The AM Forum

I am feeding the grids of my 1625s (two) with -22 volts protective bias through a 8.2 K resistor in series with a 2.5 mH choke. The choke ties to the two grids - the other end of the choke ties to one end of the 8.2 K resistor - the opposite end of the resistor is bypassed with a .001uF cap to ground - the -22 V is fed at the bypassed end.

Since the 2.5 mH choke is a very high impedance - should I bypass at the junction of the 8.2 k resistor and choke also?  Also - it would seem that the 8.2 k resistor need not be a RF type resistor (a wirewound for instance could be used) since it's purpose is to develop the proper operating bias from tube grid current.

The proper operating bias is -90 volts  - so I need another -68 volts - 8.2K x .008 ( 4 mA per tube)  = 66 volts (close enough)

Is this close?   Do I even need the choke?

Pat
N4LTA

If the grids are in push pull, you dont need the rf choke.
If they are NOT in push pull you do.
Bypass the bias after the choke.

More bias is generaly better for AM, I use 2x the normal bias which takes more drive power, but is supposed to give good modulation. its adjustable (big wirewound pot).

Some guys use a diode to provide protection bias, but use 100% grid leak bias with excitation.

Brett

Put a 3-5K 1W carbon across the choke as a swamper to prevent any LF parasitic. Bypass the bottom of the choke with a .02 to .05uF. You could probably do away with the choke entirely, use a 2K carbon instead and bypassed and then the remaining 6.2K after that with more of the same bypassing; a .001 wont do anything at low HF/MF. I dont like RFC's in the grid feed.

Carl
KM1H

edit - KM1H - I wrote this while you were posting - That is what I was thinking


The grids are fed in parallel, so I inject the fixed bias for protection via the choke and bias resistor.

The resistor is calculated to provide -90 volts at the proper grid current - 4 mA for each tube

My question is why not bypass the AC to ground at the end of the Choke?

The choke provides over a 100K of inductive reactance at 40 meters.  I can't see why the bypass should not be at the feed end of the RF choke. Most circuits show it at the feed end of the bias resistor. Right now - I have mine bypassed at both places.

Pat
N4LTA

Isnt that what I just said??

Edit: Never mind, we are leapfrogging each other ;D

Carl

Thanks Carl.

I'm not trying to drive you crazy :)

Pat