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811A tube coil

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LAN_403

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Mon Feb 20 2006, 10:09PM

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

that coil looks good, but it has about 5x the plate dissipation an 811a is capable of running at...

EDY19

Mon Feb 20 2006, 10:43PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

So what needs to be changed to run it safely?

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Tue Feb 21 2006, 12:30AM

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

You can use a smaller tank cap (c3), use a lower voltage in (use a variac to lower the voltage in the mot), use a few tubes in parallel, there are a few 304th's on ebay at ~$10 right now...

EDY19

Tue Feb 21 2006, 12:44AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Maybe a tank capacitance around 500pF or so? I'm only aiming for 1 to 2 inch sparks, and audio modulated. 1 811A should be able to handle this, right? The only reason I don't want to use those 304TLs are "They are old, and the filaments will crack from vibration alone!" What other cheap tubes are there that I could keep a watch for on ebay?

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Tue Feb 21 2006, 01:24AM

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

You should be able to get 2" from a single 811a, but I think you are going to want to run at like .2nf or so... My 811a cil uses .6 and it is almost to much for an 811a on an unrectified mot...

Really you just need to try and see, if the tubes get too hot use less and if they are not even glowing then add more (just remember to retune each time).

I am also concerned that you are not going to get the amount of sound output you want, with my 811a coil giving 5" sparks the 60hz buzz (I have confirmed that the streamers go away and reform completely on each cycle) is not very loud; noticeable across the room but not what I would consider listening level...

EDY19

Tue Feb 21 2006, 01:44AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Mine will be rectified and filtered, so It will be CW, and a voltage greater than yours i believe since yours is only taking the positive half cycle, so maybe the audio will be louder. If not, oh well, as long as i can hear it. Also, If the plates are dissipating too much, can't i just make the grid leak resistor larger to reduce power? I bet a tank cap of 470pF should be good- they are fairly common as well.

EEYORE

Tue Feb 21 2006, 07:34AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

EDY19 wrote ...

Mine will be rectified and filtered, so It will be CW, and a voltage greater than yours i believe since yours is only taking the positive half cycle, so maybe the audio will be louder. If not, oh well, as long as i can hear it. Also, If the plates are dissipating too much, can't i just make the grid leak resistor larger to reduce power? I bet a tank cap of 470pF should be good- they are fairly common as well.

I was wondering the same about the grid leak resistor...I have a large cermaic 10kv 1300pF capacitor that looks so odd....I dont feel like buying a smaller lesser one when i have a good 3inchers right now...My plate gets red within secondas, and im only using a 3.3kohm 10watt grid leak resistor. Will simple ohms law determine what size resistor to choose here?If not, how does one calculate what value to use?

Matt

EDY19

Wed Feb 22 2006, 12:24AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

The smaller the resistor, the more time the tube spends "On" I have heard around 30k should be alright...

EDY19

Thu Feb 23 2006, 02:24AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I'm looking for a transformer to get the required 200+ volts from the audio, and by a couple calculations, I figured if the amp could supply 50W into 2.5 ohms, then the voltage swing would be around 4.5V. So if I get a ~50W mains transformer for 240 down to around 5V, then I should be able to run it in reverse as the audio modulation transformer, right? Or is there a better way- It wil be difficult getting a transformer like this since i live in the US also. Maybe I can wind my own ona +50VA core?

EDIT: Just bought the tube as well as a couple of bypass capacitors- 4kv .01uf ceramics... More to come soon
EDIT2: Just got one microwave, transformer cap and diode- now just 3 more diodes for a full wave rectifier and some more filter capacitors, 1 MOT for an audio transformer and Ill be good to go
EDIT3: Got another microwave from a thrift shop with a little bit smaller MOT, and another capacitor and diode for the bridge and filtering. More to come soon.

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