Nico's amplifier - Tank Circuit Questions

[N4LTA]

Moving along in the SRS551 small linear amp. This is I think the first time that I have used a transmitting tube with the plate on the tube socket. This is what I will try to get the amplifed RF to the top where the tank circuit is.

I have mounted the plate choke and plate bypass capacitor down below the chassis and am using a high voltage feedthrough. The plate blocking cap shown in the one that I thought was 1000pF but ended up being 10pF. I think I have this covered with a care package from John  W9JSW.

There is a longer lead than I like, running from the top of the plate choke. Should I run a heavy #14 wire or use another parasitic choke? (see photo)

I can put the plate blocking capacitor at the bottom of the insulator or top. What is recommended? The photo shows it on the top of the chassis.


Anything else that looks bad, please feel free to comment. This will be a 75 meter amp. I may add 40 later or even 20 meters.

Thanks

Pat
N4LTA

[K8DI]

Any reason you can’t invert the choke, either just the connections or the actual part? Your plate to choke suppressors would be shorter, your choke to feed thru shorter, and the new “top” would be B+/more or less DC . That would possibly neaten things up a bit.

Ed

[KK4YY]

Pat,

I don't think lead length will be a problem unless they start to get very, very long. One parasitic suppressor per tube is all I'd use.

Consider a glitch resistor in the HV lead to limit current in the case of an arc. Modern power transformers with high capacitance filters can make an arc very destructive. A few ohms of resistance will limit the current before the fuse blows. Cheap insurance.

My first instinct would be to mount the plate blocking cap above the chassis, as you've done. Thinking about it some more, the only difference from one side to the other of the cap is that there's DC on one side and not the other. So you may want to mount it below the chassis to keep all the HV on one side and not subject the feed through to DC (I had a feed through arc to chassis with modulated B+ on it. It was fairly spectacular!). Operationally, it probably won't make any difference either way, so whatever works out best mechanically.

If you have a problem with neutralization you might have to poke though the chassis again to couple to the output. Time will tell, but keep it in mind.

For safety, be sure to ground the variable capacitor shafts with bushings as they pass through the front panel and/or use shaft insulators, insulated shaft extensions, etc.


Don

[N4LTA]

I  could do that but the by pass cap leads would be long. It may be better than the long lead to the  feed through though.

Pat

[N4LTA]

Thanks Don and Ed.

Mechanical it seemed easier to put it on top but it does put 1200 volts on the top.  I probably will use a shorter shaft load tube and slight the plate cap and or ut the shaft and use an insulated shaft coupler and panel bearing. I think I have some it I can find them.

Maybe rather than a strip of heavy wire would be a flat copper strip. I don't think it will matter at 75 meters , maybe at 20 meters if I decide to add that.

I have a basic 10 watt K3 transceiver that may see more use with this amp than the QRP rig.

I found the shorty plate choke in a junk box but I think it will work fine

Do you think a similar feedthrough with a plate mounted to it close to the tube would work for neutralizing cp?


Thanks for all of Yall's help.

Pat Bunn
N4LTA

[N4LTA]

Thanks Don and Ed.

Mechanical it seemed easier to put it on top but it does put 1200 volts on the top.  I probably will use a shorter shaft load tube and slight the plate cap and or ut the shaft and use an insulated shaft coupler and panel bearing. I think I have some it I can find them.

Maybe rather than a strip of heavy wire would be a flat copper strip. I don't think it will matter at 75 meters , maybe at 20 meters if I decide to add that.

I have a basic 10 watt K3 transceiver that may see more use with this amp than the QRP rig. I have plenty of resistors 3 watt 100 ohms or so or something lower in resistance?

I found the shorty plate choke in a junk box but I think it will work fine

Do you think a similar feedthrough with a plate mounted to it close to the tube would work for neutralizing cp?


Thanks for all of Yall's help.

Pat Bunn
N4LTA

[KK4YY]

Do you think a similar feedthrough with a plate mounted to it close to the tube would work for neutralizing cp?

Something like that, yes. But it may not be necessary at all. You'll find out when you get it operational.

Is this to be a grounded grid or grid driven amp?

[WD5JKO]

Pat,

   A fellow AM'er in South America, LU8JB has a ton of home brew 40m AM rigs. Some tube, some solid state, and some have a PWM modulator.

Here is the link to his AM section. Lots of eye candy!

http://www.lu8jb.com/Amplitud%20Modulada%20.htm

If you click on enough links, you will see his PWM generator board. He uses a NE555 with a lot of external parts to linearize the ramp.

Who knows, the rig you are building might get a PWM tube modulator!

