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Couple more VTTC queries - RFCs and primary coils

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J. Aaron Holmes

Thu Dec 21 2006, 12:08AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I'm back again with some more dumb VTTC questions.

Firstly:

Having gotten my 3-500ZG tube, I've been curiously pouring over various schematics of commercial amplifier gear, some of which (like the old Heath SB-2xx) actually employs this or a similar tube (yes, I know, this is no certain indication of how one ought to wire a tube up in a VTTC). One of the things I've observed, though, is the proliferation of big RF chokes on the plate and filament supplies. Have the VTTC veterans among you had any issues with RF kicking back into the HV and/or filament trannies with just bypass caps in place?

Coming from a background of SGTCs and fragile Terry-filter-needing NSTs, I'm inclined to be a little paranoid of RF kicking back into the mains or other places it shouldn't be, but perhaps that is unwarranted except in especially large VTTCs? For my own, I plan on starting with a very simple non-level-shifted MOT design which ought to be a walk in the park for this tube, I would think. No staccato yet, but someday...

Secondly:

Is there any good reason *not* to put the feedback coil *under* the primary as opposed to above like most people seem to? Various historic posts have talked about flashovers to the feedback coil from the secondary and unwanted coupling between secondary and feedback coils whilst adjusting the primary-to-feedback coupling. It stands to reason that these problems would be substantially eliminated by putting the feedback coil on the bottom. As a bonus, it probably puts the coil closer to the grid leads on the tube and avoids the need for long leads that criss-cross other high-potential wires. Hmmm...

Thanks as always,
Aaron, N7OE

...

Thu Dec 21 2006, 12:56AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The chokes are not really necessary IMHO. There is no way you are going to get any coupling at vttc frequencies back through a mot core... Just so long as you ground the core everything should be fine.

As to the secondary location, I think that its location was found to give the largest sparks. Go ahead and try it going down, but I think that you will get a better output with it above the secondary.

Have fun!

Steve Ward

Thu Dec 21 2006, 01:23AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

The chokes are not really necessary IMHO. There is no way you are going to get any coupling at vttc frequencies back through a mot core...

While true that a MOT core likely wont transfer RF to the AC input line, that is not the function of the RF choke in this circuit. The plate choke is there to prevent sustained oscillations that can occur in the event that the tube arcs internally (which causes very HF oscillations). This plate choke limits the current at those higher frequencies and helps save the tube. I have experienced tube arcing in the past due to applying around 4500V to a 2500V rated tube. I cant comment on the effect of the choke, since i never tried it withOUT the choke in place. My tube survived many of these events until i tuned the problem out.

Id suggest at least using small ceramics for RF bypassing of the plate and filament supplies. It gives a good path for RF, rather than heating up the windings in your transformers more than necessary.

J. Aaron Holmes

Thu Dec 21 2006, 05:02AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Thanks ... and Steve, I appreciate the help. Regarding the chokes, I wasn't actually talking about the chokes between the plate and the primary tank, but rather between the primary tank and HV supply (I see this in lots of linear amps, but not in any VTTC schematics). I probably wasn't too clear about that, though. The chokes between the plate and tank make sense from reading your (Steve's) VTTC FAQ and seem more than easy-to-make. If typical VTTC frequencies of 300-1000kHz are going to be eaten up by MOT and filament transformers, that sounds like goodness!

Regarding the coil placement: I was just talking about the feedback coil, not the primary or secondary. From what I understand, the purpose of the feedback coil is to drive the tube at the frequency of the primary tank, not necessarily at the frequency of the secondary. I would think, then, that it wouldn't matter if it was above or below the primary, since similar primary-to-feedback coil coupling ought to be achievable either way, right?

Thanks!
Aaron

Steve Ward

Thu Dec 21 2006, 07:45AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Doh, sorry ..., i made a bad assumption, so you had it right in the first place, the chokes are to keep stuff out of the HV supply.

Feedback placement: There is still some uncertainty it seems about the true nature of the feedback winding. Recently, someone on the TCML made a VTTC with the feedback coil raised up very high (where it likely was more strongly coupled to the secondary than the primary), and he achieved very good spark length. I have also seen designs where the feedback coil was placed below the primary, but i dont remember them being particularly good performers (but this might not say much, since many VTTCs are built and never optimized in my opinion).

If its your first VTTC, then id initially start with a design that works and put the feedback coil above the primary by abount an inch. After this, i encourage you to try other arrangements!

This reminds me once again of the 833Cs ive been neglecting... i miss my tube coils.

Steve Conner

Thu Dec 21 2006, 09:03AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi Aaron,

The choke between primary tank and HV supply is just a different way of feeding the power supply into the circuit. VTTCs usually feed it in through the primary coil itself, by having one end of the tank circuit connected to the B+. So the tank coil is kind of the RF choke too. There are various reasons why linear amp designers don't do that, some of them are (IIRC, could be wrong):

If they didn't use the RFC, either the voltage rating of the main tank cap would need to be higher because it sees B+ as well as RF (makes it bigger and more expensive) or its frame would need to be live with B+ (not a good thing for a variable cap that needs adjusted from the front panel)

Also, the RFC is better at stopping RF from feeding back into the power supply because it has a much higher inductance than the tank coil. If you don't use the RFC, the B+ end of the tank coil must be very well bypassed back to the tube cathode, to keep the RF current passed by the tube in that loop where it belongs.

We recently had a discussion about why/whether to use RFCs in Class-E SSTCs. It's exactly the same concept, so you might want to read that.

Dr. Drone

Thu Dec 21 2006, 03:27PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

J. Aaron Holmes

Thu Dec 21 2006, 05:24PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Thanks guys! Lots of good info. I'll check out the SSTC/RFC thread(s) also. I'm interested in understanding that a bit better.

Christopher_R wrote ...

You may want to build a little 811 @

coil first just to get layout and experimenting done before jumping on the 3-500Z tube.

Yeah, I saw a few 811's at the same swap meet and didn't pick them up. Probably foolish of me to pass on them. I guess I was figuring I could just as easily build a "wimpy" VTTC with a bigger tube (sort of similar to how one can build a low-power SGTC and run it off a pole pig). Plus the 3-500Z just looks so cool!!!

No doubt the filament juice will be sort of "wasted" in a smaller coil, but other than that... And I'm not planning on attacking "staccato" this time around either. Straight-up MOT AC. Unfortunately I sold most of my MOT collection (several dozen) recently, and just last night discovered that the nice-looking one I was planning on using for this VTTC has some internal arcing going on in the secondary

Regards,
Aaron, N7OE

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