Re: [TCML] help with biasing a tetrode for VTTC

[Futuret <futuret@xxxxxxx>]

Well you'll get different answers, but I like to just run the tubes in
parallel.  For 4-400A's I just connect the control and screen grids together and
run them in triode mode although I've been told that can destroy
one of the grids of each tube.  Regarding cooling, a lot depends on how hard
you run the tube, and how long you keep it running.  Normal
grid leak bias works OK.  There's no need to add any extra
biasing.  The usual standard VTTC schematic works fine
for just about any tube as long as the tube plates don't
get too red.  A single plate transformer can drive both tubes,
and a single filament transformer can power both filaments,
provided you obtain the proper 5 volts on the filaments.
I don't even bother with any fans or cooling in my coils although
I keep hearing that the seals can fail without proper cooling.
I guess it's a matter of how long the coil is run.  I don't run
my coils very long especially at high power.  When the
power is turned down, the tubes runs cooler.  

The screen grid can be run from a dropping resistor from
the HV transformer if desired, there should be a capacitor
there too, I forget where it goes... been awhile since I tried
that arrangement.  If the screen and control grids are tied
together, then probably fewer turns will be needed on the
grid coil... I think I used 12 turns.  For regular triodes I usually
needed about 20 turns.  (assuming 23 turns on the primary).

Cheers,
John

 

 


 

 

-----Original Message-----
From: Rick <hyperparasite@xxxxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Tue, Dec 28, 2010 10:04 pm
Subject: [TCML] help with biasing a tetrode for VTTC


I've been studying the websites of Steve Ward and John Freau in an effort to put 

together a VTTC.  The vacuum tubes I have on hand now are tetrodes (4-400A 

style), and I am not clear on what I need to be doing to bias the screen, and/or 

grid.  Perhaps I would have been better off starting out with some triode 

tubes...

 

If I connect two such tubes in parallel, will I be alright with one filament 

transformer matching the electrical specs for the particular tube, or does each 

tube require its own plate transformer and a different connection scheme?  

Pardon my ignorance on the subject.

 

I note that these tubes were intended to be used with an air-cooled socket.  

Any ideas out there on cooling the tubes?  Would a muffin fan be sufficient, or 

do I need a more directed airflow up from the base of the tube?

 

Any advice is greatly appreciated.

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