Quick Question on the Hybrid-A Schematic

[KyleJ]

It may be drawn a little off but...

Is the tap for the power tube biasing just taken off of one side of the PT primary? (The same tap as one of the diode triplets?)

Just want to verify as I'm building a twist of a revision of a tweak of a Dumble clone-ish thing with input from iBob.

[sgp]

I think it's showing a 50v tap for that purpose, If you don't have that tap you can use the Higher voltage tap with a bigger series resistor to drop the volts down.

[KyleJ]

The PT I'm using is a 300-0-300 @ 150mA. It has no additional tap for a second voltage on the primary.

I think this is roughly correct...

Bump the cap up to 300V or better.

Replace the 1.2k with 245k, replace the 15k with a 30k.

With the bias adjust centered I get 35k/285k = .1228. Then multiply by the 300V and I get roughly 36.84V.

Poke me with a stick and let me know if I'm right or wrong.

Thanks

[Bob-I]

The PT I'm using is a 300-0-300 @ 150mA. It has no additional tap for a second voltage on the primary.

I think this is roughly correct...

Bump the cap up to 300V or better.

Replace the 1.2k with 245k, replace the 15k with a 30k.

With the bias adjust centered I get 35k/285k = .1228. Then multiply by the 300V and I get roughly 36.84V.

Poke me with a stick and let me know if I'm right or wrong.

Thanks

Sounds about right, although you want the bias range to go up to about -50V so make sure you calculate this into the equation. -36V is about the low side of what you'll need.

Make sure you get a pair of 1ohm 1% resistors and ground the cathodes through tese. You can then measure the bias current across these resistors.

(p.s. don't place a mouser order for 2 resistors, I have some if you need them)

[KyleJ]

If I go with a 130k with the 10k pot and the 15k that's already there I can get a range of 32.1 to 57.7 volts.

Thanks