Here is a 400 watt PWM rig:

http://www.lu8jb.com/PWM_6883%20X%204_(1).htm

73,
Jim
Wd5JKO

[N4LTA]

It is grid driven and it worries me  with the grid and plate circuit all so close together. I have only seen one transmitter circuit using these tubes published. Nico pointed me to it when he suggested the tubes. It is in German but pretty straight forward. I have never build a grid driven circuit other than simple 807 and 6L6 rigs except for a 4CX1500 amp for 6 meters. That was a different animal.

I have a 5 watt 56 ohm ceramic composition resistor for the input resistor and have two more grid parasitic chokes identical to the plate ones. I have a variable bias supply already mounted and tested. I'll use a small Hammond 2.5 mH choke to feed the bias to the grids.

The power supply and the screen supply are tested under full load with resistors as the load.. The screen supply is current limited at about 80 mA. I am running 1200 volts on the plate and 400 on the screen. 1400 was more than I felt comfortable with with the max plate voltage being 1000 volts. The tube is supposed to be very rugged. I'd be very happy with 150 watts.

I have ordered a laser cut front panel for switches and pilot lamps  and four meters. Also have a timer for a step start for the primary voltage as we talked about earlier.

Thanks to all for the help and interest.

Pat
N4LTA

[kb3ouk]

I'm really following how this turns out because I'm considering building a small amp with a very similar tube, the GU-50. The couple circuits I've seen for that tube run it grounded grid, but you still are dealing with all of your connections being together on the same end of the tube. From what i can see in the GU-50s i have, they tried to use some internal shielding to isolate the plate pin from the other pins in the base. If you look around for designs using the GU-50 it might give you some pointers on what to do in your amp.

[KK4YY]

Okay, looking at the datasheet now, the beam plates are internally connected to the cathode so GG is not an option. As this is a single-ended tube, it may be necessary to construct a grounded vertical wall under the chassis to shield the input from the output. I've attached a photo of a tube socket for that tube which has that shield built right into it. If they went to the trouble of making a special socket, that's your clue that there may be an issue.

There's a lot of gain in a grid driven amp so oscillation is something to watch out for. You may need to rotate the tube sockets by 90 degrees to keep the inputs on one side, separated by a shield, with the outputs on the other.

But who knows? It may work fine the way it is. I've never built a grid-driven linear amp (much less one with a single-ended tube) so I'm just speculating here.


Don

[WD5JKO]

   Pat,

   I attach an image of the bottom side of my CE 20a after I converted the RF Finals from 6AG7 to 7591a's. On 75m, I can get 75w PEP RF Out in grid driven class Ab2 RF amplifier service. The plate tank is up top of chassis, and the input tank is also above deck with shielding between. In the 20a, there is a cathode follower driver.

   The 7591 has two screen grid terminals on the base, as well as two cathode terminals, just like the SRS551. Notice the bypassing I use of the 7591a screen grids which includes two VHF button 500pf caps, and a single .01uf in the middle. I also use an unbypassed 1 ohm cathode resistor on each tube.

   If your drive source is outside the chassis, I'd add a grid coil circuit, and link couple the drive to it. If you make the grid coil with a CT, then you can add a neutralizing circuit. The coil could be wound over a powdered iron toroidal core. A toroid is largely self shielded.

Jim
Wd5JKO

[w1zb]

I built a small 300w amp for my Central Electronics 10A exciter a few years ago. The amp uses 2 6HF5s in parallel and is grid driven. The schematic is similar to Nico's Amp. My amp does have a neutralizing circuit and regulated screen voltage. It has all the output components above the chassis and all the input components inside the chassis. I have used it on 160M thru 20M with no problems.

73 Jerry W1ZB

[N4LTA]

Thanks for the information and photos. I have the parts for the glitch resistor and will use it as a meter shunt also.  The laser cut front panel is scheduled to arrive tomorrow or Friday.

I will initially try the resistor grid input circuit as designed and if I have trouble, I'll go with the suggestion on the above with neutralization.

I need to make a few boards for the T?R relay and the shunts and the meter circuits. Shouldn't take long. Hopefully I can rob the bias transformer for the relay power.

It will be nice to have a front panel with the meter holes punched and holes for the Carling switches with the little tab there to keep the switches from rotating. Anyway that is what I hope to get.

The panel is made of .080 aluminum. I chose aluminum over stainless because I will have to cut the pi net holes myself as I fit  it.  The panel in aluminum or stainless was $29 shipped. It is 12" x 12 3/16". They accept a cad drawing and the laser cuts it. Takes about a week. I had two chassis tops cut for the Hammond walnut chassis for a couple of monoblock  audio amps and they turned out very nice.

