Plate choke exploded

[ka1tdq]

I just finished my amp and hooked it up to my single tube driver putting out around 12 watts of drive for 75 meters.  I haven't adjusted the grid tank yet for optimum and I'm testing on the air through my link coupled tuner to a dipole (tuned for resonance, and I know on the air testing is bad). Borrowing a large dummy load tomorrow.

Anyway, I started tuning the plate when the output skyrocketed to over 2kw and the SWR was pegging the meter when the plate choke literally exploded.  It sounded like a pistol went off next to my ear.

What would cause that?  Parasitics or something?  12 watts of drive shouldn't cause the amp to go over 300 watts, I'd imagine. 

More numbers:  4000 volts plate voltage (unloaded) and 4.9 volts filament. 

Jon
KA1TDQ

[ka1tdq]

...another note

Plate capacitor was at near minimum capacitance at the explosion with loading fully meshed.

[WQ9E]

Did you dip the choke for resonances once mounted?  Chokes will show series resonance at one or more frequencies and those need to be outside of the ham bands or there will be fireworks like you experienced.

[w1vtp]

Did you dip the choke for resonances once mounted?  Chokes will show series resonance at one or more frequencies and those need to be outside of the ham bands or there will be fireworks like you experienced.

Exactly!  There is a "how to" somewhere in ARRL literature.  For some reason, however, I seem to recall the need to short out the choke although that didn't make sense at the time nor does it make sense.  I bet that was your problem

[steve_qix]

Wow!  I've been there, so I truly feel your pain!

So, all chokes like that will exhibit self-resonances (as has already been pointed out).  The key is to keep them out of the ham bands, and usually they are.

I'm suspecting one of the following:

1) You had/have either an instability in the amplifier itself, and it was self-oscillating at some random frequency
2) The amp has a parasitic
3) The plate circuit was tuned to a harmonic of the input frequency, and there is a self-resonance at that frequency (less likely, but possible)

Because the output suddenly jumped up, it sounds like an instability and the amp was oscillating.

I'm going to assume you had the grid at resonance, etc.

I don't remember if you have parasitic chokes on the plate and grid leads of each tube or not.  With parallel tubes, this is an important thing to have.

[w1vtp]

Perhaps this will help

http://www.w8ji.com/rf_plate_choke.htm

GL, Al

[ka1tdq]

   There, my ears have finally stopped ringing 

This happened to be the exact same plate choke that I used with my last homebrew amp, so I know it was good for 75 meters at least.  Someone also messaged me and pointed out that this turned out to be a very good oscillator.  That would explain the over 2kw output (momentarily).

For the record too, I do have another cooling fan beneath the tube sockets.  The blue RF deck fans help cool the glass surfaces and (didn't realize this until after I installed it, but I'll take credit for the engineering) help force a spray bottle effect by pulling air from below the tube sockets.  The fan below then adds to this.  You can totally feel good air flow all around the tubes on all sides, so I'm good there.

I didn't tune the input tank either to 75 meters so that's part of my problem.  I will also borrow a grid dip meter to tune the input properly before I belch out my next oscillator.  The good news is that I just have to rewind a new plate choke.  That last one had sentimental value though since it's the one my wife and I wound back in the MA trailer.

Jon
KA1TDQ

[WQ9E]

Jon,

The plate choke needs to be mounted in place before checking for resonance.  Just because it was OK in one amp doesn't mean the resonance wasn't close to being in-band and the proximity to other components in this amp could cause it to shift.  I used a commercial choke in my homebrew triple 4CX800 amp and it was marked as having a resonance at 13.1 and that is what my Measurements 59 GDO indicated in the clear and when mounted in the amp the resonant frequency was about 200 khz. lower which was still fine but certainly different.

You stated the tune capacitor was at near minimum mesh, where should it have been with the calculations you made for operation on 75?  Any chance your driver is putting out significant 2nd harmonic energy (or a spur) and you tuned up on that?  If so you might have accidentally located an out-of-band resonance in your previous choke.

I remember seeing a photo of one of the classic National chokes where the pi layers had slammed together due to choke resonance when the amp took off on a spurious frequency.

[Jeff W9GY]

I'm with Steve, I'm thinking instability.  Out of curiosity, is there a bypass capacitor at the input side of the choke (power supply side)?  I generally use a 500 pf doorknob for this purpose and place it physically close to the input side of the choke. 

[K1JJ]

Possibly the 2KW reading was erroneous cuz when parasitics are introduced to a 50 ohm wattmeter, the swr soars and can cause high readings.  But a pair a 3-500Zs can put out 2KW when pushed hard.


Over years of building, I have had my share of imploded plate chokes too. If it's a series resonance, just rewind it slightly differently and try again. The chance of it being series res on a ham band is small, but it does happen.

