Author Topic: Strange reading for grid feedback coil voltage  (Read 1603 times)

Offline MRMILSTAR

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Strange reading for grid feedback coil voltage
« on: January 10, 2022, 10:56:48 PM »
My 833C VTTC performs OK with 20" streamers in interrupter mode but I think it could do better. I have always been guessing at the feedback voltage because I didn't have a way to measure it. I finally bought a Micsig DP20003 HV differential probe so that I could actually measure the grid feedback voltage.

The probe is connected between the grid and  ground. The reading that I got was surprising. With a MOT output of about 800 volts, about 1600 volts with doubler, the grid feedback voltage is measuring at -5000 volts peak or so it appears on the scope! This doesn't make any sense to me. The spec sheet for a 833C says the maximum grid voltage is -450 volts in Class C operation.

Just as a refresher the primary coil is tapped at 26 turns. The secondary coil is about 1000 turns. The grid feedback coil is 14 turns and sits on top of the primary coil at about 1/2 way up the secondary coil. My VTTC can be seen here.
https://highvoltageforum.net/index.php?topic=745.0

Does this make any sense? Can the grid feedback coil be getting most of its coupling from the secondary coil thus possibly explaining the high voltage reading? As I move the feedback coil down closer to the primary coil, the voltage goes down. This indicates to me that major coupling is coming from the secondary coil.
« Last Edit: January 10, 2022, 11:29:17 PM by MRMILSTAR »
Steve White
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Offline Duane B

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Re: Strange reading for grid feedback coil voltage
« Reply #1 on: January 11, 2022, 12:07:22 AM »
Right now I have a lot of questions. In the pictures on your link, it looks like the feedback coil is going to the top terminal (s) on the tube. Are the top terminals the grid connection? Can you share a schematic? Where does the low end of the feedback coil connect? What is the DC voltage on the low side of the feedback coil? A voltage of -5000 volts peak might be ok. I believe it's the minus DC voltage that is used in the specs.

There might be something wrong with your measurement technique? Is your meter capable of measuring a high voltage differentially between the probes, or will it just measure a smaller differential voltage but elevated at a high voltage?

It seems like the probe connected to high voltage is picking up some of the secondary voltage. Are you physically measuring the voltage directly on the grid, or physically measuring it at the top of the feedback coil (close to the secondary coil)? There might be capacitive coupling between the probe and the secondary coil.
« Last Edit: January 11, 2022, 12:21:20 AM by Duane B »
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Re: Strange reading for grid feedback coil voltage
« Reply #2 on: January 11, 2022, 05:25:31 AM »
The schematic is the standard Steve Ward 833A one that most VTTCs are based on. My components are identical. The only difference is that my grid feedback coil has 14 turns instead of 20.

This is a 833C tube. The 2 top terminals are grid and plate. The differential probe is a Micsig and specified to measure up to 5600 volts between the probes. One probe is attached to the grid connection at the feedback coil next to the secondary coil. The other probe is connected to ground. I am using an oscilloscope with 100 Mhz BW to measure the voltage. This is the first time that I have used a HV differential probe so I was wondering if I am seeing some sort of measurement artifact.

One odd thing as I mentioned earlier is that the voltage goes down as I slide the feedback coil down closer to the primary coil. This is the opposite behavior that I was expecting.

Attached is the schematic.
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Offline davekni

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Re: Strange reading for grid feedback coil voltage
« Reply #3 on: January 11, 2022, 06:15:08 AM »
Quote
The differential probe is a Micsig and specified to measure up to 5600 volts between the probes.
Probe may have an issue with the combination of voltage and frequency.  Most probes include specifications for voltage derating as frequency increases.  I haven't found such for Micsig.  Some probes derate voltage drastically at relatively moderate frequency.  Steve Ward fried his Micsig differential probe at lower voltage and higher frequency:
https://highvoltageforum.net/index.php?topic=1589.msg12279#msg12279
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Offline Duane B

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Re: Strange reading for grid feedback coil voltage
« Reply #4 on: January 11, 2022, 06:34:20 AM »
Thanks for the schematic and other details Steve. I don't know why most VTTC's these days use a fixed resistor for the grid bias. The first tube coil that I built had a variable rheostat here and it was part of the tuning process to adjust it. The variable resistor was very useful. Also, I'm sure that you are aware that you don't need a differential meter or probe for what you are measuring. A single high voltage probe would work just as well.

