[Amps] 2 x 4-250A Amp

[Tom W8JI]

The filament transformer is 10 volts with no center tap and 
the two five
volt filaments are wired in series. The B- is thru a meter 
to a resistor
across the filaments.The article with the amp suggests the 
bias is for class
B operation.>>


There actually is almost no such thing as class B. Class B 
by definition is 180 degrees conduction angle. That is 
actually too short for linear service unless you are really 
slack about distortion. Most amps CLAIMED to be class B are 
really class AB something. I'd bet money your amp is 
actually AB2, which is a conduction angle of more than 180 
and less than 360 and having grid current.

I want to convert the amp to a tetrode circuit running AB1. 
My first
question is the B- point. I will use the same filament 
transformer and I
want to know if connecting a meter between ground and the 
connection between
the two filaments in series will work OK. I can't see why 
not. But you folks
know a whole lot more about this stuff than I do.>>

Two resistors in series across the 10V series filaments is 
really not a smart idea. Think about what you really have. 
You have the HUM electrical equivalent of simply wiring the 
B- tap point to the mid-point of the two fialments that are 
in series. The problem is the resistance makes the bias 
sloppier so you actually have an electrically worse system, 
but uit does save a choke winding. You might consider 
trashing that resistor idea. It is poorly conceived to do 
that with series fialments. Not a well thought out idea at 
all.

I need to build a power supply and from what I can see, 
using the tetrode
board from IFWTech / G3SEK will make life a lot easier. I 
was planning on
getting the screen bias voltage for the tetrode board by 
taping across the
lower caps in the power supply. Not much current is drawn so 
I don't see
that this will cause a problem. What do you think?>>

I think it is a very poor idea unless the amp has some very 
stout equalizing resistors. In the first place it would be 
very difficult to regulate the screen, and the screen needs 
regulated. It also needs a shunt regulator, or a series 
regulator with a heavy bleeder.  It also needs current 
limited.

Personally I'd never even consider tapping down on a series 
string of electrolytics. It's just asking for problems.

73 Tom