sound good on the monitor receiver)  so far everything is cool..... But when I UNKEY the rig , the power supply growls and the plate voltage drops off as if the power supply is shorted.....Everything works at full power until I unkey....    Any ideas about what the problem might be?   Thanks....Steve KL7OF

[K1JJ]

Hi Steve,

Sounds like you're almost there - good job getting it working!

So, how are you keying the finals and modulators.... via a relay in the fil CT?

What happens to the current meters on the various tubes when you unkey?  ie, the power has to be going somewhere - is there an indication the tubes are running away during unkey?

Key up and try reducing the RF very slowly to zero and see what happens. Does it take off?

What does the RF carrier look like when you unkey on the SCOPE....   is it a clean unkeyed carrier or do you see some crude and spikes?

When this heavy current occurs, what happens to the grid voltage and current on the final and plate current on the modulators? Measure it with a VOM for suck down too. Any meter kick ups?

Can you easily disconnect the existing HV supply from the rig and connect up another one for a test? This divide and conquer technique can at least eliminate the HV supply as a suspect.

Do the tube plates show color when you unkey? - ie, make sure the bias is staying on.  How LONG does this "shorted" conditon stay on...does it go away abruptly or stay on until you kill the HV primmary AC?

Make sure the tubes are OK by replacing them - easy enuff to eliminate.

Try this stuff and get back - we'll go from there, OM.

T

[k4kyv]

How much bleeder resistance are you using on the power supply? In order to maintain critical inductance so that the power supply doesn't act like a capacitor input power supply instead of a choke input, you need 1000 ohms of bleeder for every henry of filter choke inductance at minimum load.  If you are leaving the power supply on during standby, you need a maximum resistance of 15,000 ohms load to keep the plate voltage from soaring, if your swinging choke has maximum inductance of 15 henries.  It would be more economical to unkey the primary of the plate transformer to go to standby, and let the rf final stay connected to act as a  heavy bleeder.  A 15K bleeder resistor would suck a  lot of power.  If you run the rig on cw, you need a lot more inductance in the power supply swinging choke to keep the voltage from soaring under key-up conditions.

If you are not running the power supply unloaded under any circumstances, a 50K to 100K bleeder should work OK.  If the HV is soaringr under minimum load, you could be exceeding the voltage rating of something, and so far it has shut down rather than blow up.

[W2VW]

Probably be better off putting both chokes in series. You can induce very high Voltages if the off switch time of the TX is very fast. Maybe a ceramic chassis feedthrough or H.V. connector breaking down. Do you have spark gaps on the chokes and mod xfmr? Might try putting the filter chokes in the negative supply output also to ease up hi-pot needs.

[KL7OF]

Thanks for the input fellas...Tom...I was leaving the HV on and just keying the rf driver....Fil trans ctr taps are bolted to ground...That wasn't working so now I'm keying the primary of the HV with a relay actuated when the rf driver is keyed (that's actually the way I prefer to do it) ..... The rig started working properly... I'm getting a good scope pattern and it sounds good in the monitor...I keyed and un keyed for several cycles and then something else shorted....I have narrowed the short down to something in the Mod deck...Don...I have a 100k bleeder and it works great now that I am keying the HV primary....Dave...Earlier in the project I did have problems with a cracked feed thru insulator and I will check the others now..  The swinging choke and the smoother are in series.  Each has a 4 MF cap to ground.. I may put the chokes in the neg lead later....I really want more voltage on the tubes so I may change the power supply to Cap input....  I'll let you know how it is going...Thanks again.....73....Steve

[K1JJ]

Steve,

You're looking for more voltage... good.  Mo power!   

I wud get rid of the chokes and increase the capacitors to like, 40 ufd.  Get one of those 5KV at 51 uf photoflash caps.   Size of a small loaf of bread.  Two 4uf is pretty small even wid the chokes, I think..

Chokes in supplies aren't what they are cracked up to be.  I don't use one in my 0-5KV supply and find it to be more stable under varying loads. Same with my BIG supply... 200 ufd at 10KV.. works MUCH better without the big choke in there.  The choke was rated at 1A, but needed too much current to make it past the curve. The only reason I use the choke is when I want a lower voltage, otherwise switch it out wid a vac relay.

Just use a step start- use some kind of primary winding current limiting like a 10 ohm 100w resistor for a split second during key on..

Just my opinion, OM.  Axe Dave/The International Apeman his opinion on this too.

T

[W2VW]

Just for clarification, I meant the 2 chokes directly in series with no cap between them. This would give you more inductance for the swinging choke and require less critical bleeder current. Do you have feedback around the 6L6 drivers?

[W2VW]

Vu and I posted at the same time. You won Mr. Vu. Rook at me now. I like chokes. They don't have great dynamic regulation with yesterday's old small value capacitors but with lots of cap they work quite well. I just read an email from someone who resonated the choke input choke in his TX and is extremely happy with it's performance.

[WA1GFZ]

Don't forget to step start the primary of the HV supply when you key it. This will reduce the shock wave through the system. Turn on the supply through a 10 to 50 ohm resistor. After a few hundred ms. short the resistor.