One odd thing as I mentioned earlier is that the voltage goes down as I slide the feedback coil down closer to the primary coil. This is the opposite behavior that I was expecting.

Some of the voltage picked up from your grid feedback coil might be coming from the field of the secondary coil. However, this "secondary coil" energy may not have very much driving power. The high voltage negative peaks that you see on the grid are not loaded down at all. High voltage negative peaks here sound perfectly normal. The grid coil is only loaded when the grid goes positive, pushing current through the 5k resistor, and developing a negative DC voltage across the 5k resistor in the process. This voltage of course will not be a pure DC value because the plate supply is not running in CW mode. The voltage across the 5k resistor is the grid bias voltage though. The parallel capacitor keeps the high frequency voltage off the resistor. You should be able to measure it with the scope. This is the voltage that is referenced in the tube data charts for grid operating voltage. This grid bias voltage may be controlled more by the proximity of the feedback coil to the plate tank coil instead of the secondary coil, because the plate tank coil probably has more driving power than the secondary coil.

I think the voltage you should be most concerned with is the DC voltage across the 5k resistor.

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Re: Strange reading for grid feedback coil voltage
« Reply #5 on: January 11, 2022, 04:31:28 PM »
Thanks for the response. My grid feedback resistor is an adjustable power resistor unlike what is shown in the schematic. Its currently set for about 4,000 ohms.
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Re: Strange reading for grid feedback coil voltage
« Reply #6 on: January 11, 2022, 05:19:46 PM »
Hi all,

:^) Yeah, the 833 is a weird tube with grid and plate pins both on top. Bad thing is several people in the past have accidentally mixed the connections up when they are adjusting/changing things since pins are same size.
Very bad outcome of course!

I also feel that this high voltage negative peak may also be very possible, but overall driving power low.

Several years ago, in the first tube coil of mine that I tried a tall primary I experienced the first time of noticable strange feedback winding behavior. Performance was sub par this way also despite a few changes I tried. Any changes I tried seemed to not be very responsive at all. Very little change, when it should be much more. I then immediately realized since the feedback is up so high now on the secondary that it was now influenced by the voltage rising on the secondary, instead of the primary winding that we want. And since the feedback is grounded...arc over problems too...and I see the arc shield done here, which others have resorted to also when having this problem.

Me, I knew we want to stick to feedback off of our primary. So I just took the whole primary and feedback form and I literally with no changes, just turned it upside down as is so feedback is now on the bottom. Since bottom of secondary is mainly all current, arcs to feedback problems gone. And now we always have the wanted feedback from primary only. I feel this is key to best performance possible.

If you flip this around, experiment slowly to get things tuned up correctly for the feedback. Also feedback leads may need to be switched around for correct phasing after this change.

Ever since I have run into this problem my first time, I now always build my tube coils with the feedback on the bottom below the primary, whether a short primary or tall one. My experience shows that it works much better this way once adjusted correctly of course.

I agree a variable rheostat is the way to go on the grid leak... being able to adjust on the fly is key also to best performance. I sometimes also in conjunction with the rheostat, have a fixed smaller ohm value "safety" resistor with it. That way you do not accidentally turn down to zero resistance. But these days...I mostly run just rheostat only and remember about not doing this.

Chris
« Last Edit: January 11, 2022, 05:51:43 PM by 304er »
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Re: Strange reading for grid feedback coil voltage
« Reply #7 on: January 11, 2022, 07:33:15 PM »
Hi all,

:^) Yeah, the 833 is a weird tube with grid and plate pins both on top. Bad thing is several people in the past have accidentally mixed the connections up when they are adjusting/changing things since pins are same size.
Very bad outcome of course!

I also feel that this high voltage negative peak may also be very possible, but overall driving power low.

Several years ago, in the first tube coil of mine that I tried a tall primary I experienced the first time of noticable strange feedback winding behavior. Performance was sub par this way also despite a few changes I tried. Any changes I tried seemed to not be very responsive at all. Very little change, when it should be much more. I then immediately realized since the feedback is up so high now on the secondary that it was now influenced by the voltage rising on the secondary, instead of the primary winding that we want. And since the feedback is grounded...arc over problems too...and I see the arc shield done here, which others have resorted to also when having this problem.

