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813 v.s. 803 in vttc

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LAN_403

803

Fri Apr 23 2010, 02:38AM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

hi i was wondering if the 813 is just a clone of the 803. i meen, they have almost the same ratings and made both by rca. what do you think? also which would work better?

Herr Zapp

Fri Apr 23 2010, 05:32AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

803 -

The 803 and 813 pentodes have similar ratings, with the 813 having a slightly higher plate voltage. 813's have a very heavy graphite plate structure that may give them a little more resistance to short-term spikes in plate current. As pentodes, neither the 803 or 813 are ideal for VTTC use; triodes are easier to work with. Both the 803 and 813 have been made by many different manufacturers.

Here is a link to John Freau's schematic for a small VTTC using two parallel-connected 803 pentodes powered by a microwave oven transformer:

Herr Zapp

803

Fri Apr 23 2010, 05:15PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

dear herr,

I don't think it is good to work them as a triode because i've spoke to some hams and they say that it lowers the hadling power by half. look at this guys gu81m tesla coil. he controles the 2 grid with a varaic and connects the 3 grid to the cathode like in a real pentode. if you connect all the grids together than the screen will not get enough power but the 1 grid will ment down at 2 grids power. also with the 3 grid. this is the link to the vttc plan ( sorry you will need google translotor,it's in czech)http://rayer.ic.cz/teslatr/vttc.htm

Ps how high can you run the 803? 4kv? you know that the data sheet is under rated!

Herr Zapp

Sat Apr 24 2010, 05:09AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

803 -

I think you have misunderstood me; I was NOT suggesting connecting these pentodes as triodes.

I meant that you should try to find suitably rated transmitting triodes (811A, 805, 810, 833, 250TL, 304TL, etc if you are in the West, or any of the surplus Soviet triodes ) rather then tying all the pentode grids together to operate as triodes.

If you don't mind the additional complexity of providing the correct bias voltages for the screen and supressor grids, then there is probably no technical reason that tetrodes or pentodes couldn't be successfully used in VTTCs with nearly their original plate voltage ratings.

Herr Zapp

Dr. H.

Sat Apr 24 2010, 05:27AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Good day guys

I've build a pentode driven coil in the past ... and I must say that it is not a big deal at all.

You just provide some bias for the second grid (it actually turned out that apllying the bias to first grid and connecting the feedback to second grid gave me bigger sparks).

Experimenting is the key here

Cheers

803

Sat Apr 24 2010, 03:17PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

thanks Dr.H,

that makes sence because the screen is higher rated then the 1 grid. i just have some questions- what do you think is the magic number for bias voltage and current? also do you have to externery apply voltage the the 3 grid? i thought you just have to connect that to the cathode. I love the 803 because it is cheap and is powerful( the data sheets are wrong, some hams put i kwatt and 11kv peak on them and they don't even get hot!).

Thanks,

paul

Dr. Dark Current

Sun Apr 25 2010, 09:14AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I have run a pentode coil like this- g3 to cathode, g2 directly to feedback winding (14t- turns need experimenting and measurements of peak voltages), g1 to a tap on 11th turn through a parallel RC network with R=10k 10W and C=2.2nf. The advangate of this is that you don't burn extra power in the grid leak resistor, but this driving method needs more experimentation to get it right.
Ran from a halfwave doubled MOT, pentode Wa 600W max, Wg2 120W, Wg1 10W (GU81)

Dr. H.

Sun Apr 25 2010, 08:57PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

The tubes that I've used have the g3 internally connected to the the cathode, I used something like 600V trough a resistor on g1 (I don't remember the value of the resistor but you can easily find it from the datasheets of the tubes).

Cheers

803

Mon Apr 26 2010, 02:38AM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

Dr. k

So what your saying is that both the 1 and 2 grid is connected to the feedback, just that the 1 grid is connected in sereis with a resistor ans capacitor in parallel

Herr Zapp

Mon Apr 26 2010, 04:27AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

803 -

Oops - my error. In an earlier post, I noted that the 813 had a heavy graphite plate; the 803 also has a heavy graphite plate. For some reason I thought that the 803 used a sheet-metal plate.

The 803 is an earlier design, good for use up to 20 MHz; the 813 was introduced a few years later, and is rated for up to 30 MHz.

Regards,
Herr Zapp

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