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My GK-71 HFVTTC

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Thomas W

Fri Jul 06 2012, 09:32AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Hey Folks,
this is my project thread for my HFVTTC
(and my first TC Ever!!!!

)
the parts are currently on order, some have arrived already,
im basing it heavly off RateReducers coil but with some twists on it for a permenent base and more pritty wireing

Pictures:
Vacuum Tube:

Feedback Capacitor:

Tube Holder:

Coilform (50W resistor's ceramic tube):

Winding Wire (1.70mm) & Wire for Litz (0.45mm):

Toroidal Transformer (160VA, 0 - 30 0 - 30 to be rewound):

Waiting For:

Resistors (10k, 2k both 20W):

Schematic:

Feel Free to ask questions!

~~~Thomas~~~

kimbomba

Sat Jul 07 2012, 02:54AM

Registered Member #3854 Joined: Fri Apr 29 2011, 03:45AM
Location: Mexico
Posts: 95

Nice tube! how much did you paid for it?

PhilGood

Sat Jul 07 2012, 07:30AM

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

Hello Thomas,

I'm working on the same project ^^ waiting for parts too...

I wouldn't worry about insulation with any transparent/natural plastic. Black plastic should be fine too, but I can't say 100% sure, you never know what they use to dye it (carbon?). You could solder your wires to the tube socket and then run them thru holes in your base so they don't touch the plastic.

Note that you could have problems with your plastic base becomming flabby, the tube socket can become quite hot if you run your coil for long times.

I noticed two errors on your schematic:

- there is no connection between G2 and ground
- GK-71 has a directly heated cathode, in other words filament is the cathode

I would also add a 1 to 5nF few kV cap in parallel with the filament (a blue ceramic disc one is fine).

Your feedback capacitor seems perfect !

Another important point : do you plan to fully rectify your MOT output like RateReducer did ?

I'm sure you will enjoy this VTTC as your first coil

Thomas W

Sat Jul 07 2012, 10:28AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

the schematic is the same one RateReducer did, i got the transformer today and have unwound it to 20.4v on load.. is that ok? its .4v over the rated, also how bad is it if i just turn it on at that voltage, i have no variac for such a small peice, it should be fine yeh?

- yes i am planning on full rectifying it like RateReducer

and as for the directly heated cathode, il just ground one side of the filiment then, hehe

-Thomas Williamson

PhilGood

Sat Jul 07 2012, 02:58PM

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

Tom, about the shcematic, look at yours again : by grounding G2, you just shorten your 3kV power supply with a 12k resistor. Which:
1) is nonsense
2) allows (3000/12000) = 0.25A to flow through your 12k resistor, that would need to be (0.25 x 3000) = 750W rated

!

For the cathode, yes you must ground your filament, but as you can't ground both filament's legs, it's a good thing to add a RF bypass capacitor on the filament so both filament's legs are RF grounded.

Your filament transformer voltage is fine, it's always better to have a little bit more than less. You could add a soft-start circuit, powering your filament through a resistor first, and have a relay that automatically bypasses that resistor after 30 seconds. But I don't think this is necessary with the GK-71, it's filament is not that fragile. Go ahead and enjoy the warm light of your tube

About the full wave rectification, keep in mind one thing : If your MOT has one of it's secondary's output connected to the core (as 99% of MOTs have), you cannot ground your -HV line because there is a ~1.5kV potential difference between mains ground and your -HV line.
There are two solutions:

1) Use a MOT that has two secondary outputs (no secondary connection to core). This is the best solution but these MOT's are quite rare and can be hard to find.

2) Leave the -HV of your circuit floating. In this case you must make sure your filament transformer has at least 3kV insulation between windings. As you rewound yours you must be careful with that. (But I'm not a specialist in homemade transformers and can't give you any further advice on how to correctly insulate your secondary from your primary).

Thomas W

Sat Jul 07 2012, 04:16PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

PhilGood wrote ...

