Mixed Bag of Questions

[ChrisM]

Working on an amp design and have a bunch of semi unrelated questions...

I have a PP output stage consisting of two 5840 submini tubes. How do I determine what primary impedance I should set my OT at?

I am working on a cathodyne PI. Looking at the Tweed 5E3 schem ( http://kilback.net/homebrewtweaks/amps/ ... _schem.gif ). We see the grid leak is not tied right to ground but instead to a node at the cathode. What is the purpose of the two cathode resistors in series and typing the grid leak to the node? I am thinking the two resistors kinda act like a voltage divider and the gird leak resistor is "elevated".

How do I calculate the resistance needed to drop a certain amount of voltage in my dropping string. I know this is just basic Ohms law but I don't know my current draw. I know my B+ and how much voltage I want to drop though.

How do you determine the fuse rating on your PTs primary?


Thanks!

[soma_hero]

Check this out:
http://music-electronics-forum.com/t15326/

I'm doing a similar submini project and needed to figure out grid bias voltage and plate-to-plate load. I'm using 5899s, which have pretty much the same ratings as the 5840 you're using. 260k plate resistance, 16.5mA max cathode current. Different plate curves though.

At 150V, an idle plate current, Ipq, of 5mA is about %70 dissipation. Looking at those plate curves, at 150V and 5mA and line could be drawn to the 0V grid line right at the knee of that curve. Looks like about 25mA and 60V.

RL, load resistance, is (Vpq-Vpm)/(Ipm-Ipq). So (150-90V)/(25-5mA). This gives us a RL of 4500ohm. Multiply by 4 for push pull output and that's a Zpp or 18K. So you want an 18k primary on your output transformer.

[ChrisM]

Check this out:
http://music-electronics-forum.com/t15326/

I'm doing a similar submini project and needed to figure out grid bias voltage and plate-to-plate load. I'm using 5899s, which have pretty much the same ratings as the 5840 you're using. 260k plate resistance, 16.5mA max cathode current. Different plate curves though.

At 150V, an idle plate current, Ipq, of 5mA is about %70 dissipation. Looking at those plate curves, at 150V and 5mA and line could be drawn to the 0V grid line right at the knee of that curve. Looks like about 25mA and 60V.

RL, load resistance, is (Vpq-Vpm)/(Ipm-Ipq). So (150-90V)/(25-5mA). This gives us a RL of 4500ohm. Multiply by 4 for push pull output and that's a Zpp or 18K. So you want an 18k primary on your output transformer.

Thanks for working that out, that cleared things up.

Is the 150V no load B+? My no load B+ is 170V.

This would make the Zpp 16K I think.

EDIT: the 90V is your calc should be 60V I think.

[soma_hero]

Yep you're right. 90V is the difference in 150 and 60. And according to the charts, 150V is the loaded voltage I believe. I bet your supply will come down towards 150V. It's all estimated anyway so you'll be fine.