I can't tell clearly from the pics, but is your filament choke and filament structure "looking" at the plate tank circuitry?  The entire fil structure should be shielded from the plate. This can cause a TPTG oscillator if so.   This amplifer is good for 13 to 16 dB, right?  Hi Mu triodes need good input to output isolation, esp if they are coupling at higher freqs..

It could be many, many other things...  I'm assuming this is a GG (grounded grid ) amplifier, right?  If so:  Try loading the filament to ground with a 100 pf cap in series with a carbon 100 ohm 5 watt resistor. This will help kill filament choke resonances. Add some ferrite cores to the fil leads near the tube pins.
 
If it is grid driven, is it neutralized? It will be a bear to tame if it is grid driven, so be prepared.


Try more turns on your plate suppressors.


Try increasing the width of your plate tank connections. All leads should use 1/2" wide (or wider) copper flashing, at least.  Make sure you go directly to C1 from the tube plate caps. Then directly to L1 and then to C2.  If you run a split path, this can cause VHF parasitics to arise.

Test it at a lower voltage until you are more confident of stable operation.  1500 volts is plenty for now.

Put a 50 ohm,  25 watt resistor in series with the HV lead to limit current for initial testing. This is to get you past the blow-up stage.

Use a mirror behind the rig if there is arcing. With the lights dimmed you may be able to see arcing easier if it is occurring.

Put low drive thru the rig with the precautions listed above - and then move C1 and C2 (and input tuning) all around looking for fuzzy stuff on the scope.  You should see a stable carrier if running right.

** Get some see-thru Plexiglas sheets to construct a rig cover for safety.... if you want to continue viewing the insides.  Otherwise, scrap yard aluminium sheet is cheap and FB.

There's more, but go slowly and you will find what's troubling your patient...

Post your progress.

T

[ka1tdq]

It found resonance somewhere, that's for sure. I was using a tube driver (homebrew) so I might just add a low pass filter for 75 meters as an added measure.

My last transmitter project used a commercial rig for the VFO with no harmonics. My current setup ia all homebrew so I'm probably spewing harmonics all the way to 10 GHz.

Jon

[N2DTS]

I would say part of your problem is the amp is not built very well.
For interconnections, you should use strapping, copper or silver plated flat wide thin stuff.
You can flatten copper tubing, or cut sheet copper.
Thin round wire is never used for amplifiers.

Also, you want to keep the grid and plate circuits separate, grid stuff below the chassis, plate stuff above.
For grounded grid, keeping the plate and cathode separate is a good idea.

Bypassing needs to be close and direct, with the right caps.
Common point ground is a good idea.

Most amps can have issues with some combo of grid and plate tuning with really high voltage on the tubes.

Your plate tuning cap looks way too large (capacitance) and the loading cap looks too small.
The air gap on the plate tuning cap looks to be WAY too small for 4000 volts.

When I built my 2x 4x150a rf deck, I used a nice plate choke out of an FT102, and the deck worked very oddly.  I changed the choke out to a 4 pie wound old thing in the junk box and it worked fine.
Sometimes just the place bypass cap can be changed to get an improvement.

[K1JJ]

This is a GG pair of 3-500Zs, not grid driven, correct?


You said:  "I haven't adjusted the grid tank yet for optimum."  

The grids should be strapped directly to ground through copper strap.  Use a 50 watt zener of the correct voltage in the filament center tap for cathode bias. The input tuning L/C should be connected across the filaments to ground.  Maybe this is what you meant??

T
 

[KB2WIG]

J,

Check this out.

http://www.tubebooks.org/Books/intro_orr_radio.pdf

Its the 'west coast handbook' ---  lots 'o gud stuff.  Better than the ArrlgL stuff. I've got the 14th ed, but this one should do.


klc

[WBear2GCR]

I gotta tell you.

As shown and built:

The amp is EXTREMELY UNSAFE.

The open construction looks pretty, and you said that there is no one else or pets, but that doesn't change it for a second.

All you need to do is to reach up to tune the thing and slip a few inches and you're DEAD, instantly.

Consider at least scavenging some chrome kitchen appliance grilles and re-purposing them as grounded safety shields??

Recycling yards will see you scrap metals close to scrap cost...

Never skimp on safety.

                                _-_-


Consider the plate choke as a warning...

[Pete, WA2CWA]

You can buy the RF choke that's used in the Ameritron AL-82 (pair of 3-500's) for $20.

http://www.ameritron.com/Product.php?productid=10-15197

[N2DTS]

True!
All you have to do is get to close or brush up against that and get killed.

2000 volts scares the crap out of me, so I put it all in locking racks, 4000 volts is crazy!

Brett
N2DTS

I gotta tell you.

As shown and built:

The amp is EXTREMELY UNSAFE.