Tom, about the shcematic, look at yours again : by grounding G2, you just shorten your 3kV power supply with a 12k resistor. Which:
1) is nonsense
2) allows (3000/12000) = 0.25A to flow through your 12k resistor, that would need to be (0.25 x 3000) = 750W rated

Hmm, good point... Rate Reducer? how did you combat this issue? XD

PhilGood wrote ...

For the cathode, yes you must ground your filament, but as you can't ground both filament's legs, it's a good thing to add a RF bypass capacitor on the filament so both filament's legs are RF grounded.

Ah, ok.
is this essential to have it like that or cant i just have one side grounded?

PhilGood wrote ...

Your filament transformer voltage is fine, it's always better to have a little bit more than less. You could add a soft-start circuit, powering your filament through a resistor first, and have a relay that automatically bypasses that resistor after 30 seconds. But I don't think this is necessary with the GK-71, it's filament is not that fragile. Go ahead and enjoy the warm light of your tube

Great!, thanks

[quote]
About the full wave rectification, keep in mind one thing : If your MOT has one of it's secondary's output connected to the core (as 99% of MOTs have), you cannot ground your -HV line because there is a ~1.5kV potential difference between mains ground and your -HV line.
There are two solutions:

1) Use a MOT that has two secondary outputs (no secondary connection to core). This is the best solution but these MOT's are quite rare and can be hard to find.

2) Leave the -HV of your circuit floating. In this case you must make sure your filament transformer has at least 3kV insulation between windings. As you rewound yours you must be careful with that. (But I'm not a specialist in homemade transformers and can't give you any further advice on how to correctly insulate your secondary from your primary).
[/1341674298=PhilGood]
I didnt rewind it, i just removed turns until it was right! hehe.

what if i dont ground the core? will that be better? i can put the core inside a seethrough plastic case, that could help

PhilGood

Sat Jul 07 2012, 09:27PM

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

Tom Williamson wrote ...

Ah, ok.
is this essential to have it like that or cant i just have one side grounded?

It's not mandatory, but I remember Steve advising to add it on his VTTC faq page.

Tom Williamson wrote ...

what if i dont ground the core? will that be better? i can put the core inside a seethrough plastic case, that could help

Even without grounding MOT's core, there's a risk of arc-over to MOT's primary.
And you wouldn't need a plastic bag, but 4 ceramic standoff insulators to mount your MOT on.
Furthermore it's really a bad idea to have live large and heavy metal parts in such assemblies, they are meant to be grounded.

PS: For myself, I'm trying to find one of these MOTs with two secondary terminals, if I don't have one when everything else is ready I'll go for the second solution.

Thomas W

Sat Jul 07 2012, 09:32PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

PhilGood wrote ...

Even without grounding MOT's core, there's a risk of arc-over to MOT's primary.
And you wouldn't need a plastic bag, but 4 ceramic standoff insulators to mount your MOT on.
Furthermore it's really a bad idea to have live large and heavy metal parts in such assemblies, they are meant to be grounded.

thats exactly why i said to put it into a plastic box.... keeps it safe, and also for the arc over... hmm i dont really have any idea on how to solve it... eh

PhilGood

Sat Jul 07 2012, 10:17PM

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

Tom Williamson wrote ...

thats exactly why i said to put it into a plastic box.... keeps it safe, and also for the arc over... hmm i dont really have any idea on how to solve it... eh

I explained you in my 2nd post the two possible solutions

(Let me know is something is unclear, sorry my english sometimes sucks, lol, not my native language...)

Thomas W

Sun Jul 08 2012, 01:33PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

yeh, im just trying to work out how i should fix it for myself as i dont think i have much of a chance to find a decent MOT.. i was thinking of a isolation transformer, but all ive really got that would suit this task is a 110v site transformer... mmh im in england too so we only got 240v here hehe

i think il just try, worst comes to worst i kill a MOT hehe

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