Me, I knew we want to stick to feedback off of our primary. So I just took the whole primary and feedback form and I literally with no changes, just turned it upside down as is so feedback is now on the bottom. Since bottom of secondary is mainly all current, arcs to feedback problems gone. And now we always have the wanted feedback from primary only. I feel this is key to best performance possible.

If you flip this around, experiment slowly to get things tuned up correctly for the feedback. Also feedback leads may need to be switched around for correct phasing after this change.

Ever since I have run into this problem my first time, I now always build my tube coils with the feedback on the bottom below the primary, whether a short primary or tall one. My experience shows that it works much better this way once adjusted correctly of course.

I agree a variable rheostat is the way to go on the grid leak... being able to adjust on the fly is key also to best performance. I sometimes also in conjunction with the rheostat, have a fixed smaller ohm value "safety" resistor with it. That way you do not accidentally turn down to zero resistance. But these days...I mostly run just rheostat only and remember about not doing this.

Chris

Good advice Chris! I agree with everything you say! In the early years of my coiling (about 50 years ago) I followed a two-811A tube coil construction plan and the feedback coil was on the bottom. All of the other plans I saw back then had the feedback coil on the bottom. I don't know when everyone started putting the feedback coil on top? I thought it might be due to some technique of making the coil grow longer sword sparks, but I don't know.

I think the voltage induced in the feedback coil is a complex addition of the coupling between the feedback coil and the primary coil and the feedback coil and the secondary coil. The feedback-secondary transformer having a higher impedance than the feedback-primary transformer.
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Offline MRMILSTAR

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Re: Strange reading for grid feedback coil voltage
« Reply #8 on: January 11, 2022, 08:05:27 PM »
Thanks Chris. I think that I am getting a lot of coupling from the secondary coil. As much as I hate the idea I may just make a new configuration as you suggest, with the feedback coil on the bottom. This time, I'll also add adjustable taps on the feedback coil in addition to the primary coil. I can't even remember why I put the feedback coil on the top in the first place. In retrospect it was a mistake. It still puts out 20" streamers in its current configuration so its still a decent performer, I just think that it could do better.
« Last Edit: January 11, 2022, 08:09:10 PM by MRMILSTAR »
Steve White
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Re: Strange reading for grid feedback coil voltage
« Reply #9 on: January 12, 2022, 09:09:13 AM »
Hi Duane and Steve,

I am not sure how this came about either of feedback on top?
The plans I followed around 1980 for a single 811 had feedback on top.
Did not know any better at the time... thought this is where it must go.
Saw quite a few plans like this.
But yes, I have in now more recent past come across some much older plans having this on the bottom.

Looking at Steve Ward's VTTC's he put all on top. But his coils all do not really have a tall primary, so it works still okay.
So (Steve White) you were just going by what you saw at the time. I do agree with your taller primary you did compared to what is Steve Ward's shorter on his 833. I like the increased coupling, that we "get away with" doing taller on VTTC's and it is what I currently prefer. But what worked for feedback before now becomes problematic. Definitely a lot of different things going on our coils!

You will get it and I already know it will look great also :^) when you finally get to doing this.
And I need to get going again on my 304er finalizing project again... pretty soon here :^)

Your 833 at minimum in my opinion should be 28" output with your level shifted, older bigger MOT. And I feel even more is there.
I not too long ago did a quick test on my old/new T-200 coil the same MOT setup and got 27" from this less powerful tube and the setup itself had shortcomings in it's old design. I actually just did a crude rough tuning and got this pretty quickly, but this was just a quick temporary test I did. BTW...the original old plans for this one, had the feedback also on top. It used the same wire size for both primary and feedback. Even though this has a short primary... when I fairly recently "updated" this older fella... first thing I did was rewire this so feedback now on bottom.

Chris


« Last Edit: January 12, 2022, 09:23:34 AM by 304er »
Chris Reeland
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Re: Strange reading for grid feedback coil voltage
« Reply #10 on: January 12, 2022, 03:29:04 PM »
Chris, when you put your feedback on the bottom do you make any provisions to slide the feedback coil up and down for coupling adjustment or do you rely strictly on tapping at different points on the feedback coil?