Pat
N4LTA

[KD6VXI]

That ought to be some cool fireworks if you ever need the glitch r.

Better make sure you wire a couple 10A10 across the meter.  It won't protect the meter (if you use enough diodes to keep the meter indicating correctly then it won't save the meter.  If you use less diodes than necessary you end up having an inaccurate meter.

--Shane
KD6VXI

[N4LTA]

Don't really see a problem. If the glitch resistor ever goes, it will likely come apart.

The shunt/glitch resistor will be the 10 ohm 10 watt resistor. The feed to the meter will have six 1 watt resistor totalling 1 meg - 500 K on each side of the shunt. each resistor is rated at 1.5 KV.

The meter is a low cost 50uA meter. I really don't expect great accuracy although the shunt resistors are 1% resistors.

I don't see a great probability of the 1200 volts reaching the meter movement. I will have a small PC Board across each uA meter with back to back 1n4007 diodes. It is something I have always done. All the metering will be done with 50uA and 100uA meters movements. The entire chassis will be enclosed with perforated aluminum.

In my normal occupation, I specialize NFPA 70E arc flash studies and a teach a class in arc flash safety, so I am very aware of high voltage arcs.

I would think, in a small transformer, impedance limited power supply like this - although it is DC - once the capacitor bank has discharged, the power supply AC Waveform goes to zero 120 times a second  and the bad stuff settles down. I don't think the transformer can sustain keeping the capacitor bank charged.

I would expect is would be much like a single phase AC arc which is not even included in NFPA 70 as it cannot propagate like a 3 phase arc can.


Pat
N4LTA

[KD6VXI]

My point was, without the diodes, you may get a nice burst of light from the meter itself, not that it would be all that dangerous to the guy operating the rig. 

DC arcs are nasty.  I took a 6 Kv arc to the back of my hand.  Exited out my left leg and right hand.  Through the heart in two paths. 3 strokes before I could throw myself back hard enough.

My day job I do everything I can to not cause an arc flash.  Today was working in a 2500A 208/120 panel.

--Shane
KD6VXI

[KK4YY]

It's pretty easy to over-build a HV PS with lots of capacitance, and that can make a very big bang when things go wrong. A microwave oven fuse (MOF) between the filter bank output and load doesn't sound like a bad idea to me. A glitch resistor will limit the peak surge current but it still may be north of 100 amps. I suspect that a MOF in the output will open before the slow-blow fuse in the transformer primary will. At any rate, the power transformer can't provide the kind of surge current that the stored energy in a large capacitor bank can. It's like a bomb waiting to go off. If you're lucky, it never does. If you're not, you'll be glad you had a MOF in the HV output.

[N4LTA]

The problem with a microwave oven high voltage fuse is that I can't find any specs on them. I can't find any that are not Chinese. In a microwave they don't have to interrupt much current, nothing like a capacitor bank discharging. The last large power supply that I built was a 2500-5000 volt,  1 amp supply. To get a suitable fuse required a 6" long  Bussman fuse which cost $35 -  25 years ago and the holder was $20. It is difficult for me to understand how a small glass fuse would not end up worse off than a glitch resistor, interrupting a couple hundred amps at high voltage DC.

Has anyone actually had one do so in real life? Can anyone point me to the fuse curves/specifications for one of these?

Not trying to be difficult, just an old power engineer looking for specs. Believe me, I have seen and heard of  the results of AC and DC arcs. In the class that I teach - an example is a 480 volt three phase  disconnect switch on a system with 22,000 amps available fault current (not at all unusual). A  three phase symetrical arc flash ( almost all are symetrical) delivers the energy of 3/4 pounds of TNT plus a 20,00 degree C plasma cloud. If you take it unprotected, you will likely die from burns and physical shock

Pat.

[KK4YY]

Pat,

You're right-on to ask these questions.

I used to work for a fuse manufacturer where I did the testing that generated the data that you're looking for. Interrupting DC was quite a challenge. I wasn't called upon to test any DC above 125V but even at that voltage it's hard to contain the arc and get consistent results with a particular fuse type. Most blew in the standard deviation but there were always outliers were the arc would sustain and sometimes even rupture the fuse body.

In a transmitter I built 40 years ago, I fused 2KV DC with a 250V glass fuse. When the rig arced-over, the fuse shattered into many small glass pieces and blew one ferrule right out of the clip. It was messy, but it worked.

You can probably find some very expensive fuses that will work better, but a microwave oven fuse is cheap and does offer some protection. I'd consider its use along with a glitch resistor, but not in place of one.


Don