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Registered Member #3974
Joined: Mon Jun 27 2011, 06:50PM
Location: Germany
Posts: 23
I am currently working on a tetrode based VTTC using a armstrong oszillator. I am trying to modulate the output, but not using a Statico, insted i am going to try using Grid2 on the tetrode as the modulator input into the VTTC cerket. I'v got a tube that can handel the current necesery to drive the grid and at the rated voltage of the grid.
So if anyone can tell me if this will work or not, or how i can chang the scematic to modulate the VTTC.
And im not going to use a statico. i wana use vacuum tubes.
Registered Member #3974
Joined: Mon Jun 27 2011, 06:50PM
Location: Germany
Posts: 23
I know, thats what i dident want. i wanted to modulate the thing using the vacuumetube itsself. i dont want to use a MOSFET for that. my plan was to use a crud vacuume tube amp to modulate the grid.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Use the control grid for feedback as usual, and then vary the screen grid's voltage to modulate it. A vacuum tube preamp is a good idea. Using the mosfet like Dr Kilovolt suggested usually sounds pretty rough, but a proper vacuum tube preamp will probably sound near as good as a typical vacuum tube amp driving a speaker.
Here is a schematic (halfway down) that uses a vacuum tube preamp for audio modulation: You will indeed need to use 2 tubes for the preamp, because a tube large enough to drive the screen on your QB3/300GA will require more voltage on it's grid than a typical audio source outputs.
Registered Member #3974
Joined: Mon Jun 27 2011, 06:50PM
Location: Germany
Posts: 23
my idea was to use a pcl84 audio tube, it is basikly a power pentode and a pre amp triode. thats how i wanted to do it. but the scematic looks good. i will try it i think.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The screen grid of a power tube will need several tens of Watts input power. A good solution would be to connect the screen through a power resistor to the screen supply and modulate it through a dc block capacitor from a less powerful audio amp, with just enough output voltage swing to produce the desired volume.
Registered Member #3974
Joined: Mon Jun 27 2011, 06:50PM
Location: Germany
Posts: 23
or i could just build a more powerfull amp. maby using the pl504 tube. or i culd use your method. by the way the screengrid suply is the same one used for the amp itsself. its a 15A Bridg Rectifigher. putting out about 220V DC (i hope) the input will be 220V AC
Registered Member #3974
Joined: Mon Jun 27 2011, 06:50PM
Location: Germany
Posts: 23
i think im going to try to build a litle audio amp based on the pcl84 tube, its got a pre amp stage and a amp stage. its going to be simoler to this amp: and use it with the method sugested by Dr. Kilovolt
the main reson for this is that the amp that is shown in the link by arcstarter, i got the tubes but it is to komplicated.
what wuold be a good resistanc to use in the screen grid system? and what wattag? i gues i could calkulat it from the datasheet but i dont have time for that right now. so what value shuld i use and what wattag?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I just looked at that tube and it's not that big. If you rectify 220 Vac, you get something like 320 Vdc, which looks ok for the screen. I would try a ~4.7k ~10W resistor for the g2. You can use a MOT for the plate supply.
Registered Member #3974
Joined: Mon Jun 27 2011, 06:50PM
Location: Germany
Posts: 23
actualy i was going ot use a Mot with a fullwave voltage duboler, that is varid using a internal variacc (in rev. 2). for now im using my 2KW variac to vari the plate voltage.
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Audio Modulated Tetrode VTTC
