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" 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, NĜOKS _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ________________________________________________________________________ Try Juno Platinum for Free! Then, only $9.95/month! Unlimited Internet Access with 1GB of Email Storage. Visit http://www.juno.com/value to sign up today!