4-125a amp for fusor ion trap
Posted: Tue Feb 19, 2019 12:50 pm
Hopefully, I'll get the design details in here too (at least for my own good!).
Bill found us an old homebrew ham project sometime back, most of a CW transmitter for I think 6 meters. It had two "new" 4-125a output tubes, an under-chassis 6146 driver, and a real primitive pcb in a box with a 6cl6 that used a home etched xtal in some kind of multiplier to get to frequency. A 5u4 and a couple of 6b4's rounded it out. It clearly never worked, the soldering was amazingly bad, and when I got it there was no tank coil, but a 75k resistor in series with the plates (maybe it was arcing inside those tubes, it happens with that style).
I'm working to redesign it and bring it back to usefulness it never had before. The 5u4 went and became a couple of TV flyback damper diodes (it had an L input filter with an 8uf oil cap!). Now it has a 500v 82uf panasonic cap a fraction of the physical size, and so forth. The 6b4s (2a3 type tubes more or less) are gone and a VR 150 and 105 now regulate screen volts for the 6146. I tore out the PCB and put in a perf board that doubles and triples some available AC into 70 and 100v (roughly) to bias things.
The 6146 stage now will make a few hundred volts pk-pk into the 4-125 grids and drive them nicely positive despite starting from -95v bias supply.
Input and output of the 6146 are now "modern" toroid transformers I wound that work well at the range of interest (1.5-3 mhz, but pushing it at the top end, too much Q). Based on other measurements I haven't posted yet, I think I want to be down around 1.7 mhz for now.
I did do a semi successful ion trap experiment, which seems to be showing me that like with DC, things are mushier in these conditions, and the ions see a net field a lot less than the applied one for both AC and DC.
Which is no surprise and I may wind up going lower yet (And higher volts, but it's dodgy now with 8kv AC + 25kv DC on the thing - RF in the shack). My other amp was kinda pushing it at 150w and bad matching, this should do 500 or so and this time I have matching designed a lot better.
Net with that first test was it was getting better as I ran off the edge of the graph - which is defined by how low density I can run. I'm trying to get my peak onto the paper here at a pressure and ionization percentage I can hit reliably. Initial results were encouraging with 25kvdc + 7kvrf giving similar Q to 50kv DC...though lots less neutrons at the lower voltages, it also wasn't eating power either - most went into losses in my pi network! (Glad I used pyrex for coil form...)
I need one more run with that setup to see stray ions out in the tank with the faraday probes and see how fast they're going, but it's obvious I have to go lower in F and probably somewhat higher in RF volts (which will still leave me in a good area in the mass spec equations).
The rest to come. I wish I knew if I could float the secondary of a MOT to make a bridge rectified HV supply for this, and also I'm working on how to protect the output tube screens (which would now just be tied to the 350v plate supply shared with the 6146) - they'll cook if there are various other faults, and these aren't all that cheap, even though most hams seem to think they're not that great. (I do have a 3cx1500, but overkill...and limits on my house power...)
I've been "conditioning" these to hopefully let them self-getter for awhile. Procedure is to light them up for an hour or so, then tie all the grids and plate together and put enough volts to draw rated plate current (around 50v in this case) and run that all day. This may or may not be enough, I've heard the best way is to run enough power input (somehow protecting the screen, maybe just run the grids at or near ground) to light up the plates red hot - where they will act as getters. I do plan to try seriously current-limited HV on this to see what happens...maybe with a big series R in the screen supply too. Meanwhile, a lot of tedium, adding fans and power supply control and so on. And getting my little coupling transformers right...
It's been both cold and wet here...so I'm not getting in many shop hours for awhile, but spring is on the way.
Bill found us an old homebrew ham project sometime back, most of a CW transmitter for I think 6 meters. It had two "new" 4-125a output tubes, an under-chassis 6146 driver, and a real primitive pcb in a box with a 6cl6 that used a home etched xtal in some kind of multiplier to get to frequency. A 5u4 and a couple of 6b4's rounded it out. It clearly never worked, the soldering was amazingly bad, and when I got it there was no tank coil, but a 75k resistor in series with the plates (maybe it was arcing inside those tubes, it happens with that style).
I'm working to redesign it and bring it back to usefulness it never had before. The 5u4 went and became a couple of TV flyback damper diodes (it had an L input filter with an 8uf oil cap!). Now it has a 500v 82uf panasonic cap a fraction of the physical size, and so forth. The 6b4s (2a3 type tubes more or less) are gone and a VR 150 and 105 now regulate screen volts for the 6146. I tore out the PCB and put in a perf board that doubles and triples some available AC into 70 and 100v (roughly) to bias things.
The 6146 stage now will make a few hundred volts pk-pk into the 4-125 grids and drive them nicely positive despite starting from -95v bias supply.
Input and output of the 6146 are now "modern" toroid transformers I wound that work well at the range of interest (1.5-3 mhz, but pushing it at the top end, too much Q). Based on other measurements I haven't posted yet, I think I want to be down around 1.7 mhz for now.
I did do a semi successful ion trap experiment, which seems to be showing me that like with DC, things are mushier in these conditions, and the ions see a net field a lot less than the applied one for both AC and DC.
Which is no surprise and I may wind up going lower yet (And higher volts, but it's dodgy now with 8kv AC + 25kv DC on the thing - RF in the shack). My other amp was kinda pushing it at 150w and bad matching, this should do 500 or so and this time I have matching designed a lot better.
Net with that first test was it was getting better as I ran off the edge of the graph - which is defined by how low density I can run. I'm trying to get my peak onto the paper here at a pressure and ionization percentage I can hit reliably. Initial results were encouraging with 25kvdc + 7kvrf giving similar Q to 50kv DC...though lots less neutrons at the lower voltages, it also wasn't eating power either - most went into losses in my pi network! (Glad I used pyrex for coil form...)
I need one more run with that setup to see stray ions out in the tank with the faraday probes and see how fast they're going, but it's obvious I have to go lower in F and probably somewhat higher in RF volts (which will still leave me in a good area in the mass spec equations).
The rest to come. I wish I knew if I could float the secondary of a MOT to make a bridge rectified HV supply for this, and also I'm working on how to protect the output tube screens (which would now just be tied to the 350v plate supply shared with the 6146) - they'll cook if there are various other faults, and these aren't all that cheap, even though most hams seem to think they're not that great. (I do have a 3cx1500, but overkill...and limits on my house power...)
I've been "conditioning" these to hopefully let them self-getter for awhile. Procedure is to light them up for an hour or so, then tie all the grids and plate together and put enough volts to draw rated plate current (around 50v in this case) and run that all day. This may or may not be enough, I've heard the best way is to run enough power input (somehow protecting the screen, maybe just run the grids at or near ground) to light up the plates red hot - where they will act as getters. I do plan to try seriously current-limited HV on this to see what happens...maybe with a big series R in the screen supply too. Meanwhile, a lot of tedium, adding fans and power supply control and so on. And getting my little coupling transformers right...
It's been both cold and wet here...so I'm not getting in many shop hours for awhile, but spring is on the way.