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GU81M Pentode: care and feeding in a VTTC?

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2Spoons

Tue Jul 31 2012, 12:29AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Finally got my tube! It took a month to get here, so obviously I don't want to break it doing something silly.
It will be used in a VTTC, powered by one MOT (at least initially). No dimensions finalised - its my first coil, its not built yet, so there are no pics or specs yet.

Reading around has given me some insight into safe operation:
- soft start on the filament;
- a bake in period to activate the getter.
- Operation of the plate to a dull red is OK?

Any other advice? Should it be fan cooled? Any special precautions regarding connections?
Any comments greatly appreciated.

Dr. Dark Current

Tue Jul 31 2012, 11:42AM

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

Hi
This tube is pretty much indestructible, if you don't let the grids glow red or the plate yellow...

One or two MOTs half wave doubled are the best power supply for it.

Red plate is all right.

The tube must be fan cooled if you exceed ~250W of plate dissipation. A sideways blow of air is OK, a standard 12cm PC fan wil suffice, or a MO fan. If you have an infrared thermometer, just don't let the envelope to get above 300Â°C, however the tube is rated to survive up to 350Â°C of envelope temperature.

As for driving, connecting all 3 grids together is the simplest method, then you drive it like a triode.

2Spoons

Tue Jul 31 2012, 10:26PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Many thanks for the advice.

PhilGood

Wed Aug 01 2012, 05:28PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Sorry to jump in your thread, I really enjoy VTTC's, I built a 572B VTTC as my first coil, I'm actually working on a GK-71 base fed HF-VTTC, and my next one will be a GU-81M. I saw some interesting points I'm not aware of in this thread, and would greatly appreciate some explanations :

2Spoons wrote ...

Reading around has given me some insight into safe operation:
- a bake in period to activate the getter.

I already heard about this but couldn't find any further explanation. What is the "getter" ? What is this process purpose ? How do you run this "baking period" ? Does this also applies to other tubes like 572B and GK-71 ?

Dr. Dark Current wrote ...

Red plate is all right.

Do you mean the GU-81M can run continuously with red plate ? I yes, does this also applies to 572B and GK-71 tubes (they both have a thick graphite plate) ?
( By "red" I mean something LIKE THIS )

Dr. Dark Current wrote ...

As for driving, connecting all 3 grids together is the simplest method, then you drive it like a triode.

There are different circuits for this VTTC, they mostly differ on grids connections, is the "triode mode" the most efficient one (in terms of sparks lenght) ?

Sorry for this many questions, I'm new to this hobby and still got a lot to learn.

Good luck 2Spoons on your GU-81M VTTC, It's a great tube !

2Spoons

Wed Aug 01 2012, 10:04PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

The purpose of the getter is to absorb any stray gas molecules in the tube. In a lot of small tubes its that dark, shiny, metallic deposit at the top of the tube - usually magnesium IIRC. In the GU81 it is deposited on the outside of the plate, and gets activated when the plate gets hot. Somewhere on this board was a description of how to do the 'burn in', but I've lost it.

Ahh, found it!

3rd post.

Dr. Dark Current

Wed Aug 01 2012, 10:18PM

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

PhilGood, the GU-81M can indeed run with a red plate (450 watt continuous dissipation) and probably even orange plate momentarily (600 watt momentary dissipation), all with forced air cooling of course. The GK-71 to my best understanding is rated to run with a black plate at full dissipation, the plate on this tube is also very close to the envelope, so there is a risk of envelope cracking/melting. It also needs forced air cooling at full dissipation.
The "triode mode" is usually not the most efficient configuration, and on many tetrodes/pentodes it actually works very badly. However on the GU-81M the triode connection works pretty well, probably because of its large spacing of control grid wires. On other tubes where tying all grids together doesn't work very well, it is also possible to drive just the screen and ground all other grid(s), but this needs a large voltage swing on the feedback winding and can possibly overvolt the screen.

2Spoons

Wed Aug 01 2012, 11:35PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

I'm thinking I'll run mine as a pentode: drive the control grid, ground the suppressor grid and bias the screen to 500V as per the datasheet. Its pretty simple, and gives better gain IIRC. I've always felt if you are going to do a thing you should try to do it properly. Research - plan - execute !

LuceÅ¡

Thu Aug 02 2012, 03:11AM

Registered Member #4108 Joined: Sun Sept 25 2011, 11:43PM
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Posts: 149

just as easy as driving it as a triode is connecting the suppressor grid (g3) to the cathode

PhilGood

Thu Aug 02 2012, 07:05AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks 2Spoons for the link. I knew about it, but I didn't know how it was called, lol. I understand the process, but running the tube with orange plate for 3-5 hours seems a bit unfriendly to me.
Isn't the getter supposed to fulfil its role under normal working conditions ? Afterall, these tubes were not meant to be stored for decades before being used when they were built.

And thanks Dr.DC for the detailed answer

Good to know all this...It's true on GK-71 plate is at ~5mm from enveloppe !

Dr. Dark Current

Thu Aug 02 2012, 09:05AM

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

If you bias the screen, make sure you put an amp meter in series with it and do not exceed the maximum current/dissipation. If the coil is supplied from a half wave doubled supply, the voltage on the screen should copy the plate voltage - so supplied from a half wave doubler as well, and in phase with plate voltage.

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