The open construction looks pretty, and you said that there is no one else or pets, but that doesn't change it for a second.

All you need to do is to reach up to tune the thing and slip a few inches and you're DEAD, instantly.

Consider at least scavenging some chrome kitchen appliance grilles and re-purposing them as grounded safety shields??

Recycling yards will see you scrap metals close to scrap cost...

Never skimp on safety.

                                _-_-


Consider the plate choke as a warning...

[W1RKW]

The amp looks cool but agree on the safety aspect.  Put plexi shields on it so you don't lose the looks.

[ka1tdq]

I'm finally at a computer and not on my phone so I can type a good reply.

Yes, safety aspects noted.  I'm building the plexiglass housing now and everything will be totally safe.  I've got a baby boy on the way and a Persian cat that I want to keep around (not to mention my wife and I). 

To eliminate  aspects of the whole thing, I am building a low pass filter for 75 meters to go on the input.  I will also do the 100 ohm resistor to ground with a 100 pf capacitor as mentioned above. 

I'm going to rewind the plate choke with slightly larger #20 wire.  This will still give plenty of turns and have a larger wire guage for current flow (should I need it). 

I will also drive this with just 5 or 10 watts using just my solid state driver and IRF510 amplifer and not the tube rig.  It just makes everything 50 ohms and a lot simpler.  I can very easily adjust the drive power as well.

Another thing I will do is remove the tubes during the initial test and connect a large 50 ohm resistor to the cathode.  I can then rough-in the cathode (not grid  tank for 50 ohms impedance at the proper frequency. 

This should all equal no blowy-uppy.

Jon
KA1TDQ

[K1JJ]

Jon,

Adding a low pass filter to the input will probably NOT have any effect on curing your parasitics.  The problem is most likely generated from within the amplifier itself, not from the exciter.

If your exciter is taking off due to poor neutralization or parasitics, then this is another story and should be addressed as a separate problem. It should be worked on and stable before driving a big linear amplifier.  

Try a commercial ricebox driver as a test once you get a handle on the big amp.

One way to find out if the 3-500Z amp has problems is to ground the 3-500Z amplifier input with a 50 ohm carbon resistor to ground. (no signal in) Then tune C1, C2 and the input tuning every which way.  If you see the plate meter bounce around, excessive, random plate color, or see some fuzziness on the scope, you will confirm it is generated from within the amplifier.

Do these tests at 1500 volts or so to minimize damage if it takes off. Slowly bring the HV up as you proof things out.

Good decision on adding the Plexiglas cabinet for safety.

T

[ka1tdq]

Im at Frys now getting components so Im glad you caught me. Will do...

Jon

[ka1tdq]

I wound a new plate choke using 1/4 round ceramic tile glued around a wooden dowel.  I moved the DC blocking capacitor closer to the parasitic suppresors as well. I put a couple FT37-43 cores around the filament leads going to the tubes too.  I also stuck in a 100 pf / 100 ohm resistor in parallel with the input tank to calm things down. 

Last thing I need to do is place in in-line resistor to ground with the negative plate supply lead so that I can measure plate current with a voltmeter.  I'll do testing this weekend with a dummy load and with plate voltage at around 1500 volts.  I'll start out with 5 watts drive with the solid state driver first before I add the tube transmitter in line. 

Jon
KA1TDQ

[ka1tdq]

...and as a final change I am going to use copper flashing for the grid leads to ground.  Hopefully all this will lead to no plate gunfire.

Jon

[K1JJ]

Good going, Jon.

In hindsight, you will find the explosion was the best thang that could happen. You are cleaning up the rig now and will have less chance of explosions happening over and over.

Question:  Those long bare wire leads I see in the picture... are they filament leads and part of your RF input circuit? If so, they are looking at the plate structure and may cause the amp to take off again.

Normally, all this fil/input stuff is BELOW the chassis and shielded away from the plate tank.  You should probably put an alum shield around the bifilar choke and put some coax braid around the filament leads, using heavy insulated wire for the fil leads.  Ground the braid and make sure all fil and RF in leads are shielded from the tube plates and plate tank circuitry. 

Even your antenna relay that switches input to output needs to be carefully looked at to be sure these coaxial connections are not coupling to each other.  I usually use two separate relays and think out the potential situation for coupling input to output.

Once you make the mods, take some more detailed shots of your amp so we can see other problem areas. (especially under the tube sockets)  If you do things right this time, there is a chance that amp will be very stable on the second try - just like a pair of GG 3-500Zs should be.

T

[ka1tdq]

Actually, that's a duplicate ground for the grid. Im still going to put copper flashing on though.

The filament leads are under the chassis and steel ground. Should be fine.

No antenna relays. Beverage antenna for rx and dipole for tx.

Jon - on cell with small screen. Im getting a huge Samsung Mega tonight! The wife decided to treat me.