Hi Jim,

I've spent more time modelling it now but I'm still not happy with the overshoot with sudden load changes

Plus, I've come by an "energy storage" capacitor rated 38uF at 8kV. (It's 22" tall, 6" x 10" !)

I'm not sure what parameters of a capacitor warrant the "energy storage" labelling... don't they all do that?... but it appears to be polyester film or PIO in a big metal can. Maybe originally intended for PFC applications. What's not to like?

PSUD indicates it will fix my ripple voltage problem.

Thanks for your awesome info on using chokes.

Tim

You are correct.? A MOV? from each winding...to chassis? is a wasted effort.? ?I didn't think of what happens if the spark gap arced, when the choke is part of a choke input (resonant or not).? With the choke temp shorted, via the spark gap... (or choke effectively shorted via 2 x MOV's), the B+ would skyrocket...and fry the lytics in the supply.? ?Those 16 uf? @ 7.5 kv oil caps in henry amps (they still sell em)? were notorious for arcing? to the the steel casing, when used on 6+ KV? HB B+ supplies back in the 70's and 80's.? I used 4 x 47 uf @ 4.5 kv oil caps in series-parallel...on a 6800 vdc supply. One day, the line V drifted to the high side at 247.3 vac..... and the B+ rose to 7 kv.? ?The 7 kv arced sideways, just under the insulator on the oil cap, to the side of the hole, where the threaded rod protrudes through the hole.? It blew out the red washer that sits between the steel casing....and the bottom of the? glazed ceramic insulator.? ?My? 'fix' was to use a 1/4" thick huge sheet of that red micarta (HV fiberglass)? under all 4 x oil caps...then 1/8" thick? sheets of micarta between each oil cap.? Problem permanently solved.? Oil caps insulated from each other...and the chassis.? ?

Yes, I'm a strong advocate of insulating any choke from the chassis.? ?With the choke insulated from the chassis, there is no V potential? between the choke terminals / winding....and the steel frame.? ? To arc from the windings to the chassis, it would 1st have to arc from windings to the frame...then? from frame to the chassis...and that's not gonna happen.? Think of it as a contactor, it's just then 2 x gaps in series, and you have to arc both gaps simultaneously.? ?The windings have B+ on em...and the chassis is grnded.? The choke's steel frame is between em...and floating.? ? I can't draw any leakage current at all ( windings to chassis)? with my hi-pot tester, even after 2 hrs.....but it only goes up to 15 kvdc.? ?I also tried hi-pot testing from windings to frame, then? frame to chassis...and still can't pull any leakage current with 15 kvdc.?

?

On a C-L-C setup, even with the choke shorted out via a vac relay, B+ value stays the same.? ?I put both the adjustable spark gap and also? the Kilovac SPST-NC ceramic vac relay across the choke terminals. The vac relay is there, so I can compare the calculated ripple, with and without the choke.? ?With the choke shunted, it becomes one big C filter...with C1 and C2 now directly in parallel.? ?I had bought a bunch of nib spst-nc kilovac relays yrs ago...cheap.? They have 12 vdc coils.? Nobody wanted em, since they are just spst...and also NC.? ( choke shunted with vac relay coil? de-energized).? ?The relay is there for an experiment...so I can do rapid A/B testing.??

On 2/20/2024 9:42 AM, Jim VE7RF wrote:

I was thinking about a RVS connected diode across the choke..... but
Hammond/dahl read me the riot act, said...'don't even think about
it'.? ?They are correct of course, since the normal Voltage across the
choke will swing up/down a bit like +70 to +100 vdc...then swing
negative the same amount.? The diode would conduct on 1/2
cycles....and short out the choke.

A MOV may well work, BUT if it fails, it will fail shorted every
time.? ?MOV's really should be fused...but that would require a proper
sized HV fuse.? Modeling in software, the peak V can easily be through
the stratosphere, when excess amounts of fault current are drawn
through a choke, like almost triple the B+ in some cases.

