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4-125A VTTC

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LAN_403

U4R1A

Fri Jul 15 2011, 09:46PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

I just got first light on my new VTTC. This is the first time I have used a tetrode(4-125a Eimac) and I'm semi pleased with the results so far. I used a couple 5K resistors in series so I can adjust the voltage going to the screen but everything else is the same as a triode. I have read through the data sheets but I haven't been able to find any data on how red I should left the plate get. I know on the 833A's you can let them get red and since this is a graphite plate also I assume it would be fine but I'll deffer to anyone that has experience with these. The other unusual thing I have notice is that I don't get any break out unless the gridleak resistor is >7Kohm and she seems to prefer 30-35Kohm. This seems pretty unusual to me because normally at this high of resistance it would be in sputter mode but it doesn't have the sound or spark appearance of that. I'll get some pictures up soon and if any one has any advice on my gridleak resistance issue let me know.

Herr Zapp

Fri Jul 15 2011, 11:05PM

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

U4RIA -

I am not aware of any 4-125A that was ever built with a graphite plate. Look closely; are you sure its not just thin black sheet metal, directly welded to the plate lead-in wire? Post a photo if you can.

Because of the small thermal mass of this type of plate construction, if something "goes wrong" its very easy to burn a hole in the plate before you can get your hand to the OFF switch. These smaller transmitting tubes generally run with no visible color on the plate.

Herr Zapp

U4R1A

Fri Jul 15 2011, 11:46PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

You are absolutely right. I held it up against against some other tubes and it is metal. I guess I'm going blind. Thanks for pointing that out!!

U4R1A

Sun Jul 17 2011, 01:41AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Here are my specs:
- MOT with doubler
- 4.5"X16.5" secondary with 27awg equivalent litz wire
- 4-125A tetrode with adjustable filament voltage via a variac
- Tank cap I've tried (400,650,800,1200,1950,5000)pf currently at 1950pf
- Grid leak cap current at 400pf but I've tried the above combos
- grid leak resistor 0-30Kohm seems to like >25K
- primary coil is 26 turns(12awg) with the last 6 turns are tapped
- grid coil 10-15 turns (22awg) with adjustable height
- screen voltage adjustable 0-8Kohm current at ~6.5K

So I've tried every combo I can think of and I cannot get break out >1cm without reddening the plate. If I crank her up and get the plated glowing I can get ~6 inches but as Herr Zapp pointed out this tube would quickly die. I'm open to all suggestions..

Arcstarter

Sun Jul 17 2011, 05:19AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

It has a *lot* to do with tuning. Have you tried to find the res freq of the tank circuit and compare it with that of the secondary?

Weston

Sun Jul 17 2011, 03:54PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

The 4-125a and all other radial tetrodes usually have a tantalum plate. It can be run to cherry red. If you can see the outline of the screen grid on the outside of the anode, you wanna back it down a bit though. Tantalum anodes actually need to be run red hot to act as a getter. However, you should use air cooling to keep the seal temps down. Nice coil :)

Herr Zapp

Sun Jul 17 2011, 05:17PM

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

U4RIA -

Weston is correct on the plate color of the 4-125.

(From the RCA TT-5 Transmitting Tube manual)

Types 203, 805, 838: "Plate shows no color when the tube is operated at maximum CCS ratings"

Types 802, 804, 807, 813: "Plate shows no color when the tube is operated at maximum CCS or ICAS ratings"

Types 811A, 812A, 828: "Plate shows no color when the tube is operated at maximum CCS ratings, and shows a barely-perceptible red color when the tube is operated at maximum ICAS ratings"

Types 204A, 803: "Plate shows a barely-perceptible red color when the tube is operated at maximum CCS ratings"

Type 810: "Plate shows a barely perceptible red color when the tube is operated at maximum CCS ratings and shows a cherry-red color when the tube is operated at maximum ICAS ratings"

Types 4E27A/5-125B, 808: "Plate shows a cherry-red color when the tube is operated at maximum CCS ratings"

Types 806: "Plate shows cherry-red color when the tube is operated at maximum CCS ratings and orange-red color when the tube is operated at maximum ICAS ratings"

Types 4-65A, 4-125A/4D21, 4-250A/5D22, 4-1000A, 4E27/8001, 826, 834: "Plate shows orange-red color when the tube is operated at maximum CCS ratings"

Type 833A: "Plate shows orange-red color when the tube is operated at maximum CCS or ICAS ratings"

CCS = Continuous Commercial Service
ICAS = Intermittant Commercial or Amateur Service

However, caution must be exercised when attempting to estimate plate temperature by color. The "gettering" action of a tantalum plate works most efficiently between 700 and 1200 deg C, but is reversed at higher temperatures, releasing the contaminants that had been adsorbed by the plate.

Herr Zapp

U4R1A

Sun Jul 17 2011, 06:38PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Well now that makes things very easy. I haven't cranked her up past a slight glow and that would make since why I don't have any output since I've been babying her. I need to get a copy of that "RCA TT-5 Transmitting Tube manual" because I've been hunting for data on this tube everywhere online and couldn't find anything other then the basic data sheet. I'll get some sparks pics posted soon. Thanks for the help.

Arcstarter

Sun Jul 17 2011, 08:04PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Good call Weston

I had a feeling they might have been made to run hot like that because i have seen some people say that, but i sure wasn't going to say anything, because i have also seen someone melt their plate on a bigger tube similar to this. Probably a 4-1000A or something.

But, when you start to yearn for bigger sparks, or at least better efficiency, i still suggest you try to tune the primary in relation to the secondary. I have gotten over 10" sparks with an 811A with a black plate like this, though it probably also had to do with overall duty cycle.

Herr Zapp

Sun Jul 17 2011, 08:54PM

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

U4RIA -

These may help:

Herr Zapp
]4-125a_datasheet_-_eimac.pdf[/file]
]4-125a_datasheet_-_rca.pdf[/file]

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