Hi Mark,
Your suggested meter connection will be fine.
Are you changing from cathode driven to grid driven? You may retain
the current drive configuration and run in AB-1 just by properly
biasing the grids. If you change to grid drive, you'll need to either
neutralize the tubes or swamp the grids with a low value R.
If the bleed current through the existing equalizing R's is
sufficient, say 30ma or greater, you may derive the screen voltage from
that source with a bit of circuitry. You'll need something in the
neighborhood of 600V at 25mA for those tubes, which likely means
tapping up two caps from the bottom. I'd suggest adding a shunt
regulator, made with perhaps a couple zeners and a suitable power FET,
across the lower two caps. And you'll need to change the equalizing
R's to a high value across those last two caps.
73 & Good morning,
Marv WC6W
http://wc6w.50webs.com/
****************************************
-- "Mark Kachel" <kachels@wi.rr.com> wrote:
I bought a home brew amp with 2 x 4-250A tubes on ebay. Nicely built
but it
is grounded grid.
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.
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.
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?
Mark, NØOKS
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