First VTTC: The 811a toaster project / Projects / Forums

Forums

4hv.org :: Forums :: Projects

« Previous topic | Next topic »

First VTTC: The 811a toaster project

Move Thread

LAN_403

GimpyJoe

Sun Mar 19 2006, 07:35PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Hi, I'm working on my first major learning project since joining this board. It's a VTTC based on the 811a tube. Here are some specs as the coil is now

-HV supply is a MOT, rated 2100vac. I am not using a level shifter since iirc a single 811a can't handle the higher voltage
-tank cap is three beer bottle capacitors, measured at 2.7nf, although this will change once I get some doorknobs
-filament supply is 6.1v (a wee bit too low) from a Sears battery charger
-Primary is 4 turns of 16awg stranded wire
-grid feedback coil is eleven turns of 23awg magnet wire. The primary and feedback coil are wound on a 3.5" plastic tube. Both coils can be slid up and down
-grid capacitor is 3nf 3kv ceramic disc cap
-grid resistor is 6kohm 60w
-secondary coil is 2.4" O.D. PVC wound with 7.7 inches of 23awg
-No topload right now, but breakout point is a five inch tall piece of 1/8" copper tubing with a needle crimped into the end.

Yesterday I achieved first light with 1/2" purple flower-shaped discharges on the break point, but I stopped immediately because the plate heated up and glowed orange

UPDATES:
-3/27/06 Doorknob capacitors donated by Christopher Hooper arrived today. I'm still adjusting, but with the new caps I have gotten breakout up to 1.5"
-3/28/06 Considering removing shunts and filament winding from MOT and winding a 6.3v filament supply on the extra space on the core. Would this work?

Here are some pictures. I'll change them as I work on the coil assuming I figure out how to put in pics when I edit my posts.

Michael W.

Sun Mar 19 2006, 08:10PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

Where did you get your 811A?

GimpyJoe

Sun Mar 19 2006, 08:20PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

I got my 811a at a hamfest yesterday for $8. They're pretty easy to find online, though, so even if I cook this one it was a good learning experience.

Michael W.

Sun Mar 19 2006, 09:55PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

where are all there hamfests people speak of? Why can't there be a hamfest in BC, Canada!

Steve Conner

Mon Mar 20 2006, 12:27AM

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

According to a 10 second Google search I just did, there are at least nine hamfests in British Columbia every year. Get yo snack on! And this is a project thread so don't jack it.

Michael W.

Mon Mar 20 2006, 12:54AM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

sry, no thread jacking here. Now I'm very exited,....HAMFEST!!!

GimpyJoe

Mon Mar 20 2006, 01:17AM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Yup, they're great. You can drive past convention centers and stuff and know at a glance if it's a hamfest. If it is, 98% of the cars will have a 2 meter antenna on the back!

Anyway, does anyone have any suggestions as to what values I might try? I've been fiddling for two days, but all that happens is:
A: it doesn't oscillate
B: I get a tiny breakout and the plate starts glowing orange

...

Mon Mar 20 2006, 01:30AM

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

You don't have enough primary turns, I would say you need around 20 to make it work. Same for the feedback.
I have found that a good tank cap is crucial to decent operation. I tired replacing my doorknob with a disk ceramic and I lost all output. Although I wouldn't recommend more than 1.5nf, I am running with .7 nf and get 5" discharges on my little coil. If it isn't too much trouble I would rewind the secondary with smaller wire (like 28awg or 30ish, 32awg gets pretty hot) to get the Fres lower (less losses in the tank cap).

BTW, the 811a's with a several smaller fins on the plate can handle a lot more power than the ones with 2 big ones. They also handle >4kv no problem (haven't confirmed that for the ones with only 2 fins, I sold all of those off). But you will probably need two in parallel (just parallel the tubes, don't worry about tuning) to get enough plate dissipation.

Good luck!

Vaxian

Wed Apr 11 2007, 06:24AM

Registered Member #635 Joined: Tue Apr 10 2007, 01:56AM
Location:
Posts: 85

Wondering how your coil is doing. Have you added a topload yet, this could be part of the problem if you havn't. I found with my 813 coil that it will not tune reliably without the topload.

A decent cheap topload can be made with four pvc elbows, you want 1-2" OD tube and about 6" major OD of the toroid. Then aluminum tape a piece of plastic on top, tape the whole toroid and drill a hoile for mounting.

Mine looks like this but is a bit large at 8"

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob