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In the Forum: Melquiades Amplifier
In the Thread: Bias help
Post Subject: The R5 is the key.Posted by Romy the Cat on: 4/13/2018
Gintaras,
yes, the 10.5mA was a bit too little for gas
tube, 21mA. If you look at the stability of voltage regulation graph for these
tubes then you will see an ark with 15-20ma being atop.
I do not understand why you keep asking me R13
& R14 connected to ground? Of cause they must be connected. If they not
then the whole bias supple will not work properly. They are loading resistors
and bias supple filtration work loaded to them. If they are not connected to ground
then your input choke filters are loaded only to the gas tubes which draw too little
current. As you see the same is done in case of R23. The reason for it that the
inductance of input choke that I use must be tuned to the passing current, it
call minimum inductance. If the current and instance do not tune then choke will
not go provide voltage filtration and will not provide a proper current regulation.
Google about “input choke minimal inductance”.
The current draw at the first stage is not good. If you have
~ 200V at plate, the 193V is fine, then you are fine. You want the
tube pass ~ 30mA. If you have less then very slightly change you R5 resistor. Your
-4.4v at bias is a bit too low, the schematic shows 3.4V and anything up to 4V
is fine. Try to change the tube itself, might be you get a faulty one that does
not develop current.
It is little stage and the voltages and numbers in
the schematic are very accurate. Dafter six month of experimenting with it 20
hours a day I built 2 pairs of full range Milqs and a pair of 6 channels Milqs,
I always hit the targeted voltage with no adjustments. Well, of course if I did
everything right, which was not always the case. Anyhow, ground your R13 &
R14, check everything, change the driving tube and if you still have no 25-30mA
of current then change R5Rerurn to Romy the Cat's Site