Here's an interesting historical design. This image shows the Miller RF coil on my 6L6 VTTC. Note that the feedback coil is actually on top of the secondary coil with a bakelite spark shield between them!
« Last Edit: January 12, 2022, 03:33:29 PM by MRMILSTAR »
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Re: Strange reading for grid feedback coil voltage
« Reply #11 on: January 12, 2022, 06:18:15 PM »
Hi Steve,

No, I did not do a sliding provision or taps but probably a good idea.

Me, I always did it the hard way. I would just make a slightly calculated judgement from past experiences to position the feedback (# of turns also). Then if I could not get to work good...strip off the wire and reposition it. Some coils, I had some wiggle room to move primary up or down a little instead. Then when it came to turns...I usually start with too much and cut off a turn a time and testing. And yes, I go too far sometimes removing too much and have to strip off and rewind again, but now I know from notes I took along the way where it was best for this particular VTTC. Like I said the hard way. That said I am planning something different on my 304er finalizing project for the feedback... been gathering and searching for parts that I want to try.

I actually thought about your first VTTC late last night, built way back in the day...I remembered the "pies" and yes the very top feedback "pie". This is a lower power VTTC and uses not very powerful tubes so yes it works here of course.

I have an old from about the 1960's Spellman high voltage power supply. Spellman also made these little "pies" type coils/little VTTC's and used them in these now old power supplies. The "pie" coil  form in this much higher voltage and current supply and using larger more powerful tubes...they cut this top feedback "pie" right off...I can see the cut marks...

They then did feedback  ...completely different...   I have not had the time yet to restore this PS and look at it closer on how they did feedback and study and understand what they did.

I will try to find a few pictures I took and post a link later...

Chris





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Re: Strange reading for grid feedback coil voltage
« Reply #12 on: January 14, 2022, 08:51:57 AM »
Hi Steve,

I have dug up a few pictures, that I mentioned about in above message.

Here is my Spellman power supply sitting on top of the big tube crate:

https://photos.app.goo.gl/9g2rhJWaA9GmoUS86

Some pictures of my Spellman coil inside with the feedback that was on top but cut off by Spellman themselves and the different feedback setup they did for this power supply:

https://photos.app.goo.gl/7vz9FtAbtiNNZKH28

And a catalog page from 1959:

First picture here is a close up of my Spellman coil of how it is with original feedback on top:

https://photos.app.goo.gl/QPP7E6HRQSmWmoreA

Second picture of whole page and you can see where they have on these bigger higher voltage models there is no feedback on top anymore:

https://photos.app.goo.gl/n2VVj6jkv8wpvRxYA

Chris
« Last Edit: January 14, 2022, 08:57:00 AM by 304er »
Chris Reeland
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Offline Mads Barnkob

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Re: Strange reading for grid feedback coil voltage
« Reply #13 on: January 17, 2022, 07:38:25 PM »
One odd thing as I mentioned earlier is that the voltage goes down as I slide the feedback coil down closer to the primary coil. This is the opposite behavior that I was expecting.

Attached is the schematic.

The feedback is part of a complex 3 coil coupling, I think it makes sense that moving it down from secondary, closer to primary will drop the voltage. There must be a lowest point somewhere between primary and seoncondary coil and then rising when higher coupling with either primary or secondary.

Some pictures of my Spellman coil inside with the feedback that was on top but cut off by Spellman themselves and the different feedback setup they did for this power supply:

https://photos.app.goo.gl/7vz9FtAbtiNNZKH28

I got one of the "newer" Spellman 10 kV power supplies, those flat 19" versions, properly from the 1980-1990's. It has the exactly same coil design, just uses transistors for the driver instead :)
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Re: Strange reading for grid feedback coil voltage
« Reply #14 on: January 18, 2022, 08:47:53 AM »

I got one of the "newer" Spellman 10 kV power supplies, those flat 19" versions, properly from the 1980-1990's. It has the exactly same coil design, just uses transistors for the driver instead :)

Hi Mads,

Just to add to this a little bit more of interesting history, here is another different Spellman catalog page from 1965-66 that I have in some very big older master catalogs I have in my collection.

https://photos.app.goo.gl/oUGBCKgPhreYsfjW9

Some more transistors use. And still using tubes in the rectifiers for that time though...a hybrid at the time.

