AL1200 Bias voltage

[357]

The suffix numeral on this class of tube indicates the amplification factor or "mu". That's the name of the greek alphabet character that looks like an upside-down lower-case "n".

It's the ratio of plate voltage to require 1 Volt of negative grid bias to cut off anode current. The 3-500Z is shown with a mu of 160, by way of comparison. The higher this number, the better the tube would behave with zero bias.

Eimac uses the numeral 7 for this suffix when a tube has enough voltage gain to use for zero bias. The specs above show a mu of 200 for the "D7" version of the tube. Any Eimac "D3" tube was meant for pulse amplification, or as a power oscillator. Not enough gain for an RF amp.

The one pic of a "D3" shows just what the correct one looks like, but don't buy one of these to use in a linear. It just won't behave right.

73

Happy new year Nomad.
I seen the D3 mulpitle of 24 vs 200 on the D7. the guy wants $1500 for the tube. once the "medium MU* sunk in, I kinda figured it wont work.


My amp is in "unknown" condition when I bought it and I just wanted to go over it. check the bias, do the caps to be safe. I did notice a bit of a dip in the trapizoid and I thought of giving it a bit more idle current. I did notice the cw/ssb switch is flaking out.

I also notice the a7 is obsolete so theres not alot of choice for that socket.
It does 1500 with 65w in all day so I guess I'm just bored.
I'll probably replace the switch and be done with it.

All the voltages are +volts. unless I didnt notice the 25v was negative when unkeyed

 

[linearone]

I wish the 3cx1500d7 was available easily and cost effectively.