Well then... since what we're REALLY concerned about is keeping the
voltage difference between the choke winding and iron core low enough
that it never arcs over and causes damage, what about a high-voltage MOV
(couple hundred volts or so, fused, as you suggest) between either choke
terminal and the choke frame?? Perhaps the real answer is to both
isolate the choke body from the chassis as you've done, and use the MOVs
to reduce the possibility of arcover damage? (plus use the spark gap!)
The two MOVs are effectively in series across the choke; so if they're
HV like 180 - 200V or so, then they will clip at 360 - 400 volts or so,
which is not far from what your 0.005" spark gap should arc over at.

Then again, I suppose since we isolate the choke body from the chassis
on insulators, we really don't care if it arcs between the winding and
frame. Or do we...

The reason I'm going on about this is because although I've got the new
XFMR to put in my 2K4A (haven't put it in yet), I never thought before
about the possibility of the choke having also failed. So now, when I
finally do install the XFMR, I'll also remount the choke on insulators,
as you've done (plus the spark gap).

Then again.... if those MOVs, or the spark gap, ever fire, then they are
effectively shorting out the choke... and so our B+ tries to shoot sky
high; and if it does for longer than a split second, we have Sh|7 City
inside the power supply cabinet as the electrolytics (IIRC, mine has
electrolytics instead of an oil cap for C1) blow the heck up... ===8-O

The spark gap is dead simple.? And yes, probably? ? .005" is ample.?
?Actually, .032" is good for 1100 V..so that amounts to 1100/32 =? 34
volts per .001".? ?.005" = 172 volts.? ?I believe mine is aprx .005"
The thickness of a 8.5" x 11" sheet of printer paper is typ .005"
thick. (.13mm)

Through the years, I've gotten pretty good as estimating 0.032" gaps,
since I've gapped spark plugs since I was smaller than a wee lad? 8-)

Steve, K0XP

I was thinking about a RVS connected diode across the choke..... but Hammond/dahl read me the riot act, said...'don't even think about it'.? ?They are correct of course, since the normal Voltage across the choke will swing up/down a bit like +70 to +100 vdc...then swing negative the same amount.? The diode would conduct on 1/2 cycles....and short out the choke.?

A MOV may well work, BUT if it fails, it will fail shorted every time.? ?MOV's really should be fused...but that would require a proper sized HV fuse.? Modeling in software, the peak V can easily be through the stratosphere, when excess amounts of fault current are drawn through a choke, like almost triple the B+ in some cases.

The spark gap is dead simple.? And yes, probably? ? .005" is ample.? ?Actually, .032" is good for 1100 V..so that amounts to 1100/32 =? 34 volts per .001".? ?.005" = 172 volts.? ?I believe mine is aprx .005"?
The thickness of a 8.5" x 11" sheet of printer paper is typ .005" thick. (.13mm)

?

On PSUD-2,? I don't use the resistive load, I use the constant current load.? ? And? then also have the option to toggle between any 2 x distinct current loads, like? say 200 ma, then 800 ma... OR the other way around, like 800 ma, then down to 200 ma.? ?It won't simulate? ssb,? ( but sorta).? ?When u toggle between 2 x different loads, it's like? right now, asap, instantaneous.? ?On CW, you have a 5 msec rise and decay time.? On PSUD, it's 0 msecs.?

?

PSUD-2 will not handle a resonant choke.? ?The problem with the resonant choke, like used in Henry amps is.... the B+ is only .9 X sec AC voltage.? ?With a C input filter, it's 1.414? x sec AC voltage.? ?IE: 6000 vac sec on a 8K ultra = 5400 vdc no load.? ?You coulda got the same 5400 vdc, by using? a FWB + just a (5400/1.414)? =3819 vac sec AC winding.? ?If a FWD was used, the sec is then just 1909 vdc.? ?The resonant choke setup also requires a huge amount of bleeder power at all times.? ?The 8k ultra uses? 5 x 20K @ 100 watt wire wounds in series.? ?That mess sucks 300 watts? CCS, or 60 watts per resistor...and all 5 of em run stupid hot.? If just any one of em opens up, the B+ soars to? 6000 x 1,414 = 8484 vdc.?