Chris :^)



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Re: Strange reading for grid feedback coil voltage
« Reply #14 on: January 18, 2022, 08:47:53 AM »

 


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post Re: Hi frequency ringing with GDT
[Solid State Tesla Coils (SSTC)]
CarldeLevis
May 11, 2022, 09:39:21 PM
post Thermometers
[Laboratories, Equipment and Tools]
klugesmith
May 11, 2022, 09:36:59 PM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Nicolas
May 11, 2022, 09:29:45 PM
post Re: Litz Wire?
[General Chat]
Mads Barnkob
May 11, 2022, 08:45:57 PM
post Re: Hi frequency ringing with GDT
[Solid State Tesla Coils (SSTC)]
Mads Barnkob
May 11, 2022, 08:43:38 PM
post Hi frequency ringing with GDT
[Solid State Tesla Coils (SSTC)]
CarldeLevis
May 11, 2022, 05:44:40 PM
post Re: Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
Hydron
May 11, 2022, 04:17:34 PM
post Re: Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
Robert3z
May 11, 2022, 03:10:27 PM
post Re: Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
Hydron
May 11, 2022, 01:46:57 PM
post Re: Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
Robert3z
May 11, 2022, 12:37:19 PM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Uspring
May 11, 2022, 11:33:30 AM
post Re: Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
Hydron
May 11, 2022, 11:00:59 AM
post Re: Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
davekni
May 11, 2022, 06:30:45 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
davekni
May 11, 2022, 06:11:08 AM
post Re: Litz Wire?
[General Chat]
abstruse1
May 11, 2022, 04:00:41 AM
post Driving the primary with other than 60Hz
[Spark Gap Tesla Coils (SGTC)]
Robert3z
May 11, 2022, 02:06:34 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Mads Barnkob
May 10, 2022, 07:52:45 PM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Nicolas
May 10, 2022, 10:04:27 AM
post Re: Transistor choice
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
May 10, 2022, 09:31:39 AM
post Re: Litz Wire?
[General Chat]
Mads Barnkob
May 10, 2022, 09:29:19 AM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General Chat]
Mads Barnkob
May 10, 2022, 09:23:38 AM
post Re: Transistor choice
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AstRii
May 10, 2022, 07:24:33 AM
post Transistor choice
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
May 10, 2022, 03:33:10 AM
post Litz Wire?
[General Chat]
abstruse1
May 10, 2022, 03:16:29 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
DashApple
May 09, 2022, 07:41:51 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
johnf
May 09, 2022, 05:39:51 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
davekni
May 09, 2022, 12:44:13 AM
post Re: First tesla coil building / Questions before ignition
[Spark Gap Tesla Coils (SGTC)]
Nicolas
May 09, 2022, 12:24:37 AM
post Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Nicolas
May 09, 2022, 12:21:40 AM
post Ground, Grounding, Grounded?
[General Chat]
abstruse1
May 09, 2022, 12:13:01 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 08, 2022, 06:43:31 PM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 08, 2022, 07:43:13 AM
post Re: 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters
[Transformer (Iron Core)]
Mads Barnkob
May 07, 2022, 07:59:36 PM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General Chat]
Robert3z
May 07, 2022, 06:41:41 PM
post Re: Capacitor Esoterica
[General Chat]
abstruse1
May 07, 2022, 05:57:22 PM
post Re: 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters
[Transformer (Iron Core)]
Hydron
May 07, 2022, 03:43:57 PM
post 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters
[Transformer (Iron Core)]
Mads Barnkob
May 07, 2022, 03:21:16 PM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 07, 2022, 06:52:29 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 07, 2022, 04:57:41 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 06, 2022, 08:54:52 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 06, 2022, 02:54:44 AM
post Re: DRSSTC No Breakout On Topload
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Dhaygoh
May 05, 2022, 05:26:27 PM
post Re: DRSSTC No Breakout On Topload
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Dhaygoh
May 05, 2022, 05:14:32 PM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 05, 2022, 06:29:00 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 05, 2022, 05:56:01 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 05, 2022, 05:11:45 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 05, 2022, 04:37:42 AM
post Re: DRSSTC No Breakout On Topload
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 05, 2022, 04:19:47 AM
post Re: Capacitor Esoterica
[General Chat]
johnf
May 04, 2022, 11:22:09 PM
post Re: DRSSTC driver in the chip shortage
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hydron
May 04, 2022, 11:04:19 PM

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