[KL7OF]

Tom...I like the big Cap idea...I don't have any however....Yet.......Dave....... OK on the chokes in series...with big caps!....How do I get the delay needed for the step start resistor? 

[k4kyv]

Might try putting the filter chokes in the negative supply output also to ease up hi-pot needs.

Better still, mount the filter chokes on insulation so that the frame is not grounded.  That will keep the stress on the insulation from winding to core at a minimum.  Some of that old paper insulation becomes very deteriorated after a half century.

[W2VW]

Tom...I like the big Cap idea...I don't have any however....Yet.......Dave....... OK on the chokes in series...with big caps!....How do I get the delay needed for the step start resistor? 

I use octal socketed time delay relays found for cheap on ebay. Another trick to lower wasted bleeder power is to use a plate transformer with a 240 primary but set up the T/R switching so there is 125 VAC across the plate primary during receive. Cuts the waste down to 1/4.

[W1IA]

Tom...I like the big Cap idea...I don't have any however....Yet.......Dave....... OK on the chokes in series...with big caps!....How do I get the delay needed for the step start resistor? 

I believe you can use a large resistor across a relay and use a r/c time constant to creat a couple hundred miliseconds to close the relay and short out the resistor....am I on track guys? same type of circuit I use on a couple of my rigs to stop the inrush current to the rectifier brick. 100uf in parallel with keying line, a 1k resistor and a 1n4007 in series ( cathode towards the relay) with the key line (110 volt keying) I use a 5 ohm 20 watt resistor across the relay contacts...but that can be adjusted for a much higher wattage depending on your current draw. This is placed on one side of the 220 volt supply primary.

 http://www.classeradio.com/single_voltage_power_supply.gif

This shows a typical step-start circuit used on a power supply for class-E rigs

Brent W1IA

[KL7OF]

I got the rig going tonite on the load........found another cracked feed thru ceramic.....those insulators are only 50 or 60 yrs old...The rig is in a 6 ft rack... the power supply is screwed to  a piece of 1 inch plywood in the bottom.....I am looking at a step start for the HV primary..I want more voltage too.... I have a transformer that will give me 3KV.....I have zero bias triode connected 813's in the modulators....How much voltage will they take before they need bias??

[K1JJ]

I got the rig going tonite on the load........found another cracked feed thru ceramic.....those insulators are only 50 or 60 yrs old...The rig is in a 6 ft rack... the power supply is screwed to  a piece of 1 inch plywood in the bottom.....I am looking at a step start for the HV primary..I want more voltage too.... I have a transformer that will give me 3KV.....I have zero bias triode connected 813's in the modulators....How much voltage will they take before they need bias??

Steve,

So, did the clacked insulator fix the power supply "shorting" problem, or does the HV still load down with unkey?  The Apeman suggested the feedthrus... he is vely vely intuitive about this stuff. 

When I ran my triode connected 813's, I ran the HV at about 2800V with no 813 bias. But they got a little red in the face during idle... not good for 813's.  But, that's how I ran them for years. However, on this next "Blown 813's" rig, I plan to use the 0-5kv supply and try to get away with as much as possible, maybe 3200V.  I am putting in a string of diodes in the fil CT that has a rotary shorting switch. This will adjust the idle to whatever mood I'm in.

If you go over 2100V or so, I would use some form of diode/zenor bias in the CT. Esp if you don't run air across them. 

73,
T

[KL7OF]

Tom..."The Unkey transformer short" was cured by keying the primary of the HV...The cracked insulators, (2) happened at two different times whilst I was chasing other gremlins and added to the difficulty of trouble shooting.....I've got it going on 40 now and I will do some testing on 20 today..I may have some parasitics...For the brief period I had the RF deck going on 14 mhz, I saw a little blue glowing in one of the 100th's..

[K1JJ]

Steve,

Wow.  I've had the same darn problem with feedthrus.  I know others have too....   Nowadays I usually just run a direct HV wire that has lots of air around it.

So you cured the problem by switching the HV on and off.  Seems like a band aid cure. If you leave the HV on, does it STILL get sucked down like a short when you unkey?  I would try to find the real reason for that or it will probably haunt you again, in a different form OM.   Might break some important parts.

T

[WA1HZK]

Look at the drawings for the last big rig I built. The schematic of the Step Start ckt is there along with all the other supplies.
Some Ideas anyway:
http://www.criticalradio.com/833%20Rig%20Project/Web%20Pages/Schematic%20Diagrams%20Page.htm

One more thing. Insulate all of the old Iron in the Power Supply and modulator from ground. Slide some plexi glass under the iron to reduce the shorts to ground thing. I always do that. It saves the Mod. Iron & PS when your using old swill.