With the bigger choke values, like your 10H, they require a? smaller resonating cap...which is good,? BUT the 10H choke has a lot more dc resistance vs a 1-2-4 H choke.? ?All that wire has to fit the core, so they have to use smaller ga wire = less current handling capability.? ? The issue with the resonating cap, like when a 4 H choke is used is.... the choke + cap form a 120 hz notch filter.... fine, BUT all the harmonics of 120 hz will? sail right through the resonating cap.??

On a 10H choke, it requires a .1759 uf resonating cap.? ?A 4H choke requires just .4398 uf.? ?(20 hz will sail right through a 2 uf cap).? 240 hz will sail right through a .1666 uf cap.? It just gets worse with the higher harmonics of 120 hz...like 240-360-480 hz etc.? ?Then u have the issue with now 2 x dc currents flowing through the choke, the main current, and also the circulating current in the choke.? The smaller the choke value, the higher the circulating current.? It's bad enough with a 8 H choke, but with a 4H choke, the circulating current is as high, or higher than the main current.?

To kill the harmonics, ideally the C1 cap needs to be a decent value.

John Lyles, K5PRO, when he worked for Broadcast engineering, designing single phase 5 kw? FM broadcast PA's, told us that the FM pa's had to meet this spec, called an ...."AM noise spec" .? The supply used a resonant choke.? (resonating caps were a series pair of .9 uf oil caps, like used in microwave ovens).? Caps were sent to the choke manufacturer, and the choke was hand wound, so it resonated around the supplied caps, .45 uf for the series pair).? In some cases the choke is made to a larger value, so it can be used on 50 hz.... then tapped down to a lower value for 60 hz..so resonating cap pair stayed constant.?

?

Problem was, the resonant choke would not meet the....."AM noise spec'.? (I forget how big the C1 oil cap was, think it was like 16uf to 20 uf).? ?His fix was to wire a LC filter onto the end of the existing resonant choke + C1 cap.? This time the added LC filter does? NOT have it's choke resonated.? It's there to kill the harmonics of 100/120 hz.? IE: it ends up being a (resonant) L - C1? - L2 - C2? setup.?

?

A simpler? ?C-L-C woulda done the same thing, and also coulda used a much lower secondary Vac.??

I also wanted to try yet another config... using the same? C-L-C? setup, BUT resonating the choke.? Got a buddy in W5 land to simulate it on LT SPICE for me.? It results in a lousy 4 db improvement at 120 hz.? The main drawback was, the existing, superb harmonic attenuation? was ruined by the resonating cap.? Same deal, harmonics sail through the resonating cap....and attenuated only by whatever value my C2 cap is.?

The consensus was to not mess with a resonating cap on a C-L-C config.?

As far as testing your choke for heat, like with your proposed? 2.0 to 2.5 amp? CCS load,? that's? easily tested by using a small lab supply, with current limiting, like a 0-160 vdc @ 5 amp variety.?

With 850 ma into 28 ohms =? 20.23? watts.? ?2.0 amps into 28 ohms =? 112 watts.? ?2.5 amps into 28 ohms =? 175 watts.? When more than rated current is drawn the dc resistance increases, then it slowly goes into thermal runaway.?

?

But for FT-8 use, like (I think)? 15 secs on, and 15 secs off, duty cycle is 50%, so may well get away with it.?

?

Another option is to use a lot more C....since ripple is inversely proportional to uf.? Doubling the uf results in a 6db? reduction in ripple.? ?But you would need one helluva lot of uf.?

Another option is a smaller choke, like 1-2-4 H, but with a lot higher current rating.?

Mine is rated for 3 amps? CCS, and is 4 H....and good for ( I think) 15 kv..... but it's big, 127 lbs.? Stone cold with 3 amps? CCS.? With 6 amps CCS, it runs pretty hot, (195 F)? right where the gapped cores reside.?

On Tue, Feb 20, 2024 at 04:19 PM, Jay Sturtevant, K2ZT wrote:

What is W8JI's email address for ordering parts?

?

Thanks.

?

Jay, K2ZT?

Here.? ? ?https://www.ctrengineeringinc.com/contacting-us-please-read/

DigiKey delivered my Alph 99 transformer exactly 7 weeks after ordering it for a fairly reasonable price. The new one says "Peter W. Dahl" on it and it is STOUT!. NO Hv sag at full output.

Good on ya, DigiKey!!

73

I notice in the circuit of the Henry 3K, 4K EHT supply there is a 10H choke, with a 0.2uF capacitor *in parallel*.

0.2uF // 10H makes a parallel resonant circuit at 120Hz, nice trick to up the impedance and improve ripple rejection.

Can't model it in PSUD so I'm just trying LtSpice....

On 2/19/2024 7:52 AM, Tim Fern wrote:

I will definitely insulate the choke frame from the chassis, I think
your analysis of Henry Amp choke fails is spot-on. And I’ll copy the
Acorn-nuts spark-gap across the choke. What width gap did you choose?

I wonder if a VERY careful examination of both the choke and the chassis
and choke mounting hardware, including at the mounting holes for screws,
might find some sign of soot from such HV arcing?? The soot resulting
from when my 2K4A HV transformer winding arced to its frame right
outside the winding wasn't initially visible, until I happened to wipe
my finger across that area and saw the soot on my finger. Finding such
soot might help prove this theory of the choke arcing to the chassis.

Also, I, too, was wondering what spark gap Jim uses across the choke?
I'd think you'd want it as small as possible; i.e., 0.005" if possible,
just wide enough that dust across the acorn nuts doesn't short out the gap.

Finally... I'm wondering whether a high current MOV might perform the
function of the spark gap, except more reliably?? After all, we use them
on the filament pins, where they're taking the full current of a
discharging HV capacitor through the tube. It seems to me that I've seen
MOVs connected across chokes in the past.

Steve, K0XP

I recently acquired a few Alpha amplifiers that needed repair.? Along the way I decided to test all 10 tubes I had on hand.?

I put each 3cx400A7 in one at a time, let the filament run for a while to burn in. I Applied 1500 volts for 10 minutes or so.. then Keyed up While monitoring Grid and Plate current for 5 minutes.

I noted idle current and then applied 15w RTTY drive on 20 meters and noted output power.?

Out of the 10, Two where bad, one had open filament and the other a grid short.? The other eight where all good.

Output power for those eight tubes:

175 w
175 w
175 w
150 w
140 w
120 W
100 w
75 w

I let them run each tube keyed for 5 minutes or so and power was stable, and no tuning would produce more.? All tubes ended up at pretty much exactly the same tune and load.

ONE of the new tubes made 50W but after a cycle of key OFF and Key ON power rose up. 75, then 100, then 120.? It ended up at 175 over about a 10 to 15 minute period.? Interesting to see it rise up like that.? Something was coming back to life.

ALL other tubes made power instantly.

What is interesting is that even the tubes that have lower output power and gain at 15w, make FULL power in the amplifier.? ?I can take the two weak tubes, 75 and 100, and put them in the amp and make 1500w out with a woodpecker.??

I can put the best three in the other map and make 1800w.? ?I can remove them and put the weakest three in the amp and make 1800w.

Is there a better way to test power tubes?? Or do you really need to just test them to full output?? I had reservations about applying 2700 volts to them right away, that's why I used 1500v and low drive. Some of the tubes where new and marked 1980 so some run time is probably advisable at low voltage.

I guess the drive and or added B plus is overcoming the loss of gain?

C

Thanks Jim, loads of great info here, just what I needed.

?

Your Dahl choke is a beast, wow, I thought mine was a monster, but life has taught me that somewhere, there will always be a man with a bigger one :-)

?