On the feed through "insulators" All mine in the big rig acted like they were plated with copper! Bore a larger hole & mount some Lexan or Plastic & put the insulator through that. Quick cure.
Keith

[KL7OF]

I've got the rig working very well on 40 Meters now.....I tried 20 meters and the RF deck loads and works WITHOUT the modulator turned on..With a little blue showing in one tube...When I put the modulator in the mix the 100th's won't tune and turn red almost immediately..They really take off and I hear that  squeal that a rig makes when it has parasitics... I have bypassed the filament leads at the tube sockets, bypassed the AC coming in to the rf deck and put a vhf choke in the grid lead where it hooks into the grid coil.  I got a couple of 50 ohm 2 watt carbons and have 6 turns of #24 enameled wire wrapped around them ..These are for the grid leads...I haven't installed them yet....I have another PP rig with 250th's in the final and I have the same problem with it...The rf deck will tune on 20 but it squeals when I turn the modulator on....Some rf must be getting into the modulator?  .Does anybody have any ideas why this is happening and how to cure it.??..     Thanks......73  Steve

[WA1HZK]

Lets see, coming out of the mod transformer/heising reactor you feed the HV directly to about 500 pf. bypassing to ground. Then to a Z-50 Choke (50 MHz, about 2" of 22 ga. wire wrapped around a 1/4" coil form). Then another 500 pf. to ground (RF Caps, no junk here). Then you go to the bottom of the plate RF choke which is a nice 3-30 MHz unit right? Off the finals you have some 50 ohm non-inductive resistors of 6-20 watts or so & maybe 4 turns of coil around the resistor. I use (3) 330 ohm 2 watt resistors & 4 turns of 14 gauge wire about 1" dia. here. The resistor is not real critical, around 50-150 ohms will cut it. The 50-150 ohm 2 watt resistor with 4-6 turns of 18 gauge wire coiled around it should be in each grid lead. Don't forget that your RF choke can pick up RF from your tank ckt. & screw you up. Mount the choke away from the coil and a 90 degrees away from the coil. Same thing in the grid. I burnt a couple RF chokes until I had the Homer Simpson moment, duh.....
Anyway, just rambeling about the flow of electrons out of your modulator & where they end up. Make sure all of the chassis are grounded together at one point and that goes to a decent ground system. If you still have the feedback start disabling stages in the modulator one at a time working your way back in the circuit until you can isolate where the screaming is coming from. Also, did the builder have some kind of negative feedback hooked up in the modulator? Sometimes these go high frequency positive & scream like 15 KHz at 200% modulation. Not that this ever happened to me!
And last, is this testing into a nice 50 ohm dummy load or into a bogas antenna? I have had strange results when the antenna has an exploding PL-259.
That's the thoughts. What did I miss guys?
Keith
WA1HZK

[W2VW]

Parasitic suppression resistors in the mudulator are a real good idea too while you are in there. Look at broadcast rig schematics for typical values. Most use a  of resistor in series with each mudulator plate. Class B mudulators present a dynamic load to the driver. 5 or 10k across the grids can help some as well as keep the mudulator power toobs from taking off.

[KL7OF]

Keith...Tnx for the info...I have a couple questions..  On the Z-50 choke...Is that 2 inches of 22 ga wire on a 1/4 inch form?  That only makes 3 or 4 turns...  You also had a missed word in one sentence...  "Mount the choke away from the coil and a (Missed) 90 degrees away from the coil"  I am attaching a pic of the rf deck...as you can see I mounted the choke off to the side of the coil...  I wanted to put it in the center behind the coil but ran out of room on the chassis because I didn't want to mount the coil on top of the butterfly and I wanted the tubes up front in windows so I could see them.
   There is no neg feedback in the modulator but the speech amp has neg feedback on the PP 6L6's....I don't consider this a factor however because the rig acts up with the speech amp disconnected..I am testing into a heath cantenna...I tried the dummy load in the gates BC rig but it doesn't like the higher frequency bands...

   Dave   tnx for your input as well..When you say put 5 or 10 k across the grids, do you mean to tie the 2 grid terminals at the tube sockets together with 5 or 10k ?

[K1JJ]

Wow - plug in coils with a link - P-P... quite old buzzardly. OM.....

And where did you come up with the idea to cut the outline of the 100TH tubes into the viewing window?  That's pretty slick!  Never seen that done before...   It took me a while to figure out what I was looking at....

T

[W2VW]

Wow - plug in coils with a link - P-P... quite old buzzardly. OM.....

And where did you come up with the idea to cut the outline of the 100TH tubes into the viewing window?  That's pretty slick!  Never seen that done before...   It took me a while to figure out what I was looking at....

T

Reminds me of Fred Flintstone running through a wall. Really cool look.
I meant a couple of resistors each tied to a grid and common to gnd.

[WA1HZK]

The Z50 is the "red" VHF chokes that are hopefully in your junk box. Around 1/4 to 3/8 or so diameter with about two inches of windings tightly wrapped around it. You can use any small insilated transformer wire as long as it will hold the current. I'm using a 2 amp choke that Radio Shack still sells in mine as I discovered the stock Z50 did not like high DC current. (1 amp max.) You have the plate choke mounted OK, but it's still in the field of the tank coil. You might try unbolting and moving it away from the coil if all else fails and see if that might be the problem.