It might use my choke on a big single-phase triode supply here. I like the sound of -129dB ripple filtering…. One of the problems using *any* tube amp with FT8 is that your radio neighbours report the signal is wide….. because they see the 100Hz sidebands on their decode screens even when they’re -40, -60 or -80dBc ….ask me how I know this :-(

?

PSUD is new to me...what a great tool, I’m playing around with it now. I can change the load R to simulate what happens with different plate currents as power is applied….. but were you able to simulate how the output voltage changes with sudden changes in load R? I can’t see how to do it in PSUD?

?

I will definitely insulate the choke frame from the chassis, I think your analysis of Henry Amp choke fails is spot-on. And I’ll copy the Acorn-nuts spark-gap across the choke. What width gap did you choose?

?

The DC resistance of my 10H choke is 28 ohms, I currently have a big 56 ohm glitch resistor, I might swap that for a similar 33 ohm one here, depending what C2 I end up with. I know how much energy the tube can take.

?

The 0.85A CCS rating is a bit lower than I would like. I’m wondering what would happen to it at 2A or 2.5A? Does it just risk over-heating? or might the core saturate too and stop it being an inductor? (Am pretty sure it’s not a gapped “swinging inductor”.)

?

Best regards

?

Tim

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Actually, I applied for membership to that group a few weeks ago, but still haven't heard from a moderator either way. Have been trying to read through the "back issues" of posts in the Archive, but something seems to be resetting when I try to read messages going forward in time, and instead the next message displayed is the prior one. Probly my phat phingerzes hitting the wrong key? 8-)

73,

Steve K0XP

There is an Alpha group here on "Groups.io", rather specific to the 9500, but I've seen inquiries regarding other Alpha models. In addition, Glenn Pladsen (AE0Q) often posts replies to issues posted there, and sometimes will email you direct regarding things posted on the forum.

The Alpha 9500 group is HERE: /g/Alpha9500

Good luck.
--
73,

~Alan

Hi,

I am? looking for Bandswitch Rear Wafer? E4C-N18 for my Alpha 78.

Anybody in the group either has one for sale or know where to buy it?

Thanks
73 de Arturo, LU6ETB

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Dr Alex is alive and well. I bought and paid for some doorknob capacitors just yesterday.

I try not to shop anywhere else.

73, Alek VK6APK.

On Sun, Feb 18, 2024 at 09:25 PM, Peter Voelpel wrote:

There are? Russian 4?H roller inductors..

They are available at Dr. Alex, UR4LL:

?

?

73

Peter, DJ7WW

I saw those 4 uh? roller coils. I didn't get an answer back from Dr? Alex as to? what ga wire was used, or condition of them.? I have never ordered anything from him...so have no clue what the shipping costs are to VE7.? ?At the time, the war had just started, and Dr Alex? had vanished, and had to temp relocate for his own safety.? He showed back up several months later.? They look like a viable option, albeit shaft diam is < 1/4" (6.35mm),? non issue, as an adaptor can easily be fabricated by drilling out a 3/16" coupler? to? (I think)? ?the Russian 5mm shaft.?

?

I just did this recently.? I bought a used Dahl? choke (4H,? 3 amps CCS, and 15 kv rated). Weighs 127 lbs and uses the same hypersil core as the Dahl 10 kva plate xfmrs...which also weigh 127 lbs.? Bought it across town from a ham, who got it from the? old? satellite uplink station, just north of me The date code is 1991, and it's in pristine shape.? They had a total of 19 of em.?

It's called a? C-L-C? filter setup.? ? Basically? ?you take a regular? 'high C' standard B+? power supply, and add a choke in series with the B+...followed by a cap. In software, that's called adding a 'LC' section.? ?Mine is setup for 60 hz single phase. Modelling it all in software 1st (Duncan's PSUD-2), which does NOT handle 3 phase supplies, ( but will handle 50/60/400 hz single phase)... there are some? serious concerns, and caveats.? (You would probably need Spice, or LT-Spice, to handle 3 phase).

In software, it all goes to hell, and fast, IF the added cap after the choke ( let's call it? 'C2')? is not at least the same value as the main filter cap before the choke.? It should really be 50-100% bigger than the main? 'C1' filter cap.?

If the C2 is smaller than the C1 cap, what happens is you get this severe? yo-yoing... down-up-down-up-down-up output voltage waveform, until it finally settles down after 200-600 msecs, depending on values.? For something like say FM broadcast, non issue.? For? CW, and esp SSB, it's a real mess.? ?The 1st cap? (C1 cap) still gets hammered with the high AC ripple current, same as before the LC section is added.? ?The 2nd cap has extremely little rms current flowing into it.?

With any flashover, you still need a fast, HV fuse in series with the? final B+ output, followed by a HD glitch resistor.? ?( I had that before the LC section added).? ?I got the choke for minimal cost, and so it's just a good experiment I always wanted to try out.... more to verify the software than any practical use.? End result is, it's gross overkill for ripple reduction.? ?Mine has a 0db cut off of just 1.7 hz...and a 3 db cut off of only ( I think) 2.5 hz.? Ripple reduction at 120 hz (2F on? single phase) is -129 db.? ?My L4B with it's 25 uf filter cap (8 x 200uf caps in series) is only -30 db... and 2.9% ripple.... which is blatantly obvious on any RF monitor scope, drives me nuts.??

The added LC section gets better. The harmonics of 120 hz, (240-360-480 hz etc)? ?get killed at an even greater rate.? The added LC filter attenuates above it's cut off freq at a rate of? 12 db per octave.? ? I verified all this through measurements. The software is dead on.??

Ok, here's what you don't want to do,? and that's wire the choke in series with the B-.? ? Reason is, any V drop in the B- is just extra, yo-yoing extra, unwanted bias on the GG triode.? ?And that is in addition to the normal regulated bias on the tube.?

The V drop across the choke is simply the current through the choke? x DC resistance of the choke.? ?My 4H choke is 9.56 ohms when cold, and 10 ohms when warm.? I tested it with my small lab supply, and stuffed 3 amps? CCS through it for hrs...doesn't even get warm, stays at ambient room temp.? It does heat up with 6 amps through it.... and right where the gapped cores are.? ( it also becomes the mother of all electromagnets).? Crank the current up..and u will reach a point of thermal runaway.? ?With normal 2.6 to 3.0 amps of plate current through the 10 ohm? choke, = 25-30 vdc V drop.?

I tested a 2nd choke, a new? Hammond 1H choke, which is rated at 1000 ma CCS....and has a 5.75 ohm resistance.? ?I tested it with 800 ma though it, stone cold.? The Hammond is only rated for 1 kv ( 1.5 kv hi-pot test)..so it has to be installed in series with the? B+....AND well insulated from the chassis.? Treat it like a string of 450 vdc lytics.

?

Same deal on the big, 127 lb dahl choke.? ?It's wired in series with the B+, but well insulated from the chassis.? I used 4 x of those red...? HV glastic insulators, that come with threaded? 3/8" inserts in each end.? Choke sits on top of a pair of upside down aluminum U channel's....which are in turn? insulated? ( via 4 x glastics) from the? 1.5" thick plywood? base installed at the bottom of the rack.? ?Insulated from chassis, the V stand off is a non issue.? ?You will still get the normal V drop across the choke, but at least it's not adding bias to the cathode.?

On any single phase (60 hz) FWB / FWD? B+ supply,? the raw DC ( before any filter cap)? has? 67% ripple...and ripple freq is 120 hz ( 2F).
?On 3 phase, the raw DC? has? just 5.2% ripple.? ?On 3 phase, the ripple freq is 6F (300 hz in the UK, 360 hz in NA).? ?With 3 phase, your raw DC is not much worse vs the 2.9% ripple, after the 25 uf filter in my drake amps.?

IE: caps get charged up (or topped up as folks in the UK say) 100/120 times per second on single phase......and? 300/360 times per second on 3 phase.? ?That's the HUGE advantage of 3 phase power.?

?

Before you try the choke, measure it's DC resistance 1st..then model it in software.? ?And make sure it's not a swinging choke.? See if you can measure the choke on an LCR meter.? (peter told me eons ago, that with typ 3 phase B+ supplies,? a small filter cap was used, followed by the series choke...and no C2 cap).? The ripple is then down to zip.? If it works in software, it will work? in? the real world.? ?If your DC resistance on that 10H choke is high, like (as I suspect) in the 40-60 ohm range, any normal V drop across it, will be in addition to the V drop across any glitch resistor.?

?

With the typ series string of resistors used for the multiplier string for the B+? HV meter,? you can put the? DVM across the last resistor, or 2nd or 3rd to last resistor,? and measure the AC ripple component...with a full bore load applied.?

For my setup, for safety purposes...and to protect the choke,? I installed a HB adjustable spark gap across the choke terminals. Made from solid brass? 1/4-20 hardware, mounted on a pair of 3/4" x 3/4" x 1/6" thick aluminum angles.? Angles are mounted directly to the threaded studs on the choke.? Solid brass acorn nuts used where the gap resides.? Acorn nuts are semi domed, and ideal for this application.? If they get trashed, they are easily replaced.? ?Acorn nuts face each other.? Solid brass lock nuts are used to lock the gap into place....done with feeler gauges, and also a hi pot tester.? I also installed a surplus SPST Kilovac ceramic vac relay across the choke terminals.? I can shunt out the choke at any time, for testing purposes.?

Typ, with any B+ to grid arc, or say B+ to chassis arc, the 50 ohm glitch resistor will? LIMIT the fault current to just.... B+ / 50 ohms.? Meanwhile, the fast? HV fuse before the glitch resistor assy? will open off in < 1-2 msecs every time.? The C2 cap will discharge 1st...followed by the C1 main cap. When the C1 cap discharges, all that fault current flows through the choke... with a resulting MASSIVE V spike across the choke.? Chokes impede current ( both AC + DC) by dumping voltage.? I'm 95% convinced this is why chokes fail shorted to the frame in Henry radio amplifiers that use a resonant choke input.? You can also simulate the fault current in software...it's an eye opener.? This is a problem when chokes are wired in series with the B+.....and are not insulated from the chassis and / or no spark gap used.? ?The spark gap across the choke trick was used by Harris, and several others in the past.? ?I just re-invented the wheel.?

On software, you can see the normal negative / positive swing on the choke with normal plate current drawn, either CCS cxr, or up / down like CW / SSB.? ?Typ it's not very much, < 100 vdc.? ? The spark gap across the choke is dirt cheap to implement.?

?

Jim? ?VE7RF

There is an Alpha group here on "Groups.io", rather specific to the 9500, but I've seen inquiries regarding other Alpha models. In addition, Glenn Pladsen (AE0Q) often posts replies to issues posted there, and sometimes will email you direct regarding things posted on the forum.

The Alpha 9500 group is HERE: /g/Alpha9500

Good luck.
--
73,

I've built many EHT supplies over decades but have always used capacitive only, ripple-filtering.

Sitting below my desk for many years, is an inductor rated 7,500V, 0.85A CCS, 10 Henries. It came from an industrial RF heater.

I've never seen one used in an EHT ham amplifier supply...has anyone experience of this?

10H, even with just 1uF after it, has a -3dB frequency of 50Hz, so it would be pretty effective at 300Hz (3-phase, full-wave ripple).

But how would the inductive ripple filter respond with an SSB load? What happens if the tube flashes over? Are some extra protections required?

Thanks
Tim

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RFP will not sell 3-500Z tubes to overseas buyers but I bought 4 of them through their Ebay store, no problem. They will send them anywhere in the world.

Go figure.

73, Alek VK6APK